Carbon and Nitrogen Isotope Changes in Streams Along an Agricultural Gradient

Nitrogen is a major constituent of agricultural fertilizers, and nitrogen inputs to stream water via runoff and groundwater lead to a variety of negative impacts to the ecosystem and surrounding environment. In order to quantify the movement of nitrogen through aquatic food webs, fourteen streams with varying land uses across Southern and South-Central Ontario were sampled for two species of fish, freshwater mussels, and stream water for measurement of isotope ratios of δ¹⁵N and δ¹³C. I found that nitrogen isotopes in fish, water, and mussels were related to the percentage of monoculture surrounding the stream. Though all species were enriched as monoculture increased, the rate of enrichment as the level of monoculture increased did not vary between species. This study has improved our understanding of nutrient enrichment in aquatic food webs, in addition to assessing the validity of using nitrogen isotopes to measure trophic positions in a variety of aquatic systems.