Program Element #1
3.1 The Organisms
Rationale: The negative impacts of HABs reflect not only the growth and metabolism of individual algal cells, but the ecological selection of those cells within a diverse phytoplankton assemblage. Studies at the organismal level are essen tial if we are to understand the population dynamics of HABs.
The impact of harmful algal blooms (HABs) is a function of the growth and metabolism of individual algal cells ecologically selected from a diverse phytoplankton assemblage. Growth is a general term reflecting photosynthesis, nutrient uptake and assimilation, and numerous other metabolic processes within cells. The inherent growth characteristics of species are geneti cally determined, but the realization of growth potential is often controlled by external environmental factors.
There is considerable diversity among HAB species with respect to strategies for growth and bloom formation in natural systems. Some cause harm at relatively low cell concentrations (e.g., DSP can occur with only a few hundred Dinophysis cells per liter), but in other cases, population growth of HAB species results in a monospecific bloom at high concentrations (e. g., a red tide). There are numerous explanations for that type of growth and accumulation, and many are rooted in the unique physiology of the organisms involved. For example, it has long been argued that production of toxins or other exudates allows some species to outcompete co-occurring organisms (e.g., Pratt, 1966) or to deter grazing (Huntley, 1982; Huntley et al., 1986; Ives, 1987). Practical demonstrations of these mechanisms are few, however.
Another survival and growth strategy involves the benthic resting stages of many HAB species. These cysts or spores provide a recurrent "seed" source or inoculum for planktonic populations, and this characteristic may be a critical factor in determining not only the geographic distribution of species, but possibly their eventual abundance as well (Anderson and Wall, 1978; Anderson et al., 1983). Some HAB species are motile, and under certain environmental conditions their swimming behavior may result in formation of high-density patches (e.g., Kamykowski, 1974; Cullen and Horrigan, 1981; Franks, 1992). Diel vertical movement by motile cells in a stratified environment undoubtedly has functional significance, for example, maximizing encounter frequencies for sexual reproduction, minimizing grazing losses, and allowing cells to obtain nutrients at depth and light at the surface.
These diverse issues can be incorporated into the following goal for the Organisms program element of ECOHAB:
Goal: To determine the physiological, biochemical, genetic, and behavioral features and mechanisms of harmful algal species that influence their bloom dynamics, general ecology, and impacts.
3.1.2 Research Agenda
The following issues are considered high priorities in organismal research within ECOHAB:
There is a need to rapidly and accurately identify, enumerate, and physically separate HAB species from mixed phytoplankton assemblages.
Positive identification and enumeration of specific algal species in discrete field samples collected over large temporal and spatial scales is a labor intensive, but necessary process for the characterization of HABs. A common problem in research and monitoring programs focused on HAB species occurs when the species of interest is only a minor component of the planktonic assemblage. Many potentially useful measurements are simply not feasible because of the co-occurrence of other organisms and detritus. Studies must thus rely on tedious microscope counts to enumerate the target species, and measurements of toxicity or other physiological parameters are generally not possible for just the species of interest. Another constraint arises from the difficulties in adequately identifying and distinguishing between species or strains which are morphologically simi lar. (see Box 3.1.2) Considerable time and effort are often required to identify a particular species when its distinguishing characteristics are difficult to discern under the light microscope. Such fine levels of discrimination are not generally feasible in monitoring programs or other studies which generate large numbers of samples for cell enumeration.
At present, the time lag between sample collection and the identification and counting of specific organisms severely limits our ability to follow the population dynamics of HAB species in real-time. As a consequence, we are limited in our ability to predict when potentially harmful organisms may develop in areas where they might pose a threat to public health or wildlife. Once species identification is automated and/or greatly accelerated, population data can be collected that is compatible with high-frequency measurements of chemical and physical oceanographic features.
We need to:
1. Collect, isolate, and maintain a wide range of HAB species in unialgal and/or axenic cultures.
2. Characterize key species using standard microscopic methods (e.g, light, epifluorescence, and electron microscopy as appropriate).
3. Characterize key species using molecular genetic methods (e.g., RFLP, RAPD, gene sequencing).
4. Develop molecular probes and application strategies for use in field and laboratory settings, and make these tools broadly available.
5. Detect and quantify toxins produced by HAB species using bioassays, HPLC, immunoassays, and receptor binding assays; refine those techniques for routine use on field and culture samples.
6. Develop optical sensors able to distinguish taxon-specific features such as pigments.
Approach and Technology. The problem of uncertain and slow identification can be addressed by cross-disciplinary investigations that utilize a spectrum of techniques to distinguish between species, strains of single species, and toxic and non-toxic forms. Culture collections, especially those including multiple strains of key species, are essential to this effort. A variety of identification techniques should be supported, as future applications of rapid detection methods for HAB species will likely employ multiple probe types. (see Box 3.1.2) These new technologies should be actively pursued, but traditional systematic or morphological investigations using standard microscopy or biochemistry should also be supported. The tradi tional methods are well-established, but considerable effort is needed to develop the species-specific probes and the methods to use them in a rapid and precise manner. Results from this aspect of the program will greatly accelerate progress in several other ECOHAB activities, especially those involving large-scale field programs.
Probe technologies can also be used to detect the toxins (rather than the cells) in environmental samples (see Box 3.1.3). This type of application has the potential to provide rapid and accurate information on toxin levels and distribution that can be highly useful to resource managers.
There is a need to identify the life history stages of HAB species, to determine what factors control transitions between those stages, and to establish the role of each stage in bloom dynamics.
Many marine phytoplankton species produce cysts or spores during their life histories, and these resting stages can have a significant impact on many aspects of HAB phenomena (Anderson and Wall, 1978; Anderson et al., 1983). Cyst or spore germination provides the inoculum for many blooms, and the transformation back to the resting state can remove substantial numbers of vegetative cells from the population and be a major factor in bloom decline. Cysts are important mechanisms for population dispersal, they permit a species to survive through adverse conditions, and since sexuality is typically required for their formation, they facilitate genetic recombination (Wall, 1971). They can even be important sources of toxin to shellfish and other benthic animals. Clearly, all investigations of the ecology and bloom dynamics of HAB organisms must be based on a thorough understanding of an organism's life history, as well as the factors that regulate the transitions between dormancy and a vegetative existence.
Unfortunately, the state of knowledge about resting stages and life histories is neither complete nor uniform for the many HAB species. For several (e.g., Alexandrium spp., Heterosigma carterae, Pfiesteria piscicida ), the existence of resting cysts has been documented. For many others, however, no life history information is available. The prevalence of life-cycle stages among other HAB species is not well known, and factors triggering transitions are poorly defined. Recognizing and determin ing the role of these stages in bloom initiation, growth and termination is critical to our understanding of HAB phenomena.
We need to:
1. Develop culture techniques which simulate in situ growth conditions sufficiently well that life history transitions can be induced and characterized.
2. Isolate and culture many HAB species, and support the maintenance of HAB culture collections. The importance of maintaining multiple isolates of individual species must be emphasized here, given the genetic diversity observed in regional populations of HAB "species."
3. Develop molecular probes that can assist in the identification of life history stages in natural samples.
4. Incorporate studies of resting cyst or spore distribution, abundance, and dynamics into field investigations of HAB bloom dynamics.
Approach and Technology. A combination of laboratory and field studies is required to determine the complete life histories of HAB species and to elucidate the factors that regulate transitions between life stages. Here again, culture collec tions of multiple HAB species are necessary, and multiple studies are required given the diversity of species represented by HAB organisms. Some technique development effort is needed, such as in the design and application of probes to identify life history stages of a target organism and refinement of culture techniques to permit full-cycle life history transformations to occur in the laboratory. Otherwise, methodologies are in-hand for these studies.
It is essential to understand the physiological responses of HAB species to differing environmental conditons.
The manner in which HAB species respond to a changing environment determines their survival and growth. These re sponses are governed by the physiological requirements and tolerances of each species for environmental variables such as nutrients, light, temperature, and salinity. All HAB species must be characterized with respect to these tolerances if we are to understand and predict their distribution and occurrence in natural waters. For example, an extraordinary bloom referred to as "the Texas brown tide" has persisted in the Laguna Madre for five years (Buskey and Stockwell, 1993). There are multiple potential explanations for this dominance, one of which is that the causative species out-competes other phytoplankton for essential resources. In addition to possible selection on the basis of temperature or salinity tolerances, the brown tide alga cannot use nitrate as a nitrogen source. This unique nutritional strategy may be fundamental to the success of the species in that system, but further experimental studies are clearly needed to document how this might be occur ring.
