Tom Moore, Jon Harding, Angus McIntosh & Catherine Febria.
Nitrate-nitrogen pollution is an important pathway in which agricultural land uses exert stress on stream ecosystems. Although nutrient impacts of eutrophication have been well documented, little is known how stream invertebrate communities are influenced by this stressor. Theory suggests a threshold may occur when basal resources (e.g., algae) no longer benefit from increased nutrients for growth, resulting in a more stable basal resource and stream community. To investigate this predicted threshold response, macroinvertebrate communities were surveyed from 39 lowland agricultural streams along a nitrate-nitrogen gradient (0.4 – 11.3 mg/l N-NO3-) on the Canterbury Plains. The response was assessed with traditional biotic indices and macroinvertebrate community presence/absence data. Almost all traditional invertebrate biotic indices (including the MCI) showed no relationship with nitrate-nitrogen; Taxa richness was the exception, where a weak quadratic model best described the relationship (F1,39=3.62, R2=0.12, P<0.05). In contrast, ordination of macroinvertebrate taxa presence/absence identified nitrate-nitrogen as a significant driver of community composition along with turbidity, Substrate Index and discharge were also associated. Indicator taxa most strongly correlated with the nitrate-nitrogen gradient were Elmidae larvae (positive) and Sphaeriidae (negative). These results suggest the relationship between nitrate-nitrogen and macroinvertebrate composition depends on how the biotic response is assessed. Furthermore, that the pollution-tolerant, generalist macroinvertebrate communities typical of Canterbury Plains streams either do not respond to nitrate-nitrogen, or respond weakly, which is difficult to detect. Overall, this study provides evidence to advise stream management, and highlights the tolerance of macroinvertebrates to nitrate-nitrogen pollution with implications for stream biodiversity, food-web structure, and ecosystem function.