Responses of planktonic rotifers to restoration measures-trophic cascades after liming in Lake Gårdsjön

Abstract

Major changes occurred in the rotifer community after both liming of the acidified and fishless Lake Gardsjon and after stocking with brown trout. The species number and biomass increased after liming and had its peak during the neutralised fishless period. There was a change in numeric relations between the functional groups - graspers, suspension feeders and predators - which was correlated to the experimental measures, i.e. liming and fish introduction. The development of the rotifer community is discussed in relation to effects of predation, competition and trophic cascades. During the limed fishless period, high densities of phantom midge larvae (Chaoborus spp.) and waterboatmen (Corixidae) suppressed susceptible prey organisms, mainly crustaceans. These predators were largely eliminated after fish introduction resulting in a predatory release of crustacean zooplankton. The increase of crustaceans after the introduction of fish implied a competitive suppression of rotifers. Thus, cascading effects of fish stocking were identified in several aspects of the rotifer community.