The brown tide in Texas is but one example of the need for information on physiological responses of individual HAB species to their chemical and physical environments. The same can be said for virtually all important HAB species. Why are Alexandrium blooms in the southwestern Gulf of Maine tightly linked to a coastal current of low salinity water (Franks and Anderson, 1992a)? Do the cells grow faster in that water mass due to its unique macro- or micro-nutrients? How is Gymnodinium breve, a red tide dinoflagellate from the Gulf of Mexico, able to survive during transport for 1000 km or more around the Florida peninsula and up the southeastern coast of the U.S. to North Carolina via the Gulf Stream (Tester et al., 1991)?
Experimental approaches to organismal physiology must include the following:
1. Establish new clones of key HAB species representing their entire geographical range.
2. For multiple toxic and non-toxic clones of HAB species, determine tolerance ranges and optima for growth and toxin production in response to a suite of environmental variables.
3. Conduct classical steady-state analyses of nutrient requirements and uptake rates for key HAB species.
4. Compare laboratory results with data from mesocosm and field investigations.
Approach and Technology. Species-specific physiological data can be most easily derived from experiments with unialgal cultures under controlled conditions. However, clones of a single species typically exhibit marked variation in numerous characteristics, including growth and toxin production (Maranda et al., 1985; Bomber, et al., 1989; Cembella et al., 1987; Hayhome et al., 1989; Anderson et al., 1994). Since no single isolate can be considered to be representative of a regional population, growth studies are needed for multiple strains to define the extent of genetic variability and environmental plasticity. Several laboratories in the United States have initiated "syndrome-based" culture collections of harmful marine microalgae, and these collections should be supported and exploited in this respect. As discussed below, techniques for some of the above physiological studies are not uniformly accepted. It may be necessary to convene a small working group to standardize approaches and protocols for these investigations.
In situ measurements of the rates of photosynthesis, growth, and nutrient uptake are essential for understanding the dynamics of HABs, as are assessments of the physiologial condition of cells at different times and locations.
This issue epitomizes a unique and challenging feature of HAB studies that separates them from more traditional process -oriented oceanographic investigations. Many techniques are available to assess the biological rate processes and biomass of planktonic communities (e.g., 14C-fixation, chlorophyll), but there are few methods suitable for determining growth or uptake rates or physiological condition of an individual species when it occurs in a mixed population and does not dominate the phytoplankton assemblage. Considerable methods development is thus required to fully address this autecological characteris tic of ECOHAB.
Effort is needed in the following areas:
1. Investigate the physiology, biochemistry, and molecular biology of specific processes to identify "diagnostic indica tors" for physiological condition.
2. Develop methods to estimate in situ rates of growth, photosynthesis, and nutrient uptake for HAB species.
3. Calibrate these methods carefully, and then apply them aggressively to field populations.
Approach and Technology. The necessary studies can build on molecular and biochemical techniques developed to assess growth rate (Dortch et al., 1983; Chang and Carpenter, 1991; Lin et al., 1994) and a suite of physiological processes within cells such as nitrogen fixation (e.g., Currin et al., 1990), nutrient uptake or limitation (Berdalet and Estrada, 1994; see Box 3.1.3) and photosynthetic activity (Orellana and Perry, 1992). The objective of these studies will be to develop analytical methods and diagnostic indicators that can be applied to individual cells. The more traditional bulk analyses work on commu nities rather than species. For some HAB organisms, it will be necessary to couple the above methods with identification probes and flow cytometry or cell imaging techniques to measure species-specific characteristics.
Although recent technological advances are encouraging in these fields, there is a clear need for an initial methods devel opment and calibration effort within the ECOHAB framework. The ultimate goal is to apply these techniques in ecological studies.
It is essential to know the nutrient requirements, uptake, and assimilation characteristics of HAB species.
Nutrient limitation of phytoplankton growth is a fundamental factor that places a limit on the accumulation of biomass and may determine the outcome of competition among species in mixed assemblages. It is often suggested that increasing inci dences of harmful algal blooms in coastal waters are related to changes in nutrient loading from human activities (e.g., Smayda, 1990), so verification of this linkage would have important societal implications. These nutrients can stimulate or enhance the impact of toxic or harmful species in several ways. At the simplest level, toxic phytoplankton may increase in abundance due to nutrient enrichment but remain as the same relative fraction of the total phytoplankton biomass (i.e. all phytoplankton species are affected equally by the enrichment). Alternatively, there may be a selective stimulation of HAB species by pollution. This view is based on the nutrient ratio hypothesis (Smayda 1990) which argues that environmental selection of phytoplankton species is associated with the relative availability of specific nutrients, and that human activities have altered these nutrient supply ratios in ways that favor toxic or harmful forms. For example, diatoms, the vast majority of which are harmless, require silicon in their cell walls, whereas other phytoplankton do not. Since silicon is not abundant in sewage effluent but nitrogen and phosphorus are, the N:Si or P:Si ratios in coastal waters have increased through time over the last several decades. Diatom growth in these waters ceases when silicon supplies are depleted, but other phytoplankton classes (which include most of the known toxic species) can proliferate using the "excess" nitrogen and phosphorus.
This concept is controversial, but is not without supporting data. A 23-year time series off the German coast documents the general enrichment of coastal waters with nitrogen and phosphorus, as well as a four-fold increase in the N:Si and P:Si ratios (Radach et al., 1990). This was accompanied by a striking change in the composition of the phytoplankton community, as diatoms decreased and flagellates increased more than ten-fold. As coastal communities and countries struggle with pollution and eutrophication issues, the implications of these concepts are profound and clearly deserve further investigation.
A few measurements of nutrient uptake kinetics and cell nutrient quotas of HAB species (e.g., dinoflagellates and raphidophytes) suggest that they have high nutrient requirements, indicating that they would be able to proliferate only in high nutrient environments (e.g., Eppley et al., 1969; Caperon and Meyer, 1972). Relatively few HAB species have been investigated in this context, however. Nutrient uptake and growth rate kinetics and nutrient quotas for HAB species must thus be determined to predict their growth response relative to other species. In addition, some supposedly autotrophic phy toplankton species appear to utilize dissolved organic nutrients (Cembella et al., 1984; Taylor and Pollingher, 1987) while others rely on mixotrophy to supplement their carbon requirements (Sanders and Porter, 1988). If confirmed in HAB species, these nutritional strategies may confer a competitive advantage over other phytoplankton.
Nutrient studies within ECOHAB should focus on the following:
1. Determine nutrient uptake and growth kinetics for HAB species under a range of environmental conditions. Depend ing on the species, this information is needed for N, P, Si, Se and Fe at least, but other micronutrients may need to be considered.
2. Assess the prevalence among HAB species of unique nutritional strategies such as osmotrophy and mixotrophy.
3. Develop and optimize culture techniques for fastidious HAB species.
Approach and Technology. The hypotheses that changing nutrient ratios can influence competition dynamics and that HAB species have nutrient requirements different from other phytoplankton species can be assessed using a combination of nutrient kinetics and manipulative experiments with cultures and natural populations. Several approaches have been used to derive nutrient kinetic parameters, and there is the need to standardize experimental protocols before embarking on comparative studies among HAB species. Moreover, there is no general agreement on the merits and limitations of using batch versus continuous culture methods for nutritional requirement studies. As a result, it may be necessary to convene an ECOHAB community working group to address standard experimental approaches.
The functional role of toxins and/or exudates produced by HAB species is not known.
Toxin production is a wide-spread, but not universal, characteristic of HAB species. The functional roles suggested for toxins are: 1) as deterrents to grazers (Box 3.3.1); 2) as allelopathic compounds that restrict the growth of co-occurring algal species; and 3) as storage products. It may well be that toxins are secondary metabolites with no physiological function. Field observations suggest that some fish and zooplankton avoid dense concentrations of HAB species (Huntley, 1982) and labora tory studies indicate that toxic species can be rejected by grazers (Sykes and Huntley, 1987; Ives, 1987). These studies are limited to a few species and are only a beginning. They certainly have not addressed the diversity of grazer-algal relationships necessary to evaluate the role of toxins in natural populations.
We need to:
1. Conduct laboratory and field studies to determine if there is differential grazing on toxic versus non-toxic species.
2. Determine the effects of toxic algae on ecologically significant grazers.
3. Evaluate the allelopathic activity of exudates and toxins of HAB species.
Approach and Technology. This work will depend upon a supply of appropriate isolates, our ability to manipulate them in culture, and the availability of sensitive and reliable methods of toxin analysis. In general, techniques are available to pursue this important line of investigation, although advances in 3-dimensional video analysis of grazer behavior when presented with HAB species can provide new and relevant insights.
