REFLECTANCE MODEL FOR QUANTIFYING CHLOROPHYLL-A IN THE PRESENCE OF PRODUCTIVITY DEGRADATION PRODUCTS

Marine colored dissolved organic matter (CDOM) (also gilvin or yellow substance) absorbs light at an exponentially decreasing rate as a function of wavelength. From 410 nm to about 640 nm, particulate phytoplankton degradation products including pheopigments, detritus, and bacteria have absorption curves that are similar in shape to that of CDOM. In coastal areas and areas downstream from upwelling regions, these constituents of seawater often absorb much more light than do living phytoplankton, leading to errors in satellite-derived chlorophyll estimates as high as 133%. Proposed NASA sensors for the 1990s will have spectral channels as low as 412 nm, permitting the development of algorithms that can separate the absorption effects of CDOM and other phytoplankton degradation products from those due to biologically viable pigments. A reflectance model has been developed to estimate chlorophyll a concentrations in the presence of CDOM, pheopigments, detritus, and bacteria. Nomograms and lookup tables have been generated to describe the effects of different mixtures of chlorophyll a and these degradation products on the R(412):R(443) and R(443):R(565) remote-sensing reflectance or irradiance reflectance ratios. These are used to simulate the accuracy of potential ocean color satellite algorithms, assuming that atmospheric effects have been removed. For the California Current upwelling and offshore regions, with chlorophyll a less-than-or-equal-to 1.3 mg m-3, the average error for chlorophyll a retrievals derived from irradiance reflectance data for degradation product-rich areas was reduced from +/- 61% to +/- 23% by application of an algorithm using two reflectance ratios (R(412):R(443) and R(443):R(565)) rather than the commonly used algorithm applying a single reflectance ratio (R(443):R(565)).