Great Lakes Ecological Protection and Restoration
Table of Contents
- Bioaccumulative Toxic
- Contaminated Bottom
- Diminished Wetlands
- Exotic Species
- Depleted and Volatile Fish
- Excessive Phosphorus
Great Lakes Report to Congress 1994
REPORT TO CONGRESS ON THE GREAT LAKES ECOSYSTEM
Aspects of Ecosystem Health
This chapter discusses six general problems facing the Great Lakes ecosystem:
- Contamination of fish and wildlife with bioaccumulative toxic substances
- Contaminated bottom sediments
- Lost, degraded, and threatened wetlands
- Exotic species
- Depleted native fish populations
- Excessive phosphorus
Substantial progress has been made during the last 20 years in abating several of these problems. Levels of some targeted contaminants in fish and wildlife are strikingly lower. The impacts of one especially troublesome exotic species, the sea lamprey, have been substantially reduced. Levels of phosphorus are also much lower, notably in Lake Erie where they had been most disruptive.
Yet challenges remain. Public health authorities continue to issue fish consumption advisories for each Lake. Though the present rate of habitat destruction is much less than in the past, various human activities encroach on remaining wetlands and other valuable fish and wildlife habitats. The entry of exotic species has increased since the opening of the St. Lawrence Seaway in 1959, which permitted more transoceanic shipping. Although much improved, Lake Erie continues to suffer depletion of dissolved oxygen during late summer.
Bioaccumulative toxic substances generally do not pose a problem for humans Great Lakes drinking water. Concentrations of toxic substances in the water column are extremely low because they tend to bind quickly to particles--phytoplankton or sediment--and either enter the food web or fall to the bottom where they are eventually buried. They also volatilize into the atmosphere. Open-lake concentrations of contaminants are measured in parts per billion or trillion. A part per trillion represents a teaspoon in 1.3 billion gallons of water. A person would have to drink two or three million gallons of water to be exposed to a quantity of contaminants equivalent to that ingested by eating a single mature lake trout.
Some bioaccumulative toxicants do pose problems when concentrated in the tissues of predators, such as lake trout. At the base of the food web, microscopic floating one-celled plants (i.e., phytoplankton) use sunlight, dissolved carbon dioxide, and dissolved mineral nutrients for nourishment. Microscopic animals (i.e., zooplankton) feed on such vegetation and are in turn eaten by fish. Tiny sediment- dwelling insects and crustaceans are another source of food for some small fish. Higher predators (e.g., fish and birds) consume smaller fish. Figure 2-1 provides a simplified view of the food web. The figure does not show the many different species of phytoplankton, zooplankton, bottom animals, fish, and wildlife that make up the actual Great Lakes food web, but does display the food web concept.
Phytoplankton, zooplankton, and bottom animals adsorb and retain contaminants. Fish that graze on plankton in turn ingest the contaminants that they contain. When fish ingest contaminants faster than they use or excrete them, they accumulate levels of contaminants that are higher than those in their forage. The increasing concentration of contaminants at successive levels of the food web, known as biomagnification, is repeated as predator fish feed on smaller fish, mammals and birds feed on fish, and predator birds feed on smaller birds. Figure 2-2 illustrates biomagnification in the Lake Ontario food web. In 1982, high predators, such as herring gulls and lake trout, accumulated polychlorinated biphenyls (PCB) levels that were, respectively, 6,000 and 560 times greater than those in plankton.
Fish and birds living in or around Lakes Michigan and Ontario tend to have markedly higher levels of contaminants than those of the other three Lakes. The relatively low levels in Lake Erie biota are a bit surprising since the Lake has a high surrounding population and is known to receive high loadings of toxic substances. One possible explanation is that Erie's relatively high sedimentation rate may remove toxic substances from the water column, making them less available to the food web.
There have been striking declines in some targeted substances during the last two decades. Figures 2-3 and 2-4 show declines in two substances, PCBs and the pesticide DDT (in its derivative form, DDE), in Lake Michigan herring gulls and bloater chubs. Despite these marked declines, levels of contaminants remain unacceptably high. State public health authorities issue fish consumption advisories, usually directed at PCBs, mercury, and chlordane, for species in each Lake, and in various rivers and bays. Table 2-1 shows examples of these advisories.
