Effect of an invasive fish species on nutrient cycling and on the community structure: an experimental approach

Invasive fish species can play an important role in aquatic ecosystems and have a significant impact on ecosystem dynamics, which influences primary production, planktonic communities, and aquatic macrophytes. The presence and density of invasive fish can disrupt nutrient ratios and ecological processes, leading to potential ecological consequences for invaded habitats. This experimental investigation focused on the invasive armored catfish species P. ambrosettii (Loricariidae) and postulated that high densities of armored catfish in invaded environments may affect nutrient cycling due to high N: P excretion rates. Furthermore, we sought to identify how these changes in nutrient concentration impact phytoplankton, protozooplankton, and the biomass of two invasive macrophytes. We carried out a 30-day experimental study at the Nupélia Applied Ecology Laboratory at the State University of Maringá with 20 fiber cement mesocosms with a volume of 135 L. These mesocosms had continuous water circulation and were used to simulate the natural conditions of lakes invaded by P. ambrosettii. We simulated five treatments that represented density at different levels of invasion: control (no fish), low-density (one fish), medium–low (two fish), medium–high (three fish) and high-density (four fish). The treatments affected abiotic variables such as pH, dissolved oxygen, and electrical conductivity, and nitrogen and phosphorus concentrations increased with increasing fish density, as expected. In turn, nutrient concentrations had effects on chlorophyll-a and macrophyte biomass. The chlorophyll-a and Eichhornia crassipes biomass were positively related to the increase in phosphorus, while Hydrilla verticillata showed a positive relationship with both nutrients nitrogen and phosphorous. The zooplankton community showed different density behaviors in relation to treatments. Its composition was influenced by the different treatments, and it underwent significant variations along the nutrient gradient. As predicted, the different densities of P. ambrosettii led to changes in the structure of the ecosystem. In treatments with higher fish densities, the environment was characterized by low oxygenation, high conductivity, and high concentrations of nitrogen and phosphorus. These conditions favored the dominance of phytoplankton and aquatic macrophytes, while the density and species composition of zooplankton was low. The effects observed in this experimental study contribute to insights into the field of invasion biology and its relationships with nutrient dynamics. The effects that may arise from the invasion and high population density of the armored catfish Pterygoplichthys in invaded locations reinforce the importance of understanding these processes in the functioning of the ecosystem and how this directly or indirectly impacts the dynamics of coexisting communities in natural environments.