Sea Surface Temperature image with the Coastal Mixing and Optics experiment site indicated
(click on image for better resolution)
Principal Investigator: Heidi M. Sosik and Robert
J. Olson
Sponsoring Agency: Office of Naval Research
The Coastal Mixing and
Optics ARI Program is an oceanography program to study the mixing of
ocean water on the continental shelf, and the effect of the mixing on the
transmission of light through the water. An experiment was done in 1996-97
southeast of Montauk Point, Long Island.
Sea Surface Temperature image with the Coastal Mixing and Optics experiment
site indicated
(click on image for better resolution)
Long Term Goals
The long-term goals of our component of the Coastal
Mixing and Optics project are to develop a better understanding of
the relationships between upper ocean optical properties and particulate
and dissolved seawater constituents, and to determine how these relationships
are influenced by physical processes. Specific long term objectives
include both predicting and modeling optical variability relevant for biological
processes, such as phytoplankton photosynthesis, and retrieval of information
about the biomass and activity of plankton from optical measurements.
Objectives
Spatial and temporal variability in particulate and dissolved material
is a significant source of optical variability in the upper ocean.
The primary objective of the present work is to examine the interaction
between physical processes and the properties, abundance, and optical significance
of different particle types in coastal ocean waters. Specific project
objectives are to refine individual particle measurement methods and develop
approaches to using individual particle results for interpretation of both
inherent and apparent bulk optical properties (IOP/AOP). The project
comprises a combination of instrument development and field studies in
coastal waters of the eastern US continental shelf.
Approach
The approach we have taken employs techniques for characterizing and
assessing the optical properties of particles, using both in situ
and ship-board instrumentation and both bulk and single particle methods.
Our primary tools are flow cytometry for assessing individual particle
light scattering and fluorescence properties, spectrophotometry for measuring
bulk dissolved and particulate absorption spectra (including separation
of phytoplankton pigment absorption from the bulk absorption via methanol
extraction), and spectral underwater radiometry. Our goal is to conduct
flow cytometric and spectrophotometric measurements both on discrete water
samples and with in situ instruments. In situ measurement provides
the opportunity for relatively unperturbed sampling, with generally greater
spatial resolution, while analysis of discrete water samples continues
to allow more detailed characterization of optically-active seawater constituents.
We have employed our sampling methods during the Coastal Mixing and Optics
(CM&O) field study in continental shelf waters south of Cape Cod, MA
(40o 30' N 70o 30' W).
Work Completed
With the completion of the Coastal Mixing and Optics field experiment,
effort during the last year has been focused on processing and interpretation
of the observations collected in summer 1996 (R/V Seward Johnson cruise
9610) and spring 1997 (R/V Knorr cruise 150). Our primary emphasis
has been on characterization of optically active particles, assessment
of absorption and scattering properties of particulate and dissolved material
(including size dependence for particles), and examination of apparent
optical properties. We have completed description of the general
differences between the summer and spring periods (Sosik et al. 1998) from
both a hydrographic and optical perspective. These results have been
presented in a variety of forums and we are proceeding with manuscript
preparation for peer-reviewed publication.
In addition, we have been continuing to work on particle characterization, based primarily on flow cytometric measurements. During the past year, we have refined our estimates of particle size distributions and reprocessed our entire data set from CM&O. This complete reprocessing included estimation of non-phytoplankton particle concentration and size. Our efforts have also encompassed investigation of theoretical and empirical considerations for interpreting individual particle light scattering and chlorophyll fluorescence.
Results
August/September 1996 - In late August, the water column was consistently
stratified with a persistent subsurface maximum in the concentration of
phytoplankton pigments (see Fig. 1) which was associated with peaks in
absorption and scattering coefficients for particles (Fig. 2). There
were corresponding subsurface peaks in diffuse attenuation coefficients
at blue-green wavelengths. Concentrations of picophytoplankton and
nano/microphytoplankton showed strong vertical dependence, with the smallest
cells highly abundant (>105 cells ml-1) at depths
just above the pigment maximum and the larger cells present throughout
the upper 30 m, but at approximately 10-fold lower levels. These
conditions were dramatically disrupted by the passage of hurricane Edouard
through the study site during the first days of September. Upon our
return to the site after the hurricane, pigment and phytoplankton cell
concentrations had fallen precipitously and optical properties were mainly
dependent on resuspended particulate material.