During the last few decades, researchers have observed population declines and health problems in about 15 Great Lakes fish and wildlife species. These declines and problems seem to be associated with exposure to various bioaccumulative toxic substances. Effects have usually been most pronounced at the top of the food web and across generations, as expressed in birth defects. Other problems that have been noted in fish and wildlife include loss of appetite and weight, hormonal changes, poor reproductive success, tumors, increased susceptibility to disease, and behavioral changes. With the reduction of many targeted pollutants in the food web, the populations of affected species generally seem to be improving.
Since EPA and States cancelled and restricted the bioaccumulative pesticides DDT and dieldrin in the 1970s, improved bald eagle reproductive success has led to a recovery in the national population. However, bald eagles have not recovered so vigorously along the shores of the Great Lakes. Researchers have noted that eagles do not reproduce as successfully along the Lakes as they do inland. Great Lakes fish may provide too toxic a diet for bald eagles to thrive.
During the 1970s, herring gulls around the Great Lakes were also found to have reproductive problems. Changes in behavior were a contributing factor to population decline--herring gulls neglected their nests, which caused low hatching success. During the 1980s, herring gull populations have strongly increased as PCBs and pesticides have decreased in the food web.
Also during the 1970s, scientists observed deformities in various bird species, such as double-crested cormorants, common terns, caspian terns, ring-billed terns, and herring gulls. Birds were found with crossed bills, jaw defects, and malformed feet and joints. Although the incidence of these deformities has declined in conjunction with contaminant levels, problems remain in relatively contaminated areas.
Mink have proved extremely sensitive to a diet of Great Lakes fish. In the mid-1960s, mink breeders found that their animals were experiencing high mortality rates and almost complete reproductive failure. The ranch animals were being fed fish from Lake Michigan tributaries. Laboratory toxicology experiments determined that mink are highly sensitive to PCBs. As with bald eagles, it is thought that wild mink populations are larger inland than along the shores of the Great Lakes.
Another suspected effect of bioaccumulative toxic substances on fish has been noted in bottom-dwelling fish, such as bullheads and suckers. These have been found to suffer a high incidence of dermal and liver tumors at a number of Great Lakes locations. The incidence of tumors is strongly correlated with polluted conditions, especially with the presence of polyaromatic hydrocarbon (PAH) contamination in bottom sediments. Several PAH compounds are known or suspected carcinogens. Although little is known about the significance of tumors on either the health of fish or on the health of humans who might eat these fish, visible abnormalities reduce the commercial and recreational value of fish.
EPA has established water quality criteria for about 130 substances that are known or suspected to be harmful to humans, fish, or wildlife. Criteria numerically define maximum allowable concentrations of a contaminant in water and serve as a basis for the development of State Water Quality Standards.
EPA and States have identified a set of pollutants deemed especially injurious and often present in the Great Lakes ecosystem. Table 2-2 summarizes selected priority pollutants. Pollutants listed in this table tend to concentrate up the food web. Several are the most toxic members of groups of related chemicals.
Bioaccumulative toxic substances reach the Lakes from a broad range of human activities. Some sources are more clearly measurable, such as discharges from sewage systems and industry and spills from ships and shore. Other sources are known to be significant, but are difficult to measure: deposition of contaminants from the atmosphere, movement of contaminants through groundwater, and urban and agricultural runoff. Contaminants reach the atmosphere from combustion and volatilization. They exist in the atmosphere attached to particles, associated with water droplets, and in their gaseous state. They leave the atmosphere via dry deposition of particles, rain and snow, a gas exchange to water.
In the late 1970s, studies on Isle Royale, a relatively isolated island in Lake Superior, reported PCBs, DDT, and toxaphene in the waters of its lakes. Researchers theorized that such pollution must have resulted from atmospheric deposition. Since toxaphene was principally used to reduce insects on cotton crops in the South, it was thought that toxaphene had been transported a great distance through the atmosphere.
