Bio-optical characteristics of diatom and prymnesiophyte populations in the Labrador Sea

During the spring of 1996, phytoplankton samples were collected along a tra nsect from South Wolf Island (Labrador) to Cape Desolation (Greenland). Den se blooms of diatoms were found over the shelf near the coast of Labrador, whereas high concentrations of the colony-forming prymnesiophyte Phaeocysti s pouchetii were found close to Greenland. Phytoplankton samples were separ ated into 2 major groups (diatoms or prymnesiophytes) on the basis of chlor ophyll (chl) chi c(3)/chl a ratios (determined by HPLC analysis), and the e ffects of species composition on the absorption and photosynthetic characte ristics of these 2 high-latitude phytoplankton populations were studied. At all pigment concentrations and all wavelengths examined (apart from 623 nm ), the diatom population had a much lower absorption coefficient than the p rymnesiophyte population; this was attributed to an increased pigment-packa ging effect in the larger diatom cells. Varying proportions of photoprotect ive pigments also influenced the absorption characteristics of these popula tions. The low specific-absorption coefficient of the diatom population res ulted in a higher maximum photosynthetic quantum yield relative to that of the prymnesiophyte population. The initial slope of the photosynthesis-irra diance (P-E) curve (alpha(B)) also appeared to be taxon-specific, with high er alpha(B) values being recorded for the smaller prymnesiophytes than for the larger diatom cells. The implications of species-dependent variations i n phytoplankton absorption coefficients for the retrieval of remotely-sense d chi a are discussed.