Whole stream addition of enriched stable isotopes is a powerful method for quantifying biogeochemical cycling and trophic dynamics in lotic ecosystems. However, most isotopic tracer studies have occurred in temperate North American streams, and the approach has been largely neglected in tropical running water systems. Using whole stream additions of 15N-labeled ammonium, we characterized trophic flows in two light-limited Neotropical streams before and after thinning of the riparian canopy to enhance basal primary production. Moreover, in order to understand the importance of terrestrial subsidies to these stream food webs, isotopic studies were coupled with quantification of the contribution of terrestrial invertebrates to the diet, growth and reproduction of the lone fish species, Hart’s killifish (Rivulus hartii) under ambient and enhanced levels of primary productivity. Donor-controlled, spatially-explicit modeling was used to measure fluxes of nitrogen between major trophic compartments. To corroborate the assumptions of our model we also conducted size-class-specific gut analysis of R. hartii and constructed isotopic niche diagrams. Growth, population structure and density of R. hartii were assessed using mark-recapture.

Results/Conclusions

Before the canopy thinning both streams were similar in physical and ecological characteristics. Ammonium uptake lengths were short in each stream (~ 20 m), and the majority of ammonium added to the system was assimilated by the stream (~ 80-90%), while a small fraction was nitrified (~20%). Ammonium uptake in the basal resources was dominated by benthic organic matter (~75% of in stream nitrogen uptake), followed by epilithon and coarse particulate organic matter. All major aquatic foodweb compartments were successfully labeled by the end of the experiment, with filterer/gatherers and scrapers being more enriched than shredders and predators. The addition did not label any terrestrial insects (~3.5‰ from the beginning to the end of the addition), allowing us to separate the contribution of terrestrial and aquatic organisms to the diet of R. hartii. Adults of R. hartii acquire ~90% of their diets from terrestrial sources, while smaller, juvenile R. hartii acquired 20-80% of the diet from aquatic resources. A preliminary nitrogen budget suggests that before the canopy was thinned, the majority of R. hartii reproduction was supported by allochthonous resources, while the majority of R. hartii growth was supported by autochthonous resources.