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Citation: Lundquist, M. J., and W. Zhu. 2018. Aquatic insect functional diversity and nutrient content in urban streams in a medium-sized city. Ecosphere 9(5):e02284. 10.1002/ecs2.2284Abstract. Aquatic insects play a large role in the processing of organic matter in headwater streams; however, anthropogenic disturbances associated with watershed urbanization can impact the biodiversity and abundance of these insects. Aquatic insects function on multiple trophic levels (i.e., shredders, collectors, scrapers, and predators) and interact in the nutrient cycling within lotic systems. The degradation or loss of any of these functional groups could disrupt stream biogeochemistry and the flow of energy in stream ecosystems. We used a paired reach site design to assess the insect functional group representation in five streams impacted by urbanization with downstream urban and upstream rural reach sites. We also sampled two non-urban reference streams with rural upstream and rural downstream reach sites. We assessed functional group richness and abundance and measured insect biomass, insect body nitrogen (N) content, phosphorus (P) content, and N:P ratio. We found that collectors were numerically the most abundant and taxonomically the richest insect group in all site types. There were much fewer insects collected from urban sites than rural sites and individual insects were generally smaller in the urban sites. Collectors represented the most insect biomass in urban sites and predators the most insect biomass in rural sites. Insect N and P concentrations were not affected by urban landuse, ranging from 88.45 AE 12.50 to 121.74 AE 15.80 mg/g and 10.60 AE 2.10 to 18.36 AE 7.05 mg/g, respectively; and N:P ratios from 3.48 to 20.97. However, lower total insect biomass, particularly of predators and shredders, led to significantly lower total N and P mass in urban sites. Urbanization had clear effects on the diversity of functional groups, insect biomass, and their nutrient mass in this medium-sized city.

Aquatic insects are important components of stream food webs and are greatly impacted by anthropogenic disturbances, including urbanization. Successful restoration of urban rivers is contingent on the growth of aquatic insect populations from colonizers from local, less-disturbed streams. However, rivers in highly urbanized watersheds may not have nearby source populations, and therefore must rely only on individuals already surviving in the river. At the regional level, urbanization is a homogenizing process, but the impacts could be heterogeneous at the local level. Therefore, some sites within highly urban rivers might support higher local biodiversity and provide source populations for restoration projects focused on other sites in the river. In this study, we collected aquatic insects from sites within the Bronx River, a highly urbanized river in the New York City metropolitan area, NY, in the summers of 2021 and 2022. We found that while taxonomic richness was similar among sites, insect abundance and dominant taxa, particularly members of Hydropsychidae (Trichoptera), varied significantly among sites. These findings suggest sites within the Bronx River are not homogenous and that some sites within the river harbor larger populations of aquatic insects and may be integral to the success of future conservation projects. This pattern of within-river heterogeneity may exist in other urban rivers and deserves consideration in determining conservation goals and the planning of stream-restoration projects.

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