Researchers have subsequently tried to estimate the extent of atmospheric deposition of contaminants to the Great Lakes. Atmospheric deposition may be the largest path for some contaminants to enter Lake Superior, for instance, because of the Lake's relative lack of adjacent development and its large surface area. Yet there are substantial uncertainties surrounding such estimates.
Recent research in Minnesota and Wisconsin has concluded that the atmosphere is a significant pathway for mercury, which is emitted by garbage incinerators and coal-burning power plants, among other sources. In the last several years, Michigan has issued advisories regarding fish consumption for thousands of its inland lakes based on levels of mercury, while Minnesota and Wisconsin have issued similar advisories for hundreds of their inland lakes. The issuance of these advisories partly reflects the expansion of fish monitoring programs. Though there are atmospheric loadings of mercury across the entire region, differences in water chemistry and bacteria between waterbodies cause mercury levels to be more of a problem in the fish of some lakes than in others. Mercury levels in walleye and lake trout have sharply fallen in areas of the Great Lakes where they were highest two decades ago following the modification or closure of pulp and paper mills and chloralkali plants that were then the major source of loadings. In general, there are no clear indications that mercury levels are rising in Great Lakes fish, although the evidence of atmospheric loadings to the region warrants continued monitoring.
Bottom sediments that hold such substances as PCBs and DDT are probably the principal cause of the continuing contamination of fish and wildlife with these now restricted chemicals. The transfer of sediment-bound contaminants to the base of the food web takes place both directly, through accumulation of contaminants in bottom-dwelling organisms, and indirectly, through resuspension of contaminants to the water column and their ensuing adsorption by phytoplankton. Contaminated sediments are also toxic to bottom-dwelling organisms, killing them or impairing their normal functioning. Sublethal effects associated with contaminated sediments include tumors in bottom fish. Brown bullheads, a variety of bottom-feeding catfish, have been found with a high incidence of facial tumors in the Buffalo River in New York and the Black River in Ohio where they are exposed to contaminated sediments.
Contaminated sediments also impose economic costs. Special steps are required to dredge and dispose of contaminated sediments, which increase the cost of maintaining waterways for navigation. In a number of locations, including Indiana Harbor, Indiana; Ashtabula River, Ohio; Sheboygan Harbor, Wisconsin; and Menominee River, Michigan, navigational dredging has been delayed for years because of issues surrounding disposal of dredged sediments. Reduced dredging increases transportation costs because industries must find alternative transportation methods or reduce ship loads.
Yet, the natural sedimentation process can also cover old contamination with cleaner sediments. This can be an important natural means for the recovery of the ecosystem. The rate of burial differs from location to location and from lake to lake, with Lake Erie having a relatively high rate of sedimentation, and Lakes Michigan and Superior low rates.
EPA and States have designated 31 harbors and rivers in the region, all of which have contaminated bottom sediments, as Areas of Concern. Bottom sediments in these areas contain a wide range of contaminants, including toxic metals, such as copper, lead, nickel, and zinc, as well as chemicals. Figure 2-5 illustrates the geographical zone of sediment contamination in one Area of Concern, the Detroit River.
Another indication of the scope of the contaminated sediment problem is that in recent years, to maintain navigation channels, the Army Corps of Engineers has dredged a large volume of sediment from the Lakes that is too contaminated for open-lake disposal. As directed by the Water Resources Development Act, the Corps places such material in confined disposal facilities (CDFs), which are structures designed to hold and isolate contaminated dredged materials. There are 43 CDFs completed or under construction; one-third are on land, and two-thirds displace water. The Corps adds about two million cubic yards of sediments to them annually. This represents about one-half of the total volume of sediment dredged by the Corps each year in the Great Lakes. Although they may lower the transfer of contaminants to the Great Lakes food web that would otherwise take place if contaminated bottom sediments remained in place, CDFs encroach on the Lakes and require ongoing monitoring and periodic maintenance.
