Light availability mediates the impacts of exotic snails on nutrient cycling in tropical streams

Background/Question/Methods Few studies have examined the impact of invasive species on ecosystem processes in tropical aquatic ecosystems, and even less in known about how habitat degradation may mediate the influence of invaders. In Trinidad, West Indies, we examined nutrient remineralization by two invasive aquatic snails (Tarebia granifera and Melanoides tuberculata) as a function of light availability in two streams and in light-manipulated experimental channels to investigate potential impacts of riparian vegetation removal on snail-mediated nutrient recycling. We collected snail abundance, biomass, and excretion (NH₄ and SRP) data in streams to determine snail-derived nutrient remineralization per (1) area of streambed and (2) snail biomass. Results/Conclusions Where riparian vegetation had been removed and direct sunlight reached the streambed, both snail species were more abundant and per-area remineralization of NH₄—but not SRP—by snails was significantly greater than in closed canopy conditions. Patterns of per-biomass NH₄ excretion rates in streams were less consistent among light conditions, despite consistently higher epilithon C:N in locations where the canopy had been removed. However, results from light manipulations in experimental channels suggest that high light availability increased epilithon C:N and decreased Tarebia biomass-specific NH₄ remineralization relative to low light conditions. Thus, light availability—and therefore riparian canopy presence—can influence both biomass- and area-specific nutrient recycling by exotic snails in these tropical streams. Our findings have important implications for whether human-mediated destruction of riparian vegetation facilitates proliferation of aquatic invaders and augments their influence on ecosystem-scale processes in tropical streams.