Is the elemental composition of stream invertebrates a determinant of tolerance to organic pollution?

Many running waters are polluted by organic effluents resulting from agricultural, industrial, and urban activities. Organic pollution consists of the release of high levels of labile C, which usually is rapidly consumed by heterotrophic microbes, and large quantifies of N and P, which influence algal productivity and elemental composition (nutrient:C ratio). The nutrient:C ratio of periphyton is somewhat flexible and can be linked to nutrient level in the water. In contrast, elemental stoichiometry of stream macroinvertebrates is constrained within narrower ranges. This observation leads us to the question whether macroinvertebrates with higher nutrient:C ratios, and presumably high nutrient requirements, are more likely to be found in more nutrient-rich (i.e., polluted) sites, containing food (e.g., periphyton) with higher nutrient:C ratios. The capability of macroinvertebrate taxa to thrive in polluted streams often is referred to as tolerance. Empirical data show that taxa have different tolerances to organic pollution. However, mechanisms that explain their tolerance are largely unexplored. We used published data to test the degree to which tolerance to organic pollution is linked to the elemental composition (nutrient:C ratios) of freshwater invertebrates and their functional feeding group. We showed that the nutrient:C ratios of macroinvertebrate taxa are statistically linked to taxon-specific tolerance to organic pollution. This relationship is strongest among scrapers. The observed relationship between pollution tolerance and macroinvertebrate stoichiometry indicates that macroinvertebrate elemental stoichiometry can determine species distribution along an organic pollution gradient.