Impact of hydrodynamics and seagrass habitat structure on the suspension feeder Cerastoderma edule

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Schanz, A. , Asmus, R. and Asmus, H. (2005): Impact of hydrodynamics and seagrass habitat structure on the suspension feeder Cerastoderma edule , Lecture, BioFlow Workshop IV, Flume designs and scaling of biological processes, 5-9 Sept., Sylt, Germany. .
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Impact of hydrodynamics and seagrass habitat structure on the suspension feeder Cerastoderma eduleThe edible cockle Cerastoderma edule, one of the most common suspension feeder on the tidal flats along the European Atlantic Coast, shows large annual as well as site specific fluctuations in the survival and growth of juvenile and adult cockles. Amongst the main factors that may induce these variations are abnormal climatic conditions, predation by crustaceans and fish, intraspecific competition, and spat fall transport by near bottom flow. However, little attention has been paid to the effect of hydrodynamic exposure and habitat structures, as well as the interaction of these factors which may influencing the cockle population structure.The effect of different hydrodynamic exposure and seagrass habitat structure on the abundance, biomass and growth of C. edule was investigated in intertidal Zostera noltii beds and adjacent sand flats in the Wadden Sea, near the island of Sylt. Regardless of the seagrass density and hydrodynamic exposure, the abundance and biomass of cockles were generally higher inside the seagrass beds in comparison to the adjacent sand flats. However, transplantation of cockles of equal size into sheltered and exposed seagrass beds and sand flats resulted in lower growth rates of individuals in the seagrass beds. Furthermore, shell lengths and body weights of C. edule were distinctly higher in exposed than in sheltered seagrass beds, where individuals seemed to have starved. Highest growth rates were measured in areas with highest current velocities. Experiments carried out with an in situ three-currents-flume, which modifies tidal current velocities within the same seagrass site, showed an increase in cockle shell length and body weight at mean current velocities between 4 and 8 cm s-1. Corresponding measurements of seston content and the percentage of POC and PON within the water column of the different flume lanes revealed highest seston values in lanes with the lowest current velocities. In contrast, values of the percentage of POC and PON increased with increasing current speeds. Thus it is concluded that seagrass vegetation positively effects the abundance of cockles, whereas the growth and condition of C. edule is controlled by current velocities, promoting the quality of food for C. edule at higher current velocities.

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