Seasonal water-column dynamics of dissolved inorganic carbon stable isotopic compositions (δ13CDIC) in small hardwater lakes in Minnesota and Montana

The carbon stable isotopic value of dissolved inorganic carbon (delta C-13(DIC)) was measured over several years at different depths in the water column in six carbonate-precipitating temperate lakes. delta C-13(DIC) 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 delta C-13(DIC) 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 delta C-13(DIC) values of individual lakes differ between lakes with similar delta C-13(DIC) 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 call be made that allow lakes to be grouped by delta C-13(DIC) behavior. We recognize three modes in the relationship between delta C-13(DIC) values and DIC concentration ([DIC]) of individual lakes: (A) delta C-13(DIC) values decreasing with increasing [DIC]; (B) delta C-13(DIC) values increasing with increasing [DIC]; (C) delta C-13(DIC) values decreasing with increasing [DIC] but increasing again at the highest [DIC]. This approach is useful both in understanding delta C-13(DIC) dynamics in modern hardwater lakes and in reconstructing the environmental changes recorded by sedimentary delta C-13 components in the lacustrine pateorecord. (c) 2006 Elsevier Inc. All rights reserved.