Perspectives on the eutrophication of the Yahara lakes

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Abstract
Eutrophication of the four Yahara lakes—Mendota, Monona, Waubesa, and Kegonsa—near Madison, Wisconsin, has been dramatic since the mid-1800s. For Lake Mendota, the erosion of sediments from higher water levels established by the damming of the lake's outlet, plus the agricultural expansion of its watershed, resulted in blue-green algal growths. These impacts, however, were dwarfed by water quality problems stemming from Madison's wastewater inputs that directly entered Lake Monona from the late 1800s through 1936, and then Lake Waubesa until 1958. Blue-green algal blooms were so bad in the lower Yahara lakes that the Madison Public Health Department conducted major copper sulfate treatments during 1925–1954. During the wastewater input years, inorganic nitrogen (N) and especially dissolved reactive phosphorus (DRP) concentrations in the surface waters were very high (particularly in Waubesa and Kegonsa), indicating neither nutrient was limiting algal growth. No P legacy from the wastewater inputs was found in Waubesa and Kegonsa's sediments; minimal P-binding potential due to low iron (Fe) availability is the hypothesized reason. Mendota's algal blooms were not a problem until the mid-1940s when wastewater inputs from upstream communities increased as well as the agricultural use of N and P fertilizers. This increase in eutrophication symptoms coincided with an increase in indices of DRP and inorganic N concentrations in the lake. After wastewater diversion in 1971, blue-green algal blooms persisted in Lake Mendota, and the onus of the problem shifted to agricultural and urban nonpoint source pollution. While much progress has been made in recent years to control these pollution sources to Mendota, manure runoff during late winter continues as a management problem. As evidence, P loadings during January to March constituted 48% of total loadings measured for 1990–2006 in the Yahara River subwatershed. Much of this runoff P was dissolved and not associated with high sediment loads, whereas during other months, more of the runoff P was bound to sediments that could settle out in lower stream reaches prior to entering the lake. However, low P-binding potential of recently deposited sediments in Mendota along with signs of water quality improvements following periods of drought indicate the lake could respond rapidly to nutrient input reductions. Finally, DRP and inorganic N concentrations since 1980 have indicated that algal growth in the Yahara lakes during July–August may have been limited by not only P, but N (especially in the lower Yahara lakes). Aggressive programs to reduce inputs from both nutrients will be important to prevent scum-forming blue-green algal blooms and filamentous algal growths that could become problematic once zebra mussels become established in the Yahara lakes.