Evidence of recovery from phosphorus enrichment in Lake Michigan

During the last century, diatom production responses to increased phosphorus loading in Lake Michigan resulted in increased silica sedimentation losses and a consequent decline in the silica content of the lake. In light of recent reductions in phosphorus loading to the lake, we examined long-term monitoring data to determine if this trend might be reversing. Spring total phosphorus concentrations, although highly variable, declined approximately 1 mug.L-1 throughout the lake between 1983 and 2000. Spring soluble silica concentrations, an indicator of total in-lake reserves, increased from 1.1 to 1.6 mg.L-1 during this period. Summer epilimnetic silica concentrations in the southern basin were at apparently limiting levels of approximately 0.15 mg.L-1 until 1991 and then increased sharply. A similar trend was seen in the northern basin. Summer diatom populations increased in both basins after 1991, and ordination analyses suggest that the species composition of both spring and summer communities has responded to these silica increases. These results document for the first time a reversal of the silica depletion sequence and provide the most compelling evidence to date that phosphorus load reductions are having an impact on the Lake Michigan ecosystem.