It is essential that we define the genetic basis of toxin production, elucidate toxin biosynthetic pathways, and determine how toxin accumulation in cells is regulated.
Toxin production is a distinguishing characteristic of many HAB species. However, the prevalence of toxin synthesis among these organisms is continuously being re-evaluated. For some species the toxins are well described (e.g., saxitoxins, domoic acid, brevetoxins; Hall et al., 1990; Shimizu, 1993; Falconer, 1993), although non-toxic strains or sub-species have been documented for several such taxa (e.g., Yentsch et al., 1978; Smith et al., 1990). Other species such as Heterosigma carterae and Pfiesteria piscicida are known to produce toxins, but the actual compounds remain uncharacterized (e.g., Burkholder et al., 1992; 1995). Still, other species are considered likely to be toxigenic based on an association with events such as fish kills, but their toxicity has yet to be confirmed. Our lack of information about this basic trait for many HAB species limits our ability to ascertain the nature and extent of HAB impacts or to evaluate mechanisms underlying trends in HAB incidence.
While relatively few organisms have been examined, the available data suggest that the amounts and forms of toxin con tained in a cell vary with its physiological status. For the saxitoxins, environmental factors such as nutrient concentration and temperature can influence the expression of individual toxin derivatives (Hall, 1982; Anderson et al., 1990a,b). This is an important ecological consideration, because various derivatives can differ markedly in their potencies (Oshima et al., 1989). Production of algal toxins can also be modulated by co-occurring bacteria (Bates et al., 1995), and in certain cases, bacteria themselves represent autonomous sources of phycotoxins (Kodama et al., 1988; Doucette and Trick, 1995).
Nutrients have a clear and significant influence on the production of toxins by some algae. In species from a variety of taxonomic groups producing different toxins, cellular toxin content varies dramatically during nutrient starvation in culture. For example, the abundance of saxitoxins in Alexandrium species can vary by more than an order of magnitude depending upon whether phosphorus or nitrogen is limiting (e.g., Hall, 1982; Boyer et al., 1987; Anderson et al., 1990). Likewise, domoic acid production in Pseudo-nitzschia species varies with silicate availability (Bates et al., 1991; Bates and Douglas, 1993), and Chrysochromulina polylepis, the chyrsophyte responsible for massive fish and invertebrate mortalities in Sweden and Norway in 1987, has been shown to be more toxic when phosphorus is limiting (Edvardsen et al., 1990; Granéli et al., 1993).
These and other demonstrations of the effects of nutrient availability on toxicity have major implications with respect to our efforts to understand the manner in which HABs are influenced by, and impact their environment. There are many unknowns remaining, however, as studies to date have only demonstrated the nature of the linkage between nutrients and toxicity, and then only for a few species. Biochemical and cellular mechanisms remain to be elucidated, as does the extent to which the nutrient limitations that alter toxicity are actually occurring in natural waters. Without more detailed information about the physiology of toxin production for a wider range of HAB species, it is very difficult to assess the ecological role of toxins in population and community dynamics (see sections 3.2 and 3.3).
The biosynthetic pathways for the production of several toxins have been described to the extent that elementary "building blocks" have been identified (Shimizu et al., 1984; Douglas et al., 1992; Box 3.1.4), but in no case have complete pathways, including all intermediates involved, been elucidated. Isolation of the enzymes involved in toxin synthesis or interconversions is also at a very early stage of development (Sako et al., 1995). At the most basic level isolation of the genes and enzymes directing the production of algal toxins remains an important but elusive goal.
Organismal studies of toxin production and its genetic control will need to include the following:
1. Determine the prevalence of toxin production among HAB species in culture and the time-varying concentrations of toxin at different stages of growth.
2. Determine the linkage between bacteria and toxin production.
3. Isolate and purify poorly characterized or unknown toxins and determine their chemical structures.
4. Elucidate toxin biosynthetic pathways and characterize the genetics and regulation of toxin production.
5. Determine the nutrient assimilation and partitioning pathways which permit toxin synthesis, and determine the factors which influence toxin production at the cellular level.
Approach and Technology. A number of well-established experimental approaches are available and appropriate to address patterns of toxin production, typically involving the growth of an HAB species under a suite of environmental conditions and monitoring the manner in which toxicity varies. The isolation of toxin genes/enzymes is a critical first-step toward identifying the actual mechanisms underlying environmentally-induced toxin variability. Additionally, studies at the molecular level will allow us to evaluate the intracellular trade-offs between toxin production and maintenance of "normal" cellular metabolism, ultimately leading to a clearer understanding of "why" it might be ecologically beneficial for toxigenic organisms to synthesize toxins. Technological strategies for implementing these studies will include: classical laboratory culture methods, application of toxin probes, assays and analyses; biosynthetic feeding/label-incorporation experiments; standard chromatographic techniques; and, methods incorporating mutagenesis and gene expression protocols.
We must understand the importance of motility and other behaviors of HAB species.
Some motile HAB species exhibit directed swimming behavior such as vertical migration or orientation towards prey, as has been demonstrated for the ambush dinoflagellate, Pfiesteria piscicida which is capable of detecting and swimming towards its preferred food. Vertical migration is thought to be a response to light, salinity, nutrient gradients, and even gravity (e.g., Holmes et al., 1967; Eppley et al., 1968; Kamykowski, 1974; Cullen and Horrigan, 1981). This mechanism is fundamental to the population dynamics of many motile HAB species, as it can result in dense concentrations of cells that affect grazing losses, light harvesting, nutrient availability, and encounter frequencies for sexuality. The aggregation of cells is also directly related to the scale of the adverse impact from the blooms. Non-motile algae are also capable of orienting themselves vertically by changing their relative buoyancy.
We need to:
1. Conduct vertical migration studies at a variety of scales, from tube cultures to mesocosms to natural populations.
2. Characterize the influence of environmental variables such as salinity, light, and nutrients on these behaviors.
3. Refine models of swimming behavior and examine how different strategies interact with physical features such as pycnoclines, fronts, or internal waves.
Approach and Technology. A combination of laboratory and field investigations is necessary to address the significance of these behaviors. Much of the technology required for such studies exists, but new approaches such as fine-scale sampling techniques, (e.g., Donaghay et al., 1992) will be needed to measure biological, chemical and physical parameters with appropri ate resolution. Rapid detection and enumeration techniques for HAB species will be required (see Box 3.1.2), and mesocosm strategies will be important as well.
The Organism program element of ECOHAB reflects the fundamental importance of physiological, genetic, and behavioral studies in an initiative designed to develop an understanding of the population dynamics and trophic impacts of harmful algal species. We take it as a given that studies of HAB blooms require a thorough understanding of genetic variability and regulation, nutritional and environmental tolerances and responses, behavioral adaptations, life history transforma tions, toxin physiology and function, and numerous other processes and features that will vary among HAB species. Given the diverse array of HAB species in the U.S. and the many different environments in which they occur, this program element will likely be dominated by small research programs conducted by individual investigators or small teams. Focused, multi-investigator proposals are also envisioned on specific issues where coordination and comparisons between organisms would be beneficial. In many cases, the technology exists for the studies that are proposed, but a focused methods develop ment effort will greatly accelerate progress on numerous other ECOHAB elements. For example, rapid and automated detec tion and enumeration of HAB species using probe technologies would eliminate a major constraint to field programs namely the time required to process cell count samples collected at spatial and temporal frequencies similar to those for hydrographic and chemical parameters. Such techniques would also make cell counts and even the physical separation of HAB species from co-occurring organisms fast and accurate, permitting measurements that otherwise would not be possible. Prioritization within the research issues highlighted above was not attempted, as the list already reflects an effort by work shop participants to include only the most important and timely research topics.
Rationale: Concurrent with escalating influences of human activities on coastal ecosystems, the environmental and economic impacts of HABs have increased in recent decades. It is therefore imperative to know if present trends of human activities and HABs will lead to unacceptable consequences, and if the means can be developed to mitigate impacts. The key to this knowledge is an understanding of the ecology and oceanography of harmful algal blooms. An important facet of this complex topic is environmental regulation, that is, the influence of environmental factors on the population dynamics of harmful algal species and their competitors.
