摘要：Autochthonous calcite precipitation is an important process for Cfixation in hardwater lakes, which is mainly induced by the photosynthesis of planktonic microorganisms Among these, the widespread calcifying green alga Phacotus lenticularis (Ehrenberg) Diesing contributes to biogenic calcite precipitation in temperate regions Its role in carbonate precipitation needs to be investigated, because there are no studies dedicated to the quantitative contribution of Phacotus shells to long term carbonate sequestration in hardwater lake sediments In order to fill this gap, the Phacotus shell content in the sediments of four German hardwater lakes was determined and compared to the fraction of Phacotus shells in the total suspended autochthonous calcite of the euphotic zoneIt was found that the Phacotus shells contributed at least 10% to the autochthonous carbonate precipitation in the upper water column in three investigated lakes During a Phacotus mass occurrence with a cell density of 18 × 106 Ind L1 in Lake Hopfensee, even 59% of the 36 mg L1 total carbonate concentration consisted of Phacotus shells In contrast to this high amount, the topmost basinal sediment contained a Phacotus shell content between 80 and 36,252 individuals per mg dry sediment, representing only 002% to 228% of the total carbonate sediment content In a gravity core from Lake Grosser Ostersee, dating back ~150 years, the Phacotus shell content was continuously below 024% whereas the shell diameters remained equal to those of living individuals found in the water column proving that Phacotus shells are capable to persist in the sediment after depositionA main reason for the large discrepancy between Phacotus shell abundance in the euphotic zone and in sediment was found to be the gross authigenic carbonate precipitation, which dilutes the sedimenting Phacotus shells that accumulate exclusively during short and intensive population peaks in summer Additionally, dissolution of the carbonate shells during sedimentation was proven to be a relevant factor in Lake Igelsbachsee by means of reducing the number of Phacotus shells reaching the lake bottom These facts explain that shortterm high Phacotus carbonate contents of the total suspended carbonate in the water column do not mirror the contribution of Phacotus shells in the sedimentary record.
关键词：Microfossil;calcifying green alga;quantitative sediment analysis;autochthonous calcite precipitation;calcite shells;hard-water lake
