Stable Carbon and Nitrogen Isotopic

Abstract

The perennially ice-covered lakes in the McMurdo Dry Valleys, Antarctica, are part of the coldest and driest ecosystem on earth. To understand lacustrine carbon and nitrogen cycling in this end-member ecosystem, and to define paleolimnological proxies for ice-covered lakes, we measured the stable carbon and nitrogen isotopic composition of particulate organic matter (POM) and benthic organic matter (BOM) within the lakes of Taylor Valley. The δ13C compositions of seasonally ice-free edges of the lakes (moats) are enriched relative to under-ice organic matter. Thus, the organic carbon isotopic composition of buried sediments may be a proxy for sample position within the lake. In the moats, δ13C values are governed by limited CO2diffusion across benthic cyanobacterial cell membranes. During a high glacial melt (2001–2002) season, both δ13CPOM and δ13CBOM in the moats were more depleted than during previous low melt years. We propose that this occurred in response to higher [CO2](aq) and/or reduced growth rates resulting from turbidity-induced light limitation. Though moats and under-ice environments are usually poorly connected, during the 2001–2002 season, the enrichment of the δ13CPOM values at 6 m depth in the stream-proximal sites relative to deep-profile sites implies enhanced connectivity between these environments. The δ13C compositions of BOM and POM profiles in Lake Hoare and Lake Fryxell indicate that these lakes are dominated by benthic productivity. In contrast, in Lake Bonney, the similarity of the δ13C values of BOM and POM indicates the pelagic component dominance in the carbon cycle.