The return of the Great Lakes to a more oligotrophic condition—as evidenced by a reduction in the abundance of blue-green algae and by a reduction in the annual occurrence of anoxic conditions in the bottom waters of central Lake Erie—is certainly desirable from a water-quality management perspective and is also desirable from a fisheries management perspective, as long as increasing oligotrophy does not result in a substantial reduction in fish production. Fisheries and water-quality management strategies for the Great Lakes have evolved more or less independently; although the two types of management strategy have generally benefited the environment, they do not have identical goals or approaches. In the future, a more ecologically oriented approach that considers both fisheries and water-quality management goals and that better integrates management activities should be aimed for.
7.1.7 Nearshore Nutrients
Nutrients important to algal growth (phosphorus and nitrogen) are added to the lakes in the nearshore zone through combined sewer overflows (CSOs), sewage treatment plants (STPs), and rivers. The Ecosystem Health Division of the Environmental Conservation Branch of Environment Canada's Ontario Region is responsible for conducting the Great Lakes Surveillance Program. Since 1967, open-lake cruises have been conducted on the Great Lakes to assess transboundary pollution, and to fulfil Canada's obligations under the Canada–U.S. Great Lakes Water Quality Agreement. The surveillance program now focuses on one lake each year (except Michigan), conducting multiple water sampling cruises for organics, nutrients, and physical parameters. The program's objectives are to ensure compliance with water-quality objectives, evaluate trends, identify emerging issues, and support the development of LAMPs.
Although the surveillance program is an offshore monitoring program, some of the stations routinely sampled fall within the SOLEC definition of "nearshore" (i.e., less than 10 m in Lake Superior, less than 30 m in all other Great Lakes). None of the Great Lakes Surveillance stations on Lake Superior meet the criteria. Conversely, almost all the Surveillance stations on Lake Erie are within the 90 percent of the lake classified as "nearshore" according to the criteria used in Table 1. For Lakes Huron and Ontario, only the stations closest to the shoreline are within the 30-m contour.
Using the most recent surveillance data (1991 for Lake Superior; 1994 for Lake Huron; 1995 for Lake Erie; 1993 for Lake Ontario), surface distribution maps of spring total phosphorus (Figure 7), spring soluble reactive phosphorus (Figure 8), spring filtered nitrate-plus-nitrite (Figure 9), and summer chlorophyll a (Figure 10) were constructed to illustrate nearshore–offshore gradients. Although the stations that fall within the SOLEC definition of "nearshore" are still, in general, 1 km to 2 km from shore, elevated concentrations of phosphorus and nitrate-plus-nitrite, as well as the highest concentrations of chlorophyll a, are observed. In Lake Ontario, where the spring phosphorus guideline is 0.010 ppm, exceedances are observed within these "nearshore" stations. Similarly, in Lake Erie, where the total phosphorus guideline is basin-specific (0.015 ppm for the Western basin, 0.010 ppm for the Central and Eastern basins), exceedances are also observed, both at stations that meet the "nearshore" criteria and offshore. Algae and macrophytes require nutrients for growth.
Figure 7. Surface Distribution of Phosphorus Concentrations
Figure 8. Surface Distribution of Soluble Reactive Phosphorus Concentrations
An overabundance of nutrients leads to nuisance algal populations in the water and also leads to algae attaching themselves to rocks and structures. Nutrients stored in sediment stimulate macrophytes, which may cause navigation problems for recreational boaters in shallow-water areas. Figure 10 shows that chlorophyll, the algal indicator in this case, tends to follow the total phosphorus concentrations in the nearshore of the lower lakes. Thus, the nutrient sources are bioavailable. This is consistent with STP sources of nutrients rather than less available nutrient forms in natural soils, which may be in suspension near shore.
Figure 9. Surface Distribution of Spring Filtered Nitrate-plus-nitrite
Figure 10. Surface Distribution of Chlorophyll a Concentrations
Clearly, the ubiquitous STP outfalls and CSOs still influence nearshore water quality near population centres. Though sewage plants reduce the phosphorus in sewage, they do not eliminate phosphorus. Many STPs operate with effluents in the range of 1000 µg P/L, which is 100 times the desired concentration in Lake Ontario. Thus, nearshore–offshore gradients are to be expected. Experiments conducted in 1991 by M.N. Charlton (unpublished data) between Burlington and Toronto are typified by the results shown in Figure 11 (Charlton Nearshore TP Gradient). The nutrients are introduced to the lake at the shore side of the shore boundary layer both by sewage sources and by rivers. Thus, relatively high concentrations can occur locally even though control programs have caused low concentrations generally in the lake.
