| The lacustrine carbon cycle incorporates heterogeneous watershed inputs and complex internal lake processes, lending ambiguity to paleorecord interpretation. This study used multiple proxies to investigate carbon stable isotopic ratios (delta13C) in the water column and sediments of three hardwater lakes in Minnesota on three different time scales: first, the modern-day carbon cycle over several growing seasons; second, in the past 200 years, which includes the period of human impact; third, over several thousand years in the early and middle Holocene, under well-defined conditions of changing climate and vegetation.; In a comparison of the Minnesota lakes with three in Montana, lake circulation and dissolved inorganic carbon pool size were found to be significant factors in determining delta13CDIC behavior. Lakes with overlapping source water composition strongly diverged in delta13 CDIC, and internal variability was frequently large enough to obscure any signal from changing landscape vegetation type (C3/C4). The most productive lakes did not necessarily exhibit the highest epilimnetic delta13C DIC: in this study the least eutrophic lake was the most 13C-enriched. Hypolimnetic delta13CDIC values were also affected by processes like methanogenesis and carbonate mineral dissolution that can add 13C.; Historical records of human impacts to one urban and two rural/suburban lakes (McCarrons, Green, and Spectacle lakes, respectively) show the effects of 19th century logging and agriculture and 20th century urban and residential development. Sedimentation rates in all three lakes increased in response to land clearance, and eutrophication occurred as nutrient inputs increased. Green Lake and Lake McCarrons were evidently most heavily impacted in the mid-20th century, and have since improved, while Spectacle Lake is at its worst today. An annual-resolution study of Lake McCarrons sediments showed a sensitive response to drought cycles and land-use changes in the watershed.; Early and late Holocene sediments of Spectacle Lake are carbonate-free, while Green Lake sediments are carbonate-rich throughout. In the mid-Holocene "prairie period" Spectacle Lake produced and preserved authigenic calcite in multidecadal cycles with a strong isotopic and lithological signature. These cycles are probably related to water column stratification and lake residence time. Green Lake delta13C is highest in the mid-Holocene, suggesting increased productivity and/or residence time. |