The geographic range, persistence, and intensity of HABs are determined by both physical and biological factors. For example, the initiation of a bloom requires successful recruitment of a population into a water mass. This may result from excystment of resting cells during a restricted set of suitable conditions (e.g., Alexandrium in the Gulf of Maine; Anderson and Keafer, 1987), transport of cells from a source region where blooms are already established (e.g., Gymnodinium catenatum in northwest Spain; Fraga et al., 1988), or exploitation of unusual climatic or hydrographic conditions (e.g., Pyrodinium bahamense and ENSO events in the Indo-West Pacific; Maclean, 1989). Once a population has begun growing, its range and biomass are still affected by physical controls and nutrient supply. Physical controls include long distance transport of populations (e.g., Franks and Anderson, 1992a), accumulation of biomass in response to water flows and swimming behavior of organisms (Kamykowski, 1974; Cullen and Horrigan, 1981), and maintenance of suitable environ mental conditions (including temperature and salinity, stratification, irradiance, and nutrient supply; Whitledge, 1993). Aspects of nutrient supply include not only the amount of macro- and micronutrients, but also their ratio and mechanism of supply. Thus, physical forcings, nutrient supply, and behavior of organisms all interact to determine the timing, location, and ultimate biomass achieved by the bloom, as well as its impacts. The first goal of the Environmental Regulation program element of ECOHAB is thus:
Goal: To determine and parameterize the environmental factors that govern the initiation, growth, maintenance, dissipation and impacts of HABs.
Physiological responses and life histories of HAB species are varied, as are local and regional physical environments where HABs occur. Thus there is considerable variability in the relationship of HABs to their environment. In spite of this complexity, however, it is usually possible to elucidate the patterns underlying recurrent blooms in an area. Generalizations to other regions is not usually appropriate, however. An understanding of the relationship between an HAB species and its physical and biological environment is critical to predicting environmental and economic impacts and to the formulation of mitigation strategies to minimize those effects. Since it is impractical to study all of the ecosystems in which HABs occur, a second goal is:
Goal: To formulate principles that explain similarities between ecosystems during HABs and to understand how these systems are unique with respect to the types of blooms that occur.
3.2.2 Research Agenda
The following section outlines specific issues defining high priority field, modeling and experimental studies required in the Environmental Regulation program element of ECOHAB.
To what extent do HABs reflect increases in growth rate versus physical transport and immigration? Is there a specific suite of physical factors associated with many HABs?
Physical factors in the environment influence HAB population dynamics both directly by moving and aggregating cells, and indirectly by influencing the cells' physical and biological environment. Factors that can influence the population dynamics and physiology of phytoplankton include: nutrients, temperature; salinity; irradiance; stability of the water column; turbulent mixing; currents; vertical advection, dispersion or dilution; wind stress; bottom stress and bathymetry. Temperature, salinity and irradiance directly influence growth rate, physiology, and in some species, behavior (e.g., Watras et al., 1982; Tyler and Seliger 1981). High shear associated with turbulent mixing may alter growth, behavior, and in extreme cases induce mortality (e.g., Pollingher and Zemel, 1981; Thomas and Gibson 1990; Berdalet 1992). Water motions (a complex interaction of most of these factors) determine the losses and gains from advection as well as losses to dispersion (Kamykowski 1979, 1981). In addition, water motions determine in large part an alga's ability to exploit light and nutrients. Finally, stratification of the water column allows weakly swimmming algae to interact with current shear and thereby drastically alter immigration rates. The success of individual HAB species is associated with different hierarchies of these influences, and we expect these associations to vary among ecosystems.
There are a wide variety of flow regimes in which HABs occur, from well-mixed estuaries to upwelling regimes and highly stratified river plumes. Among these diverse settings, vertical and horizontal transport processes play important roles in regulating bloom development, although the physical mechanisms and rates may vary considerably between environments. Characterizing transport processes in these stratified and/or spatially non-homogeneous regimes represents a challenging basic research problem. Our progress in understanding HABs thus requires advances in coastal physical oceanography. Some of these unresolved physics problems that have an important bearing on HABs include:
Turbulence and Vertical Transport. While there are many turbulence closure models that parameterize the effects of stratification (e.g., Martin, 1985), such models may not apply across the range of conditions found in coastal environments. A particularly difficult problem is the mixing in stratified shear layers removed from boundaries, such as those found at the base of buoyant plumes. The rate of vertical mixing must be accurately quantified and related to measurable mean properties in order to interpret observations and develop models of HABs. Vertical transport processes associated with upwelling, frontal processes and secondary circulations may be comparable to (or more important than) vertical mixing. These motions facili tate vertical exchange, but they also play an important role in the aggregation of plankton.
Horizontal Dispersion Processes. As is the case with parameterizing vertical mixing, the rate of horizontal dispersion is difficult to quantify at the scales relevant to HABs. In estuarine environments, there have been a number of attempts to relate flushing rate to easily measurable physical parameters. There has been some success in this regard, but there remains consid erable room for improvement in parameterization of flushing. In coastal settings, Okubo's (1971) mixing diagrams still pro vide the benchmark for estimation of small-scale horizontal exchange. Dispersion theory has advanced considerably with the contributions of Young et al (1982) with respect to shear dispersion and Zimmerman (1986) for chaotic dispersion. However, there are few observational studies that provide the requisite measurements of small-scale velocity variation required to turn these theoretical ideas into estimates of horizontal dispersion. Given recent progress in measurement of small-scale variations of velocity (e.g., Geyer and Signell, 1992; Prandle 1991), there is great potential to make substantial progress on this important research area, and one mechanism for such study would be through ECOHAB.
Buoyant Plumes. HABs are frequently observed in association with buoyant plumes (Therriault et al., 1985; Franks and Anderson, 1992a; Tester et al., 1991). While the bulk characteristics of buoyant plumes are well established, the details of the velocity and density structure and their variability are not adequately understood to explain the transport, aggregation and dispersion of algal cells within plumes. The strong vertical and horizontal shears occurring within plumes result in a complex advective regime that may concentrate cells within the front or disperse them, depending on the interaction between the relative motion of the organisms and the flow field. Secondary circulations associated with winds, planetary rotation and topography also contribute to the complexity of the flow within these environments. Numerical models simulating theoretical coastal buoyant plumes under surface wind stress have been formulated (e.g., Chao, 1987), but no one has yet incorporated behavioral or physiological models of HAB species into such physical models.
Estuarine Circulation. The classic paradigm of two-layer estuarine circulation is a gross oversimplification of time-depen dent and three-dimensional motions in estuaries. Wind-driven motions provide a large perturbation that in many environ ments may dominate the horizontal exchange, and tidal dispersive effects often control small-scale transport and in some cases regulate the estuary-scale exchange. Complex, three-dimensional motions due to the interaction of stratification, tides and winds play an important role in horizontal dispersion and vertical exchange, and they provide critical controls on the spatial distribution of plankton cells. These processes and their interactions are certainly complex, but careful consideration of both physical and biological features can lead to important insights into the population dynamics of phytoplankton in an area. The work of Seliger et al., (1970) and Tyler and Seliger (1978) are noteworthy in this regard.
Each of the physical processes described above is an active area of research in physical oceanography, but much of this work is taking place without reference to phytoplankton populations. With respect to the ECOHAB program, it would be unrealistic to strive for a complete understanding of how each these processes can drive ecosystem response. There are, however, tractable physical problems that can be addressed, and large advances in our understanding can be obtained with collaborations between HAB biologists, physicists, and modelers.
Priority acitivites should be to:
1. Describe and model the dynamics of HABs in relation to their physical environment.
2. Determine how variations in population growth rate and biomass depend on small scale turbulence through its influence on nutrient uptake, grazing, cell division, accumulation, and bloom structure.
3. Determine how the vertical distribution of HAB populations regulates bloom development and dissipation, and how vertical distribution relates to physical (e.g., horizontal and vertical advection, vertical mixing and stability) and biological processes (e.g., motility, buoyancy control).
Approach and Technology. To determine the influence of environmental factors on the development of HABs, it is essen tial to describe their distributions in time and space, and this will require coordinated and multidisciplinary studies. Informa tion on the distributions of physical and biological variables should be synoptic in space (local to regional scales) and highly resolved in time (hours to days) covering successive bloom periods (interannual variability). This level of coverage would be ideal, but it is expensive and probably unrealistic in the context of ECOHAB alone, given the many sites where HABs occur. Excellent results have been obtained, however, with focused field programs that involve significant physical components (e.g., Seliger et al., 1970; Tyler and Seliger, 1978,1981; Franks and Anderson, 1992a). Future collaborative efforts between physical and biological oceanographers should thus be emphasized and encouraged.
