Print PageMarine phytoplankton are responsible for close to half of global primary productivity. Climate change poses a major threat to this productivity, through the effects of temperature on cell physiology as well as ocean stratification (and therefore nutrient supply). Understanding how rising temperatures are likely to affect phytoplankton communities is a requisite step in the accurate prediction of future carbon sequestration. We used a trait-based approach to investigate the direct effect of changes in ocean temperature on global patterns in phytoplankton productivity and diversity.
We assembled a database of published thermal tolerance curves of phytoplankton strains isolated from across 150 degrees of latitude. Using ocean temperature data obtained from NOAA, we examined whether phytoplankton were adapted to their environmental conditions (rarely demonstrated in microbes outside extremophile communities). We then used an eco-evolutionary modelling framework to identify the optimal thermal strategies at the isolation locations in our dataset, under current and future climatic conditions. To determine how rising temperatures would affect existing phytoplankton strains, we assessed how changes in the pattern of environmental temperature variation (mean, range and skewness) across the oceans would affect the fitness of strains at the locations they are currently found.
Results/Conclusions
We found strong evidence for local adaptation in marine phytoplankton, both to the mean and the temperature variability. The optimal temperature for growth is explained well by variation in the mean annual temperature (p<0.001, R2=0.70) and there is a weaker relationship between temperature niche width and annual temperature range (p<0.001, R2=0.10).
Tropical strains were found to perform best at temperatures at or below those that they currently experience, while temperate and polar strains perform best at temperatures above those currently prevailing. A uniform rise in temperature was therefore found to lead to a decrease in phytoplankton fitness in the tropics and an increase at higher latitudes. This suggests that, in the absence of adaptation, phytoplankton productivity in the tropics may decrease, though this may be counterbalanced by the increases experienced elsewhere. This decline in fitness for most tropical strains also suggests that the genetic diversity of the tropical oceans will decrease as a result of rising ocean temperatures. We are currently incorporating local-scale temperature projections to make more accurate projections for ocean productivity.
