Convergence of Trophic State and the Lower Food Web in Lakes Huron, Michigan and Superior

The open waters of Lake Huron and Lake Michigan have shown signs of oligotrophication in recent years.  Spring total phosphorus (TP) and the relative percentage of particulate phosphorus in both lakes have declined, and spring soluble silica concentrations have increased significantly in both lakes, consistent with decreases in productivity. Transparencies in Lakes Huron and Michigan have increased, and in most regions are roughly equivalent to those in Lake Superior. Seasonality of chlorophyll, as estimated by SeaWiFS satellite imagery, has been reduced in lakes Huron and Michigan. The spring bloom is now largely absent from both lakes with a seasonal maximum occurring instead in autumn, as is the case in Lake Superior. As of 2006, the loss of cladocerans and the increased importance of calanoids have resulted in crustacean zooplankton communities in Lake Huron and Lake Michigan closely resembling that in Lake Superior. Decreases in Diporeia in offshore waters has resulted in abundances of non-dreissenid benthos communities in these lakes that approach those of Lake Superior. These changes have resulted in a distinct convergence of the lower food webs in the three lakes, with Lake Huron currently as – or more - oligotrophic than Lake Superior by many measures.