Allelopathic interactions between phytoplankton species: The roles of heterotrophic bacteria and mixing intensity

Toxin-producing phytoplankton species may compensate for competitive disadvantages by secreting chemicals that affect toxin-sensitive phytoplankton species. Heterotrophic bacteria, however, may, in turn, degrade the toxins produced by allelopathic phytoplankton, thus confounding allelopathic interactions between phytoplankton species. Moreover, recent theoretical studies suggest that incomplete mixing of the water column might also modify allelopathic interactions. Here we analyze a model where phytoplankton species, bacteria, nutrients, and a toxin are linked through material cycling. The model considers a toxin-producing and a toxin-sensitive phytoplankton species and two species of heterotrophic bacteria. The model is analyzed for two scenarios: a simple well-mixed aquatic ecosystem is contrasted with an aquatic ecosystem with low mixing intensity. The results show that (1) the winner of competition between toxin-producing phytoplankton and toxin-sensitive phytoplankton species may depend on the species that becomes dominant first, (2) heterotrophic bacteria able to degrade allelopathic toxins will facilitate dominance of toxin-sensitive phytoplankton species, (3) heterotrophic bacteria unable to degrade allelopathic toxins may to some extent counter the facilitating effect of toxin-decomposing bacteria, due to competition between the bacterial species, and (4) there is a major effect of mixing intensity on these species interactions: when turbulent mixing rates are low, toxin concentrations are less diluted whereas degradation activities of heterotrophic bacteria are more localized. As a result, this model study predicts that weak mixing, especially when combined with the presence of bacteria unable to degrade allelopathic compounds, will favor the development of allelopathic phytoplankton populations.