There is considerable potential for the use of moored optical sensors in red tide/toxic algae research and monitoring (see Box 3.2.2). When properly designed and calibrated, these sensors measure radiometric quantities that should be particularly appropriate for long time-series observations in coastal systems. That is, unlike measurements of chlorophyll, floristics, and stimulated fluorescence, which are somewhat dependent on equipment and methods, records of irradiance and radiance should be completely comparable over many years, documenting changes associated with eutrophication or remediation, for example. If such observations can be made autonomously and interpreted reliably, they would be ideal for the detection of HABs in coastal waters, even in remote locations. Because some harmful species can exert profound negative effects on coastal resources without dominating the phytoplankton and changing the color of the water, there are limitations to the usefulness of optical instruments for detecting HAB phenomena. Nonetheless, continuous optical measurements in coastal waters would be extremely useful for describing bloom dynamics and long-term trends. Furthermore, with the development of species-identification technologies described in section 3.1.2, it should become feasible to use moorings to obtain long-term records of HAB species distributions and associated environmental variables.
A hierarchy of remote sensing platforms would provide frequent, synoptic, near-surface spatial information (see Box 3.2.3). Aircraft-mounted units are needed to provide high-resolution distributions on local to regional scales (e.g., Millie et al., 1992). Satellite sensors, such as SeaWiFS, will provide lower temporal and spatial resolution, but over regional to global scales. Calibration and deployment of these instruments during HAB events is essential to the development of a remote sensing capability for such phytoplankton blooms.
Shipboard research programs are also essential to the elucidation of HAB dynamics, not only to obtain direct measurements of rates and standing stocks of key components, but also as a means of relating component variability to information from moorings and remote platforms. Field programs aimed at understanding the interactions between HAB species and their physical environment are typically hampered by the enumeration of HAB individuals in a diverse assemblage of phytoplank ton, microzooplankton and detritus. Small, low cost profilers are needed that can be rapidly deployed in the vicinity of HABs to define their spatial structure and temporal evolution. These profilers can be deployed with self-contained CTD/fluorom eter/transmissometer/optical backscatter packages for measurement of key physical parameters that may directly or indi rectly control bloom development.
Analytical and numerical models are important tools for studying physical-biological interactions in the ocean. Models have been used for decades to understand how physical forcings influence the distribution and production of HAB species. New archi tectures for physical models which incorporate turbulence-closure formulations for the small-scale motions (e.g., Blumberg and Mellor, 1987) now provide important platforms for the investigation of physical-biological interactions over scales of meters to hundreds of kilometers. The incorporation of behavioral and physiological models of HAB species into these physical models is a necessary and important step in elucidating the couplings between nonlinear physical flows and time-dependent biological re sponses (see Box 3.1.6). The formulation of theoretical models investigating the nature of interactions between physical flows and organism behaviors must be encouraged.
How do physical and ecological processes control the partitioning of nutrients within a system and the relationship between nutrient inputs and the population dynamics of HAB species?
The availability of nutrients (inorganic and organic) to individual organisms ultimately regulates the growth rate and net biomass of blooms. Physical forcings, such as vertical mixing, stratification or advection can be significant factors in determin ing the availability of those nutrients. It is also clear that the relationship between nutrient inputs and population dynamics is complex and reflects many other, interacting processes. One of the explanations given for the increased incidence of HAB outbreaks worldwide is that these events are a result of increased pollution and nutrient loading of coastal waters. Some argue that we are witnessing a fundamental change in the phytoplankton species composition of coastal marine ecosystems throughout the world due to the changes in nutrient supply ratios from human activities (Smayda, 1990). There is no doubt that this is true in certain areas where pollution has increased dramatically. It is perhaps real, but less evident in areas where coastal pollution is more gradual and unobtrusive. In Tolo Harbor, Hong Kong, human population within the watershed grew 6-fold between 1976 and 1986, during which time the number of red-tide events increased 8-fold (Lam and Ho, 1989). The underly ing mechanism is presumed to be increased nutrient loading from pollution that accompanied human population growth. A similar pattern emerged from a long-term study of the Inland Sea of Japan, where visible red tides increased steadily from 44 per year in 1965 to over 300 a decade later, matching the pattern of increased nutrient loading from pollution (Murakawa, 1987). Japanese authorities instituted effluent controls in the mid-1970's, resulting in a 50% reduction in the number of red tides that has persisted to this day.
As coastal communities and countries struggle with pollution and eutrophication issues, the implications of the trends in Hong Kong, Japan, and other countries are profound. The public, the press, and regulatory officials are concerned about whether this is happening in the U.S. as well, and are asking for predictions and answers about HAB incidence that exceed our present capabilities. Unfortunately, competitive outcomes in phytoplankton species selection and succession cannot yet be predicted, nor can the relative effects of natural versus anthropogenic factors be resolved . A variety of important issues involving nutrients and the manner in which they are supplied to and utilized by HAB species must thus be addressed.
We need to:
1. Determine how changes in the magnitude and elemental ratios of nutrient inputs to coastal ecosystems can influence ecological responses, especially those that favor HABs.
2. Determine whether the frequency and duration of harmful algal blooms are increasing in coastal waters relative to increases in phytoplankton production in general.
3. Investigate the extent to which HABs are indicators of local (point-source) or regional (diffuse input) increases in nutrient loading.
4. Investigate how climatic variability from local to global scales influences the development and dispersal of HABs.
5. Learn whether HABs are indicators of environmental or habitat changes induced by nutrient over-enrichment or other anthropogenic effects (e.g., alterations in freshwater or contaminant inputs).
Approach and Technology. The potential stimulatory influence of anthropogenic nutrient inputs on HAB incidence is certainly one of the more pressing unknowns we face, and will require a focused commitment of resources and effort greatly in excess of that previously devoted to the topic. Time-series analysis of existing data bases for phytoplankton communities and of variables such as major nutrients or pollutants are required. Where such data are lacking, long-term monitoring programs must be initiated in key regions where anthropogenic changes are anticipated. Moored instrument packages includ ing optical sensors and other devices to resolve HAB classes or species within the plankton would be highly effective in this regard. Laboratory studies of the stimulatory effects of chemicals contained in effluents or terrestrial runoff are also needed, as are kinetic studies and other experiments that can quantify the nutritional requirements and uptake capabilities of HAB species (see section 3.1.2 for a more detailed list of approaches to nutrient issues).
Are there specific physical, chemical, and biological regimes or processes that are associated with HAB events? Are some ecosystems more susceptible to HABs than others?
Population dynamics, including the rate processes required in predictive models of harmful blooms, cannot be adequately described or predicted, although this information is of fundamental importance to effective resource management.
Information on bloom dynamics can be gained through laboratory and field studies that define nutrient uptake kinetics, growth rates, loss terms, and life cycle dynamics. While field conditions such as circulation, meteorology, and water chemistry have long been recognized as critical elements in blooms of some toxic species, neither the initial boundary conditions, nor the hydrographic regimes within which harmful blooms occur are clearly understood. The comparative ecosystem approach adopted by ECOHAB will permit common features to be identified, such as coastal currents, upwelling, and nutrient enhance ment of biomass levels. Additional insights will be obtained through numerical modeling efforts. Despite the fundamental importance of predictive models for harmful algal blooms in different regions, no such models exist for U.S. problem species (see Box 3.2.4). The ultimate goal is to couple population dynamics with physical circulation models for a given hydrographic regime, and to refine physically/biologically coupled models using field bloom observations and toxicity patterns.
We need to:
1. Identify environmental and biological cues or characteristics that can be measured and used to predict the onset and magnitude of HABs for the purposes of research and management.
2. Determine biological rate processes and initiate studies of coastal hydrography and water circulation for development of physically/biologically coupled models at temporal and spatial scales appropriate to harmful algal blooms.
Approach and Technology: Here again, shipboard observations, field programs, satellite remote sensing and moored instrument arrays can all provide the level of detail required for the identification of the mechanisms underlying HAB out breaks. The key is to obtain data at appropriate time and space scales for the blooms under study, and this will require careful planning and considerable advance study so that programs are mounted in the proper place at the proper time.
Theoretical and heuristic models that can be used to guide the formulation and testing of hypotheses and to evaluate the causes and consequences of variability in nature should be developed as an integral part of these multidisciplinary field studies. Models are required to represent the broad range of environmental dependencies that contribute to HABs. Since HABs reflect physical and biological dynamics over a broad range of time and space scales, a hierarchy of models will be required. On small scales, models that examine the vertical experience of HAB populations over the diurnal cycle are needed to eluci date cell dynamics. On larger scales, models that examine bloom transport and dispersion are essential. Model dynamics and parameterization must be driven by field and laboratory data that are sufficiently detailed to allow independent testing and corroboration. Such robust models may then have predictive capability. Numerous data sets exist that can be used for testing hypotheses regarding HAB dynamics, but often such data have not been examined in depth nor have they been examined in terms of potential interactions among physical, chemical and biological variables as they relate to HABs. Retrospective analy ses of historical data and information may provide important insights at a relatively low cost.
