Current state of freshwater phytoplankton diversity and ecosystem functioning research

Patrick Venail, Ph.D., Département F.A. Forel, University of Geneva, Switzerland

Freshwater biodiversity is under major threat by human driven activities. Despite being one of the most vulnerable ecosystems, we have little knowledge on the consequences of diversity loss in freshwater lotic systems. We performed a review of the existing literature linking phytoplankton diversity to ecosystem functioning in freshwater lakes and ponds. We found 23 studies including a total of 109 experiments that directly manipulated any aspect of phytoplankton diversity and measured its influence on ecosystem functioning. Taxonomy based metrics of diversity were by far dominant, accounting for 88% of the experiments manipulating phytoplankton diversity. Our revision revealed a plethora of ecosystem functions such as chlorophyll-a, biovolume, biomass and resource use efficiency on which the influence of diversity has been tested. Overall, half of the relationships between phytoplankton diversity and ecosystem functioning are linear positive and about another quarter are non-linear positive. Negative or complex impacts of phytoplankton diversity on ecosystem functioning accounted for 4% of the results only. Finally, no effect of diversity was observed in 23% of the experiments. Our results reveal that while not universal, loosing phytoplankton diversity in freshwater lotic systems is detrimental for their overall functioning. More studies based on other measures of diversity such as functional or genetic diversity are urgently required to get a better picture of the importance of phytoplankton diversity on the functioning of freshwater lotic ecosystems.