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

Abstract

One of the ongoing debates around meta- community 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 phyto- plankton assembly in a floodplain river. A probabilis- tic co-occurrence model revealed the presence of 94.1% random and 5.9% non-random species pairwise associations. The latter were higher at both hydrolog- ically 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 repre- sented (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 stochas- tic processes. The higher percentage of random co- occurrence and the larger assemblage variability observed in isolated environments suggests that ran- dom dispersal, ecological drift, and priority effects could promote stochasticity. We concluded that both processes affect the structure of phytoplankton meta- communities in a floodplain system and suggest the preponderance of stochastic organisation.