Evidence to date indicates that plankton community dynamics are consistent with the species sorting model, where dispersal functions to distribute species across habitats, but local community structure is driven by the local environment. Neutral theory, however, challenges the functional distinctiveness among species and emphasizes the role of stochastic processes in structuring communities. Indeed, stable co-existence requires shared ecological attributes among species, which is the impetus behind the use of functional groups as ecological indicators. Our purpose was to evaluate ecological drift in a community subject to weak competition and a low immigration rate. We examined the phytoplankton dynamics in a warm monomictic reservoir during winter circulation and assessed the strength of species relationships to environmental gradients as well as ecological drift. During this time period, nutrient levels were high and increasing, zooplankton biomass was low, and algal abundance (biovolume) was strongly correlated to light availability. We used canonical correspondence analysis to evaluate the significance of species response to ecological gradients.This was contrasted to the predictions of ecological drift, where the proportion of the total abundance contributed by any species in a local community at time t+1 is a function of its proportion at time t.
Results/Conclusions
The first and second CCA axes accounted for 66% of the variation of the phytoplankton data matrix. The two axes distinguished an environmental gradient from a homogenous water column with high light extinction and relatively higher temperature to a slightly stable water column with lower water temperature. We also evaluated the relative abundance of two diatoms and two cryptomonads versus ecological gradients and found distinct patterns in response to water column stability and light extinction. With few exceptions, the proportional abundance of all species over time was outside of the 99% confidence interval predicted by ecological drift, accounting for inflow (immigration) into the system. This was true for both dominant and non-dominant species. In addition, there appears to be no underlying pattern in the few occasions when the relative abundance of a species was similar on successive dates. While prior studies have demonstrated that the evolution of functional equivalence is possible, and that stochastic processes play a role in community dynamics, we suggest that ecological drift is a minor factor in phytoplankton abundance patterns at the local scale.
