Monitoring phytoplankton assemblages in estuarine waters: The application of pigment analysis and microscopy to size-fractionated samples

In the context of a monitoring program developed in the Nervion River estuary, the structure of the phytoplankton community was assessed by means of microscopy and high-performance liquid chromatography (HPLC) analysis of pigments. Four size fractions were considered: <3 mm (picoplankton), 3e8 mm (small nanoplankton), 8e20 mm (large nanoplankton) and >20 mm (microplankton). Samples were taken in spring and summer at three sites located in the marine extreme of the estuary. Fucoxanthin, chlorophyll b and alloxanthin were the most abundant signature pigments. Fucoxanthin alone explained more than 95% of the total chlorophyll a variance. Most of the chlorophyll a and the major signature pigments were included in the nano- and microplankton size fractions, denoting the high trophic status of the water. The picoplankton, dominated by chlorophyll b containing algae or eustigmatophytes, accounted always for less than 5% of the total chlorophyll a. The fucoxanthin/chl a ratio of natural communities was highest in the microplankton and large nanoplankton whereas that of alloxanthin/chl a was generally highest in the small nanoplankton and that of chl b/chl a reached the highest values in the picoplankton.