Food web interactions along a marine and freshwater gradient within a subarctic estuary
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Date
2022-03-29
Authors
St. George, Jillian
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Abstract
Food webs describe the trophic relationships between species within ecosystems. The trophic structure and interactions can vary in space and time, which can lead to changes in the food web. Ecosystems are linked together by the movement of nutrients, prey, and consumers. This movement between ecosystems is known as habitat coupling, where ecological dynamics are driven by the input of nutrients and energy from distant or adjacent habitats. Estuaries are an important link between freshwater and marine systems since they are a transition zone where species can acquire resources from both systems. The objectives of this thesis were to (1) quantify the diet composition between freshwater-and marine-derived nutrients and niche size of mobile consumers (13 fishes and 2 seal species) within the lower Churchill River, (2) quantify the trophic positions of the Churchill marine/freshwater food web for mobile consumers and to understand the ontogenetic effects on trophic position, and to (3) quantify changes in trophic structure with certain members of the fish community between 1993-1995 and 2019-2020. Results indicated habitat coupling for cisco, lake whitefish, and northern pike. These species were also found to have the greatest niche size, indicating a broader use of resources. I also found species that mainly foraged on freshwater resources occupied the secondary consumer position, whereas species that foraged on marine resources occupied the tertiary consumer position. Trophic position increased with age for cisco, fourhorn sculpin, Greenland cod, and northern pike. Six community-wide metrics of the fish assemblage (cisco, fourhorn sculpin, and lake whitefish) revealed more trophic redundancy in the 2019-2020 community. Greater trophic redundancy means that individual species are now playing similar trophic roles within the food web, which may help promote ecosystem stability and reduce vulnerability to secondary extinction events. Overall, my thesis findings have provided an understanding of the trophic structure and interactions within and between a freshwater-marine gradient within the lower Churchill River. This study highlighted how estuaries are a vital link between freshwater and marine systems and understanding trophic dynamics and connections among species in these habitats is critical for their management.
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Keywords
Churchill, Food web, Stable isotopes, Niche size, Estuary