1. Despite non-point-source (NPS) pollution being perhaps the most ubiquitous stressor affecting urban streams, there is a lack of research assessing how urban NPS pollution affects stream ecosystems. We used a natural experimental design approach to assess how stream macroinvertebrate community structure, secondary production and trophic structure are influenced by urban NPS pollution in six streams.
2. Differences in macroinvertebrate community structure and secondary production among sites were highly correlated with stream-water specific conductivity and dissolved inorganic phosphorus (DIP) concentrations. Macroinvertebrate richness, the Shannon diversity index and the Shannon evenness index were all negatively correlated with specific conductivity. These patterns were driven by differences in the richness and production of EPT and other intolerant taxa. Production of the five most productive taxa, tolerant taxa, non-insect taxa and primary consumers were all positively correlated with stream-water DIP.
3. Despite the positive correlation between primary consumer production and DIP, there was no correlation between macroinvertebrate predator production and either total or primary consumer macroinvertebrate production. This was observed because DIP was positively correlated with the production of non-insect macroinvertebrate taxa assumed to be relatively unavailable for macroinvertebrate predator consumption. After removing production of these taxa, we observed a strong positive correlation between macroinvertebrate predator production and production of available prey.
4. Our results suggest that urban NPS pollution not only affects macroinvertebrate community structure, but also alters secondary production and trophic-level dynamics. Differences in taxon production in our study indicate the potential for altered energy flow through stream food webs and potential effects on subsidies of aquatic insect prey to riparian food webs.