Effects of omnivorous tilapia on phytoplankton and water quality in an ecosystem with submerged macrophytes

Nile tilapia [Oreochromis niloticus (Linnaeus, 1758)] is a widespread invasive fish, now present in lakes, rivers, and reservoirs around the world, and its dominance has led to serious ecological problems in many freshwater systems. However, little is known about its effect on macrophyte-dominated lake ecosystems. We conducted a 10-week mesocosm experiment to test the impact of tilapia on phytoplankton and water quality in model ecosystems with submerged macrophytes [Vallisneria natans (Lour.) Hara] and the presence and absence of tilapia. Tilapia led to an increase in concentrations of total phosphorus, total nitrogen, total dissolved nitrogen, ammonia nitrogen, and total suspended solids and had negative impact on submerged macrophyte and light intensity, thus causing an overall deterioration in water quality. Tilapia increased the abundance of rotifers and changed the zooplankton community structure. Tilapia increased the total biomass of phytoplankton but the effect depended on the phytoplankton size, i.e., the overall proportion of micro-phytoplankton increased, while that of pico-phytoplankton decreased. Thus, tilapia also caused changes in the phytoplankton community structure. Our research shows that tilapia deteriorates water quality and modifies phytoplankton and zooplankton Communities of shallow lakes.