Lakes exhibit variable phosphorus concentrations through time and through space, in part due to natural hydrogeomorphic features such as forest and wetland cover and lake morphometry. Paleolimnologic studies conducted in North America have shown that lakes have not only increased in total phosphorus (TP) since European settlement, as might be expected, but have also decreased in TP. The reason for the latter TP declines are less well understood, but could be related to alterations of the hydrologic connectivity and spatial pattern of wetlands in relation to lakes, which we know have undergone significant changes. Incorporating a landscape ecology and historic perspective on wetland-lake interactions may help to explain how lake TP has changed from presettlement to present. Our research questions were: 1) How has the extent, connectivity, and spatial pattern of wetland types changed from pre-European settlement (1800s) until present-day?; and 2) In the 1800s landscape, how does lake hydrology and wetland extent, type, and connectivity influence lake TP? We measured fossil diatom assemblages in the tops and bottoms of lake sediment cores to quantify diatom-inferred TP (DI-TP) for the 1800s (bottom) and present-day (top) for 31 Michigan lakes. We also measured past and present wetland cover using available geographic information databases. Lakes were grouped by their hydrologic setting into one of two categories, seepage and drainage.
We found that changes in DI-TP from the 1800s to present were variable with 12 lakes exhibiting no change, 9 showing an increase, and 10 lakes showing a decrease. Wetland spatial patterns in the lake catchments have changed from the 1800s to present with an overall loss in total wetland extent, however the connectivity has changed as well. In particular, wetlands more connected with lakes had a higher proportion of loss in comparison to all wetlands within the lake catchment. The percent wetland type within the total lake catchments showed no significant relationship with historic lake TP concentrations. However, wetlands within the 100m buffer around lakes were significantly related to historical TP. In addition, drainage lakes showed a significant relationship with wetlands within the 100 m buffer and historic lake TP, whereas seepage lakes did not. These results indicate that both wetland connectivity to surface waters and wetland spatial pattern are important in determining lake TP levels. These analyses are relevant to wetland conservation and mitigation by identifying wetlands vulnerable to loss and wetlands that may be more effective at retaining phosphorus.