Stoichiometric constraints on secondary production in a headwater stream food web

Background/Question/Methods Most consumers must maintain body tissue Carbon:Nitrogen:Phosphorus within relatively narrow constraints, even though these ratios often vary markedly among basal resources. These elemental constraints might be a major factor governing resource selection by, and production of, consumers. As a consequence, elemental imbalances between resources and consumers could constrain the flux of matter through food webs, and hence their structure. In this study we aim to characterise and quantify the distribution of C, N and P within the nodes of a quantified food web of a headwater stream ecosystem (i.e. species populations and basal resources). The experimental reach was surveyed in spring and autumn. The standing stock and elemental composition of epilithic algae, particulate organic matter fractions (collectively referred to as POM), macroinvertebrates, and fish were quantified. The feeding links of both macroinvertebrates and fish were identified and quantified using gut content analysis.

Results/Conclusions The results show remarkably little variation in carbon to nitrogen ratios among the consumers in the system (macroinvertebrates and fish). This applied across taxonomic groups as well as on a temporal scale. In contrast, the carbon to phosphorus ratios of the consumers in the system displayed a much larger degree of variation within and between taxonomic groups, as well as on a temporal scale. The basal resources (POM and primary producers) had substantially higher carbon to nutrient (N & P) ratios than the consumers, with the autochthonous resources (primary producers) being richer in nutrients (lower C to nutrient ratios) than the allochthonous resources (POM). Despite the poor elemental quality of allochthonous resources, gut content analysis showed that they provided 95% of the biomass of basal resources entering the food web. Therefore large stoichiometric imbalances between consumers and the basal resources exist in this ecosystem. The fixed elemental ratios of carbon to nitrogen in the consumers in combination with the high carbon to nutrient ratios of the allochthonous basal resources potentially constrain the secondary production of the stream ecosystem.