Testimony of Professor David A. Culver, Ph.D.,
Dept. Evolution, Ecology, and Organismal Biology and
The Graduate Program in Environmental Science,
The Ohio State University,
1735 Neil Ave.,
Columbus, OH 43210. 28 July 2002
Anoxia in central Lake Erie
The Problem: Lake Erie water quality affects drinking water, swimming, and fish survival
High availability of phosphorus decreases Lake Erie water quality. Low water quality increases the amounts of taste and odor causing compounds and even toxic compounds from algae in drinking water. Toxic algae tend to float to the surface in later summer and can be blown to shore, increasing the likelihood they will be taken in by potable water intakes and causing risks for swimmers, and for wildlife, livestock, and pet animals that may drink from the shore of the lake. Toxic algae have been shown to negatively affect the food chain upon which fish depend. Bacterial contamination from combined sewer overflows similarly affects these groups.
Causes: The thin Central Basin hypolimnion makes it susceptible to anoxia The cool layer at the bottom of the lake (the hypolimnion) receives too little light for much photosynthesis, and is cut off from atmospheric oxygen because it is denser than the warm layer (epilimnion) floating on top. Because of the shape of Lake Erie, its central basin hypolimnion is only 2 or 3 m deep, whereas its epilimnion is 18 m deep. As the lake decreases to water levels closer to the long-term average, the hypolimnion can become even thinner. Algae and animals produced in the epilimnion die and release feces that settle into the hypolimnion, where they decompose, consuming oxygen. The more nutrients available in the epilimnion, the greater the algal growth there. The more algae produced, the faster the rate of consumption of oxygen in the hypolimnion. It is a race between the rate of consumption of oxygen and the occurrence of the totalcirculation of the lake in September, which is caused by cooling of the surface waters.
Effects: Low oxygen in the Central Basin bottom waters decreases fish habitat Most fish species cannot tolerate oxygen levels less than 3 ppm (e.g. walleye, yellow perch), and some require 4 ppm or more. Because the central basin is very flat, an increase in the area where concentration at the bottom is less than 3 ppm will greatly decrease the area useable by game fish and small fish upon which they depend for food. Lower concentrations yet will kill the benthic insects (e.g., mayflies) and plankton that these fish eat.
Effects: Low oxygen in the Central Basin bottom waters recycles phosphorus, producing more algae Phosphate ions in the sediments are bound by iron and clays fairly well under aerobic conditions. When sediments become anoxic, however, the ferric iron is reduced to ferrous iron and the phosphate is then much more soluble and diffuses out of the sediment. This phosphate can be mixed up into the surface waters when the lake circulates in September, causing additional algal growth.
Effects: Algae decreased in abundance from 1970 to 1997, but have increased since then
Central Basin algae biomass declined from 3 to 0.6 g/m3 from 1970 to 1997, but 2001 abundances (2.0 g/m3) (please see Figure 1) are now as high as they were in the early 1980s, suggesting that water quality improvements are being reversed. This is all reflected in the planktonic animals in the lake (Please see Figure 2). Algae increases are made up in part by toxic strains of Blue-green Algae, which had become rare in the early 1990s. EPA phosphorus data also show this trend. There is no evidence that increases in inputs from the watershed have occurred, although accurate estimates of inputs are difficult to obtain.
Possible Causes: Zebra mussels have recycled phosphorus Zebra mussels have recycled phosphorus and nitrogen in algae that otherwise would have settled to the sediments and stayed there. They consume algae all year round, providing continuous recycling of nutrients that can encourage algal growth. Their effects will be particularly felt in the western basin and near shore, but these waters also flow into the central basin where the anoxic hypolimnion occurs.
Possible Causes: Quagga mussels are replacing zebra mussels Quagga mussels (another introduced species) are replacing zebra mussels in the whole lake. Our preliminary data suggest quagga mussels excrete more phosphate and ammonia than do zebra mussels for equivalent-sized individuals.
Possible Causes: Combined sewer overflows bypass nutrient removal at sewage treatment plants
Phosphorus and nitrogen inputs to the lake are increased by storm-induced overflows from combined storm water and sanitary sewers.
Solutions: zebra or quagga mussels cannot be removed There is no way to remove zebra or quagga mussels from the lake.
Solutions: decrease human input of nutrients If recycling by animals in the lake is increasing, our only solution is to decrease inputs of nutrients, particularly phosphorus, from point and non-point sources. As the human population increases in the Lake Erie watershed, it will require even greater efforts to decrease nutrient inputs.
Solutions: support better nutrient modeling of the lake Scientific studies of the interactions among water circulation, nutrient inputs, and the plants and animals in the lake are hampered by incomplete information on the sources and amounts of nutrients coming in from rivers and direct discharge into the lake. I recommend increased efforts in monitoring inputs of nutrients, especially phosphorus and nitrogen into the lake.
Figure 1. Seasonal
(May-September) averages of phytoplankton algae wet weight (g/m3) in
the western (WB), central (CB), and eastern basins (EB) of Lake Erie. The toxic Microcystis bloom in 1998
caused a very high algal weight (4.6 g/m3) in the western basin.
This value was not included in the regression line. Names above the regression lines indicate the sources of data.
Figure 2. Seasonal (May-September) averages of crustacean zooplankton dry weight (g/m3) in the western (WB), central (CB), and eastern basins (EB) of Lake Erie. Contributions of rotifers and zebra and quagga mussel larvae are not included. Names above the regression lines indicate the sources of data.