Abstract
Small shallow lakes are important systems for modern waterfowl production and for their longer-term potential as carbon sinks. Here, we present age-dated downcore records of elemental information (TOC, TN, and C/N), stable isotopic values (delta (super 13) C and delta (super 15) N), and organic carbon (OC) burial rates for three shallow lakes in west-central Minnesota to explore questions related to sedimentation and paleoecology in these systems: 1) What are the main sources of organic matter? 2) How has sedimentation changed with increasing human influences? 3) Can geochemical proxies in sedimentary organic matter (SOM) serve as useful recorders of stable state transitions in shallow lakes? 4) How much OC is typically buried? Shallow lakes can exist in two alternative stable states: either a clear-water state dominated by macrophytes with little phytoplankton abundance, or a turbid-water state where conditions are opposite. Two of these lakes (Mavis East and Mavis West) are small (<25 ha), closely-spaced ( approximately 250 m apart), and currently in opposite states. Lake Christina is a larger (1600 ha) lake that has been subject to biomanipulation efforts to keep it in a clear-water state. SOM in the lakes is a mixture of phytoplankton, macrophyte, and terrestrial sources. Prior to human settlement ( approximately 1880 A.D.), macrophytes and terrestrial material appear to have made up a greater proportion of the OM buried in the lakes. We find that while the proxies can perhaps record longer-term trends in the prevailing stable state, higher frequency transitions are probably not discernable. The lake records reveal a notable increase in OC burial since the time of settlement with max. burial (10.5-14 mg/cm (super 2) /yr) reaching 2-4 times the pre-settlement rates. Assuming these rates apply to similar lakes across the Prairie Pothole region, current burial estimates are approximately 1.5-3 Tg OC/yr.