Anthropogenic sources of nitrogen resulting from landscape development in watersheds remain primary sources of negative impacts in aquatic ecosystems. Measurements of nitrogen stable isotopes (δ15N) in aquatic biota are often used as indicators of these influences. This study measured δ15N ratios in aquatic insects and periphyton collected from 12 stream sites in Rhode Island and Massachusetts in 2012, 2013 and 2018 to compare their utility for indicating anthropogenic impact in streams and to assess their temporal variability. Site selection covered a wide geographical range and encompassed relatively pristine streams and others from more highly developed areas in the Narragansett Bay watershed. Sites were sampled once in 2012 and seasonally in 2013 and 2018. Aquatic insect samples were collected by nets. Periphyton was collected at each stream site, and δ15N was measured in samples of dried insects and periphyton by isotope ratio mass spectrometry. Organisms were separated into two feeding groups: Predators and Detritivores.
Results/Conclusions
Comparisons of δ15N in periphyton (δ15N Periphyton) among years and seasons showed a high level of correspondence, indicating low temporal variability. Across the 12 stream sites sampled seasonally in 2013 and 2018 comparisons of regression lines of annual mean δ15N in the feeding group as a function of annual mean δ15NPeriphyton were: δ15NPredators = 1.08 δ15NPeriphyton + 1.42 (p <0.001, R2 = 0.84, n = 12) in 2013, and δ15NPredators = 1.03 δ15NPeriphyton + 1.59 ( p < 0.001, R2 = 0.73, n = 12) in 2018. For Detritivores the relationships were: δ15NDetritovores = 1.03 δ15NPeriphyton + 0.69 (p <0.001, R2 = 0.88, n = 12) in 2013, and δ15NDetritovores = 1.10 δ15NPeriphyton + 0.43 (p < 0.001, R2 = 0.85, n = 12) in 2018. Comparisons of these lines showed 95 % confidence intervals overlapped for the slopes and intercepts for predators and detritivores indicating no significant difference in these lines over the six year time interval. Overall, the results show a general constancy of exposure conditions over the 6 years and provide strong support to the utility of analysis of δ15NPeriphyton in streams as a simple, cost-effective indicator of anthropogenic development, which traditionally has been accomplished by the collection, identification and enumeration of organisms (e.g. insects).