Phytoplankton diversity and photosynthetic acclimation along a longitudinal transect through a shallow estuary in summer

Diversity, nutrients and irradiance acclimation of phytoplankton were investigated along a longitudinal transect through the shallow Darss-Zingst estuary (southern Baltic Sea). Sampling in the pelagial during a stable midsummer clear-sky period served to search for basic longitudinal patterns. Cyanobacteria dominated the phytoplankton biomass. The phytoplankton biomass in the innermost basins decreased with increasing proportions of mesotrophic Baltic Sea water. Individual-based phytoplankton diversity reached maximum values at highly productive sites and was not inversely related to biomass concentration. Marked gradients of dissolved plant nutrients were not observed. Nutrient concentrations and element ratios suggested Redfield limitation by phosphorus. In contrast to element ratios in the particulate pool, dissolved iron concentrations indicated potential iron starvation at several sampling sites. The relatively high concentrations of non-reactive soluble phosphate at many sampling sites made chemical interactions of phosphates and iron probable. The availability of photosynthetically active radiation (PAR) limited the photic zone to a water depth of less than 2 m in highly productive areas, but a gradual acclimation response of phytoplankton to the longitudinal gradient in irradiance was undetectable. Significant correlations of chla-normalised carotenoid content and saturation irradiance of photosynthesis (Ek ) as well as chla-normalised carotenoid content and daily incident PAR (sunrise to sampling time) suggest that high photon flux densities (PFDs) in midsummer clear-sky periods prevent effective photosynthetic acclimation of phytoplankton to average low irradiance availability in the turbid water columns. Low variable photosystem II fluorescence after dark incubation indicates inhibitory cross-effects of severe nutrient limitation and stress due to high PFDs at the water surface and/or high oxygen oversaturation in the pelagial. Thus, the photosynthetic machinery may be hampered under certain weather conditions, even in a highly eutrophicated estuary.