Distribution of phytoplankton pigments in nine European estuaries and implications for an estuarine typology

Abstract

Phytoplankton pigments were studied by Liquid Chromatography (HPLC) in nine West European estuaries. Three estuaries, i.e. the Rhine, Scheldt and the Gironde were sampled four times to cover the different seasons, whereas the other six estuaries were sampled once. Pigment distributions in estuaries reflect both riverine inputs as well as autochthonous blooms. Fucoxanthin was the most common accessory photosynthetic pigment showing that Diatoms were the most common group in the studied estuaries and were particularly dominant during autumn and winter. In the very turbid Gironde estuary, degradation processes were predominant between salinities 1 and 20, while Diatoms, Dinoflagellates and Crypto- phytes bloomed above 20 salinity during spring and summer. This contrasted with the highly eutrophic but less turbid Scheldt, where phytoplanktonic blooms occurred at low salinities close to the city of Antwerp. In the Scheldt, we observed both a tenfold fluctuation of phytoplankton biomass and a fluctuating pigment diversity index. In contrast, chlorophyll a was always low in the Gironde, but we observed large variations of pigment diversity among samplings during different seasons. Distribution of pheopigments showed that the maximum turbidity zone (MTZ) was a highly reactive region for heterotrophic phytoplankton degradation. The Scheldt and the Thames were the most anthropogenic influenced estuaries contrasting with the Gironde estuary that has a less urbanised watershed. An estuarine typo- logy is proposed based on three clusters emerging from a correspondence analysis of pigment variables and variables characterising the anthropogenic impact and physical forcing.