Abstract
Small shallow lakes and wetlands have significant potential as carbon sinks. Here, we present age-dated downcore records of elemental information and stable isotopic values for C and 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 much OC is typically buried? 3) How has OC burial changed with increasing human influences? 4) Is there a connection between the dominant plant type existing in these lakes (algae, macrophytes) and the efficiency of OC burial? 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. The lake records show increasing algal sources of OM since settlement. The lake records reveal a notable increase in OC burial since the time of settlement with max. burial (10.5-14 mg/cm2/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 2-3 Tg OC/yr. Based on early findings, we also suspect that lakes in the clear-water state are more efficiently burying OC.