The twenty experimental ecosystems were inoculated with the natural plankton community from the local Lake Greifen and were then planted either with only the watermilfoil, only the mussels, neither organism or both. Every two weeks, increasing amounts of nutrients (phosphorus and nitrogen) were added to the ponds. There were four ponds that contained neither of the foundation species and did not receive nutrients to track the baseline dynamics.
After three months, the effects were evaluated: in ponds containing only the mussel or only the watermilfoil, algal growth occurred after the addition of nutrients, but then also rapidly recovered. So, ponds having just one of the two species maintained clearer water overall over the course of the experiment than the ponds that had neither.
Much more surprising, however, according to Anita Narwani, was another observation: in the ponds containing both species together, these results were reversed. In ponds with both species together, the algal biomass and turbidity was significantly higher after nutrient additions than in the ponds with neither, and recovered more slowly as the nutrient inputs got larger. In other words, algal biomass remained persistently high. In addition, in this case, a small cyanobacteria consistently took over and completely dominated these algal blooms, resulting in a loss of taxonomic and trait-level biodiversity. This had knock-on effects on other components of the pond ecosystems, including reduced zooplankton grazer abundances, and higher oxygen concentrations.
How is this to be explained? Narwani says: “The mussels and aquatic plants clear the water, but their effects may have mainly targeted the larger species, including the green algae. We believe that cyanobacteria are more resistant to the grazing by mussels and chemical warfare by the plants, and so their growth increases as the nutrients are added to the system and their competitors are suppressed by the “double whammy” of control by grazers and plants.” This hypothesis was supported by experiments subsequently carried out by Narwani in the laboratory.