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Long-term records relating to Lake Erie's nutrient status suggest a process of reduced nutrient status. EPA's water quality data show a downward trend of eutrophy (the Carlson Trophic State Index) for the period 1983-2000. Furthermore, concentrations of total phosphorus in the water, averaged over the whole year have been falling by about 0.2 mg/m3/yr. However, the amounts of nutrients present in the water in early spring have continued to rise, extending to 8 years a trend that was first seen in 1995. Much of the among-year variation in the amount of phosphorus entering the lake over the last few years is due to the intensity and timing of storms, which cause flooding and erosion, rather than to municipal inputs. Data from the last several years indicate that more phosphorus is leaving Lake Erie in the waters of the Niagara River than is entering the Lake from the major tributaries.
The period of water turbidity associated with spring is persisting longer than formerly. The planktonic algal cells are smaller than they were in the 1980s, and there seem to be slightly more algae during the spring than in the late 1990s. However, zooplankton are no more abundant than previously. Over the period 1983-2000, the biological demand for oxygen in the bottom waters of Lake Erie's central basin has been rising by about 0.03 mg/m3/yr, when averaged over the whole year. Biological oxygen demand of the sediments seems to increase over the course of the summer.
In summertime, light is penetrating deeper into the water - algae are now growing (and producing oxygen) in the deep layers of the central basin and on the western and central basin lake bottoms. Extensive layers of the filamentous algae, Cladophora are common along rocky shorelines around the Lake. There is also more bacterial activity deep in the water, but there are very few planktonic algae in the shallow water near shore, where zebra mussels are most abundant. There is only limited evidence that the scarcity of planktonic algae is due to nutrient limitation, either in the spring, or later in summer. Microbes in the water are more likely to be limited by the availability of carbon than by either phosphorus or nitrogen. Studies to determine if the scarcity of trace metals such as iron, copper, or zinc may be limiting algal production have been inconclusive. The picoplankton are most responsive to experimental additions of these metals.
Populations of dreissenid (zebra and quagga) mussels and Hexagenia mayflies are steady or declining. The development of thick mats of algae along shorelines, especially in the eastern and central basins, reduce the living space available for dreissenid mussels. Zebra mussels have all but disappeared from eastern and central basins, being supplanted by quagga mussels. Overall mussel densities seem to be lower than in recent previous years, possibly because there are so many gobies now in the lake. The diversity and abundance of invertebrate animals, especially mayflies and net-spinning caddisflies in the wave-washed zone of the shoreline has dropped markedly since the last time they were surveyed in the 1970s.
The goby population in Lake Erie is large, but the numbers are quite a bit fewer than they were two years ago. Most of the gobies occur in rocky and sandy areas in all three basins. There are very few in the centre of the lake.
Walleye populations are not growing. The abundance of their preferred food (herrings and soft-fin fishes) has not changed since the 1970s. Gobies will likely become an acceptable source of food for walleye. Gobies are common in the diets of almost all of the Lake Erie sports fish.
Evidence seems to suggest that we are seeing new pathways of internal cycling of nutrients, likely caused by the activities of dreissenids, which may be altering the size structure and dynamics of particles in Lake Erie. However, the consequences of physical (weather-related) influences cannot be ruled out as an accompanying explanation for the apparent increasing frequency and extent of central basin anoxia events. The persistent periods of spring turbidity may be due to the effects of heavy fall and winter storms, which contribute more sediment for a given amount of precipitation than summer storms. Also, cold water is more viscous than warm water, causing particles to settle more slowly. Spring water temperatures in 2002 and 2003 have been among the coldest on record, perhaps partly accounting for the greater concentrations of spring turbidity and possibly associated nutrients.
There is now stronger evidence than ever of human-induced climate change. For example, the average water temperature of Lake Erie has risen by 0.4 degrees C over the past 18 years. Between now and 2090, our climate is expected to continue to become warmer, and this will result in significant reductions in lake level. Lake Erie water level is expected to become as much as 85 cm lower over the next 70 years, and its surface area may shrink by 15%. Total amounts of precipitation may not change on an annual basis, but storms will become less frequent and more intense. Strong winds will also become more common. These physical events will have noticeable effects on the Lake Erie shoreline habitats.
These physical events will have noticeable effects on the Lake Erie shoreline habitats. We should anticipate the changes in habitat structure that will accompany these modifications and their consequences for both coastal and lakewide processes. The changes in timing and amounts of precipitation and runoff will require different strategies for water management.
New methods are being developed to monitor the condition of the land next to the lake and its likely effect on the nearshore water. GIS and the advent of more powerful computer technology are improving our ability to map and interpret the characteristics of the water and lake bottom, and to understand their importance to the biota. A project is underway to produce a single, integrated map of habitat types and condition for the entire Lake Erie watershed. The success of this initiative will ultimately depend on continuing participation of Network agencies through data sharing and support for funding requests. Such information is crucial if we are to anticipate the changes in habitat structure and their consequences for both land and water management in the Lake Erie basin.
