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

Citation

Demi, Lee; Benstead, Jonathan; Rosemond, Amy; Maerz, John (2020), Experimental N and P additions relieve stoichiometric constraints on organic-matter flows through five stream food webs, Dryad, Dataset, https://doi.org/10.5061/dryad.n02v6wwt4

Abstract

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.

Methods

Methods for data collection and processing can be found in the metadata tabs of data spreadsheets and in the associated manuscript and citations.

Funding

National Science Foundation, Award: DEB-0918904

National Science Foundation, Award: DEB-0918894

National Science Foundation, Award: DEB-0823293