4.0 Limnology of Long Point Bay

4.2.4 Limnology of Lake Erie

Figure 4.1 Municipal Phosphorus Loadings to Lake Erie, 1972-1990.

Table 4.2 Municipal phosphorus loadings throughout the Great Lakes Basin between 1986 and 1990. Values are in tons per year.

Lake Erie, with a surface area of 25,657 km², is the 9th largest freshwater lake in the world by area and the 15th by volume (Herdendorf 1990a, 1992). It is the warmest of the Great Lakes (5.6°C warmer than Lake Superior), and being the shallowest, has the largest annual range of surface water temperature. The average summer temperature is 22°C and the average for winter is 1°C (Brown et al. 1974; Philips and McCulloch 1972). Prior to the 1980s, due to its shallow depth, warm water, and substantial point source loadings of nutrients and contaminants, large portions of Lake Erie were eutrophic and heavily polluted. The lake was oxygen starved and supported dense concentrations of algal blooms (Beeton 1969; Francis et al. 1985).

Legislation to reduce the allowable levels of phosphorus in laundry detergents and the upgrading of sewage treatment plant operations to include chemical precipitation of phosphorus (Phosphorus Management Strategies Task Force 1980; Nicholls and Hopkins 1993) resulted in substantial reductions in the amount of phosphorus entering Lake Erie during the 1970s (Figure 4.1)(Nicholls et al. 1977, 1980). In particular, substantial declines in the annual total phosphorus loading to Lake Erie were achieved through improved sewage treatment and phosphorus removal at the Detroit Metro Wastewater Treatment Plant during the 1970s and early 1980s (Hartig 1983). The result was a substantial decline in the production of phytoplankton and blue-green algae and a reduction in the eutrophication of Lake Erie. Nevertheless, municipal phosphorus loading to Lake Erie still exceeds that received by all the other Great Lakes (Table 4.2).

Zebra mussels were introduced to Lake Erie in 1988. They feed by indiscriminately filtering biotic and abiotic material from the water column and this has resulted in a net decline in Lake Erie plankton, increased water transparency and further oligotrophication of the lake (Holland 1993; Leach 1993; Stanczykowska 1984).

The orientation of Lake Erie along the same axis as prevailing winds results in rapid changes in lake levels (wind set-up and seiches - see Chapter 2). The consequent oscillating volumes of water that enter Long Point Bay and associated wetlands cause fluctuations in the chemical and biological nature of the water being transported into the Bay (Bedford 1992). Consequently, the Inner and Outer Bays and their associated wetlands are strongly influenced by the nutrient and contaminant load carried by Lake Erie, as there is a constant flux of water and suspended material between the lake and Long Point Bay.

Table 4.2 Municipal phosphorus loadings throughout the Great Lakes Basin between 1986 and 1990. Values are in tons per year.