Often the main limitation of models is the paucity of data available to formulate, force and test them. As discussed above, field programs must be combined with coupled physical-biological models to gain the most from limited resources and to test hypotheses concerning bloom initiation, transport, and patterns of accumulation and dispersal. Much can be accomplished with limited data if it is obtained at the right places and the right times.
The Environmental Regulation program element of ECOHAB addresses factors that act on harmful algal populations and regulate their distribution, abundance, and impact. Despite the diverse array of HAB species and the many hydrographic regimes in which they occur, one common characteristic of such phenomena is that physical oceanographic forcings play a significant role in both bloom dynamics and the patterns of toxicity or adverse impacts. Furthermore, the interplay or coupling between physical variables and biological "behaviors", such as swimming, vertical migration, or physiological adaptation, holds the key for understanding many HAB phenomena. This physical/biological coupling can occur at both large and small scales, and includes processes of great interest to both physical and biological oceanographers.
Understanding the small- and large-scale physics underlying HAB phenomena is a clear priority, but this need should not be restrictive. Observational and modeling studies of physical processes need not be massive in scale, cost, or complexity to provide useful information. Significant insights on HAB dynamics have been obtained from field programs with modest but focused physical components (e.g., Seliger et al., 1970; Tyler and Seliger, 1978; Franks and Anderson, 1992a).
The potential stimulatory influence of anthropogenic nutrient inputs on HAB incidence is a key unknown and time-series analyses of existing data bases are required, as are laboratory studies of the stimulatory effects of chemicals contained in effluents or terrestrial runoff.
This program element will require investigations spanning the spectrum from large-scale field studies to mesocosm and laboratory experiments. Modeling has a major role to play as well. In many cases, the technology exists to address the questions that are asked, but development is needed to permit biological data to be obtained on time and space scales similar to those currently possible with physical and chemical measurements. This challenge is compounded by the need to focus on individual species rather than communities.
The issues highlighted in this program element are entirely complementary to those of the Organism element, and together they outline a direct path toward the goal of understanding HAB dynamics and impacts. Managers must recognize the urgent need for better information about how the environment, and especially how human alterations to the environment, can alter coastal ecosystems and lead to harmful blooms.
Rationale: The negative impacts of HABs are the result of complex interactions that begin at the phytoplankton commu nity level and extend to upper trophic level compartments. Habitat physics, life cycles, community structure, growth and grazing processes all combine to regulate the dynamics of the HAB event. Therefore, studies on the impacts of trophic interactions in the selection and dynamics of HABs, and conversely, the impacts of HAB events on trophic structure, processes and interactions are essential if we are to understand the ecology and oceanography of harmful algal blooms.
Phytoplankton blooms develop through a sequence of stages termed initiation, growth, maintenance and decline. A key to understanding bloom dynamics is the identification of processes leading to transitions between these stages; that is, what factors in the biology of harmful algal species and their grazers lead to changes in growth and loss processes at different phases of the HAB cycle? Of the terms included in the population growth equation given in Box 3.2.1, we consider in this section only the trophic interactions. Specifically, it is imperative that we understand how competitive interactions between harmful algal species and other phytoplankton contribute to the formation of blooms. Likewise, we must evaluate how grazing controls, or fails to control, HAB development. These issues define the first goal of the Food-Web/Community Inter actions program element:
Goal: Determine the impacts of trophic interactions on selection for, and dynamics of, HABs.
Harmful algal blooms involve multiple interactions among predators, competitors and the harmful algal species within an ecosystem. Many routes have been demonstrated by which HABs can impact food-webs (Box 3.3.1), yet little is known about the nature, extent, and ramifications of many of those pathways. We must therefore determine the relative importance of each of these interactions over appropriate spatial and temporal scales. Implicit in this task is the elucidation of the pathways and fates of HAB toxins in the food-web. The mechanisms by which the timing and frequency of HABs (both toxic and high -biomass types) affect community and trophic structure also need to be identified. Progress in these areas is essential to realizing our second goal:
Goal: Determine the impacts of HABs on trophic structure, processes and interactions.
3.3.2 Research Agenda.
The following section outlines specific issues defining high priority field and experimental studies required to establish how trophic interactions regulate HAB species' selection and population dynamics, and how HAB events, in turn, influence community/trophic structure and trophodynamics.
It is essential to know the extent to which bloom formation results from a breakdown of grazing or from harmful species outcompeting other phytoplankton for limiting resources.
Interspecific competition influences HAB dynamics. The presence of co-occurring phytoplankton species reduces the capacity of the environment to support one species' requirements from a common pool of limiting resources (e.g., nutri ents). Species competition coefficients are modified continuously by changes in growth parameters such as temperature, light and nutrient availability, and are further altered by changes in grazing pressure, community structure and allelochemical effects. Enhanced growth or physical accumulation alone does not always explain HABs, as some taxa secrete allelopathic substances that inhibit or stimulate the growth of competing and co-occurring algal species (e.g., Pratt, 1966; Gentien and Arzul, 1990) or inhibit grazing (Smayda, 1992).
Grazing control of HABs depends upon both the local abundance of grazers and their ability to ingest the harmful algal species (Box 3.3.2). Low grazer abundance can be critical in the early phases of bloom development by providing times or regions where grazing losses are less than increases from cell division. Low grazer abundance can result from a variety of external biological factors (e.g., predation on grazers), or physical factors (e.g.,spatial separation of HAB species from benthic grazers). Reductions in grazer abundance may also occur in direct response to an HAB event (e.g., avoidance or mortality induced by HABs; Fiedler, 1982; Huntley, 1982), or as a result of the effects of past HAB events on grazer populations. In cases where grazers are abundant, grazing control may still not be exerted because toxins or small size can reduce the chance that HAB species will be ingested. If harmful algal species are consumed, grazers may be unaffected, impaired or killed. The response of zooplankton and benthic grazers to toxic algal occurrence is often species-specific in terms of behavioral responses and toxin susceptibility.
Grazing control of HABs can also depend on the population density of the harmful alga, as, for example, when suppression of grazing occurs above a threshold concentration of the alga, as demonstrated for the Narragansett Bay brown tide in 1985 (Tracey, 1988). A threshold effect may also occur if the daily production of new HAB cells becomes large enough to saturate the ingestion response of the grazers and the ability of grazers to increase their populations. In that case, population growth can accelerate dramatically (Donaghay, 1988). A breakdown of grazing control has been implicated in the brown tides in Narragansett Bay (Tracey, 1988) and in Texas (Buskey and Stockwell, 1993) and removal/loss of the grazer population has been reported to precede or accompany bloom development (Montagna et al., 1993). There is, however, little information on how the nature of the grazer response influences the timing, magnitude and duration of HABs.
In order to address the issue of trophic influences on HAB formation we need to:
1. Determine the role of differential growth rates, nutrients and nutritional strategies in competitive interactions among phytoplankton species.
2. Determine the nature and extent of allelopathic interactions.
3. Determine the importance of spatial and temporal separation between harmful algal species and grazers, and the relative contribution of pelagic and benthic grazing.
4. Determine the role of harmful algal species' behavior, toxicity and food quality in reducing or avoiding grazing controls, as well as the importance of density-dependent processes (e.g., grazing thresholds).
5. Determine the effects of mixed (toxic/non-toxic) assemblages on grazing control (e.g., does breakdown of grazing only occur once harmful algae become a dominant component of the phytoplankton?).
Approach and Technology. A combination of field, mesocosm and laboratory studies will be required to elucidate the nature and extent of species' interactions and grazing regulation in HAB phenomena. Quantitative data for growth rates and grazing-related mortality rates of harmful algal species are needed, as are measurements of population recruitment rates for both HAB taxa and their grazers. In situ estimates of growth and grazing rates, obtained in the context of sampling programs that define the temporal and spatial variability of an HAB and its potential grazers, are essential for quantifying the role of grazers in controlling HAB dynamics. Mesocosm experiments are needed to determine the patterns and dynamics of interac tions between HAB species and grazers, and the population growth characteristics of competitors in the presence and absence of harmful algal species. Laboratory investigations are required to determine interactions between HAB species and individual grazers, to measure growth and grazing rates on harmful algal species in the presence of "organics" secreted by HAB species, and to elucidate allelopathic impacts, mechanisms and pathways involved in HABs.
Development and/or improvement of the following technologies are required to implement the approaches described above: species-specific molecular probes for HAB identification; methods for in situ detection and quantification of HABs during all bloom phases; diagnostic indicators of grazer physiological status; methods for assessing grazer food quality; video techniques for measuring in situ grazing and avoidance behavior; high-resolution sampling of fine-scale HAB and grazer distributions; and suitably scaled microcosm and mesocosm experimental strategies and systems.
