Abstract
Abstract. The elemental stoichiometry of marine phytoplankton plays a critical role in global biogeochemical cycles through its impact on nutrient cycling, secondary production, and carbon export. Although extensive laboratory experiments have been carried out over the years to assess the influence of different environmental drivers on the elemental composition of phytoplankton, a comprehensive quantitative assessment of the processes is still lacking. Here, we synthesized the responses of P:C and N:C ratios of marine phytoplankton to five major drivers (inorganic phosphorus, inorganic nitrogen, inorganic iron, irradiance, and temperature) by a meta-analysis of experimental data across 366 experiments from 104 journal articles. Our results show that the response of these ratios to changes in macronutrients is consistent across all the studies, where the increase in nutrient availability is positively related to changes in P:C and N:C ratios. We found that eukaryotic phytoplankton are more sensitive to the changes in macronutrients compared to prokaryotes, possibly due to their larger cell size and their abilities to regulate their gene expression patterns quickly. The effect of irradiance was significant and constant across all studies, where an increase in irradiance decreased both P:C and N:C. The P:C ratio decreased significantly with warming, but the response to temperature changes was mixed depending on the culture growth mode and the growth phase at the time of harvest. Along with other oceanographic conditions of the subtropical gyres (e.g., low macronutrient availability), the elevated temperature may explain why P:C is consistently low in subtropical oceans. Iron addition did not systematically change either P:C or N:C. Overall, our findings highlight the high stoichiometric plasticity of eukaryotes and the importance of macronutrients in determining P:C and N:C ratios, which both provide us insights on how to understand and model plankton diversity and productivity.
Highlights
Elemental stoichiometry of biological production in the surface ocean plays a crucial role in the cycling of elements in the global ocean
Our meta-analysis represents an important bottom-up approach in predicting how elemental stoichiometry of phytoplankton may evolve with climate change
We conclude that macronutrient availability is the most significant and shared environmental driver of C : N : P
Summary
Elemental stoichiometry of biological production in the surface ocean plays a crucial role in the cycling of elements in the global ocean. The elemental ratio between carbon, nitrogen (N), and phosphorus (P) in exported organic matter expressed in terms of the C : N : P ratio helps determine how much atmospheric carbon is sequestered in the deep ocean with respect to the availability of limiting nutrients. The N : P ratio reflects the relative availability of nitrate with respect to phosphate, both of which are externally supplied from the atmosphere via nitrogen fixation and/or continents via river supply and lost by denitrification and burial (Broecker, 1982; Lenton and Watson, 2000; Redfield, 1958; Tyrrell, 1999). Phytoplankton may alter resource allocation between the Prich biosynthetic apparatus, N-rich light-harvesting appara-
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