An isotope tracer study of nitrogen cycling and foodweb structure in a neotropical stream

Background/Question/Methods

Tracer applications of stable isotopes have become widely used in nutrient cycling studies. Tracer approaches have become particularly popular in stream studies, especially in temperate North America. Far fewer studies have used ¹⁵N tracer approaches to quantify N cycling in Tropical streams. We and others recently initiated research investigating links between ecological and ecosystem characteristics and the trajectory of guppy life history evolution in Trinidad’s Northern Range. As part of this larger effort, we have initiated ¹⁵N additions to evaluate the ecosystem effects of light alteration, species addition (guppies), and the rapid evolution of life history traits. In this presentation, we report results from our pre-manipulation experiments in 2 adjacent streams. ¹⁵N-NH₄⁺ and rhodamine-WT were added to 2 streams at a constant rate for 10-days. Longitudinal samples of water column ¹⁵N-NH₄⁺ and several hydromorphological variables were used to quantify NH₄⁺ uptake kinetics. Spatially explicit sampling and modeling were used to estimate N flux into algal and microbial assemblages and consequent movement into several benthic consumers and Rivulus hartii, the lone fish species.

Results/Conclusions

Stream discharge was similar between streams (23 vs. 28 L s^-1) and ammonium uptake lengths were short in each stream (18m and 23m). Low ambient NH₄⁺ concentrations and similar depths resulted in similar estimates for uptake velocity and gross areal uptake. Longitudinal production and loss of ¹⁵N-NO₃ was modeled to estimate the nitrification rate of water column NH₄⁺ and nitrate uptake rates. Nitrification ate varied by <20% between streams. Preliminary modeling results indicate that inadequate reach length prohibited NO₃^- uptake estimates. ¹⁵N label accumulated in all benthic compartments and often exceeded 300‰ by day 10 in epilithon samples. Bulk detrital pools (fine and course bethic organic matter) accumulated less 15N on a mass-specific basis but were the largest biotic pool of 15N at the end of the labeling period. Similarly, all aquatic invertebrates became enriched in 15N by day 10 and remained enriched in post-enrichment sampling. We are currently using a donor-controlled modeling approach to estimate N flux between food resources, invertebrate consumers and Rivulus. Results from the first year of this project indicate that our experimental streams operate much like their temperate counterparts with respect to ammonium cycling rates and its movement within the stream food web. Between stream similarities should aid our ability to document how N cycling in neotropical streams is altered as bottom-up (light) and top-down (guppy introduction) forces are altered in upcoming manipulations.