Freshwater ecosystems (i.e. lakes, ponds, rivers, streams) are main providers of one of the most important goods for humanity, freshwater. Still, human driven activities leading to contamination, eutrophication and reduction of its associated biodiversity are putting these ecosystems under major stress. Within the plethora of organisms living in freshwater ecosystems phytoplankton is the base of the entire trophic web and is known to play a major role in the fluxes of matter and energy. Despite the recent efforts to better understand the structure, composition and functioning of phytoplankton communities in freshwater systems we still have a pretty narrow mechanistic understanding of the impact of phytoplankton diversity on ecosystem functioning, specially on how its diversity influences nutrient use and subsequently biomass production. We first performed a literature review to summarize the previous studies linking any aspect of phytoplankton biodiversity to any facet of ecosystem functions in lentic freshwater systems (lakes and ponds). Then, we conducted a controlled laboratory experiment in which we simultaneously but independently manipulated different aspect of phytoplankton diversity and measured their relative influence on resource use efficiency and biomass production that would provide a clear mechanistic understanding of this relationship.
To date, over a hundred laboratory and over sixty field studies explored the influence of phytoplankton diversity on freshwater ecosystem functioning, with a large majority focusing on taxonomy based metrics of diversity. Functional or genetic diversity was present in less than ten percent of such studies. Overall, in controlled experiments near 70% of studies showed a decrease in ecosystem functioning as diversity decreased whereas in field studies only over 40% of studies showed this trend. In either lab or field studies true mechanistic descriptions of these effects were almost completely absent. Our controlled laboratory experiment showed that both taxonomic and functional diversity, expressed as the variance in cell size, increased biomass production via a higher use efficiency of nitrogen and phosphorus. On the contrary phylogenetic diversity had no effect on neither resource use efficiency nor biomass production. We advocate for more studies providing a mechanistic understanding of how phytoplankton diversity connects to resource use efficiency and biomass production. Only such a proper mechanistic description of the impact of biodiversity on ecosystem functioning would allow us to evaluate the real consequences of diversity loss and the capacity of freshwater lentic system to provide important ecosystem services.