CLIMATE CHANGE IMPACTS: WATER QUALITY RESPONSES .1 Water quality concerns

Specific water quality concerns in the Great Lakes have included eutrophication (Schertzer &

Lam, 1991) and delineation of pathways for toxic contaminants (see Schertzer & Murthy, 1994), both issues which are dependent on the accurate simulations of hydrological, thermodynamical and hydrodynamical processes. Water quality has been a focus of research on the Laurentian Great Lakes for decades and consequently there is an accumulated wealth of physical, chemical and biologicL1 data (Schertzer et al. 1993). Such data is essential for model development and verification, and is also invaluable for extended analyses such as climate scenario testing. Current water quality models are applied in large lakes to predict the transport and transformation of such parameters as turbidity, salinity, nutrients (e.g., nitrogen and phosphorus), dissolved oxygen and concentration of dissolved metals, and organic chemicals and others. Atkinson et al. (1999) provides a detailed discussion on modeling of water quality issues on the Great Lakes. Such models have utility for resource management and as research tools. As management tools, they have direct application for comparison of system response to alternative management strategies and for extended specialized assessments such as evaluation of the system water quality response to climate induced changes which may result from changes in thermal or hydrodynamical processes.

Many models (e.g., Sonzogni et al., 1987) have been developed to address a wide range of issues in the Great Lakes. These include long-term build-up of dissolved solids (chloride), nutrient-eutrophication, toxic chemicals, contaminated sediments, and fish production (e.g., Bierman et al., 1992; DePinto et al., 1993, 1994a,b; Endicott et al., 1992a; MacKay, 1989, 1991; Thomann & DiToro, 1983; Stepien et al., 1987; Halfon & Oliver, 1990). Each of these models include process. formulations which can be affected by climate change, largely through temperature dependence. We provide here, two case study examples in which potential climate change responses have been considered.

6.2 Eutrophication : Lake Erie and Lake Ontario

Water quality-eutrophication concerns have been most prevalent in Lake Erie and Lake Ontario as well as specific embayments in other lakes. Atkinson et al. (1999) provided a qualitative description of the relative distribution of changes in water quality in Lake Erie and Lake Ontario for a CCC-II climate change scenario. Base case and climate changed case meteorology were used to drive a fully mixed two-dimensional hydrodynamical model (e.g., Simons, 1974) to produce a computed circulation. The currents are used in a transport model in an advection-diffusion framework along with a primary production submodel (e.g., Simons

& Lam, 1980; Lam et al., 1987b) to generate spatial distributions of water quality variables for base (1 x COz) and climate change (2 x COz) cases (Fig. 3). Verification of the preliminary model simulations were compared with observations sampled over space and time (e.g., Simons & Schertzer, 1987) for Lake Ontario.

This preliminary analysis indicated that in Lake Ontario, the uptake of soluble reactive phosphorus increases more than the base scenario especially to the eastern part of the lake.

Particulate phosphorus which includes algae, also increases in Lake Erie suggesting an increase in productivity. While changes in western Lake Ontario were small, pronounced changes occur in the central and eastern basins of Lake Erie. Increased water temperature in the climate change scenario was the primary influence on these changes. The results of this simple hypothetical simulation demonstrated that climate change effects on lake hydrodynamics can have a significant effect on the water quality nutrient concentrations in large lakes. The extended implication from this analysis is that for some lakes, oxygen depletion, which is linked to temperature and nutrient conditions, may become more acute.

Uncertainties in temporal boundary conditions used in the current generation of GCMs and the validity of assumptions on the dates of early spring warming and the occurrence of fully- mixed conditions etc. means that the current simulations are considered preliminary. More intensive investigations of the climate change impacts on the lower lakes are currently being undertaken to assess the climate impacts on water quality on the lower lakes (e.g., Schertzer

& Lam, 1996).

Figure 3. For Lake Erie and in early summer (June), predicted particulate phosphorus (PP) and soluble reactive phosphorus (SRP) contours for qualitative visualization and comparison of lxCO2 and 2xCO2 climate change scenarios.

6.3 Toxic contaminant exchange (PCB): Green Bay mass balance study

An example of strong coupling between a hydrodynamic and water quality model with significant wind effects was the application of the TOXFATE model (Halfon & Oliver, 1990) coupled with the hydrodynamical model of Sirnons & Schertzer (1990) to evaluate contaminant transport in Lake St. Clair (Halfon et al., 1990). Other than the hydraulic flow associated with the inflows and outflows of the St. Clair and Detroit Rivers, respectively, wind-driven circulation is a major component in controlling the fate and transport of

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chemical species. Changes in the meteorological forcing as would occur under climate changed conditions (e.g., wind speed, temperatures, flows) could have significant impact on the distributions of chemical species in this lake.

Atkinson et al. (1999) describes the Green Bay Mass Balance Study (GBMBS) as one of the largest coordinated studies of toxic chemical sources, transport and fate undertaken in a large lake. One of the major goals of this investigation was development of a mass balance modeling framework for use in the management of pollutant loading and impacts on the Great Lakes ecosystems. An integrated exposure model framework (GBTOX) was developed. GBTOX is a series of linked models that permit accurate simulations of sources, transport and fate of chemical sorbents in the large lake system. In essence there is a water balance, a sorbent mass balance supported by a separate eutrophication model that generates autochthonous loading of sorbents (phytoplankton production), and finally a toxic chemical mass balance that integrates all three. Each of these models is a process-orientated, deterministic mass balance model, and there are climatic dependencies throughout the framework. This model framework was tested to determine the PCB concentrations within the Bay as a result of different loading scenarios. It is recognized that climate changes might affect a wide variety of processes important to water quality models. The Green Bay modeling framework includes most of these processes and would, therefore, represent the cumulative effect of climate change on the transport and fate of contaminants in large lakes.

Although the Green Bay modeling framework has not been applied to the question of climate change, Table 5 contains a list of processes included in the Green Bay modeling framework along with a qualitative description of how each major process might be impacted by climate change. These qualitative descriptions may have relevance to other large lake systems.

Table 5. Potential impacts of climate change on hydrophobic organic chemical modeling framework base on the Green Bay case study example (modification from Atkinson et al., 1999).

Process Description of Climate Change Impact

Primary production Increased water temperature will increase the rate of overall primary production by increasing algal growth rates and increasing nutrient remineralization rates. Also, increasing epilimnion depth is likely to cause an increase in depth-integrated production, the timing of algal blooms and on timing and relative magnitude of seasonal algae succession.

Algal respiration A temperature-dependent process which will increase in response to increasing temperature.

Organic carbon decay Rate of microbial utilization of organic carbon is directly proportional to temperature.

Sediment deposition and Solids in the water column (and associated contaminants) may be resuspension more closely coupled with bottom sediments under decreased water levels as both deposition and resuspension rates are likely to increase.

PCB tributary loading Changed tributary flow rates will affect direct loading as well as possible resuspension of in-place contaminant.

Air-water exchange of PCBs Temperature and wind-induced mixing are strongly dependent features of climate, and control air-water transport of PCBs and other contaminants.

Partitioning and phase Temperature affects the equilibrium partitioning coefficient, distribution having a direct result on phase distributions, with associated

impacts on modeling and on management decisions regarding potential risk and clean-up.

7 OTHER LARGE LAKE SYSTEMS, SECTORS AND DECISION MODELS