Unravelling the role of determinism and stochasticity in structuring the phytoplanktonic metacommunity of the Parana River floodplain

One of the ongoing debates around metacommunity ecology is to what extent stochastic and deterministic processes act on community assembly. We explored the influence of both determinism, mediated by environmental filters, and stochasticity, mediated by dispersal and ecological drift, on phytoplankton assembly in a floodplain river. A probabilistic co-occurrence model revealed the presence of 94.1% random and 5.9% non-random species pairwise associations. The latter were higher at both hydrologically isolated (4.42%) and connected environments (2.2%). Variation partitioning analysis showed similar significant explanations by the unique environmental (7.7%, Secchi, conductivity, vegetation, phosphorous) and spatial (7.2%, watercourse distance, longitude) components. Temporal variability was poorly represented (2.4%) because we only considered two low-water periods. Species co-occurrence patterns showed that most taxa coexist randomly. The environmental explanation is in line with niche-assembly models (species sorting), but the similar proportion explained by spatial organisation related to random dispersal guides the evidence to both deterministic and stochastic processes. The higher percentage of random co-occurrence and the larger assemblage variability observed in isolated environments suggests that random dispersal, ecological drift, and priority effects could promote stochasticity. We concluded that both processes affect the structure of phytoplankton metacommunities in a floodplain system and suggest the preponderance of stochastic organisation.