Anthropogenic nutrient enrichment increases the supply ratio of N and P to aquatic ecosystems and can affect the identity of the limiting nutrient. Here we focus on how stream communities change along gradients of N and P supply and stream catchment landuse intensity. We used a survey approach in 41 southern New Zealand tributaries to investigate how much changes in water N and P concentrations are reflected in periphyton C∶nutrient ratios (C∶N or C∶P) and how much food quality (high food quality corresponds to low periphyton C∶nutrient) is reflected in the abundance and taxonomic richness of benthic invertebrate primary and secondary consumers. We measured streamwater nutrient state, periphyton nutrient ratios, biomass (as chlorophyll a in µg/cm2), algal taxon richness, and macroinvertebrate abundance, taxonomic composition, and richness. We also estimated stream habitat and catchment characteristics, such as current velocity, shading, substrate, geology, and landuse intensity. We calculated the Akaike information criterion (AIC) for each possible multiple linear regression model to select the best predictive models for each response variable. C∶nutrient ratios were more strongly negatively related to water-column N than P availability. Neither N nor P availability covaried with periphyton biomass. Lower periphyton C∶N partly explained higher grazer, but not predator, abundance. Increased % runoff from pasture and periphyton N∶P co-occurred with a decrease in invertebrate taxon richness. For example, a 4× increase in periphyton N∶P was related to the loss of ∼½ of invertebrate species, but with high uncertainty (R2 = 0.13). We conclude landuse intensity affects these southern New Zealand streams, and these effects are mediated by agricultural N runoff into streams (among other factors). Further shifts toward high-intensity farming within stream catchments may lead to losses of benthic species at all trophic levels.
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