Sedimentation of dry matter, phosphorus and phytoplankton biovolume in tworeservoirs of different trophic state

The mesotrophic reservoir Saidenbach (SB) has an up to six fold higher tota l phosphorus (TP) load than the oligotrophic reservoir Neunzehnhain (NH). B oth are P limited. The TP of SE, originating from water bodies located upst ream, agricultural land and domestic waste water is to a greater extent inc orporated in biomass or loosely adsorbed than its counterpart in NH and the refore better bioavailable. In NH, the TP is stronger bound to minerals or humic substances. This results in about 30 times higher phytoplankton abund ances and 40 times larger sedimentation fluxes in SE (4.3) than in NH (0.1 cm(3)/m(2).d). With an average of 7.0 (SB) and 2.7 (NH) mg P/m(2).d, howeve r, the P-flux of NH is relatively higher than in SE (in relation to the ext ernal load). This can be the result of a stronger accumulation effect in NH or of different longitudinal sedimentations. Otherwise, the sedimentation losses of particulate organic matter (POM) are similar (SB 0.68, NH 0.42 g/ m(2).d). It is assumed, that in NH the import of humic substances is higher because of the forested catchment and that this organic matter precipitate s, induced by the pH increase in the reservoir. Additionally, there is a co -precipitation by Al: the soluble Al in the acidic inflowing water changes by the increasing pH to particulate Alhydroxides adsorbing P and organic su bstances. The results show, that the knowledge of the external nutrient loa d and the in-lake concentration alone does not allow reliable statements co ncerning the expected sedimentation losses of nutrients and POM.