Response of Particulate Optical Properties to Coastal Mixing Processes

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
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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.
 

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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.
 

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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.
 

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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.

 

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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).
 


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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.
 


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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).
 
 


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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).
 

 

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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.
 

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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.
 

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