Doctor of Philosophy
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.
Summary for Lay Audience
Nutrients (i.e., nitrogen and phosphorus) are resources that are essential for the growth of plants (i.e., primary producers) in streams, lakes, and coastal areas. But too much nutrients can cause the excessive growth of primary producers and harm aquatic ecosystems. Nutrient pollution is common in streams, lakes, and coastal areas that are nearby agricultural and urban landscapes. Because agricultural and urban landscapes have increased with growing human populations, we need to better understand the effects of nutrient pollution to help protect aquatic ecosystems. The purpose of this thesis was to examine the extent of nutrient pollution and the effects of human activities on stream ecosystems in Great Lakes-St. Lawrence basin. Findings from this thesis were used to provide recommendations on how to better protect stream ecosystems from the effects of nutrient pollution. Four studies were conducted to address this research purpose. First, nutrients were measured in streams throughout southern Ontario to provide information on the extent of nutrient pollution. Results showed that both nitrogen and phosphorus pollution were widespread throughout the region. Second, common actions used to prevent nutrient pollution from agricultural landscapes were evaluated to determine their success. Findings suggested that natural vegetation nearby streams could reduce nutrient pollution, but actions to remove water from farm fields increased nutrient pollution. Next, this thesis assessed the effects of nutrient pollution from agricultural and urban activities on stream ecosystems with two studies. In real streams, nutrient pollution from urban sewage treatment facilities had a greater effect on stream primary producers than nutrient pollution from agricultural activities. But in a controlled environment, continuous nutrient pollution from urban sewage treatment facilities had an equal effect on stream primary producers as short-term nutrient pollution from agricultural activities. Actions to reduce nitrogen and phosphorus pollution from human landscapes are needed to help protect stream ecosystems, but future research is required to better understand the effects of short-term nutrient pollution to help target management actions. Re-establishing natural vegetation in agricultural and urban landscapes was identified in this thesis as one potential strategy to help protect stream ecosystems from nutrient pollution.
Pearce, Nolan, "Spatial and temporal variation of nutrients in the Great Lakes-St. Lawrence basin: Implications for primary production in stream ecosystems" (2020). Electronic Thesis and Dissertation Repository. 7139.
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.
Available for download on Saturday, August 06, 2022