Benthic and Pelagic Coupling of Diatoms Bacillariaceae) in Response to Hydrodynamic Forces in the Wadden Sea

Abstract

Within the microbenthic community, diatoms play an important role in terms of food source for grazers and suspension feeders. Furthermore, they contribute significantly to the primary production on intertidal flats and in coastal waters world wide. Numerous studies have now documented microalgal biomass, primary production and other aspects of their ecology in many habitats. However, knowledge on the resuspension of diatoms or the effect of hydrodynamic forces at the species level is sparse but a better understanding of processes at the sediment/water interface and of potential consequences for the ecosystem regarding climate change is urgently required. In addition, knowledge of the effect of hydrodynamics in the intertidal is important because this force links and alters various processes from small scale areas at the sediment/water interface to ecosystem as a whole.The microscopic algae inhabit the sediment and water column and a terminology exists for referring to the habitats occupied by the diatoms, including pennate and centric forms, concerning their life mode: epipsammic (attached to a substratum), epipelic (cells are associated with the sediment, mainly motile) and pelagic diatoms that will occur mainly in the water column. A variety of diatoms are frequently found in both habitats, and often described as tychopelagic: a life mode with pelagic forms that are likely to have settled from the plankton and conversely, benthic diatoms that are readily resuspended. One aim of this thesis was to investigate the diatom community in the water and on the sediment surface and to establish the life mode of selected species in order to gather background information for further studies concerning bentho-pelagic coupling. The increased resolution of the identification and cell counts revealed a discrimination at the species level. Several species, notably of the genera Diploneis and Achnanthes, were found to react to altered flow conditions by presenting enhanced cell numbers on the sediment under increased flow or reduced flow, respectively. Furthermore, Diploneis, previously assumed to be a benthic species, is now shown to adopt a tychopelagic life style. Settling processes during slack water phases and resuspension caused by wind-induced waves and tidal currents lead to a blurring of the boundaries between the two communities. The present study investigated for the first time the influence of altered flow conditions and turbulence on diatoms in situ. This enabled us to take into consideration the effects of wind direction, wind force and tidal currents on the coupling. For this purpose a three-current-flume construction was deployed modifying the flow velocity. The study was carried out on a Wadden Sea intertidal flat at Sylt island in the North Sea, Germany, on 6 dates between July and September 2003. Analyses of chlorophyll a (chl a), suspended matter (SPM), cell counts, particulate organic carbon (POC), particulate organic nitrogen (PON) and species analysis on the sediment surface and in the water column served to characterise the relationship between the two communities. Flow characteristics were measured using the Acoustic Doppler Velocimeter (ADV) technique to calculate the physical parameters describing bottom shear stress and turbulence in order to assess the impact of hydrodynamics on diatom species and cell-size groups.Results showed that wind direction, à ?à ¯Ã ?à ¿Ã ?à ½velocity and fetch had the greatest influence on the variability of the samples, superimposed as they were on tidal forces. An increase in flow velocity reduces turbulence, whereas under reduced flow conditions turbulence exerts stochastic resuspension events by lowering the critical bed shear stress. This affects small diatoms (< 20 à ?�à ?à µm) of the benthic community that are entrained into the water column.