Are biological controls (e.g., grazers, pathogens) the cause of bloom termination?
The role of biological mechanisms in contributing to bloom termination remains largely unknown. In some instances, HAB impacts on grazers are so severe that these organisms may be of little consequence in the termination of blooms (e.g., Bricelj and Kuenstner, 1989; see Box 3.3.1). The occurrence of viral particles in cells of a harmful algal species has also been observed (Sieburth et al., 1988; Milligan and Cosper, 1994), but the efficacy of this mechanism to control natural HABs remains to be demonstrated. In addition, it has been suggested that bacteria may play a role in regulating the population dynamics of HAB species (Doucette, 1995).
Assessing the involvement of biological controls in terminating HABs requires investigation of:
1. The effects of HABs on the grazer community in terms of functional groups and their concentrations.
2. Alternative biological mechanisms for HAB decline and termination (e.g., pathogens).
Approach and Technology. Several approaches, including field studies and mesocosm and laboratory experimentation, are needed to determine how specific biological mechanisms can contribute to the termination of HABs. The majority of tasks described earlier as essential to defining the influences of trophic factors on bloom formation are also required here. In addition, the impacts of pathogens such as viruses and bacteria on HABs need to be assessed in a quantitative fashion in the context of both field studies and mesocosm manipulations. Sampling schemes must take into account the appropriate tempo ral/spatial scales relevant to these interactions, as well as environmental factors potentially regulating the distribution and abundance of the pathogens. When possible, pathogenic organisms should be isolated and examined in the laboratory to provide a mechanistic understanding of their effects on HAB taxa.
Strategies for elucidating factors involved in the formation and termination of HABs overlap considerably. Most of the technologies required to investigate bloom termination are well established in the phytoplankton and zooplankton literature. To evaluate the role of pathogens in terminating blooms, several new techniques must be developed or improved: taxon -specific molecular probes and their application for identifying and quantifying pathogens in situ; sampling methods for concurrently resolving HAB and pathogen distributions over a range of temporal/spatial scales; techniques for measuring negative impacts of pathogens on HAB species; identifying the underlying mechanisms, and assessing the taxonomic specific ity of these effects; and appropriately "contained" experimental microcosm and mesocosm systems.
It is essential that we learn the manner in which the effects of HABs on the food-web are controlled by toxin dynamics, routing pathways, and the differential susceptibility of species at higher trophic levels.
The toxins of HAB species may have evolved to release these species from grazing pressure. Similar anti-herbivore defenses are well-documented in terrestrial plants, but have received scant attention in marine systems. Many algal toxins (e.g., PSP and DSP toxins) are endotoxins that affect planktonic and benthic grazers after consumption. Susceptibility to ingested toxins and, thus, the ability to accumulate toxins, vary markedly within and among taxa (Twarog et al., 1972), as suggested by reports that finfish appear to be more sensitive to PSP toxins than crustaceans or molluscs (Robineau et al., 1991). If the grazing species are not killed, accumulated toxins may be transferred to other components of the food-web and affect other organisms at higher trophic levels. This is an area where our knowledge is rudimentary at best, as subtle, ecosystem -level effects are probably pervasive, affecting many different trophic levels, depending on the toxin involved. Recruitment rates or year-class sizes of important commercial fish species may well be directly affected by brief exposures of larval or juvenile stages to toxic algae.
Zooplankton impaired by ingesting harmful algal species may be more susceptible to predation, and thus may become an important vector for transferring toxins in the pelagic food-web. Alternatively, zooplankton killed outright may sediment and allow toxins to enter benthic food-webs. Zooplankton fecal pellets may also be important sources of toxin to benthic commu nities. Thus, zooplankton can act as vectors of HAB toxins resulting in events such as fish kills (White, 1981; Smayda, 1992). Herbivorous fish can also accumulate and transfer toxins, and even cause mass mortalities of the marine birds that consume them (Work et al., 1993). Mortality of marine mammals linked to trophic transfer of HAB toxins has also been reported (Geraci et al., 1989). During their food-web transfers, toxins may be bioaccumulated, excreted, degraded or structurally modified, as in the case of enzymatic bio-transformation of PSP toxins in some bivalve molluscs (Cembella et al., 1993).
In order to define the role of toxins in mediating the effects of HABs on food-webs, it is necessary to:
1. Identify target species and their life-history stages that are adversely affected by toxic algae, and those that act as vectors of toxin transmission through the food-web.
2. Determine pathways, transfer rates and mechanisms for bioaccumulation, transformation, degradation and elimina tion of algal toxins.
3. Characterize modes of action of various phycotoxins (e.g., neurotoxic, cytotoxic, hemolytic) on marine fauna and determine their differential susceptibility.
Approach and Technology. Field studies as well as supporting laboratory and mesocosm studies using algal toxins as tracers are needed to describe changes in toxin concentrations and transformations of toxins from one trophic level to another. Predictive and heuristic models of food-web transfer of algal toxins should be developed. These models might be analogous to those formulated for anthropogenic contaminants (heavy metals, radionuclides, organic pesticides). A comparative, experi mental approach is also needed to determine dose-dependent behavioral (e.g., swimming avoidance), physiological (e.g., grazing inhibition) and cellular (e.g., toxin inactivation, compartmentalization) responses of marine organisms to toxic algal species.
In order to carry out the research on toxins associated with HABs outlined above, rapid, standardized toxin assays, with detection based on both chemical structure and toxic activity, must be developed to elucidate toxin pathways. Toxin probes (e.g., antibodies specific for individual forms of a toxin), employed in conjunction with these assays, may be required to quantify and localize toxin derivatives in target organisms.
Are chronic, sublethal impacts of HABs more significant than acute (lethal) impacts in altering food-webs or causing trophic dysfunction?
Blooms of harmful algae may be recurrent in some areas (e.g., red tides of PSP-and NSP-producing dinoflagellates in the Northeast U.S. and Gulf of Mexico, respectively), episodic (e.g., 1987-88 Gymnodinium breve red tide in North Carolina estuaries) or, more rarely, persistent (e.g., brown tides in Laguna Madre, Texas). Chronic, sublethal effects of HABs on marine biota have been documented, as in the case of brown tide persistence being linked to gradual reductions in eelgrass and shoalgrass meadows (Dennison et al., 1989). More often, however, it is the effects of brief, but acute blooms that have received the most attention because of their immediate impacts on ecosystems or humans (e.g., Shumway, 1988; Bates et al., 1989; Smayda, 1992; Burkholder et al., 1992). Episodic HABs are often associated with acute, lethal effects on adult stages of commercially important species (Tracey, 1988; Summerson and Peterson, 1990; Burkholder et al., 1992; Taylor, 1993). Re moval of parental stocks may cause recruitment failure of some natural populations with limited dispersal capabilities (Peterson and Summerson, 1992). However, it is more likely that HABs adversely affect recruitment success by exerting sublethal, chronic impacts on reproduction (e.g., reduced fecundity), growth and behavior (Bricelj et al., 1987; Buskey and Stockwell, 1993). These chronic effects, which may have long-term consequences for year-class strength and persistence that are critical in the recovery of natural populations to pre-bloom levels, have received little attention and merit serious consideration.
Characterizing the relative importance of chronic, sublethal versus acute, lethal impacts of HABs on food-webs and their components requires that we:
1. Determine lethal and sublethal effects of HABs on life-history stages of key species in the food web.
2. Identify mechanisms of recruitment failure, or reduction in affected species.
3. Investigate the extent, time frame and mechanisms of recovery of natural populations impacted by HABs.
4. Characterize differential effects of episodic, recurrent and chronic HAB events on food-webs.
5. Identify HAB-induced changes in ecosystem energy/nutrient pathways.
Approach and Technology. Field and mesocosm studies, as well as laboratory experimentation, are essential for evaluating the impacts of chronic versus acute exposure to HABs on food-web structure and trophodynamics. This suite of approaches is needed to determine the species-specific effects of harmful algal species on egg, larval, juvenile and adult stages of target species. Comparisons of HAB impacts are needed both among different habitats and within the same system where HABs occur with different frequencies, duration and intensities. We must quantify the effects of recurrent, episodic and persistent HAB events on recolonization rates of species from representative trophic levels, which vary in their life-history strategy, generation time and susceptibility to HABs. We also need to investigate the impacts of HABs at aquaculture sites, where stocks are concentrated at high densities and are routinely monitored for growth, mortality and disease incidence.
