Why is Anoxia again occurring in the central basin of Lake Erie? There is no one answer to this question. Scientists do know that the bottom waters in the central basin of Lake Erie became anoxic (without oxygen) in late summer last year. Aquatic creatures need oxygen in the water to survive. When the oxygen is depleted, organisms suffocate, and a barren area, often referred to as a “dead zone,” is all that remains of once thriving aquatic populations.
Anoxia has been a historical problem since the 1930s. The problem was thought to have peaked in the late 1960s and early 1970s. However, it was largely alleviated at that time by pollutant and discharge regulations that included phosphorous controls, the banning of phosphates in detergents, and encouraging the construction and upgrading of sewage treatment plants around the Great Lakes.
Last summer, researchers were alarmed by increased phosphorus concentrations in Lake Erie as well as by anoxia in the cold, deep waters of the central basin. There are a variety of factors, which may be contributing to this problem. The configuration of the central basin of Lake Erie is partly responsible for the problem, the discharge of too many nutrients from human activities is also a major factor, and the problem may also be intensified by lower water levels,warmer water, and clearer water.
Clearer water allows sunlight to penetrate further, which contributes to algae growth. Lake Erie waters are clearer since the invasion/colonization of the lake by zebra and quagga mussels, which consume and filter floating debris. Massive die-offs and decay of exotic species unsuited to ecosystem conditions in the lake may also be consuming oxygen.
Zebra and quagga mussels may be contributing to the increased phosphorus concentrations of Lake Erie. Work at the Great Lakes Environmental Research Laboratory (GLERL) suggests that zebra and quagga mussels may be responsible for recent water quality problems in the Great Lakes www.glerl.noaa.gov/pubs/brochures/ mcystisflyer/mcystis.html.
The GLERL has shown that zebra and quagga mussels influence nutrient availability. Where phosphorus is found in higher concentrations, such as in the relatively phosphorus-rich waters of Lake Erie, mussels increase phosphorus availability by immediately excreting the phosphorus contained in the phytoplankton they ingest. In other systems, where phosphorus is found in very low concentrations, phytoplankton contain little phosphorus, and the mussels retain the phosphorus in their tissues, because they need a certain amount of phosphorous to survive.
The anoxia problem of Lake Erie may be related to weather, mussels, human influences, or some combination of these factors. In an effort to unravel this mystery and determine the cause of oxygen depletion in central Lake Erie, the United States Environmental Protection Agency in cooperation with universities and other agencies in the U.S. and Canada, began an intensive study in June of 2002. Stay tuned for the conclusions of this important study.