Figure 11. Phosphorus Gradient in Lake Ontario
In the late 1960s, the Ontario Water Resources Commission began monitoring planktonic algae in samples collected weekly from a number of municipal water-supply intakes in the province, including several on the Great Lakes. The program was expanded in 1976, when the (then) Water Resources Branch of the Ontario Ministry of the Environment increased the number of Great Lakes sampling locations to 13 and began measuring several trophic state variables, including nitrogen, phosphorus, silica, and chlorophyll. Five additional intake sampling locations were added between 1978 and 1985. Data from this program have been useful for measuring the response of the nearshore Great Lakes to the international phosphorus control program (Nicholls et al. 1980) and are essential for the fulfilment of terms of the Great Lakes Water Quality Agreement (IJC 1988). More recently, the data have proven useful in demonstrating some water-quality effects of the zebra/quagga mussel invasion relative to phosphorus management (Holland 1993; Holland et al. 1995; Johengen et al. 1995; Nicholls 1996; Nicholls and Hopkins 1993; Nicholls and Standke 1996). The following is a brief synopsis of some of the recent Ontario findings.
Total phosphorus (TP) and chlorophyll concentrations ranged from the lower limits of analytical detection (0.001 mg P/L or 1 µg P/L, and 0.2 µg chl/L) in many of the Lake Superior samples to maximum concentrations two orders of magnitude higher in the Bay of Quinte. For the May through November periods of all years, the relationship between monthly mean TP and chlorophyll a was well defined for the pre–zebra/quagga mussel years (Figure 12). The invading mussels remove chlorophyll at a higher rate than they remove total phosphorus; this removal-rate difference has led to a decrease in the summer chlorophyll-to-TP ratio of more than 60 percent (Nicholls and Standke 1996; Figure 12).
Figure 12. Total Phosphorus (TP) and Chlorophyll Concentrations
Long-term declines in total phosphorus are evident in all of the Great Lakes, but considerable variability has characterized many of the sampling locations. It is apparent that many years of data are needed to identify trends, which are best defined in western Lake Erie and eastern Lake Ontario (Figure 13). At the Union location in western Lake Erie, TP concentrations rose steadily from 1976 to 1983 and then declined at a rate of about 0.003 mg/L per year through 1994. The rate of decline of TP at the Kingston and Brockville locations was about three times higher, averaging about 0.010 mg/L per year between the middle 1970s and the middle to late 1980s. No further declines have been apparent so far during the 1990s at any of the Lake Ontario locations (Figure 13).
Figure 13. Long-term Trend in Total Phosphorus Concentrations in the Great Lakes
Long-term chlorophyll data from all locations are highly variable; only after 1988–89 in Lake Erie is there a major reduction (Figure 14), which is attributed to the establishment of zebra/quagga mussels. A reduction of between 30 percent and 50 percent at the Grand Bend location in 1993–94 (Figure 14) is consistent with the delayed establishment of mussels in parts of Lake Huron (Johengen et al. 1995). Similarly, large recent reductions in chlorophyll at Kingston and Brockville are consistent with the establishment of invading mussels in eastern Lake Ontario and the Bay of Quinte in 1992–94.
Figure 14. Long-term Trend for Chlorophyll a Concentrations in the Great Lakes
In the short term (10 years), the zebra mussel affected Lake Erie planktonic algae dramatically in all three basins of the lake (Figures 15 and 16a). In the western basin, however, a longer-term view of the data (30 years) provides a very different perspective relative to the phosphorus loading control effects. Over a three-decade period, the declines in chlorophyte plankton (including several "weedy" species of the genera Pediastrum and Scenedesmus) that occurred during the 1970s and 1980s were of much greater importance than the decline experienced in 1988 attributable to zebra mussels (Figure 16b). By the late 1980s (before the mussel invasion), total chlorophyte density was only 6 percent of late 1960s–early 1970s levels, so further reductions brought about by invading mussels were relatively minor. This was not the case in the lake's central and eastern basins, where phosphorus loading controls have apparently been less effective (as evidenced by relatively unimportant declines in algae before the mussels invaded during the 1988–90 period). The decline in western Lake Erie phytoplankton was well under way by the time the chlorophyll sampling started in 1976. As well, the phytoplankton data demonstrate a continuing decline through the 1980s, apparently in response to decreasing phosphorus loads (Figure 16), while chlorophyll levels remained fairly constant at about 5 µg/L (Figure 15, Union data). This apparent discrepancy may relate to the changing chlorophyll contents of algal cells—a change that would result from a shift from N limitation to P limitation brought on by phosphorus loading controls and by rising nitrate concentrations through the 1970s and 1980s (Figure 18). Because cells' chlorophyll contents depend on the availability of inorganic nitrogen as well as on other factors, greater care may be needed in interpreting long-term chlorophyll data than in interpreting data on phytoplankton biovolume and density.
