An Experimental Study of the Plankton Community Impacts of Two Omnivorous Filter-Feeding Cichlids, Tilapia galilaea and Tilapia aurea

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Abstract
We conducted laboratory selective grazing experiments and outdoor mesocosm experiments assessing impacts of two cichlids, Tilapia galilaea and Tilapia aurea, on plankton from Lake Kinneret, Israel. Laboratory feeding rates of both fish increased for larger particles, reaching maximum values for zooplankton and Peridinium cinctum. Tilapia galilaea had higher feeding rates on Peridinium elpatiewsky and on intermediate-sized nanoplankton. Outdoor mesocosm experiments examining fish impacts on plankton community structure included two 21-d spring and summer experiments of replicated 2 .times. 2 factorial design (T. galilaea .times. T. aurea). Both fish suppressed crustaceans and rotifers. In the spring, fish also suppressed chlorophyll concentration and the dominant phytoplankter P. cinctum. In the summer, when nanoplankton dominated the phytoplankton and the smaller P. elpatiewsky was the most abundant dinoflagellate, only T. galilaea suppressed Peridinium spp., while presence of T. aurea was associated with increased chlorophyll concentration. Overall, T. galilaea suppressed more and enhanced fewer nanoplankton taxa than did T. aurea. Production of each fish species was lowest when both species were together, suggesting potential competition for plankton resources.