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Abstract
Microscopical and molecular analyses were used to investigate the diversity and spatial community structure of spring phytoplankton all along the estuarine gradient in a macrotidal ecosystem, the Baie des Veys (eastern English Channel). Taxa distribution at high tide in the water column appeared to be mainly driven by the tidal force which superimposed on the natural salinity gradient, resulting in a two-layer flow within the channel. Lowest taxa richness and abundance were found in the bay where Teleaulax-like cryptophytes dominated. A shift in species composition occurred towards the mouth of the river, with the diatom Asterionellopsis glacialis dramatically accumulating in the bottom waters of the upstream brackish reach. Small thalassiosiroid diatoms dominated the upper layer river community, where taxa richness was higher. Through the construction of partial 18S rDNA clone libraries, the microeukaryotic diversity was further explored for three samples selected along the surface salinity gradient (freshwater - brackish - marine). Clone libraries revealed a high diversity among heterotrophic and/or small-sized protists which were undetected by microscopy. Among them, a rich variety of Chrysophyceae and other lineages (e.g. novel marine stramenopiles) are reported here for the first time in this transition area. However, conventional microscopy remains more efficient in revealing the high diversity of phototrophic taxa, low in abundances but morphologically distinct, that is overlooked by the molecular approach. The differences between microscopical and molecular analyses and their limitations are discussed here, pointing out the complementarities of both approaches, for a thorough phytoplankton community description.
Highlights
Estuaries form transition zones linking freshwater and marine biomes
As confirmed by the Principal Component Analysis (PCA) analysis including nine environmental variables, three main area can be discriminated along the estuarine continuum
An ‘‘upper surface layer’’ in the river channel [samples As( = VIRE River), Bs, Cs( = ESTUARY)], defined by oligohaline (0.5–5) to mesohaline (5–18) waters, high nutrients from terrestrial runoff, warmer temperatures (13.8–16uC) and high phytoplankton abundance (8.5–226103 cells mL21), was distinguished from the bottom brackish layer, which was characteristic of the estuarine ‘‘silt plug’’
Summary
Estuaries form transition zones linking freshwater and marine biomes. Due to mixing of both distinct water bodies, they are characterized by pronounced gradients of physical and chemical components [1]. The rise of molecular microbial ecology has opened the possibility of studying protist diversity independently of morphological considerations Such molecular environmental surveys revealed a high diversity of eukaryotic lineages and contributed to our current understanding of microbial food web structure and biogeochemical processes in aquatic systems [5], [6]. This approach has been applied in a wide variety of ecosystems, including oceanic/coastal waters, freshwater ecosystems, and many extreme environments such as anoxic systems or deep-sea vents [7]–[10]. Insight of many novel eukaryotic lineages divergent from known protist sequences suggests that a large fraction of these communities still remains to be discovered [11]
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