The Bay of Quinte Remedial Action Plan (RAP) has set an interim phosphorus concentration objective of 0.030 mg P/L for the upper Bay of Quinte. Significant declines in phosphorus concentrations have occurred in the Bay of Quinte since 1977 (Figure 19a), mainly in response to optimized secondary sewage treatment and phosphorus removal at municipal sewage treatment plants discharging to the Bay of Quinte. A few more years of data will likely be required to be certain that concentrations at Station B in the upper bay are (after 1995) consistently below this target concentration. Declines in phytoplankton biomass have generally followed the decreasing phosphorus concentrations (Figure 19b) and have been reflected in improved water quality in the upper bay for drinking-water supply and recreational uses.
Nutrient loads to the lakes have been reduced, not eliminated. The problems caused by nutrients, therefore, may be reduced but are still present. For example, the attached alga Cladophora grows on rocky bottom areas in shallow water. Formerly, the growths, when they broke off and drifted to shore, created widespread problems by their unsightliness and unpleasant odour. During July 1995, a survey of Lake Erie's east basin that was conducted by the Ontario Ministry of Environment and Energy found shoreline fouling at four areas. In addition, growth still attached to the bottom was abundant at 16 locations between Fort Erie and Port Dover. The reason for the widespread abundance of Cladophora in the shallow littoral zone in July 1995 is unclear. The minimum phosphorus concentrations predicted to sustain growth are relatively low (Jackson and Hamdy 1982). Neilson et al. (1995) predicted that SRP concentrations in Lake Erie's nearshore were sufficient to sustain Cladophora growth. Several studies have shown that Cladophora growth responds to phosphorus concentration variations in the range of concentrations now found in the lakes. Thus, intermittent loads or even small sources can stimulate this nuisance. Local shoreline or tributary inputs of nutrients to the littoral zone probably contributed to the greater-than-average abundance of Cladophora in some areas. But the extent to which local sources of nutrients were a factor in the overall abundance of Cladophora is not known. Increased water clarity in the eastern basin may also contribute to the observed abundance of Cladophora by reducing the degree of light limitation on growth. A more speculative question is whether Cladophora benefits from the presence of dreissenid mussels by scavenging nutrients released from the mussels' waste products (faeces and pseudofaeces).
Figure 15.Algal Response to Phosphorus Loading in Western Basin of Lake Erie
To identify areas of concern and monitor contaminant trends over time, the Ontario Ministry of Environment and Energy initiated a contaminant surveillance program using juvenile fish as biomonitors in the nearshore waters of the Great Lakes in 1975. This program's findings have been widely reported (Suns et al. 1991).
Figure 16. Cumulative Decline of Algal Populations in Western Lake Erie
Figure 17. Phytoplankton Density in Western Lake Erie
A variety of organochlorine contaminants and metals are known to bioaccumulate in fish. Contaminants that are often undetectable in ambient water samples may be detected in young-of-the-year forage fish. Because fish integrate spatial and temporal changes in water quality and in contaminant availability, body burdens provide a good basis for assessing environmental change. A common forage fish, the spottail shiner (Notropis hudsonius), was selected as the principal biomonitor (Suns and Rees 1978) for assessing temporal trends in contaminant levels in nearshore waters, determining the spatial extent of pollution throughout the Great Lakes, identifying sources of contamination, and assessing the effectiveness of pollution control. Among the criteria used in selecting spottail shiners were its limited range in its first year of life, its undifferentiated food habits in early life stages, its importance as a forage fish (Scott and Crossman 1973), and its presence throughout the Great Lakes. Forage fish also provide an important link in assessing contaminant transfer to higher trophic levels (e.g., fish-eating birds, mammals).
Figure 18.Nitrate Trend in Central Basin of Lake Erie
Figure 19.Phosphorus Concentrations in the Bay of Quinte
The significance of the contaminant levels in the forage fish is assessed using wildlife protection guidelines. Specifically, the Forage Fish Contaminant Index (FFCI) developed by Suns et al. (1991) assesses risk to piscivorous wildlife for 7 organochlorine compounds. The FFCI is calculated as the sum of individual contaminant concentrations divided by individual wildlife protection guidelines or objectives. The concept of additivity inherent in the FFCI has been used by the USEPA (1989) to establish risk factors for chlorinated dibenzo-p-dioxins and dibenzofurans. Bishop (1989) has shown that the sum of total organochlorine body burdens, rather than specific compounds, was related to biological effects.
Guidelines used for calculating the FFCI were the most stringent available and included the IJC Aquatic Life Guideline (GLWQA 1978) and the NYSDEC Fish Flesh Criteria (Newell et al. 1987) for the protection of piscivorous wildlife. Contaminants and guidelines used were polychlorinated biphenyls (PCBs) (100 ng/g), dichlorodiphenyl trichloroethane (DDT) (200 ng/g), hexachlorocyclohexane (BHC) (100 ng/g), hexachlorobenzene (HCB) (330 ng/g), octachlorostyrene (OCS) (20 ng/g), and chlordane (500 ng/g). Because the mirex guideline is below detection limits, a value of 1 ng/g was used in calculations.