A wetland is an area such as a marsh, swamp, bog, or fen. A vital component of the Great Lakes ecosystem, wetlands serve a variety of important functions, providing nursery, resting, feeding, and breeding grounds for a rich diversity of birds, fish, and wildlife. Wetlands protect a variety of fish species from waves and predators. Coastal wetlands offer fish warmer temperatures than open-lake waters. Larval and juvenile fish harbored by wetlands are an important food source for waterfowl. Ducks consume plants that extend above and below the water, and geese graze on plants above water. Wetlands also protect shorelines from erosion, store flood waters with their dense vegetation, and trap sediments that can pollute waterways. Many of the wetland areas of the Great Lakes watershed have been lost during the last two centuries. On the Canadian side, it is estimated that between 1800 and 1982, more than 60 percent of the wetlands in southern Ontario were lost. In southwestern Ontario, more than 90 percent have been converted to other uses. Similar losses have occurred in the United States. On a statewide basis, Illinois and Indiana have each lost more than 80 percent of their original wetland acreage. Ohio is believed to have lost 90 percent of its wetlands, with the 1,500 square mile Black Swamp of northwest Ohio almost entirely converted to farmland by the 1920s (Figure 2-6).
The most extensive losses took place in the nineteenth and early twentieth centuries when many wetlands were drained for agricultural use. Remaining wetlands continue to be threatened by building construction, waste disposal, and mining of sand. Consumption of groundwater has diminished recharge of certain wetlands. There are also indications that wetlands have been disrupted by nonnative plants, such as purple loosestrife, and fish, such as carp.
During the past 200 years, humans have introduced more than 130 exotic (nonnative) species to the Great Lakes, many of which have profoundly affected the populations of native species. Exotics damage native populations through direct competition for food, displacement from physical environments, direct attack, and alteration of the chemical or physical conditions needed by other species.
Some introductions have been intentional, such as those of carp and Pacific salmon (chinook and coho). Since the 1960s, salmon have been regularly stocked by States and the Province of Ontario to provide an additional predator to control the numbers of smelt and alewife (which are also exotic species). In addition, salmon provide sport fishing alternatives to greatly diminished lake trout populations. Many other introductions of exotics have been unintentional, such as sea lamprey, alewife, zebra mussel, and smelt.
Introductions of exotics have accelerated during the last 30 years, as shown by Figure 2-7. Of the exotics introduced to the Lakes since 1810, about one-third have appeared since 1960. This increased pace is largely due to greater transoceanic shipping traffic on the Great Lakes since completion of the St. Lawrence Seaway in 1959. Oceangoing vessels have often taken on ballast water in a distant port and later discharged it into the Lakes to compensate for the on-loading of cargo. Ballast water can sustain exotic organisms until it is released into the Lakes. Thus, oceangoing vessels have often spanned saltwater barriers to freshwater species from other continents.
Figure 2-8 shows the routes by which exotic species are believed to have entered the Great Lakes. Nearly one-third of exotics have been stowaways on ships in cargo, ballast tanks, or attached to hauls. Organisms that can survive in ship ballast tanks are frequently very adaptable and aggressive. When released to an ecosystem in which they have few natural predators, they can proliferate and severely affect the existing balance between native species. The transfer of exotics through ballast water can be prevented if ships take on ballast water at sea before entering the Great Lakes. Saltwater organisms are unlikely to survive in the Lakes.
Exotics have also made their way into the Lakes via canals. Species that had been barred from the upper Lakes by Niagara Falls were able to enter them after the Welland Canal was completed or enlarged.
Fish species are among the best known of the exotics. Yet, numerous other exotics have also been introduced. Plants represent about 45 percent of exotics, fish 18 percent, and algae 18 percent (Figure 2-9).
Zebra mussels may prove to be the most harmful exotic yet introduced to the Great Lakes. Named for their distinctive black and yellow bands, this tiny barnacle-like shellfish (up to two inches long) is found throughout Europe. Zebra mussels are prolific breeders; female mussels produce as many as 400 surviving offspring each year. Zebra mussels were first noted in Lake St. Clair in 1988.
Since then, they have been found in numerous locations, from Duluth to the entrance of the St. Lawrence River. They have infested Lake Erie with impressive speed, colonizing nearly every available surface in just two years. It is expected that the species will occupy most of its suitable living environments within the Lakes over the next several years. It also seems inevitable that the zebra mussel will eventually spread through much of America, via pathways such as the Chicago River to the Mississippi River system and carried by ships and recreational boats. Adult mussels cling to boat hulls from which they detach during journeys. As of fall 1991, mussels had already been noted in the Hudson, Susquehanna, and Mississippi drainages.
