Urbanization and agriculture influence stream dissolved organic matter quality variability more than decomposition rates and macroinvertebrate diversity across seasonal time scales

In the era of the Anthropocene, humans have impacted over half of the Earth’s surface. Urbanized and agricultural land use pressures have easily become some of the dominating forces shaping ecosystems today, revealing similar impacts on freshwater ecosystems. Streams and rivers are among the most heavily impacted due to the influence of catchment land use on stream water quality and ecological condition. Structural and functional indicators collected by biomonitoring programs are underused as tools for targeting stream restoration efforts. In the present study we applied a novel combination of indicators—dissolved organic matter (DOM) composition, cotton strip decomposition, and benthic invertebrate sampling—to determine if streams highly impacted by urbanized and agricultural land use across Windsor-Essex (southwestern Ontario, Canada) were consistent by season, anthropogenic land use or some combination of both. Overall, our results suggest that agricultural and urban streams are indeed degraded at a similar level, with high decomposition rates and low levels of macroinvertebrate diversity. Moreover, DOM quality proved to be the most effective indicator, integrating insights from both decomposition and macroinvertebrate indices while remaining stable seasonally. Microbial humic-like DOM correlated positively with decomposition rates, and negatively with invertebrate species richness. Our findings show that function changes in stream ecological condition can be effectively tracked by structural indicators like DOM composition. We suggest that these measures should be incoporated into monitoring programs to develop functional indicators for targeting stream restoration.