The two most significant water quality issues related to inland waters in large parts of eastern Canada remain lake acidification and eutrophication. Water chemistry records that document pre-disturbance conditions are unavailable for most aquatic systems, making it difficult to evaluate the impact of human activities on water quality. Biogeochemical models offer a mechanistic understanding of the important processes involved that, once sufficiently understood, can be extrapolated to other systems.
Biogeochemical models related to acid deposition have served several purposes: they have been used to hindcast historical conditions in lakes and streams in the absence of measured data so that the degree and rate of acidification can be assessed, to predict future water quality as a function of changes (increases or decreases) in acid deposition rate resulting from emission variations, and to estimate critical loads of acidifying substances (S and N) below which harmful impacts of acidification will not occur, e.g. acid neutralizing capacity, exceeding critical threshold values.
The lakeshore capacity model (LCM), which relates phosphorus inputs to trophic status characteristics (Dillon et al. 1994), has been used extensively in many regions of Canada and the US. The model has been used to evaluate the potential effects of new development and/or to ascertain development capacity, given defined threshold water-quality criteria. In recent years, new components have been added to the LCM, specifically to address the response of hypolimnetic oxygen levels to nutrient inputs (Clark et al. 2001), and to assess changes in cold-water fish habitat (Dillon et al. 2002). In some jurisdictions (e.g. Ontario), the LCM is used to calculate historic nutrient levels, in the absence of development, and allowable increases in nutrient inputs are set as a proportional increase above the background level. Because the LCM deals with a wide variety of land uses and geological settings, lake morphometries, and variable hydrology, the model should be useful in eastern Canada after suitable modification.
Soil data required for model parameterization will be collected through an extensive field program, focussing on low-order lakes with relatively small catchments. The results, including the hindcasting of lake chemistries back to pre-1830, the estimation of the degree of acidification of the lakes, as well as the estimation of critical sulphur loads to these lakes, and predictions of future conditions based on scenarios describing proposed changes in deposition, will be compared with results generated through complementary paleolimnological work performed by researchers at Queen’s University.
In addition, the model provides the means of estimating future conditions based on deposition scenarios, and of estimating the critical deposition that cannot be exceeded without the lake passing pre-determined critical chemical thresholds. Because some of the characteristics of the investigated lakes are substantially different from others that MAGIC has been used for in the past, notably the high DOC, we will revise MAGIC by building an organic acid or DOC module that incorporates our current knowledge on the role of DOC in lakes. At present, the role of DOC in acid-base chemistry is represented in MAGIC in a very elementary way using simple dissociation constants, with no formation or loss of DOC within the system considered. A revised model with a detailed DOC module will be useful in other areas as well, and will almost certainly lead to better hindcasting and forecasting.