Contrasting response of a plankton community to two filter-feeding fish and their feces: An in situ enclosure experiment

Abstract

The responses of plankton communities and nutrient loads to different biomass levels of silver carp and tilapia (0, 25, 70, 130g/m3) were evaluated in situ, using enclosures in Lake Taihu, and the effects of fish feces on algal biomass were assessed using in situ dialysis culture. Both silver carp and tilapia significantly suppressed zooplankton biomass, after which zooplankton grazing was too low to suppress algal populations. Both fish also played an important role in accelerating nutrient circulation, and their feces directly contributed to algal growth. In fact, after passing through the digestive tract of silver carp, the cyanobacteria Microcystis remained viable, and their photosynthetic activity was even stimulated; however, no viable algae were detected in the tilapia feces. Overall, algal biomass increased with increasing biomass levels of the tilapia, whereas the medium biomass level of silver carp (70g/m3) was most effective in controlling cyanobacteria. Our results indicate that stocking with silver carp, rather than with tilapia, is an appropriate strategy for reducing summer cyanobacteria blooms in eutrophic lakes that lack large crustaceous zooplankton, such as Lake Taihu. In addition, stocking with 70g/m3 of silver carp was most effective for suppressing cyanobacteria blooms and improving water quality, which suggests that this moderate biomass level can be used to achieve a beneficial balance between the positive effects of filter feeding and negative effects of feces-derived nutrients on the growth of algae. Statement of relevanceGood for algae control and aquaculture yields increase.