Life strategies, dominance patterns and mechanisms promoting species coexistence in phytoplankton communities along complex environmental gradients

This paper analyses the life strategies, the dominance patterns and the diversity in phytoplankton communities in large and deep lakes. The study was carried out on the largest Italian Lake (Lake Garda) from 1995 to 2000. Different statistical analyses were applied. For phytoplankton the time variable represents a complex environmental gradient driving annual succession; this gradient was made explicit by the application of PCA analyses to the environmental data. The use of Non Metric Multi Dimensional Scaling applied to Bray-Curtis dissimilarity matrices revealed an ordered and cyclic development of phytoplankton every year; the Bray-Curtis index, calculated between pairs of chronologically contiguous samples, was also used as a measure of the community change rate (beta(t)) over the temporal succession. A significant relationship between beta(t) and the complex environmental gradient was assessed. Finally, for every phytoplankton species, the optimum conditions for growth and the realised niches were determined. The positioning of the species on the complex environmental gradient, and the contemporaneous application of cluster analysis based on the different specific environmental optima, highlighted primarily the existence of two groups at the extreme of the complex environmental gradient. The first group included the large late winter/spring diatoms, which developed during high water turbulence and strong physical control, high nutrient concentrations, low light conditions and reduced competition. The second group was composed by many heterogeneous summer species characterised by the ability to contrast losses by grazing and sinking in stratified and stable conditions, and the ability of tolerating nutrient deficiency. A third group of species developed during environmental conditions in the middle of the two previous extremes. These included the three master species Mougeotia sp., Fragilaria crotonensis and Planktothrix rubescens/agardhii. The endogenous and exogenous mechanisms promoting species coexistence are discussed, along with the applicability of competitive and equilibrium/non-equilibrium theories to phytoplankton dynamics.