The filter-feeding exotic clam, 'Corbicula fluminea' and omnivorous African blue tilapia, 'Oreochromis aureus' have been found in Florida since the 1960's. It was hypothesized that both exotic species may significantly alter the planktonic food web (first-order effect) as well as organisms that depend on smaller particles for food (second-order effect).

In 37-liter laboratory experiments, 'Corbicula' was able to reduce turbidity levels and bacterioplankton, phytoplankton, and rotifer biomass significantly (first-order effect). Free from fish predation, the planktonic structure of the water column shifted almost exclusively to larger-bodied cladocerans and copepods. It was found that a second-order effect also occurred when fathead minnow ('Pimephales promelas') larvae suffered significant mortality in microcosms containing 'Corbicula', presumably from starvation (microzooplankton virtually absent).

Three month-long experiments depicting summer, winter, spring/fall were run in twelve outdoor mesocosms, each containing 1420 liters of water. During winter, in both fishless (control, clams) and fish-containing mesocosms (fish, clams+fish), cladoceran and copepod biomass increased and phytoplankton decreased. However, in water temperatures above 24 degrees C, cladoceran and copepod biomass again increased but phytoplankton increased as well. Thus, removing predacious fish with the hope that increases in the larger crustacean zooplankton would lead to oligotrophication is not realized in subtropical waters.

These experiments demonstrated that to achieve successful biomanipulation of waters in the subtropics to reduce algal biomass, the addition of a filter-feeding mollusk and a phytophagous fish are required, unlike temperate systems.