A bridge between ocean optics and microbial ecology

Phytoplankton cell size is believed to be closely regulated by the nutrient regime of water masses in the western North Atlantic Ocean. Since particle size affects attenuation of light in ocean water, we argue that a bridge between classical ocean optics and microbial ecology has formed whereby the physics of ocean color must include consideration of ecological factors important to the diversity of phytoplankton species. Observations of the specific absorption coefficient, a*, have been made during 10 oceanographic cruises in the western North Atlantic. The range in Chl a concentration for ocean samples from the various water masses in this region was 0.1–8.0 µ g liter −1 . Diffuse attenuation spectra of particles measured by a glass‐fiber filter technique show strong correlations between extracted chlorophyll and blue (440 nm) or red (670 nm) absorption. Of the two, a *440 is the most variable. Average specific absorption coefficients are calculated to be 0.049 m 2 mg −1 at 440 nm and 0.028 at 670 nm, but both relationships are seen to be nonlinear. The nonlinearity observed is attributed to differences in cell size “packaging” within natural phytoplankton populations. Variation in cell size and a* are primarily related to the availability of nitrate‐N. In addition, other factors contribute to the variability in a *440, such as detritus and short wavelength UV‐absorbing pigments.