Hydrologic controls on the trophic status of shallow lakes on the Boreal Plain

The trophic status of shallow lakes on the Boreal Plain of Alberta exhibits high natural variation in space and time. It is not currently known what the dominant controls on this variation are at regional scales and over longer time periods. The aim of this thesis is to characterize the natural variation in trophic status, as measured by the concentration of chlorophyll a (CHLa), and assess the importance of hydrology, captured by climatic and landscape metrics, in explaining the spatial and temporal variation in trophic status. Indicators of trophic status were derived from Landsat images acquired during late summer (August) over a 20 year period from 1984 to 2003. A two-way analysis of variance revealed that of the total variation in CHLa, approximately 50 percent was in the spatial component, 10 percent was in the temporal component and 40 percent was in the spatio-temporal component, which also included the error term. Variation in the temporal component was related non-linearly to climate, as measured by effective precipitation (precipitation minus evapotranspiration). The relation between CHLa and climate indicated that under wet conditions CHLa decreased possibly due to phosphorus dilution and that under dry conditions CHLa increased possibly due to phosphorus concentration. Variation in the spatial component was related to indicators of groundwater and surface water connectivity. Lakes, situated in regional groundwater flow systems, exhibited higher CHLa than lakes situated in local groundwater flow systems. For the majority of lakes, situated in local groundwater flow systems, high wetland cover in their catchments corresponded to low CHLa whereas low wetland cover corresponded to high CHLa. For the minority of lakes, located in regional groundwater flow systems, higher stream connectivity was associated with higher CHLa. Overall, groundwater and surface water variables explained 74 percent of the spatial variation and climatic variables explained 78 percent of the temporal variation. These results underscore the importance of considering hydrology when understanding the sources of variation in trophic status measured at regional scales over long time periods. Incorporating hydrologic knowledge in resource management may help in mitigating impacts of resource activities.