SEASONAL WATER-COLUMN DYNAMICS OF DISSOLVED INORGANIC CARBON STABLE ISOTOPIC COMPOSITIONS (?13CDIC) IN SMALL HARDWATER LAKES IN MINNESOTA AND MONTANA

Abstract

The carbon stable isotopic value of dissolved inorganic carbon (?13CDIC) was measured over several years at different depths in the water column in six carbonate-precipitating temperate lakes. ?13CDIC behavior in three of these lakes departed from the conventional model wherein epilimnetic waters are seasonally enriched relative to all hypolimnetic waters, and in general ?13CDIC values in the water column were not readily correlated to parameters such as lake stratification, algal productivity, hydraulic residence time, or water chemistry. Additionally, the processes implicated in generating the ?13CDIC values of individual lakes differ between lakes with similar ?13CDIC compositions. Each lake thus initially appears idiosyncratic, but when the effects of carbonate mineral equilibria, microbial activity, and lake residence time are viewed in terms of the magnitude of distinct DIC pools and fluxes in stratified lakes, generalizations can be made that allow lakes to be grouped by ?13CDIC behavior. We recognize three modes in the relationship between ?13CDIC values and DIC concentration ([DIC]) of individual lakes: (A) ?13CDIC values decreasing with increasing [DIC]; (B) ?13CDIC values increasing with increasing [DIC]; (C) ?13CDIC values decreasing with increasing [DIC] but increasing again at the highest [DIC]. This approach is useful both in understanding ?13CDIC dynamics in modern hardwater lakes and in reconstructing the environmental changes recorded by sedimentary ?13C components in the lacustrine paleorecord. ? 2006 Elsevier Inc. All rights reserved.