Effect of mosquitofish (Gambusia affinis) and sestonic food abundance on the invertebrate community within a constructed treatment wetland

The effect of mosquitofish (Gambusia affinis) predation on the invertebrate community in a hypereutrophic constructed treatment wetland in southern California was investigated at two nutrient levels that influenced sestonic food abundance. Gambusia affinis and insect predators in the wetland had a significant impact on larval mosquito density in the wetland irrespective of nutrient level. At the end of the 5-month study, cladoceran abundance in predator exclusion enclosures was 2-3 orders of magnitude greater than in the treatments that allowed access by planktivores. Chironomids were the most abundant insect group collected in emergence traps, and midge production from the high nutrient location of the wetland was greater than from the low nutrient location, but was not affected significantly by G. affinis. The presence of abundant alternative prey in this highly enriched wetland may have weakened the predation impact of G. affinis on mosquitoes. The abundances of six invertebrate groups in dipper samples and of four insect groups in emergence trap collections were analysed using a multivariate distance-based linear model. Fish treatment and location interactions with sampling date explained significant amounts of the variation in the abundance of invertebrate groups. Multivariate multiple regression analysis showed that chlorophyll-a concentration explained a large portion of the variability in non-predatory insect and zooplankton abundance at the high nutrient location, whereas bacterial density explained a large portion of the variability in the abundances of these taxa at the low nutrient location. Predatory insects were not directly coupled to the bottom-up influence of bacterial abundance and chlorophyll-a.