Biotic and abiotic controls on the ecosystem significance of consumer excretion in two contrasting tropical streams
JONATHAN P. BENSTEAD1,
WYATT F. CROSS2,
JAMES G. MARCH3,
WILLIAM H. McDOWELL4,
ALAN P. COVICH2
Article first published online: 14 JUN 2010
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 lmol NH4-N shrimp)1 h)1, 0.23–5.76 lmol total
dissolved nitrogen (TDN) shrimp)1 h)1 and 0.002–0.186 lmol 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
5. Large between-stream differences in shrimp biomass drove differences in total excretion
by the two shrimp communities (22.3 versus 0.20 lmol NH4-N m)2 h)1, 37.5 versus
0.26 lmol TDN m)2 h)1 and 1.1 versus 0.015 lmol 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 lowbiomass
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.