LAKE EUTROPHICATION IN WESTERN WISCONSIN: NATURAL VS ANTHROPOGENIC PHOSPHORUS?

Lake eutrophication events in western Wisconsin are characterized by large algal blooms that have negative social, health, and economic impact. It is apparent that the problem is becoming increasingly worse, despite recent changes in agricultural practices. Lake eutrophication events are not consistent across the upper Mississippi River watershed, but seem to vary with bedrock geology, hydrogeology, and land use. Lake eutrophication results from excessive nutrient loading (phosphorous, nitrogen), and mitigation requires quantification of the ultimate nutrient source.

Lake eutrophication is assumed to be the product of nutrient overload from agricultural runoff, which is supported by a regional surface water study showing elevated concentrations of phosphorus throughout western Wisconsin. In addition, seasonal variations have been documented between peak agricultural months (July: ~160-180 ppb), and winter months (November: ~20-95 ppb).

The same regional study analyzed groundwater and documented elevated levels of phosphorus up to an order of magnitude greater than surface water levels. These groundwater phosphorus values are curious, since it is believed that agricultural phosphorus is absorbed by the soil profile. Therefore, the ultimate source of phosphorus in groundwater is unknown, and the relative contribution of anthropogenic vs natural phosphorus is unclear.

The role of natural phosphorus in lake eutrophication is an outstanding issue. Aquifers in western Wisconsin are largely hosted in by Paleozoic sandstone which, on average, (~2000 ppm) hold more phosphorus than groundwater (~ 0.5-1.6 ppm), and surface water (<0.2 ppm). Sequential extraction analysis demonstrates that phosphorus is bound with the iron-manganese oxides and is easily liberated under anoxic conditions. These data suggest that temporal variations in anoxic conditions in subsurface aquifers could lead to an influx of phosphorus directly from Paleozoic bedrock. It is possible that phosphorous naturally derived from Paleozoic bedrock may lead to elevated concentrations of phosphorus in the groundwater system, in turn elevating surface water concentration. The addition of agricultural phosphorus to naturally elevated phosphorous in surface water may be driving lakes past the tipping point to lake eutrophication.