Avian Botulism: Avian botulism is a bacterial disease that has been causing large die-offs of fish and fish-eating birds during late summer and early fall. Dreissenids and gobies are suspected to be involved in this new phenomenon, but the role they may play is not well understood. Dreissenids may be responsible for nearshore habitat changes (such as the development of Cladophora algal blooms), promoting local anoxic conditions where the Clostridium botulinum bacterium can multiply. Perhaps the dreissenids collect bacterial spores while they are feeding. Possibly, when gobies eat the mussels, they become sick and may be at greater risk of being caught by fish-eating birds. Infected birds become paralyzed by the botulinum toxin and drown. Effort is needed to refine working hypotheses and to develop and undertake key investigations. We must also consider the socioeconomic implications of effects on the bird and fish populations both inside and outside of the Lake Erie basin.
Blue-green Algal Blooms: Blooms of blue-green algae (Cyanobacteria) are becoming noticeable at certain places and times. Some species produce chemicals (microcystins) that are potent toxins to humans and wildlife. We need to do more research to understand the biology of these algae and the causes of their blooms. Samples collected in various open-water areas revealed a correlation between locations where blue-green algal pigments were most abundant and places where dreissenids were abundant. There is a need to track the distribution and incidence of such blooms to improve our understanding of their risk to human and animal health.
Pharmaceuticals: Pharmaceuticals and personal care products are another class of compounds of concern. Analgesics, anti inflammatory drugs, birth control chemicals, Prozac-like drugs, and cholesterol-lowering drugs have all been found in the effluent from water treatment plants discharging into the Detroit River, although at low concentrations. We must continue to monitor the concentrations of these compounds and anticipate their increased use and discharge into the ecosystem as a consequence of an aging human population. We should consider how to develop preventive technology to control the release of these chemicals.
The use of GIS technology is helping us understand the distribution and likely sources of many persistent pollutants. PCBs, PAHs, and mercury are all problem chemicals. PCBs, other organochlorine compounds, and mercury are at excessively high levels in fishes, and exceed sediment quality guidelines at many locations. A newly developed Sediment Quality Index has helped us quantify and communicate the condition of sediments within the basin, and will be a valuable reporting tool. Overall sediment condition is poorest in the lower Detroit River, but condition improves from west to east and from south to north in Lake Erie.
Although we can map the distribution of chemicals in the water and sediments, reduced frequency, spatial extent, and sensitivity of tributary monitoring has made it increasingly difficult to accurately calculate the amount of chemicals entering Lake Erie or its tributaries. Therefore, even though we know that phosphorus levels within Lake Erie waters are rising, we are unsure of the degree to which the phosphorus is coming from loadings vs. from internal recycling.
The detection limits for persistent organic pollutants and trace metals now used by some agencies are higher than the levels suspected or known to cause harmful effects to the biota. In general, contaminant levels of PCBs, pesticides and mercury in biota (invertebrates, fishes, birds) have declined over the last 20 years. There are many exceptions and discontinuities in these trends. These can largely be explained by changes in the food web. We must understand food web patterns as well as contaminant patterns if we wish to solve contaminant problems in biota. The Detroit River is likely a conduit rather than a source of some contaminants. We must examine the Huron-Erie corridor in its entirety if we wish to better understand the sources and effects of pollutants.
A comprehensive report by J. Hartig et al. (2003) has documented our knowledge and uncertainties regarding the distribution and ecological effects of PCBs in sediments of the Detroit River and nearby Lake Erie. There are marked local effects of PCBs where they are monitored, but the 'far-field' effects are poorly understood. Major needs are a restoration of resources for monitoring, greater analytical sensitivity of the samples that are collected, and strategy to ensure that we evaluate the effectiveness of the remediation efforts that have been undertaken.
New hydrodynamic and trophodynamic models are improving our understanding of the present distribution and transport of contaminants such as PCBs, PAHs, and mercury. Long-term records of these contaminants in the sediments and the biota show dramatic declines from the 1970s. There has been relatively little change through the 1990s. The changes that we have seen likely represent biological adjustments to the flow of energy through the food web as much as to contaminant loading patterns. The modelled distribution of contaminants in sediments and the biota is consistent with observed contaminant patterns. When diverse biota are collected from different locations, zoobenthos, and forage fish body burdens best reflect the amount of sediment contamination in those areas. Improved methods are being developed to distinguish between the contaminants becoming bioavailable through resuspension and contaminants newly entering the ecosystem.
New non-native species continue to be found in Lake Erie. Zebra and quagga mussels, spiny waterfleas, and gobies continue to have the greatest impact on the ecosystem among these invaders. The densities of dreissenids appear to be stable or declining in most areas. Nearshore zones are showing evidence of reduced density possibly because extensive mats of filamentous algae (Cladophora) are covering the hard substrates that the dreissenids formerly used for attachment. Predation by gobies may also be affecting dreissenid abundance. Zebra mussels have been almost completely replaced by quagga mussels in the eastern and central basins. Quagga mussels are also becoming more dominant in the western basin, but zebra mussels remain common there.
Goby densities have become reduced since 2000. They are most abundant in the western basin, where their growth and excretion represent a significant source of phosphorus to the ecosystem budget. The major new threats facing Lake Erie may be bighead carp and viruses. The prospect of the eventual appearance of ruffe continues to be a concern.
Major efforts are underway to study ship ballast transport and to develop regulations to control release of new exotic species into the Great Lakes. New theoretical models of transport and colonization are beginning to give us a new understanding of the most significant ballast water threats. Research needs include improving our understanding of invader effects, socioeconomic studies of the true costs of invaders, legislation that will stop the import of living organisms that can become invaders, and examining alternatives to saltwater shipping in the Great Lakes.