Consumer-resource elemental imbalances in a nutrient-rich stream

Pronounced stoichiometric imbalances (C:N:P) between consumers and resources reported from nutrient-poor systems potentially constrain key ecological processes, but such imbalances should be less marked when more nutrients are available. In a headwater stream rich in nutrients (total P = 208 mu g/L; total oxidizable N = 7 mg/L), we determined the elemental composition and standing stock of the consumer species and basal resources in relation to taxonomic identity, feeding mode, and season (spring and autumn). Compared with previous studies, basal resources had low elemental ratios (C:N and C:P), reflecting the high concentrations of inorganic nutrients in the water. Nevertheless, elemental imbalances were still evident between consumers and these basal resources, particularly for organisms feeding on detritus. Some of the variation in elemental ratios among consumers could be attributed to taxonomic identity. Furthermore, detritivores typically were depleted in N and P compared to taxonomically related species with different feeding modes. Elemental ratios of primary consumers differed between the 2 sampling occasions. Collector-gatherers and scrapers had lower C:P and N:P and shredders had higher C:N in October than in May. Basal resources (fine and coarse particulate organic matter and periphyton) made up most of the standing stock of organic N and P, but quantities varied between May and October. The elemental composition of consumers of basal resources appeared to track changes in resource availability. Even with a plentiful supply of inorganic N and P available to primary producers, the availability of elements from food (a combination of quality and quantity) may influence the elemental composition of consumers.