Recent Trends and Possible Causes

Trends in Lake Erie water and ecosystem quality since the early 1990s are not well understood. Recent research paints a confusing picture of simultaneously positive and negative trends in water and ecosystem quality (Table 5). Considerable year-to-year variations in scientific observations also inhibit identifying cause-and-effect linkages that can guide resource management and policy decision-making. For example:

• Springtime phosphorus concentrations have begun to increase and summertime levels of dissolved oxygen are depleting in the lake's central basin, even though there is no firm evidence of increases in external phosphorus loading.³ Recent calculations suggest minimal increases of phosphorus from point sources. However, as noted in the Commission's Tenth Biennial Report, uncertainty exists about phosphorus discharges into tributaries because of cutbacks in monitoring programs and less sensitive detection limits of phosphorus in sewage treatment plant discharges.⁴



• An increase in phosphorus should stimulate the growth of phytoplankton (tiny, free-floating plant life), which is a key component of the food web. However, phytoplankton concentrations generally remain low in offshore waters.⁵



• Invasive species continue to enter and become established in Lake Erie, causing economic damage and ecosystem disruptions. Scientists suspect that zebra and quagga mussels and the round goby (Fig. 7) are causing major changes in the Lake Erie ecosystem, perhaps including the springtime increases in phosphorus in lake waters. Non-native species may in fact be altering the way the natural ecosystem functions, as changes in the food web and rising phosphorus concentrations have coincided with the arrival and population boom of non-native zebra and quagga mussels. Whether there is a relationship between these events or if they are mere coincidences remains unclear.



• The walleye population recovered dramatically during the 1980s and developed into one of the most financially important sport fisheries in North America (Fig. 6). However, walleye and other fish populations (such as rainbow smelt) have declined in recent years, raising concerns among sport fishers and the fishery management community that changes in phosphorus and the food web may be responsible.⁶ Again, the causes for these changes are unclear.



• In some nearshore waters, especially around the western Lake Erie islands, greater water clarity has resulted in a dramatic increase in rooted aquatic plants.⁷ This has improved habitat diversity for such fish as the smallmouth bass. (Fig. 6)⁸ Concurrently, blue-green algae (cyanobacteria) blooms periodically erupt in the open waters of western Lake Erie, causing a soupy, green scum on surface waters. Closer to shore, sheets of Cladophora, macro algae, are growing at excessive rates on rocks and other hard surfaces, sloughing off in wind and waves, then dying and rotting on beaches. (Fig. 7)⁹ These conditions prevailed when eutrophication was at its worst in the late 1960s and early 1970s, yet phosphorus levels in the western Lake Erie basin do not suggest eutrophication is occurring.¹⁰



• Hexagenia, a large burrowing mayfly, serves as an important indicator of high water and sediment quality. This once-abundant insect spends its immature (nymph) stage in the lake and emerges only briefly as an adult, when it serves as a food source for many fish. This species disappeared from the lake during the 1950s, presumably due to oxygen depletion, but has recovered dramatically in the western and near shore portions of the central and eastern basins of Lake Erie since the early 1990s. Its reappearance after four decades - sometimes in great clouds of adults - can be hailed as an indicator that the Lake Erie ecosystem is recovering. (Fig. 6) ¹¹



• The predominant bottom-dwelling organism in the deeper, colder waters of the eastern basin of Lake Erie has been the deepwater amphipod, Diporeia, a small shrimp-like organism. It also is an indicator of good water quality and an extremely important food source for fish. Diporeia populations declined dramatically in the late 1990s (Fig. 7), and the species is now virtually absent.¹² The lake whitefish, once the mainstay of the Lake Erie fishery during the 19th and early 20th centuries but a minor part of the fish community for decades thereafter, had undergone a recovery in the eastern basin during the 1990s. One of its main sources of food is Diporeia, and as that prey species declined, so did the short-lived recovery of the lake whitefish in the eastern basin. However, the lake whitefish population is still rebounding in the central basin and occurs in the western basin during the colder months of the year.¹³



• Episodic die-offs of bottom-feeding fish and fish-eating birds from botulism poisoning are being reported, mainly in the eastern basin of Lake Erie, with lesser outbreaks noted in the western and central basins as well as in lakes Huron and Ontario. During and after the die-offs, rotting fish and bird carcasses litter beaches and shorelines (Fig. 7). Toxins from the bacterium Clostridium botulinum and specifically Type E botulism, which is found in fish-eating birds in the Great Lakes, cause these die-offs. Type E botulism is one of seven botulism types identified with the letters A through F, each characterized by the neurotoxin it produces. The last substantial Type E botulism outbreak occurred in Lake Michigan during the 1960s. The neurotoxin is produced in the absence of oxygen and with suitable temperature and nutrient conditions. It remains unclear which factors trigger the bacterium to produce the neurotoxin and the ensuing fish and wildlife die-offs. However, Type E botulism outbreaks have occurred as the round goby population, another invasive species, has increased. Researchers are looking for clues that triggered the botulism outbreak in Lake Erie, the source of the toxin, and its transfer among fish and other aquatic organisms, waterfowl, and fly maggots on carcasses.¹⁴



• Two other factors may be influencing or contributing to Lake Erie's ecosystem alterations, perhaps in similar or different ways on the other Great Lakes. Both short-term storms and long-term climate change may be influencing the ecosystem's dynamics. As already discussed in previous chapters, the same changes in land use, shoreline hardening from buildings, roads and parking lots, and wetland loss also may be triggering changes.

Table 5. Summary of Recent Trends in Lake Erie Ecosystem Quality

Figure 6 ( click on figure to enlarge )

Figure 7 ( click on figure to enlarge )