Section 3: Phosphorus (Annex 3)

Observation

Calculations suggest that limits on external phosphorus loads are generally being achieved and that known phosphorus sources were not the cause of increased phosphorus concentration levels in Lake Erie during the 1990s. Despite this, the concentrations of phosphorus in Lake Erie exceed limits, and suggest that eutrophication (excessive nutrient enrichment) is occurring. The Parties must invest in new basic research, monitoring and information sharing if they are to achieve the binational goal for the management of phosphorus under Annex 3 for Lake Erie. As reported in newspapers of late, the discovery of a "dead zone" in Lake Erie is puzzling scientists, with causes currently undetermined.

Discussion

Twenty-five years ago, numerous scientific studies conducted by the Commission's Pollution From Land Use Activities Reference Group resulted in the Parties adopting policies and programs to manage phosphorus for each lake basin through a variety of point and nonpoint source control measures. This linkage of science and policy resulted in programs to reduce phosphorus loads to the Great Lakes based on reduction targets as set out in Annex 3. The achievement of a target load for each lake is currently represented by a specific outcome: concentrations of phosphorus in the open waters. For lakes Superior, Huron, Michigan and Ontario, concentrations indicate that progress has been sustained.¹ In the case of Lake Erie, however, open water concentrations of phosphorus often exceed the guideline, indicating that phosphorus is being released into the lake by sources or processes not fully understood.

Better information, derived from monitoring programs that can confirm the sources and loadings of phosphorus to the lake, could lead to improved policy decisions. As municipal wastewater discharges appear to decrease in importance due to technological improvements and infrastructure upgrades, reduced water quality monitoring efforts associated with this sector are often perceived as justified. However, increased effort in point source and tributary monitoring may be warranted to account for the tremendous changes that have occurred in the basin over the last 25 years, particularly as a result of urbanization and the increase in pavement and other practices that create land that is impervious to water.² The impact of large point sources, such as Detroit's sewage treatment plant, is difficult to assess because suitable phosphorus loading data are not readily accessible for independent or binational interpretation. Although municipal sources may not provide the greatest contribution to the total phosphorus inputs to the lake, their impact can be significant, since the phosphorus they discharge is in a form that is more readily utilized by living things than is the predominant form transported to the lakes from nonpoint sources.

Tributaries are large sources of total phosphorus to the lake; however, because some forms of phosphorus are not readily bioavailable, good data are needed to distinguish phosphorus species and their relative contribution to increasing concentrations in the open waters. Current tributary monitoring efforts do not provide such loading estimates. Some streams are sampled no more than monthly and others have not been regularly sampled since 2000. The number of unmonitored streams is increasing, adding additional uncertainty in determining the primary sources of phosphorus to the lake.

Major tributaries to Lake Erie, such as the Maumee River, have achieved notable decreases in suspended sediment discharges and reductions in phosphorus loads as a result of improved agricultural practices.³ However, these tributaries are still very large sources of phosphorus with year-to-year loads varying with the frequency and intensity of flooding. For example, phosphorus stored in the sediment of tributaries can build up during dry or average rainfall years and can serve as a substantial load to the lake during a single flood event. Such major events could become common in the Great Lakes as a result of climate change, adding a further management challenge to achieving target loads. The Commission is evaluating the potential impacts of climate change based on different predictive models, and will report on this during the next few years and in its Twelfth Biennial Report.

Scientifically, the phosphorus cycle is well understood. However, detailed information to assess options to manage phosphorus effectively in Lake Erie has not been available.⁴ Because of the level of uncertainty, scientific knowledge based on the past should not be assumed to be adequate to serve as a basis for decision-making for the future. New investment in basic research, monitoring, and information sharing for Lake Erie needs to include improved long-term monitoring of point and nonpoint sources and tributaries, calculation of the rate of inter-basin transfer of nutrients, and development of better tools for making predictions.