Stable C and N isotope record of short term changes in water level in lakes of different morphometry: Lake Anastazewo and Lake Skulskie, central Poland

We present the stable C isotope record of the changes within the past 20 yr in water level of two morphologically different lakes in central Poland. The aim was to explain the relationship between lake water level and the δ13C signature of bulk sedimentary organic matter (δ13CTOC) and to assess the potential of δ13CTOC as a paleolimnological proxy of lake level change. This was done by comparison of the fossil δ13CTOC record with instrumental data for lake level change in a shallow and small lake as well as in one large and deep basin. In both lakes the water table varied greatly between 1980 and 2000 AD. The δ13CTOC data were supplemented with δ15N and bulk geochemistry data, as well as paleoecological data. We show that δ13CTOC reacted to short term and low amplitude fluctuation in water level, but the response was highly dependent on the morphometry of the lake. In the shallow and small basin, δ13CTOC decreased along with lake level drop due to oxidation and greater input of organic matter from macrophytes colonizing the lake bottom. On the contrary, in the deep/large lake δ13CTOC decreased with increasing water level due to enhanced delivery of soil-derived OM to the lake during highstands. Our results have broad paleolimnological implications as they show that δ13CTOC cannot act as a universal paleohydrological proxy. Its interpretation for a particular lake can be ambiguous and must be supported with additional geochemical and paleocological information.