The use of pigment signatures to assess phytoplankton assemblage structurein estuarine waters

The seasonal dynamics of chlorophyll a and the main accessory pigments acco mpanied by microscopic observations on live and fixed material were investi gated in the Urdaibai estuary, Spain, Fucoxanthin was the dominant pigment during the peak in chlorophyll a, with which it was strongly correlated. Co ncentrations of fucoxanthin (81-30 mug l(-1)) in the upper estuary were amo ngst the highest found in the literature, and were mainly associated with d iatoms and symbiotic dinoflagellates. In the lower estuary, fucoxanthin sho wed values typical of coastal waters (<5 <mu>g l(-1)) and was mainly due to diatoms and prymnesiophytes. Chlorophyll b concentration was high along th e estuary, followed the same seasonal pattern as chlorophyll a, and was ass ociated with the presence of euglenophytes, chlorophytes and prasinophytes. High values of 19'-butanoyloxyfucoxanthin were often measured, but no orga nisms containing this pigment were observed in live or fixed samples. Allox anthin and peridinin were found in low concentrations which was in agreemen t with cell counts of cryptophytes and peridinin-containing dinoflagellates . Two main patterns of phytoplankton assemblages were observed along the es tuary. In the upper segments, during the chlorophyll a maximum fucoxanthin containing algae masked the other algal groups, which were relatively more abundant during or after enhanced river flows. In the lower estuary, althou gh dominated by fucoxanthin-containing algae, the other algal groups were i mportant all year around. In this study, the use of diagnostic pigments has provided considerable insight into the temporal and spatial dynamics of ph ytoplankton assemblages by detecting phytoplankton taxa generally underesti mated or overlooked by microscopy. (C) 2001 Academic Press.