Controls on the seasonal and interannual dynamics of organic matter stable carbon isotopes in mesotrophic Lake Holzmaar, Germany

Abstract

We investigated the particulate organic matter (POM) flux and its stable carbon isotope composition in response to changes in phytoplankton primary production and abiotic environmental affects in mesotrophic Lake Holzmaar. POM fluxes were determined by sediment traps emptied biweekly from 1995 to 2004. Water chemical measurements and meteorological observations were done concurrently. POM fluxes within and directly below the lake’s epilimnion were relatively low during winter (<0.5 g m-2 d−1) and increased to 1.0–1.5 g m-2 d−1 during summer months because of enlarged phytoplankton production. In contrast, in the lake’s hypolimnion a much higher POM flux was observed during winter as result of resuspension of particulate material from the lake’s bottom. C:N (weight, total organic carbon : total nitrogen) ratios of settling organic matter vary around 8 at all three depths, indicating a predominant phytoplankton origin of POM. The annual epilimnic POM flux correlates with the epilimnic phosphate concentration early in the seasonal cycle, indicating that the supply of phosphate is the major control on the amount of algal biomass. A recurrent annual oscillation of the stable carbon isotope composition of POM with low δ13C values during winter and more positive values during summer is interpreted as the result of phytoplankton primary production. A close δ13CPOM-POM flux relationship was observed, testifying to the primary dependence of d13CPOM on lacustrine primary production. Contrary to POM flux, the epilimnic δ13CPOM signal is transferred to the lakes bottom notwithstanding sediment resuspension and microbial alteration. Our results demonstrate that sedimentary stable carbon isotopes are a valuable proxy for lacustrine paleo-productivity in Lake Holzmaar.