Abstract
Whole-ecosystem manipulations in shallow lakes effectively demonstrate threshold ecological responses, namely a clear water, macrophyte dominated regime and a turbid, phytoplankton dominated regime. Here we show that C burial (total organic carbon mass accumulation rate; TOCMAR) in shallow Lake Christina (Minnesota, USA) is governed more by the long-term trophic state and settlement history of the lake rather than short-term oscillations between clear and turbid regimes. Paleoecological remains of diatoms, along with proxies of primary production (total chlorophyll a and TOCMAR) and trophic state (total phosphorous) from sediment records clearly show a single dramatic shift of the lake in the mid-1950s. During the modern, anthropogenic period of the lake (post-1950), three top-down manipulations (fish kills) induced temporary clear water regimes which allowed for macrophyte growth. While these manipulations induced profound ecological shifts, the temporary regimes yielded insignificant changes in the overall sedimentation and C burial in the lake; instead C burial changed most dramatically following the shift in the 1950s. Preliminary results among multiple shallow lakes in western Minnesota suggest that patterns of TOCMAR are not driven by the prevailing ecological regime. It appears that landscape changes and settlement history have a greater influence on C burial, either through allochthonous contributions of C or changes in nutrient cycling which affect lake primary production.