Eutrophication is the process by which lakes acquire high concentrations of nutrients which promote excessive algae growth. Beginning in the early 1970’s, point sources of phosphorus (P) in tributaries to Lake Erie dropped precipitously and there was a continuous improvement in water quality. Since the mid-1990’s there has been a clear process of re-eutrophication. Algal biomass has increased in the western and central basins along with water column oxygen demand and total phosphorus concentration. Nonpoint sources now make up the largest, although highly variable, source of total phosphorus.

Dave Baker, Professor Emeritus and founder of the National Center for Water Quality Research (NCWQR) at Heidelberg University in Ohio opened the MI-SWCS conference in East Lansing, A Matter of Balance: Feeding our Crops and Protecting our Water in a Changing Climate with his presentation “Long-Term Trends in Agricultural Runoff to Lake Erie: Causes, Consequences and Remedies.” The NCWQR has decades of water quality data gathered at 16 water sampling stations in tributaries in eight agricultural watersheds contributing to the western basin of Lake Erie. Automatic samplers collect water samples every eight hours and each sample is analyzed for at least eleven water quality parameters including total phosphorus (TP), dissolved reactive phosphorus (DRP), various forms of nitrogen and suspended sediment.

Total annual phosphorus loading in Lake Erie is greatly influenced by storm events. Most runoff and nutrient loss occurs from December through March. Water quality monitoring shows that particulate phosphorus has decreased over time, but DRP has increased. This is alarming because DRP stays in the water column and is 100 percent bioavailable for algae and aquatic plants. Particulate phosphorus is linked to erosion and is only about 26 percent bioavailable and tends to settle out of the water column. The reasons for the increase in DRP are not clear.

Various agencies have suggested target load reductions to curtail the hypoxia problem. The current estimate is that DRP in runoff is only about two percent (1.5 pounds per acre-year) of what is applied annually. Farmers are not losing much P but the goal is for a 40 percent reduction in DRP in the western basin and an 80 percent reduction in the central basin. Possible sources include soil P stratification from long-term no-till or reduced tillage, the extensive network of tile drainage throughout the watersheds, changes in soil pH, fall surface broadcast applications of P on agricultural fields, urban point sources and combined sewer over-flows. ‘Hot spots,’ fields with excessively high soil P test levels could be contributing but there is ample evidence from water quality data that tile drains are also contributing N and DRP. It is also possible that the lake is now more sensitive to DRP loading because of zebra mussel activity and warmer lake temperatures. The solution to the problem is neither simple nor obvious.

As a Michigan State Universy Extenstion educator I asked Dave Baker if there is one or two things farmers could do to have the greatest impact on improving water quality. I thought he might suggest avoiding fall broadcast applications of P fertilizers or some other production practice. Instead, he emphasized the need for continued water quality monitoring, research, and education. Because there are so many potential contributing sources of phosphorus, and losses are so greatly influenced by timing and intensity of rainfall, there really is no single change in production practice that will be most effective every year.