Biotic and abiotic controls on the ecosystem significance of consumer excretion in two contrasting tropical streams

1. Excretion of nitrogen (N) and phosphorus (P) is a direct and potentially important role for aquatic consumers in nutrient cycling that has recently garnered increased attention. The ecosystem-level significance of excreted nutrients depends on a suite of abiotic and biotic factors, however, and few studies have coupled measurements of excretion with consideration of its likely importance for whole-system nutrient fluxes. 2. We measured rates and ratios of N and P excretion by shrimps (Xiphocaris elongata and Atya spp.) in two tropical streams that differed strongly in shrimp biomass because a waterfall excluded predatory fish from one site. We also made measurements of shrimp and basal resource carbon (C), N and P content and estimated shrimp densities and ecosystem-level N and P excretion and uptake. Finally, we used a 3-year record of discharge and NH₄-N concentration in the high-biomass stream to estimate temporal variation in the distance required for excretion to turn over the ambient NH₄-N pool. 3. Per cent C, N, and P body content of Xiphocaris was significantly higher than that of Atya. Only per cent P body content showed significant negative relationships with body mass. C:N of Atya increased significantly with body mass and was higher than that of Xiphocaris. N : P of Xiphocaris was significantly higher than that of Atya. 4. Excretion rates ranged from 0.16-3.80 μmol NH₄-N shrimp⁻¹ h⁻¹, 0.23-5.76 μmol total dissolved nitrogen (TDN) shrimp⁻¹ h⁻¹ and 0.002-0.186 μmol total dissolved phosphorus (TDP) shrimp⁻¹ h⁻¹. Body size was generally a strong predictor of excretion rates in both taxa, differing between Xiphocaris and Atya for TDP but not NH₄-N and TDN. Excretion rates showed statistically significant but weak relationships with body content stoichiometry. 5. Large between-stream differences in shrimp biomass drove differences in total excretion by the two shrimp communities (22.3 versus 0.20 μmol NH₄-N m⁻² h⁻¹, 37.5 versus 0.26 μmol TDN m⁻² h⁻¹ and 1.1 versus 0.015 μmol TDP m⁻² h⁻¹), equivalent to 21% and 0.5% of NH₄-N uptake and 5% and <0.1% of P uptake measured in the high- and low-biomass stream, respectively. Distances required for excretion to turn over the ambient NH₄-N pool varied more than a hundredfold over the 3-year record in the high-shrimp stream, driven by variability in discharge and NH₄-N concentration. 6. Our results underscore the importance of both biotic and abiotic factors in controlling consumer excretion and its significance for nutrient cycling in aquatic ecosystems. Differences in community-level excretion rates were related to spatial patterns in shrimp biomass dictated by geomorphology and the presence of predators. Abiotic factors also had important effects through temporal patterns in discharge and nutrient concentrations. Future excretion studies that focus on nutrient cycling should consider both biotic and abiotic factors in assessing the significance of consumer excretion in aquatic ecosystems.