B. Greg Mitchell, Dariusz Stramski, and Rick A. Reynolds

Scripps Institution of Oceanography, University of California San Diego

"Optical measurements and modeling to estimate concentrations and fluxes of organic matter within the Southern Ocean"

Our field program entailed participation in three Southern Ocean JGOFS cruises during the period November, 1997 through March, 1998. One cruise was within the Ross Sea (Process IV) and the remaining two cruises were located within the Antarctic Polar Front Zone (Survey II, Process II). On these cruises, we measured continuous in situ profiles to a depth of 200m with our integrated optical profiling system, and collected water samples from the ship's CTD-Rosette for laboratory analyses. Several of these profiles coincided with overpasses of the SeaWIFS satellite. This data set is unique in the sense that we measured various optical quantities in situ (radiometric quantities describing the light fields, apparent and inherent optical properties) in parallel with detailed optical measurements and other analyses of water samples from discrete depths. Key measurements of our program include:

1. Daily downwelling spectral irradiance at the sea surface
2. Downwelling spectral irradiance and upwelling spectral radiance measurements
3. Spectral absorption coefficients and beam attenuation coefficients
4. Spectral backscattering coefficients
5. Spectral absorption coefficients for particulate and dissolved materials
6. Particle number and size distribution

Because of the recent completion of our field campaigns, we are presently in the early stages of raw data processing, data quality control, and generation of an optical database. We anticipate to have the majority of these tasks completed during the Fall of 1998, at which time we will continue with more advanced data analysis and modeling efforts. The anticipated results of these efforts include the parameterization of inherent and apparent optical properties in terms of planktonic community biomass (e.g. Chl a, POC), the development of suitable bio-optical models for use in models of radiative transfer and primary production, and the generation of ocean color algorithms for satellite applications (e.g. SeaWIFS).

********************

PROJECT SUMMARY

This proposal is a collaboration between Dr. Dariusz Stramski of the University of Southern California in Los Angeles and Dr. Greg Mitchell of Scripps Institution of Oceanography, University of California in San Diego. Our overall goal is to conduct optical measurements and modeling to estimate concentrations and fluxes of organic matter in the Southern Ocean in support of the U.S. JGOFS Process Study in this region.

Key variables and processes of high relevance to accomplish the JGOFS goals include time and space resolution of phytoplankton pigments, particulate organic carbon, and the formation and export of organic carbon. Our proposal will establish the fundamental relationships for parameterization of these variables and processes in terms of the optical properties of sea water, and will also seek to understand why the Southern Ocean differs from other systems, or has differentiation within. Our approach builds upon historical observations that optical properties provide a useful proxy for key reservoirs and rate processes of relevance to the JGOFS objectives. We will carry out detailed studies of in situ and water sample optical properties including spectral reflectance, absorption, attenuation and scattering coefficients. Specifically, we will evaluate the ability to estimate chlorophyll and phytoplankton absorption coefficients using the spectral reflectance of the ocean (ocean color), particulate organic carbon from light scattering, and particulate carbon export from ocean color and sea surface temperature. Our first hypothesis regarding algorithm differentiation between the Southern Ocean and low latitude systems derives from our previous work on pigment and carbon flux algorithms for polar regions. We will evaluate differentiation within the Southern Ocean by studying both the Ross Sea and the Antarctic Polar Front Zone (APFZ). As a consequence of different physical forcing, and ecological response to the forcing, we expect differences for these two relationships between the Ross Sea and the APFZ region, and across the APFZ. However, based on our laboratory experiments and theoretical considerations, and field work of others, we expect that particulate organic carbon-light scattering relationships will not vary between the Southern Ocean and other systems, or within the Southern Ocean (this is our second major hypothesis). Our third hypothesis postulates that an understanding of the algorithms relating bulk optical properties that are easily measured to reservoirs and processes of interest can be derived from fundamental theory and measurement of the constituent optical properties. To test this hypothesis we will measure particle absorption, scattering and size distribution, absorption by soluble material and absorption of single particles. The data we collect on bulk and individual component optical properties will be analyzed in the context of fundamental optical theory. The understanding we develop will directly improve our ability to extrapolate the time-space scales of the system dynamics using moored or remote optical sensors. Our parameterizations of pigment concentrations, carbon concentrations and carbon fluxes will be directly relevant to development of system models, and long-term monitoring of the Southern Ocean.