Phosphorus dynamics in neotropical stream food webs
Small, Gaston Elvin
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Humans have dramatically altered global biogeochemical cycles, resulting in widespread nutrient pollution in freshwater ecosystems. The harmful effects of eutrophication are well-documented in lakes and estuaries, but less in known about the effects of sustained nutrient pollution in streams, mostly due to the short-term nature of most experiments. In this study, I use a series of streams in lowland Costa Rica that naturally vary in dissolved phosphorus (P) levels due to inputs of solute-rich groundwater to investigate the effects of sustained, high levels of P-loading on stream food webs. I ask two major questions: (1) What are the effects of sustained P-loading on consumer physiology? and (2) What role do consumers play in supplying limiting nutrients in these stream ecosystems? In the naturally high-P streams, invertebrate consumers increased two-fold in body P-content, showing that the elemental composition of consumers can reflect their environmental conditions as much as their phylogeny. For one important group of invertebrate detritivores, chironomid larvae, I found that P-demand reflected food quality in streams across this heterogeneous landscape. As a result, chironomids from low-P streams circumvented P-limitation, and did not respond to P-enriched food resources with increased growth rates, which may act to stabilize this detritus-based food web against perturbations caused by nutrient enrichment. I found that all fish species in high-P streams excreted P at high rates, but in a low-P stream, P-excretion rates for species with higher body P-content were negligible. In the low-P focal stream, one species accounted for 90% of P recycled by the fish assemblage, even though it only represented 18 % of the total fish biomass, due to its high-P diet and low body P-demand. Finally, I quantified the potential importance of terrestrial insects as a nutrient subsidy in rainforest streams, showing that this nutrient pathway may supply a significant fraction of nutrient demand in small headwater streams. This dissertation improves our understanding of how rates of nutrient retention and recycling by organisms are affected by ecosystem nutrient availability, which ultimately determines the resilience of ecosystems to anthropogenic nutrient pollution.