Phytoplankton Bloom and Photosynthetically Active Radiation in Coastal Waters

The effect of a phytoplankton bloom on the change with depth of photosynthetically available radiation (PAR), the spectral downwelling irradiance in coastal waters, and the phytoplankton ability to absorb PAR in the sea is estimated. The 10-fold increase in the chlorophyll a concentration (from 0.4 to 4.0 mg/m 3 ) during a diatom/ dinoflagellate bloom leads to a decrease in water transparency and a narrowing of the photosynthesis zone. As a consequence, the average PAR within the phytoplankton habitat layer is almost halved. A diatom/dinoflagellate bloom is accompanied by changes in the downwelling irradiance spectrum at the same PAR level (optical depths). The penetrating irradiance maximum near the bottom of the euphotic zone shifts to longer wavelengths (~550–600 nm) as compared to background conditions (~500–550 nm). The increase in diatom/dinoflagellate biomass and the concomitant change in the downwelling irradiance spectrum are accompanied by a decrease in the specific absorption coefficient of phytoplankton pigments up to about three times that under background conditions at the same optical depths. The downwelling irradiance spectrum at fixed optical depths and the ability of cells to absorb light change slightly as compared to background conditions when coccolithophores bloom.