ALTERNATIVE STABLE STATE TRANSITIONS IN SHALLOW NORTHERN MINNESOTA LAKES: PRELIMINARY GEOCHEMICAL RESULTS FROM LAKE CHRISTINA

Shallow lakes are known to exhibit two alternating stable states. The first state is characterized by clear water, submerged vegetation, and strong communities of fish and invertebrates. The second state, equally stable, is less species-rich and less diverse, with an absence of submerged vegetation, fish communities, and invertebrates. This state is recognized by the turbidity of the water and strong algal blooms. The primary controls on these stable state transitions are poorly understood, but both physical and biological mechanisms appear to be important. During March, 2004 we collected a 144 cm sediment core from Lake Christina, a large, shallow, eutrophic lake in northwestern Minnesota to study the occurrence of stable state transitions over the past few centuries. Since vascular plants and algae have distinct stable carbon isotopic values (δ¹³C), we measured these values from lake sedimentary organic matter in Lake Christina as a proxy for past stable state transitions. Throughout the record the measured δ¹³C values are generally high (-13 to -17‰) and indicative of a significant contribution from vascular plants. There are several sharp changes of 2-3‰ in δ¹³C values that may signal periodically occurring rapid changes (interannual- to decadal-scale) in the prevailing stable state of the lake.