Effects of carp, gambusia, and Australian bass on water quality in a subtropical freshwater reservoir

Freshwater systems are frequently compromised by excessive macrophyte biomass and cyanobacterial blooms associated with eutrophication and possibly exotic fish. The effects of piscivorous Australian bass (Macquaria novemaculeata), planktivorous gambusia (Gambusia holbrooki [Girard]), and benthivorous carp (Cyprinus carpio L) on water quality in Emigrant Creek Dam (ECD), Australia, were studied in 20 experimental enclosures (depth 1 m, volume 3.2 m3), stocked at a density of 1875 kg/ha. Fish additions had immediate effects on water quality, except for bass, which merely recycled preexisting water column nutrients. Water turbidity, total suspended solids (TSS), total nitrogen (TN) and total phosphorus (TP) concentrations, and chlorophyll a (Chl-a), were all significantly higher in gambusia, but more so in carp enclosures. Assuming a linear relationship, mean daily Chl-a increases in carp enclosures were 4.6 μg/L/100 kg carp/ha. Moreover, Chl-a (\u3e90 μg/L) was positively correlated with TP (r2 = 0.91), TN (r2 = 0.89), TFe (r2 = 0.96), and TMn (r2 = 0.83) in carp enclosures; however, Fe and Mn resuspension from sediments does not seem to be a principal mechanism affecting total phytoplankton biomass. Macrophyte, epiphyte, and periphyton biomass were also significantly lower in carp enclosures, a consequence of sediment and nutrient resuspension, dietary excretion, and macrophyte damage. In the absence of carp, epiphytes, periphyton, and macrophytes competed with phytoplankton for available nutrients, providing a critical stabilizing role. Exotic fish removal may prove to be an effective management tool in subtropical systems to maintain high water quality