Zebra mussels cement themselves to hard surfaces, building grape-like clusters more than six inches thick; densities up to 700,000 to the square meter have been found in Lake Erie. The lifespan of the species is three to five years. Zebra mussels favor relatively warm, nutrient-rich, shallow water (6 to 30 feet deep). Microscopic mussel larvae float freely for 10 to 15 days, carried by currents before finding a suitable hard surface to which they attach and mature into the familiar mussel form.
The zebra mussel poses many ecological problems. One adult mussel filters the suspended phytoplankton from one liter of water per day. A large population of zebra mussels can devour a vast quantity of phytoplankton, the foundation of the Great Lakes food web, and may in time create a food shortage for other phytoplankton grazers and ultimately reduce the food supply of predators, such as lake trout, salmon, walleye, and bass. Zebra mussels also threaten the spawning sites of native fish. Many species, including walleye, prefer rocky shoals for spawning and must compete with zebra mussels which favor this habitat for colonization. In addition, zebra mussels coat clams and crayfish, making it difficult for them to open or move.
The mussels have economic impacts as well, because they clog municipal and industrial water intakes. Many hundreds of millions of dollars will have to be invested in the redesign of intakes to reduce their vulnerability to mussel fouling, extension of pipes into deeper water, and periodic mussel removal. Mussels also encrust and slow ships and infiltrate and clog their ballast and cooling systems. Beaches can be fouled by the odor of decaying zebra mussels, and bathers at some beaches have to wear foot protection to prevent cuts from mussel shells. Dead mussels also give off methane gas, imparting a foul taste and smell to water. In addition, the mussel attaches to navigational buoys, breakwater rocks, piers, and fish nets.
Freshwater drum, also known as sheepshead, is a native fish species that feeds on zebra mussels. Scaup, a diving duck that migrates through the Lakes, is another mussel predator. Yet, scientists think that these natural predators will be unable to arrest the explosive growth in numbers of zebra mussels.
The sea lamprey was one of the first exotic species to devastate native populations. This small, parasitic, eel-like fish attaches to larger fish and lives off their bodily fluids, often killing the host. Native to the Atlantic Ocean, sea lamprey may have made their way into Lake Ontario via the the St. Lawrence River or the Erie Canal. By the mid-nineteenth century, they were present in Lake Ontario but were barred from the other Lakes by Niagara Falls. In the l920s after enlargement of the Welland Canal, they escaped into the upper Lakes and during the next three decades spread throughout them.
Partly as a result of lamprey depredations, lake trout populations in Lakes Huron, Michigan, and Superior collapsed; commercial catches in the 1950s were only one percent of those 20 years earlier., Whitefish and burbot populations were likewise decimated, and walleye and sucker populations were also attacked. As large prey disappeared, lamprey turned to smaller fish, virtually extinguishing several of the larger species of cisco in the upper three Lakes.
The sea lamprey has wreaked less destruction on Lake Erie fish populations. This may be because Erie is warmer and the lamprey prefers the cold environment of the upper Lakes. Or it may be that the lamprey has lacked spawning areas in Lake Erie.
In 1961, the United States and Canada began to apply a chemical to sea lamprey spawning grounds. This lampricide application program has decreased numbers of lampreys by about 90 percent (Figure 2-10). However, complete eradication of the lamprey is probably not feasible, and the control program will need to continue indefinitely to keep the lamprey's predations in check. Today, the lamprey is concentrated in northern Lakes Huron and Michigan and in Lake Superior. The strong currents of the St. Marys River lessen the effectiveness of lampricide application. As a result, a large population of lamprey lives in the river and in nearby reaches of Lake Huron. Lamprey continue to exact a heavy toll, for example, on lake trout in Lake Superior, particularly west of the Keweenaw Peninsula. In the late 1980s, lamprey were estimated to have killed about one-half to one million pounds of lake trout per year in the U.S. waters of Lake Superior.
