Past studies have shown that diffuse nutrient loading from agricultural activities is an important cause of lake eutrophication. The degree to which this relationship can be scaled-up (e.g. at an inter-regional scale) has not, however, been widely addressed. My thesis objectives were therefore to define the generality and the impact of agriculture land use and lake morphometry on lake water quality. Analyses along spatial and temporal scales were conducted to evaluate the significance of these effects.
In the first manuscript, we tested whether agricultural activities explain a significant proportion of the variation in lake water quality at a broad inter-regional scale. The degree to which lake mean depth modulates this response was also assessed. From our meta-analyses of 358 lakes, we noted a significant correlation between total phosphorus concentration and the extent of agricultural catchment development. This relationship was further strengthened by including lake mean depth as a second predictor. We also observed among-study variability in the relationship between these three variables. Thus, although there is a general relationship between total phosphorus concentrations and our two predictors, agriculture catchment development and lake mean depth, regional baseline nutrient differences modify this relationship.
To address the issue of lake morphometry more closely, we adopted a spatio-temporal approach to investigate whether the effect of agricultural catchment development on water quality differed between dimictic and polymictic Albertan lakes. We found that the correlation between surface water total phosphorus concentration and the percent of agriculture in a lake's catchment was strongly modified by lake mixis patterns (which in turn are related to morphometry). Furthermore, with our paleo-indicators of lake water-quality, the chironomid communities, we detected opposing responses between the dimictic and polymictic basins to temporal land-use change. We suggest that external nutrient loading exerts a more notable effect on dimictic lakes, whereas internal loading is more important in polymictic systems.