Impact of three-spined stickleback Gasterosteus aculeatus on zooplankton and chl a in shallow, eutrophic, brackish lakes

Abstract

The 3-spined stickleback Gasterosteus aculeatus often dominates fish communities in eu- trophic, brackish, north-temperate lakes where it is thought to contribute significantly to the low zoo- plankton biomass and low grazing pressure on phytoplankton. To elucidate its effect on zooplankton community structure and phytoplankton biomass (chl a), we conducted a 5.5 mo enclosure experiment with sticklebacks at different densities in high nutrient concentrations and a salinity of 2 psu. Increas- ing fish density (0 to 10.2 sticklebacks m -2 ) led to substantial changes in the zooplankton community structure, an increase in phytoplankton chl a, and a reduction in water clarity. At densities below 4 to 6 sticklebacks m -2 , large-bodied cladocerans dominated the zooplankton biomass, the zoo- plankton:phytoplankton biomass ratio was high (>1), and chl a was accordingly low (<20 µg l -1 ). At higher stickleback densities, large cladocerans disappeared and the abundance of small cladocerans was low. Instead, cyclopoid copepods and rotifers dominated, the zooplankton:phytoplankton biomass ratio was well below 0.5, and chl a exceeded 200 µg l -1 in late summer. The fish density level for a shift from a clear to a turbid state is low compared to the densities observed in several eutrophic, brackish lagoons. We conclude that fish have a strong impact on water clarity in shallow, eutrophic, brackish lakes with low salinity, which may explain the usually low zooplankton:phytoplankton biomass ratio and the turbid state of such aquatic environments. However, our results also indicate that once the density level declines to below 4 to 6 sticklebacks m -2 , a shift to a clear state is likely to occur.