Mechanisms of microalgae selection during the assembly of a planktonic community

We attempt to ascertain the selection mechanisms that affect algal species’ abilities to thrive in a given environment, and how the variability of the response is reflected in the community structure (total biomass, diversity) and also in community function (photosynthesis and respiration). For that, we formed algal communities, assembling eight species of freshwater cosmopolite algae with distinct morphologies (Cosmarium contractum, Cryptomonas ovata, Euglena gracilis, Limnothrix redekei, Monoraphidium contortum, Pediastrum tetras, Planktothrix agardhii and Scenedesmus acutus) and performed four treatments combining low and high nutrient concentrations (N and P) in the culture media and a presence or absence of herbivores (a mixture of Daphnia magna, Keratella cochlearis and Brachionus calicyflorus). Competition between pairs of algae was also studied, and the viability of the species as inocula after a senescence period. The presence of herbivores (predation) and nutrient concentration (competition) are the mechanisms affecting (individually or synergistically) microalgae assemblages, originating different alternative states from the same pool of species. However, the effect of these mechanisms may vary, depending on algal properties such as size, growth rate and viability as an inoculum. The presence of herbivores and oligotrophic conditions reduce the primary producer biomass but increase diversity due to relaxation of competition and increase the uncertainty of final states. The variations in the community structure directly reflect on community function, affecting primary production and respiration.