The net effects of grazing and nutrient remineralization on algal biomass and primary productivity by an exotic grazing fish

Background/Question/Methods Species invasions provide a unique opportunity to ask fundamental ecological questions and test the impacts of individual species on ecosystem function. Recent studies have demonstrated that invasive species can alter energy pathways and shift patterns in nutrient recycling. Grazing fishes can potentially affect ecosystem structure and function in contrasting ways. Initially, fishes can depress resources by cropping algae and by reducing the amount of bioavailable nutrients by acting as a nutrient sink and sequestering nutrients in their bodies. Conversely, high densities of fishes can create localized areas of rapid nutrient remineralization, or biogeochemical hotspots, which can enhance primary production in aquatic habitats. We measured the effects of nutrient remineralization and grazing by an invasive armored catfish (Siluriformes: Loricariidae: Pterygoplichthys) on algal biomass and primary productivity using a series of mesocosms in the Chacamax River in Chiapas, Mexico. We employed three treatments: control (no fish), low catfish density, and high catfish density. Each mesocosm contained a fish exclusion in the center, which was exposed to nutrient remineralization, but was protected from grazing in the fish treatments. We measured algal biomass on rocks in each mesocosm using fluorometry and estimated primary productivity using closed chambers during the study period. Results/Conclusions Rocks protected from loricariid grazing but exposed to nutrient remineralization had significantly higher algal biomass than the rocks grazed by loricariids and the rocks from the control treatment lacking fish. Additionally, samples collected from rocks exposed to loricariid grazing had lower algal biomass than corresponding controls. Primary productivity was significantly greater on rocks taken from pools with high densities of loricariids when the rocks were protected from grazing. However, the same increase was not seen in rocks exposed to grazing in high-density loricariid pools. These results indicate that exotic grazing fishes may stimulate primary productivity via nutrient remineralization in invaded systems. Yet, these effects may be masked when the invaders attain high densities and graze the available standing crop of primary producers.