Experimental N and P additions relieve stoichiometric constraints on organic-matter flows through five stream food webs

1. Human activities have dramatically altered global patterns of nitrogen
(N) and phosphorus (P) availability. This pervasive nutrient pollution is
changing basal resource quality in food webs, thereby affecting rates of
biological productivity and the pathways of energy and material flow to
higher trophic levels. 2. Here, we investigate how the stoichiometric
quality of basal resources modulates patterns of material flow through
food webs by characterizing the effects of experimental N and P enrichment
on the trophic basis of macroinvertebrate production and flows of dominant
food resources to consumers in five detritus-based stream food webs. 3.
After a pre-treatment year, each stream received N and P at different
concentrations for two years, resulting in a unique dissolved N:P ratio
(target range from 128:1 to 2:1) for each stream. We combined estimates of
secondary production and gut contents analysis to calculate rates of
material flow from basal resources to macroinvertebrate consumers in all
five streams, during all three years of study. 4. Nutrient enrichment
resulted in a 1.5´ increase in basal resource flows to primary consumers,
with the greatest increases occurring among diatoms and wood, which
experienced the largest reductions in carbon:P (C:P) ratios. Flows of most
basal resources were negatively related to resource C:P, indicating
widespread P-limitation in these detritus-based food webs. Nutrient
enrichment resulted in a greater proportion of leaf litter, the dominant
resource flow-pathway, being consumed by macroinvertebrates, with that
proportion increasing with decreasing leaf litter C:P. However, the
increase in efficiency with which basal resources were channeled into
metazoan food webs was not propagated to macroinvertebrate predators, as
flows of prey did not systematically increase following enrichment and
were unrelated to basal resource flows. 5. This study suggests that
ongoing global increases in N and P supply will increase organic-matter
flows to metazoan food webs in detritus-based ecosystems by reducing
stoichiometric constraints at basal trophic levels. However, the extent to
which those flows are propagated to the highest trophic levels likely
depends on responses of individual prey taxa and their relative
susceptibility to predation.