Are terrestrial insects an important nutrient subsidy in Neotropical headwater streams?

Background/Question/Methods

The exchange of nutrients between terrestrial and aquatic ecosystems in temperate ecosystems has received increasing attention by ecologists, but little is known about the magnitude of this cross-boundary exchange in the tropics.  In tropical rainforests, terrestrial insects falling from the forest canopy have the potential to represent an important nutrient flux in nutrient-poor headwater streams, if these insects are efficiently captured by in-stream predators.  We used a combination of approaches to quantify the contribution of terrestrial insects to the nutrient budget of two headwater streams in lowland Costa Rica.  We used pan traps to quantify the total areal input rate of terrestrial insect biomass.  We also measured nitrogen (N) and phosphorus (P) excretion rates for the fish assemblage, dominated by the insectivorous species Priapicthys annectens (Poecillidae), and used gut contents and stabile isotope analyses to partition terrestrially- and aquatically-derived nutrients.  To quantify how effectively fish captured drifting terrestrial insects, we added a known number of ants along with a “conservative tracer,” and we also quantified terrestrial insect drift in pools, both before and after removing fish.   

Results/Conclusions

Priapicthys had an average density of 8 individuals m^-2 in these streams, and terrestrial insects (predominantly ants) constituted 50% of their diet.  Based on pan trap data, terrestrial insects constituted nutrient fluxes of 90-142 µg N m^-2 h^-1 and 2.4-3.7 µg P m^-2 h^-1.  We estimated mean fish capture efficiency over six different 5 meter reaches ranging from 44-74% based on ant additions, and from 32-93% based on ambient drift measurements.  Terrestrially-derived N-excretion ranged from 12.5-47 µg NH₄⁺ m^-2 h^-1, and terrestrially-derived P-excretion ranged from 0.3-1.1 µg total dissolved P m^-2 h^-1.  Based on measured N-uptake rates in one of these streams, we estimate that terrestrially-derived N-excretion by Priapicthys can supply 30-100% of stream NH₄⁺ demand.  These results indicate that high input rates of terrestrial invertebrates in low-solute headwater streams in a tropical rainforest can comprise a significant nutrient subsidy.  Insectivorous fishes such as Priapicthys play an important role in mediating this subsidy by retaining these terrestrially-derived nutrients and converting them into dissolved form, readily available to the rest of the stream ecosystem.