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Sedimentary Losses in the Reservoir Saidenbach: Flux and Sinking Velocities of Dominant Phytoplankton Species

Sedimentation of phytoplankton was studied in the meso/eutrophic reservoir Saidenbach for two years and measured as biovolume in a sedimentation trap near the bottom. It comes to an annual average of 2.76cm3/m2 × d (0.4… 10.9) and is statistically significant dependent on the free water concentration measured 14 days before. This allows flux to be reliably calculated without any direct measurement. The bottom is reached above all by diatoms which form 90% of the deposited algae. The sinking velocity of the diatoms is mainly determined by their physiological state: During growth phases low sinking velocities (0.1 … 2m/d) were found, while in decline phases they increased considerably (>6m/d). The highest average sinking velocities found among large diatoms were calculated for Fragilaria crotonensis (3…4m/d), the lowest for Melosira italica (1.5…2m/d). The values for Asterionella formosa, Synedra acus and Diatoma elongatum were between them. Turbulence has no influence on sinking velocity (usually, spring full circulation sees the highest sedimentary losses), but plays an essential part in the survival of the population in free water. Permanent redistribution prevents diatoms from sinking out from the euphotic layer, this “inoculation” making further development possible. On an annual average, phytoplankton forms approximately only one quarter of the whole trap sediment (max. 62%). The flux of the dry weight of seston (2.33g/m2 x d on an average) reflects the changes in phytoplankton flux negligibly only and does not reveal any relation to it. so that seston flux is not suitable for determining phytoplankton sedimentation. But it is possible to calculate this at a probability of 65 to 94%, by using the free water concentration measured 14 days before.