Temperature and the biogeography of algal stoichiometry

Aim: The average carbon-to-nitrogen-to-phosphorus ratio (C:N:P) of marine algae is known to be tightly coupled to that of the inorganic pools of C, N and P in the ocean interior (i.e. the Redfield ratio), and therefore plays a key role in regulating the C and N cycles in the ocean. The C:N:P ratio of algae also varies substantially, both within and among taxa, in response to variation in the abiotic environment, raising the possibility that biogeochemical controls on the marine C and N cycles may shift as a result of climate change. However, the role of temperature in driving phenotypic variation in stoichiometry within algal taxa, as well as biogeographic variation in particulate C, N and P among oceanic regions, remains largely unresolved. Location: Global. Methods: To assess the extent to which temperature controls algal stoichiometry we performed two complementary meta-analyses. First, we characterized the global temperature dependence of algal stoichiometry by analysing field data that encompassed 767 estimates of C:N:P from 22 oceanic sites spanning over 130° of latitude. Second, we characterized the within-species acclimation responses of C:N:P stoichiometry to temperature by analysing data that encompassed 17 experiments, 9 species and 4 taxonomic classes. Results: The geographic analyses demonstrated that the N:P and C:P ratios of marine algae were best predicted by latitudinal variation in average sea-surface temperature, and that both ratios increased 2.6-fold from 0 to 30°C. These global-scale temperature responses, which largely reflect geographic variation in the species compositions of algal assemblages, were of similar magnitude to the average within-species response of the N:P and C:P ratios to experimental temperature manipulations. Main conclusions: The congruence between field and experimental observations suggests that temperature-dependent physiological mechanisms operating at the subcellular level play an important role in determining the stoichiometry of algae in the world's oceans.