April/May 1997 - During the spring cruise, stratification was weaker
than during the previous August and some mixing or advective events occurred
during the first half of the cruise (see Fig. 1). In contrast to
the first cruise, picophytoplankton abundances were very low and maxima
in phytoplankton pigment concentrations and cell abundance were found in
the surface layer. This vertical distribution of phytoplankton was
associated with surface layer maxima in absorption, scattering and diffuse
attenuation coefficients (Figs. 1 and 3). The latter half of the
sampling period was characterized by weak but persistent stratification
which was associated with a phytoplankton bloom in the surface layer, with
increases in particle absorption and scattering and in diffuse attenuation.
Interestingly, the period of highest pigment concentration during this
period (days 126-128) was associated with relatively low phytoplankton
cell concentrations.
(click on image for better resolution)
Figure 1. Time series of depth profiles observed during intensive
sampling conducted at 40o 30' N 70o 30' W during
late summer 1996 (left panels) and spring 1997 (right panels). Panels
top to bottom: Density, chlorophyll a + phaeopigment concentration (data
provided by Dr. Collin Roesler, U. Conn.), diffuse attenuation for downwelling
irradiance at 490 nm, picophytoplankton (genus Synechococcus) cell concentration,
nano/microphytoplankton cell concentration.
(click on images for better resolution)
| Figure 2. Time series of absorption coefficients for particulate (ap) and soluble (as) material and scattering coefficients for particulates (bp), all at 440 nm. Data were collected with in situ instruments deployed from a ship at the central experiment site during late summer 1996. Data credit: Scott Pegau and Ron Zaneveld, OSU. | Figure 3. Time series of absorption coefficients for particulate (a-p) and soluble (as) material and scattering coefficients for particulates (bp), all at 440 nm. Data were collected with in situ instruments deployed from a ship at the central experiment site during spring 1997. Data credit: Scott Pegau and Ron Zaneveld, OSU. |
Optically active particles were found to be highly variable during the
experiment. Some of the most dramatic changes were associated with
passage of the hurricane, which resulted in a decrease in abundance of
picophytoplankton and an increase in larger (>2 microns) cells (Fig. 4).
Before the hurricane, non-phytoplankton particles were major contributors
to particle volume only near the bottom, but afterwards they dominated
in the chlorophyll maximum layer as well. The non-phytoplankton particles
were smaller after the hurricane, with few particles > 10 microns present.
In the spring, picophytoplankton were much less abundant than in summer,
and there was a bloom of >10 microns cells at the surface.
Non-phytoplankton particles in the upper water column were more abundant
in the spring than in the pre-hurricane summer samples, but the spring
surface bloom of large phytoplankton cells was not reflected in an increase
in other large particles.
(click on image for better resolution)
Figure 4. Selected examples from >1000 size distributions of
phytoplankton and non-phytoplankton particles estimated from flow cytometric
measurements of forward light scattering by particles in 5-ml water samples.
Time series of mean phytoplankton properties show diurnal variations associated with cell growth and division patterns and larger scale changes related to a combination of physical processes and physiological acclimation (Fig. 5). The largest and most highly pigmented cells were usually found below the mixed layer in summer, while in spring they sometimes occurred near the surface. The cells which reached highest abundance (see Fig. 1) were relatively small (low scattering cross-section) and had little pigment (low fluorescence). Towards the end of the sampling period, bulk pigment concentrations declined slightly as cell concentrations increased; mean cellular fluorescence and forward scattering cross-section declined during this period. We are currently exploring the significance of these changes in particle properties for interpreting optical signatures during this period.
(click on image for better resolution)
Figure 5. Mean scattering cross-sections (forward angles ~3-20o)
and chlorophyll fluorescence per cell for nano- plus microphytoplankton
as measured by flow cytometric analysis.
Analysis of inherent and apparent optical properties observed during
the CM&O experiment has revealed that particles played a dominant role
in determining water column optical properties. This is shown clearly
by comparison of variations in diffuse attenuation (Kd) with
absorption coefficients (Green et al. 1998, Figs. 6 and 7). Absorption
by dissolved material (as) exceeded that of particles (ap)
at ultraviolet wavelengths; at visible wavelengths, however, as was usually
lower and much less variable with depth and time than ap.
Interestingly, as was somewhat more variable under the stratified
conditions present in late summer (Fig. 7) when values were consistently
lower in the surface mixed layer compared to the rest of the water column
(Sosik et al. 1998).
(click on image for better resolution)
Figure 6. Relationships between diffuse attenuation for downwelling
irradiance (Kd) and absorption coefficients for dissolved (as)
and particulate (ap) material at three wavelengths during August-September
1996. Kd estimates were derived from irradiance measurements
collected with a tethered free-fall profiler (Satlantic SPMR) and the absorption
data was collected using ac-9 sensors (WetLabs) mounted on the OSU Slowdrop
profiling package. Data were averaged in 10-m bins and Kd and
absorption profiles collected within two-hour windows were used.
