Global marine phytoplankton and zooplankton diversity shifts under climate change.

Plankton diversity and community composition govern important marine ecosystem services related to biogeochemical cycles and higher trophic levels. Yet, the responses of phytoplankton and zooplankton species diversity to future climate changes remain poorly understood. Here, we estimate global surface plankton diversity by modelling the monthly spatial distributions of more than 800 species based on in situ occurrences, climatological environmental observations and an ensemble of statistical habitat models. We project the habitat models into the oceans’ future conditions using climatological anomalies from the GFDL-ESM2M Earth System Model for changes between the 1971-2000 and 2071-2100 periods, and thus explore the impacts of climate change on the species richness (alpha diversity) and composition (beta diversity) of phytoplankton and zooplankton. Temperature is the primary driver of plankton diversity. Zooplankton richness increases with temperature until its peak around 23°C and then decreases towards higher temperatures. Phytoplankton richness increases with temperature across almost the full temperature range. At the second order, phytoplankton richness covaries with biomass and nutrient availability, while zooplankton richness does not. As a result, contrary to what is found for terrestrial ecosystems, the two groups display non-overlapping diversity peaks and contrasting responses to future climate change. Between the 1971-2000 and 2071-2100 time periods, phytoplankton diversity is projected to increase by nearly 25% everywhere but at the poles, with spatially constrained areas of high species turn-over. Zooplankton diversity decreases in the tropics and increases in temperate ocean regions, with a relatively lower amplitude than phytoplankton diversity, as species turn-over buffers climate change driven species losses. Our results suggest that differential changes in plankton diversity across multiple trophic levels need to be taken account in climate change impacts studies on marine ecosystems functioning.