The river ruffe, a small (typically six to eight inches) perch-like fish from northern Eurasian freshwaters, entered Duluth harbor around 1986 probably from the discharge of ballast water from an oceangoing vessel. The ruffe is hardy and a rapid breeder. A growing population has been noted in the relatively warm and nutrient-rich St. Louis River estuary. In 1989, the ruffe's population was estimated at 300,000; a year later, its population was estimated to have doubled. Scientists doubt that the temperature or food supply of Lake Superior will be a barrier to the ruffe. They think the ruffe will spread eventually, although its pace will not rival that of the zebra mussel. If the ruffe spreads, it may injure desirable native species. It competes for food with native fish, such as yellow perch, and feeds on the eggs of whitefish.
As a first attempt to control the ruffe population in Duluth harbor, fisheries managers stocked walleye and northern pike. Early indications are that walleye are not effective in controlling ruffe if alternative prey is available. Limited sampling of burbot, a voracious member of the cod family, and northern pike show that some had eaten ruffe. Further work is underway to assess the potential of these predators to control the ruffe population.
Spiny Water Flea
Another recent invader to the Great Lakes is the large zooplankton Bythotrephes cederstroemii (spiny water flea). At up to one-half inch in length, it derives its name from its long spiny tail. First noted in Lake Huron in 1984, the spiny water flea is native to Eurasian freshwater.
The effects that the spiny water Flea may have on the Great Lakes ecosystem are not yet apparent. The flea feeds on a few species of Daphnia, another form of zooplankton. Daphnia are an important food source for young fish, such as the bloater chub, and its decline might also bring about an increase in algae on which it feeds. Recent evidence, however, indicates that alewife may consume the spiny water flea, providing a constraint on its population.
The sardine-like alewife is a 4- to 11-inch long member of the herring family. Alewife are native to the Atlantic Ocean and entered Lake Ontario presumably through the Erie Canal in the mid-1800s. Alewife spread to the other Lakes from l931 to l954, after enlargement of the Welland Canal allowed the species to bypass Niagara Falls. Alewife have become a favored food of lake trout and salmon. With the precipitous decline of lake trout populations, alewife populations exploded. In 1967, millions of alewife in Lake Michigan died and washed ashore because of the effects of cold temperatures and hunger. The species may be more vulnerable to such stresses in the Great Lakes than in its native Atlantic waters. It has experienced other occasional die-offs in Lakes Huron and Ontario. Such instability can abruptly decrease available food for valued sportfish that feed on alewife. Stocking of salmon and lake trout have subsequently helped to control alewife numbers, and the species has been harvested commercially for fertilizer and pet food. Alewife are believed to have damaged the populations of several native species through competition for food. Among these are lake herring and emerald shiner, whose numbers have never recovered since the control of alewife.
Before 1800, about 170 fish species existed in the Lakes. Smallmouth and largemouth bass, channel catfish, muskellunge, northern pike, and sturgeon lived nearshore. Blue pike, freshwater drum, grayling, lake herring, lake trout, lake whitefish, sauger, walleye, and white bass inhabited deeper waters. Sturgeon lived 90 years, frequently exceeding six feet in length and 100 pounds; lake trout lived 75 years.
The species mix varied between lakes. A large population of Atlantic salmon was confined to Lake Ontario. Eastern Lake Erie supported lake trout, whereas Erie's warmer, shallower western basin did not. Lake trout and lake whitefish were prevalent and were staples in the diets of Native Americans. Fish populations were richly abundant. Around 1890, commercial fishermen took about five million pounds each of lake trout and lake whitefish from Lake Superior each year. From l920 through 1960, they harvested more than ten million pounds of lake herring from Superior each year.
Today, fish populations are very different. Fish are generally smaller and do not live as long they did two centuries ago. The populations of many native species are not as plentiful and their numbers are much more volatile. Populations surge and fall abruptly.
