Microphytobenthic dynamics in a Wadden Sea intertidal flat - Part I: Seasonal and spatial variation of diatom communities in relation to macronutrient supply

The benthic diatoms composition of the surface sediments were sampled at two-weekly intervals from May 2008 to May 2009 at ten stations along a transect across the intertidal zone of the Solthorn tidal flat (southern North Sea, Germany). Inorganic macronutrients (nitrate, nitrite, silicic acid and reactive dissolved phosphate) in the overlying water showed a distinct seasonal cycle, with high winter and low summer values. The benthic diatom assemblages exhibited both seasonal and spatial distribution patterns, showing distinctive differences in species composition among summer, autumn, winter and spring communities. Independent of season, all structures were characterized by the dominance of small epipelic species such as Navicula phyllepta, N. digitoradiata, N. gregaria and N. salinicola, with seasonal successions of secondary species, including several larger benthic (e.g. Pleurosigma angulatum, Gyrosigma peisonis) and pelagic species (Plagiogramma and Thalassionema). In addition, the communities contained epipsammic species, such as Achnanthes and Cocconeis, characterizing mixed sediments. Epipsammic species showed a significant decrease along the transect. Thus, small epipelic forms dominated the near-shore part of the transect within the investigation period while epipsammic species were only found in smaller numbers here. N-limitation was observed during summer 2008 in the seaward part of the transect, while during autumn, winter and spring the diatom assemblages in the landward part were P-limited. The results of statistical analysis indicate that the distribution patterns of some epipelic and epipsammic diatom taxa are related to nutrient availability, particularly of silicate and ammonium.