Linear regression results show the importance of dissolved material at
412 nm and the strong dependence of Kd variations on ap at 443
and 488 nm.
(click on image for better resolution)
Figure 7. Relationships between diffuse attenuation for downwelling
irradiance (Kd) and absorption coefficients for dissolved (as)
and particulate (ap) material at three wavelengths during April-May
1997. Data were collected and processed as described for Fig. 1.
Linear regression results show strong dependence of Kd variations
on ap, with even less contribution of dissolved material than
observed under conditions of greater stratification in late summer the
previous year (Fig. 1).
Based on flow cytometric analysis, it is possible to examine the optical role of different types of particles in greater detail (DuRand et al 1998). We have extended conventional flow cytometric measurements of marine particles, which has focused exclusively on phytoplankton, to include quantitative examination of other particles. This broad class of “non-phytoplankton” material presumably includes particles of organic detritus and of mineral origin, as well as heterotrophic organisms. We have enumerated and estimated optical properties of three types of particles: 1) picophytoplankton of the genus Synechococcus, 2) eukaryotic phytoplankton (~2-30 microns) and 3) other particles in the ~1-30 microns size range.
Results of flow cytometric analysis show that non-phytoplankton particles
dominated numerically in both late summer and spring and were less variable
in space and time than the phytoplankton. In late summer before hurricane
Eduoard, Synechococcus abundances were very high with a strong subsurface
maximum present (> 105 cells ml-1), while in spring
these cells were 10-fold less abundant (DuRand et al. 1998, Sosik et al.
1998). Eukaryotic phytoplankton did not differ in abundance between
the two seasons, until the bloom at the end of the spring sampling period
when concentrations increased 3-4 fold (Fig. 8).
(click on image for better resolution)
Figure 8. Time series of particle concentration observed during vertical
sampling in spring 1997. Results for three general particle classes
are shown, Synechococcus (top panel), eukaryotic phytoplankton (middle
panel), and other particles in the same size range (bottom panel).
(click on image for better resolution)
Figure 9. Time series of relative contributions of different particles
to forward light scattering observed during vertical sampling in spring
1997. Results for three general particle classes are shown, Synechococcus
(top panel), eukaryotic phytoplankton (middle panel), and other particles
in the same size range (bottom panel).
Particles were found to have substantial differences in their particle
specific optical properties that affected the overall contribution to bulk
water column optical properties. This is evident when relative contributions
to light scattering are considered; the significance of both Synechococcus
and the non-phytoplankton particles decreases relative to their numerical
abundance (compare Figs. 8 and 9, Fig. 10). In the spring, in particular,
the importance of eukaryotic phytoplankton often exceeded the other particles
in surface waters, but decreased rapidly with depth.
(click on image for better resolution)
Figure 10. Summary of abundance and integrated forward light scattering
for different types of particles sampled by flow cytometry in spring 1997.
Mean +/- standard deviation is shown for three time periods and three particle
types: Synechococcus (red), eukaryotic phytoplankton (green) and other
particles in the same size range (black).
Current work on these topics is focused on deriving absolute (rather than relative) estimates of particle scattering cross-sections and estimating backscattering contributions. Preliminary results investigating differences in spatial and temporal variability between diffuse attenuation and spectral reflectance suggest that changes in the particle size distribution and associated scattering effects are important in regulating apparent optical properties of the water column.
Impact/Applications
This project includes the development of improved techniques for analyzing
marine particles and characterizing their optical properties. Our
ability to independently quantify size distributions for phytoplankton
and non-phytoplankton particles is a new contribution that will lead to
better understanding of optical variability in the ocean.
Acknowledgements
We wish to thank Dr. Collin Roesler (UConn) and Drs. Ron Zaneveld and
Scott Pegua (OSU) for sharing data collected during the CMO fieldwork.
References
DuRand, M. D., H. M. Sosik and R. J. Olson. 1998.
Size-dependent light scattering and absorption by particles in continental
shelf waters: flow cytometric analysis during the Coastal Mixing and Optics
Experiment. Abstract, 1998 AGU/ASLO Ocean Sciences Meeting.
Green, R. E. and H. M. Sosik. 1998. Vertical and temporal variability in apparent optical properties during the Coastal Mixing and Optics Experiment. Abstract, 1998 AGU/ASLO Ocean Sciences Meeting.
Sosik, H. M., R. E. Green and R. J. Olson. 1998. Optical
variability in coastal waters of the Northwest Atlantic. In: Ocean
Optics XIV, S. G. Ackleson (ed.). In press.