Evidence for invasive carp mediated trophic cascade in shallow lakes of western Victoria, Australia

Abstract

Lakes of western Victoria are typically shallow (2-3 m), slightly saline (3-8 ppt) and eutrophic (total phosphorus 0.1-5 mg l -1 and total nitrogen 3-8 mg l -1 ). These lakes are prone to algal blooms in late spring and summer. We conducted experiments in ponds to determine whether lake zooplankton exert enough grazing pressure to control algal blooms and also if carp predation can regulate zooplankton communities and increase algal biomass. Zooplankton grazing (high zooplankton, low zooplankton and control) and carp predation (carp, nutrient and control) experiments had three treatments and each treatment had three replicates. Algal numbers and biomass (28 μg l -1 ) were uniform in all ponds before zooplankton stocking. When stocked with lake zooplankton, algae were consumed efficiently and after 30 days algal numbers and biomass were significantly lower than those in control ponds (0.4 μg l -1 and 33 μg l -1 , respectively). Ponds stocked with low zooplankton numbers were intermediate. Complete elimination of Cladocera and subsequent increase in algal biomass (from 4 to 45 μg l -1 ) occurred after 30 days in carp treatment ponds. Zooplankton and algal populations were unaffected when nutrients but no carp were added and also in control ponds with no added nutrients or carp. The pond experiments illustrated the mechanics of a carp mediated trophic cascade when stocked with species and densities of zooplankton naturally occurring in lakes of western Victoria. Confirmation of whether this process occurs in turbid Australian lakes is urgently required.