Assessing the potential of shallow mesocosms to facilitate the consumption of cyanobacteria by the snail Viviparus georgianus

Abstract

Background: In a rapidly warming world, cyanobacterial harmful algal blooms (cyanoHABs) are becoming a pressing issue on a global scale. Pelagic cyanoHABs’ toxic effects on the well-being of ecosystems and detrimental impact on animal and human health demand economical, environmentally friendly solutions. Viviparus georgianus is a benthic snail ubiquitous to many eutrophicated Canadian bodies of water that can consume the cyanobacterium Microcystis aeruginosa. Although V. georgianus are already in the lakes where cyanoHABs are present, there is still the problem of how to overlap the interaction between these benthic snails and pelagic cyanobacteria. In our study, we assessed if V. georgianus could ingest naturally occurring, bloom-forming, multi-species cyanobacteria and discussed the necessary conditions to successfully implement shallow mesocosms to facilitate the snails’ consumption. Methods: We created two types of shallow mesocosms: in the first set, we placed equal biomasses of snails as well as equal volumes and concentrations of cyanobacteria but varied the depth to evaluate: 1. consumption, 2. the efficiency of consumption by small vs. large snails and 3. depth at which snails have a greater consumption. In the second set, we kept the volumes, depths, and concentrations of cyanobacteria constant but varied the snail biomass to evaluate the amount required to reduce cyanobacteria concentrations from high to moderate. Results: Mesocosms with snails had significantly stronger declines in cyanobacteria than mesocosms with no snails (F1,30 = 81.97, P < 0.0001) indicating that the snails consumed substantial amounts of cyanobacteria during the 48 hrs trial. Using equal snail biomass, large and small snails consumed similar amounts of cyanobacteria (F1,18 = 0.064, P = 0.804) indicating that snail size does not have an effect on consumption. The rate at which cyanobacteria concentration declined over time was similar at all depths (F16,60 = 1.509, P = 0.13). The minimum biomass of snails needed to reduce the concentration of cyanobacteria from high to moderate is 20 g of snails per litre. Conclusions: V. georgianus consume naturally occurring, bloom-forming cyanobacteria in relatively short periods of time in aquaria, indicating the potential for its use as a bioremediation method for in-situ studies. Within the range that we tested, depth does not have a significant impact on the snails’ consumption nor do differences in size. Future research is required to test their effectiveness in a natural setting.