Phytoplankton functional diversity increases ecosystem productivity and stability.

The effect of biodiversity on ecosystem functioning is one the major questions of ecology. However, the role of phytoplankton functional diversity in ecosystem productivity and stability under fluctuating (i.e. non-equilibrium) environments remains largely unknown. Here we use a marine ecosystem model to study the effect of phytoplankton functional diversity on both ecosystem productivity and its stability for seasonally variable nutrient supply and temperature. Functional diversity ranges from low to high along these two environmental axes independently. Changes in diversity are obtained by varying the range of uptake strategies and thermal tolerances of the species present in the community. Species can range from resource gleaners to opportunists, and from cold to warm thermal tolerances. The phytoplankton communities self-assemble as a result of species selection by resource competition (nutrients) and environmental filtering (temperature). Both processes lead to species asynchrony but their effect on productivity and stability differ. While the diversity of temperature niches has a strong and direct positive effect on productivity and stability, the diversity of uptake strategies has a weak and indirect positive effect. These results show that more diverse communities lead to higher and more stable ecosystem productivity than less diverse communities. However, the strength of the biodiversity effect on ecosystem functioning for microbial communities depends critically on the type of environmental gradient.