Effects of Nutrients and Dissolved Organic Matter on Ecosystem Metabolism in Southern Ontario Streams

Anthropogenic actions have affected the structure and function of freshwater ecosystems through conversion of land for agriculture, increased pollution, and altered nutrient and carbon loadings. However, the effects of nutrients and dissolved organic matter concentration (DOC) and composition (DOM) in shaping stream metabolism remain uncertain. Our study examined metabolism in 12 streams across southern Ontario representing gradients in total dissolved nitrogen (TDN) and phosphorus (TDP), DOC, and DOM composition. For each stream, gross primary production (GPP) and ecosystem respiration (ER) were estimated daily for three distinct temporal periods of 11 days in early summer (June), late summer (August), and fall (October). Water samples from each stream were collected during these same periods to measure ambient nutrients, DOC, and DOM. We used a generalized linear mixed model approach to describe the association between stream metabolism and TDN, TDP, DOC, and DOM composition, accounting for variation across streams with a random effect of stream ID. We found stream GPP was not affected by nutrients but associated negatively with DOC (p < 0.05) and positively with Freshness Index (β/α; p < 0.05). Stream ER was not affected by nutrients or DOM composition but associated negatively with DOC (p < 0.05). Our findings demonstrate inconsistent effects of nutrients on stream metabolism and suggest elevated GPP drives greater autochthonous production of fresh DOM within these streams, highlighting a strong relationship between composition of DOM and stream metabolic activity.