These changes are caused by a variety of reasons. Food chains have been disrupted (e.g., increased phosphorus levels altered the plankton communities). Nonnative species prey on or compete with indigenous ones (e.g., lamprey feed upon large fish, while alewife and rainbow smelt have displaced lake herring). Fish habitat has been lost or disrupted (e.g., wetlands have been drained, spawning beds have been covered with silt, and dams have impeded passage to spawning grounds). Sport and commercial fishing have sometimes reaped excessive harvests. Fishermen have also done incidental damage to fish populations (e.g., sturgeon were killed because of the damage they did to nets). And, some pollutants are suspected of hindering fish reproduction (e.g., contaminants in lake trout may reduce its reproductive success).
The decline of once native prolific fish populations is profound. Grayling were extirpated by forestry practices that polluted their spawning streams. Atlantic salmon disappeared from Lake Ontario in the nineteenth century, and blue pike vanished in the 1950s. Several species of deepwater cisco in the upper lakes were eliminated by the sea lamprey. Sturgeon and the once predominant forage fish, lake herring, survive in much depleted numbers. Though more thriving, walleye, white bass, and yellow perch are also much reduced from nineteenth century abundance. Hatchery-reared lake trout must be stocked to maintain ecological balance as well as losses to sea lamprey and sport fishing. Lake whitefish in Superior and parts of Lakes Michigan and Huron are sufficiently plentiful to support commercial fishing. Stocked, nonnative Pacific salmon--coho and chinook are the most abundant top predators, except in western Lake Erie where the top predator is walleye.
The depletion and vulnerability of fish populations have brought both ecological losses and economic costs. Populations of fish-eating birds and other wildlife have declined in part because of loss of forage. Programs to stock fish and reduce lamprey have costs. Employment in commercial fisheries has withered.
Nevertheless, heartening progress to improve Great Lakes fish resources has been made. The control of sea lamprey and the stocking of valued fish, notably lake trout and Pacific salmon, have bolstered fish resources and permitted the growth of an important sport fishing industry.
By the late 1960s, various areas of the Lakes exhibited eutrophic conditions, marked by thick algal blooms that imparted unpleasant odors and taste to the water and depleted dissolved oxygen following its decay in late summer. These conditions were most pronounced in Lake Erie, which, as the shallowest, warmest, and biologically most productive lake, is most susceptible to nuisance levels of algae. Lake Erie has also been vulnerable because it surpasses other Lakes in receipt of effluent from sewage treatment plants and of sediment from the rich farmland in its watershed. Both effluent and sediment carried phosphorus to the Lake, altering its chemistry and, as a result, its algae populations. To a lesser degree, eutrophic conditions were also evident in Lake Ontario and in shallow, naturally productive embayments, including Saginaw Bay, Green Bay, and the Bay of Quinte.
During the last two decades, the United States and Canada have generally reduced phosphorus levels across the Great Lakes. Lake Erie's improvement, in particular, has been visible and dramatic. Scientists determined that lowering phosphorus concentrations would have the greatest limiting effect on algal productivity. The United States and Canada passed laws limiting phosphorus content in household detergents and constructed more effective municipal sewage treatment plants, cutting their phosphorus discharges (Figure 2-11). As a result, open-lake phosphorus concentrations have declined (Figure 2-12).
Phosphorus levels have also declined in Saginaw Bay and Green Bay. A facility that draws drinking water from Saginaw Bay has not found taste or odor problems since 1980. This facility had 56 days of such problems in 1974. During the 1980s, phosphorus levels in lower Green Bay decreased by about 25 percent from the average during the 1970s.
The bottom waters of Lake Erie 's central basin continue to suffer depletion of dissolved oxygen during late summer. During the summer, the central basin stratifies by temperature, forming a thin bottom layer. When algae die and sink to the bottom, their decay exhausts the limited supply of dissolved oxygen in that layer, creating during late summer a zone that cannot support bottom-dwelling fish.
However, in many other respects, Lake Erie has recovered. Increased catches of sport fish, such as walleye, are indirect evidence of this rebound (Figure 2-13). Another indication of Lake Erie's improved quality is that the rate of oxygen depletion in the bottom layer of the central basin has steadily declined and in 1989 was at its lowest rate in 20 years (Figure 2-14). This reduction means that the period of oxygen depletion is shorter than in the past.