Several technological advances/improvements are needed to augment those currently employed for acquiring the types of information cited above: remote sensing technology and in situ high frequency optical devices to obtain more rapid, efficient measurements of phytoplankton abundance and composition; computerized motion analysis to study behavioral effects; toxin assays and probes, including applications development, for quantifying and localizing toxins in target species.
Are HAB impacts are controlled by the degree of temporal and spatial overlap between blooms and critical life cycle stages of affected species.
The impacts of HABs on sensitive life cycle stages of affected species higher in the food-web depend upon their co -occurrence in time and space, which varies dramatically with the degree of vertical stratification and exchange with surround ing waters. In some well-mixed estuaries and lagoons, HABs may be sufficiently persistent and dispersed throughout the system so that all species are equally exposed, as in the case of brown tides in Laguna Madre, Texas (Buskey and Stockwell, 1993). Such even exposure greatly simplifies the task of relating observed changes to the effects of HAB populations. However, in stratified waters, reduced vertical mixing may allow weakly swimming plankton, including HABs and potentially affected groups (i.e., microzooplankton, macrozooplankton and fish larvae) to form highly concentrated layers surrounded by regions of low or undetectable concentrations of the organisms (Donaghay et al., 1992). In stratified systems exposed to current shear from winds and tides, the distributions of both HABs and target plankton may also vary dramatically in response to lateral advection of layers and interactions between organismal swimming behavior and current shear. Regardless of the prevalent physical system, however, it is highly unlikely that HAB impacts can be predicted from average concentrations of HAB and affected species.
In order to assess the extent to which temporal and spatial factors control the impact of HABs on other species, we need to:
1. Investigate the temporal and spatial coincidence of susceptible life-history stages of key species (e.g., grazers) with HABs.
2. Determine the physical processes and biophysical interactions that control bloom development and grazer responses.
Approach and Technology. Determining the degree to which HAB impacts are regulated by temporal and spatial factors requires evaluation of impacts on affected species from actual time-space abundance measurements. The typical approach has been to estimate responses based on statistically derived average dispersion and abundance measurements, often on an annual basis. Yearly, and probably even seasonal means, are, however, inadequate to assess impacts of HAB species, which often form short-term, high-impact blooms in areas that provide critical spawning/nursery habitat for higher trophic levels. Consequently, we need to determine the small-scale, temporal/spatial distribution patterns and abundances of HAB taxa.
Resolution of co-occurrence effects in stratified waters will require both field and mesocosm studies. Field programs must be aimed at quantifying temporal and spatial scales, as well as in situ concentrations of HAB and co-occurring, affected species. This will require application of high resolution sampling techniques, both to detect changes in abundance on sub -meter scales, and to link those changes to physical structure and processes in the system where the HAB occurs. Such field investigations must be complemented by mesocosm experiments designed to elucidate the underlying mechanisms that lead to observed patterns of co-occurrence or avoidance.
Accurate, fine-scale characterization of the temporal/spatial aspects of HAB interactions with affected species will depend on: adapting spectral optical and video sensors for deployment on vertical profilers or towed systems to detect HAB distribu tions in real-time; designing "smart" sampling systems triggered by these sensors to collect discrete samples for simultaneous identification of and experimentation on HAB and affected species; developing techniques for measuring in situ swimming behavior of motile HAB taxa and affected species; and improving methods for rapid quantification of HAB toxin concentra tions.
Do high biomass (non-toxic) HABs adversely impact the food-web directly through reduced food quality, or indi rectly through environmental effects?
Harmful effects of algal blooms may occur in the form of anoxic/hypoxic events. Such incidents result from increased sedimentation of organic matter coupled with enhanced microbial decomposition of phytoplankton on the bottom, and/or via transient increases in water column respiratory demands of the phytoplankton (Box 3.3.3; Falkowski et al., 1980). Mass mortalities of benthic fauna associated with these events are widespread and affect a broad range of taxa (e.g., Swanson and Sindermann, 1979), but their connection to HABs is often circumstantial or speculative. High microalgal biomass and result ing light attenuation are also known to cause marked declines in biomass and distribution of seagrasses. Both effects have been noted in eelgrass and shoalgrass communities exposed to picoplanktonic brown tides in New York and Texas waters. Reductions in irradiance levels may also induce shifts in macrophytie species' composition toward less desirable forms. Blooms of picoplanktonic microalgae show enhanced light-scattering properties and are thus particularly likely to cause these light-related effects, yet the prevalence and magnitude of the problem have not been adequately characterized. Because seagrasses provide an important nursery habitat for many commercially valuable shellfish and finfish species as well as their associated fauna, issues related to these habitats are of special concern.
Nutrient-mediated macroalgal blooms can also lead to the decline of seagrass as well as coral reef ecosystems (LaPointe and O'Connell, 1989). In addition, high biomass HABs may limit growth and recruitment of grazers if the dominant algal species is poorly predated upon due to its unpalatability, small size and indigestibility, or because of physical impairment of feeding (e.g., Bass et al., 1990). High algal densities per se may also interfere with food uptake and utilization by many suspension feeders. Sublethal effects of food quantity/quality, which remain poorly understood (Donaghay, 1985), are potentially important deter minants of recruitment success in grazer populations.
Evaluating the direct and indirect effects of high biomass HABs on food-webs requires that we:
1. Determine the relative importance of oxygen depletion from HABs in the water column vs. surface sediments.
2. Understand the extent and ecological consequences of light attenuation from HABs, including relative effects on phytoplankton, epiphytic algae, seaweeds, seagrasses and their supporting fauna.
3. Investigate the effects of HABs on food quality available to consumers (e.g., via changes in size spectra, chemical composition).
4. Define the mechanisms and threshold levels at which high algal biomass interferes with food capture and utilization by grazers.
5. Understand the controlling mechanisms for restructuring marine communities during recovery.
Approach and Technology. Elucidation of high biomass HAB impacts on food-webs will require a multi-faceted approach, including field studies, experimental mesocosm manipulations and laboratory investigations. Field work should be aimed at quantifying organic sources and rate processes within the water column and in sediments to allow development of an oxygen budget. This will involve continuous monitoring of dissolved oxygen and irradiance levels at spatial and temporal scales relevant to HABs. Assessment of macrophyte and faunal coverages, as well as seasonal successional patterns, before and after bloom events are likewise needed, and should incorporate synoptic aerial mapping of the macrophyte communities. The effects of changes in food quantity and quality must be evaluated in field and mesocosm experiments to determine their impacts on ingestion, growth and reproduction for critical life stages of target species. Mesocosm studies are required to assess the conse quences of removal or perturbation of target species on food-web structure and processes. Laboratory and mesocosm experi ments will provide insights into the mechanisms and linkages between high biomass HAB events and associated food-web /habitat responses.
Implementing the variety of studies needed to characterize the effects of high biomass HABs on food-webs will rely on: remote sensing approaches to aid in determining the occurrence and distribution of HAB events and high biomass coverage; moored instrumentation for continuous monitoring of impacted vs. non-impacted sites; techniques for automated, simulta neous sampling of the diel changes in vertical structure of light, oxygen, phytoplankton (distinguishing HAB taxa from other phytoplankton) and vertical mixing; regional geographic information system data bases for high-biomass blooms; computer ized predictive models for oxygen deficits from HABs, relating water-column dissolved oxygen, BOD and algal abundance with sediment BOD and algal abundance; improving available methods for assessing food quality, both biochemically and by comparison to other known quality foods.
As algal toxins move through marine food-webs, they can have a broad spectrum of effects on marine organisms in inshore, offshore, pelagic, and benthic habitats (Box 3.3.4). The scope of these effects, resulting from both chronic and acute exposure to the toxins, has become more evident in recent years, since a wide variety of animals are now known to accumulate biotoxins and act as intermediate vectors to consumers at higher trophic levels. Algal blooms can also have harmful effects not related to production of toxins, such as overgrowth and shading by seaweeds, oxygen depletion of the water column from high biomass blooms, fish mortalities from over-stimulation of gill mucus production, and mechanical interference with filter -feeding structures. The Food-Webs/Community Interactions program element of ECOHAB recognizes the diverse nature of these processes, and highlights key areas for focused investigation. What is needed is a recognition by managers and regula tory officials that harmful algal bloom impacts extend far beyond the obvious manifestations of poisonous shellfish and dead fish, and include subtle, sub-lethal effects that can alter or even destroy ecosystems through time. Identifying such impacts and determining their extent and magnitude is a significant challenge for ECOHAB scientists.
This program element also emphasizes research in the other direction the effects of grazers and other organisms on the harmful algal blooms, since in many cases, the bloom reflects the supression or absence of grazing. This again is an area of obvious importance to the dynamics of HABs, but one which has received only rudimentary study thus far.