Sedimentation - The main loss factor in waters dominated by diatoms. Results of long-term investigations

In the mesotrophic reservoir Saidenbach, diatoms are dominating the phytopl ankton. The long-term mean percentage (1989 to 1996) was 77% of the total p hytoplankton biovolume. sedimentation is the major loss process for these m ainly microplanktic algae. In the sediment trap they contribute, on average , 95% to the biovolume, i.e. they are overrepresented because all other phy toplankton taxa reach the bottom in a non-recognizable state or do not arri ve there at all. The mean phytoplankton biovolume sedimenting during the period 1989 to 1996 was 3.5 cm(3) m(-2) d(-1). With a time delay of two weeks the flux showed the same high variations (about 0.2 to 20 cm(3) m(-2) d(-1)) as the abundan ce in the pelagic. The diatom concentration in the open water has the highe st influence on dynamics and amount of the phytoplankton flux, thus explain ing 82% of its variance. In the annual average the calculated sedimentation rates for selected diato ms are always 2 to 2.5 times higher than the positive rates of change, wher eas the grazing rates are about 10% of these positive rates only. The compa rison of yearly biomass yields and losses shows similar results: The sedime ntation of diatoms amounts to 1217 cm(3) m(-2) a(-1) (mean of 8 years), i.e . 2.5 times higher than the positive yearly diatom net production of 494 cm (3) m(-2) a(-1) observed in the reservoir for the same period. Provided the re are no further essential loss components and the yearly gross production can be assumed to be the sum of annual sedimentation and grazing losses, s ome 13% only of the phytoplankton gross production (or 9% of diatom product ion) are grazed in one year, while 87% (or 91% of the diatoms) sediment. The long-term measurements show the great significance of the diatom sedime ntation and confirm their function as a so-called lake internal tertiary tr eatment step (GRIM, 1967). Under the prerequisites of a diatom water (i.e. P limitation with sufficient Si supply and adequate turbulence) high sedime ntation losses guarantee the maintenance of a low trophic state. Moreover, they are the cause for the high buffering capacity, even for unusually high nutrient loads within restricted periods.