| The filter-feeding Asian clam, Corbicula fluminea, and omnivorous blue tilapia, Oreochromis aureus, have been established in Florida waters since the early 1960s. They are known for their ability to reach great abundance and to feed effectively on small particles such as detritus, phytoplankton, and microzooplankton. 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 microcosm experiments, Corbicula was able to reduce turbidity levels and bacterioplankton, phytoplankton (as chlorophyll-a), and rotifer biomass significantly. Freed from fish predation, the planktonic structure of the water column shifted almost exclusively to larger cladocerans and copepods.; Once fish larvae have absorbed their nutritional yolk sac, after approximately seven days, they must immediately feed on microzooplankton to survive. In laboratory microcosms, a second-order effect from Corbicula 's filter-feeding was a reduction of useable food available in the water column. Because of this, fathead minnow (Pimephales promelas ) larvae suffered significant mortality in microcosms containing Corbicula, presumably due to starvation.; Although each clam can filter approximately 600 mL–800 mL hr −1 of water, not all planktonic organisms are assimilated. Corbicula produce pseudofeces that encase whole algal cells and small rotifers. With the aid of a microbial technique based on reduction of 2( p-iodophenyl)-3-(p-nitrophenyl)-5(phenyl tetrazolium chloride) (INT), active cyanophytes, Anabaena and Microcoleus, were found in pseudofeces. This encased viable material may represent a valuable nutritional addition to benthic and pelagic organisms, as well as a nutrient source to the benthos.; The cornerstone assumption of whole lake biomanipulation experiments in temperate areas is that large cladocerans and copepods will increase significantly if freed from fish predation. Three month-long experiments (depicting summer, winter, spring/fall) were run in twelve outdoor mesocosms, each containing 1420 L of water. Oreochromis and Corbicula, separately or together, were not able to reduce algal biomass during warm water periods. This also confirms these small-bodied Florida forms are not able to control algal biomass effectively. Thus, removing predaceous fish with the hope it would lead to oligotrophication is not effective in subtropical waters. (Abstract shortened by UMI.)... |
