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

During the spring of 1996, phytoplankton samples were collected along a transect from South Wolf Island (Labrador) to Cape Desolation (Greenland). Dense blooms of diatoms were found over the shelf near the coast of Labrador, whereas high concentrations of the colony-forming prymnesiophytePhaeocystis pouchetiiwere found close to Greenland. Phytoplankton samples were separated into 2 major groups (diatoms or prymnesiophytes) on the basis of chlorophyll (chl) chlc₃/chlaratios (determined by HPLC analysis), and the effects of species composition on the absorption and photosynthetic characteristics 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 prymnesiophyte population; this was attributed to an increased pigment-packaging effect in the larger diatom cells. Varying proportions of photoprotective pigments also influenced the absorption characteristics of these populations. The low specific-absorption coefficient of the diatom population resulted in a higher maximum photosynthetic quantum yield relative to that of the prymnesiophyte population. The initial slope of the photosynthesis-irradiance (P-E) curve (αB) also appeared to be taxon-specific, with higher αBvalues being recorded for the smaller prymnesiophytes than for the larger diatom cells. The implications of speciesdependent variations in phytoplankton absorption coefficients for the retrieval of remotely-sensed chlaare discussed.