Tidal impact on planktonic primary and bacterial production in the German Wadden Sea

Tidal variation of biological parameters was studied at three anchor statio ns in selected inlet channels of the northern German Wadden Sea in May and July 1994. Concentrations of bacteria, chlorophyll a and suspended matter a s well as primary and bacterial production were assessed over a period of 2 5 h in the surface and in the bottom water. Diurnal variation in primary pr oduction was found both under in situ light conditions and under constant i llumination. Tidal turbulence caused the introduction of detritus, bacteria and pigments from the sediment into the water column. The impact of sedime nt resuspension was most evident in the bottom water, leading to tidally os cillating bacterial production rates which were high during high stream vel ocity and low during the slack times. Estimations of the areal daily phytop lankton production and corresponding bacterial carbon demands were unbalanc ed. Primary production accounted for only 25-45% of the total bacterial car bon requirement. This discrepancy is due to the shallow euphotic depth in t he Wadden Sea, allowing net primary production only in the upper 2-3 m of t he water column, while the relatively high levels of bacterial activity do not show a vertical decline. Assuming that the specific biological activiti es in the water columns over the tidal flats are similar to those found in the inlet channels, it was found that production processes dominate in shal low areas whereas decomposition processes dominate in the deep channels. Mo reover, the predominance of heterotrophic processes in the inlet channels m eans that additional organic carbon sources must contribute to the heterotr ophic metabolism in the deep parts of the Wadden Sea, and that the horizont al flux of material is important in this turbid mesotidal ecosystem.