Spatial and temporal variation of nutrients in the Great Lakes-St. Lawrence basin: Implications for primary production in stream ecosystems
Abstract
Growing human populations and associated land use activities has increased the amount of nutrients delivered to surface waters. Eutrophication from the over-enrichment of nitrogen and phosphorus has degraded ecosystem conditions in streams, lakes, and coastal areas worldwide. Thus, the management of anthropogenic nutrient loading is a global concern. This thesis employed a combination of field and experimental research to provide watershed managers with information on the spatial and temporal patterns in stream nutrient enrichment, and the associated ecological effects of anthropogenic nutrient loading in the Great Lakes-St. Lawrence basin. Four studies were completed to address this research goal. First, I evaluated spatial and temporal patterns in stream nutrient enrichment and the potential for stream nutrient limitation in the lower Great Lakes-St. Lawrence basin. Second, I assessed the nutrient mitigation benefits of agricultural best management practices promoted throughout the Great Lakes basin to reduce stream nutrient concentrations. Third, I examined patterns in ecosystem metabolism among three streams exposed to human activities that differed in nutrient exposure. Lastly, I conducted a stream mesocosm experiment to compare the response of benthic algae communities to temporal patterns of phosphorus loading associated with point and nonpoint source phosphorus enrichment. Findings from this thesis indicate that nutrient enrichment is widespread in streams of the lower Great Lakes-St. Lawrence basin and that the management of both nitrogen and phosphorus is needed to minimize the risk of stream degradation by eutrophication. Converting agriculture to natural land covers and some agricultural best management practices appear to be effective strategies to reduce stream nutrient concentrations. However, agricultural practices designed to improve farm drainage may counter the nutrient mitigation benefits of these best management strategies. Moreover, results from this thesis indicate that episodic and continuous nutrient enrichment can support stream primary production to a similar extent. Therefore, management strategies should consider remedial actions that provide the greatest return on investment in nutrient load reduction. However, variation in extraneous physical-chemical conditions of streams can influence the response of primary producer communities to nutrient enrichment, thus future research is needed to better understand the ecological implications of episodic nutrient loading in streams.