POTENTIAL FUTURE RESPONSES OF MARINE NUTRIENT BIOGEOCHEMICAL CYCLES TO OCEAN ACIDIFICATION

Anthropogenic CO₂ uptake and acidification accompanied by other global change-mediated shifts in key environmental variables are likely to have large but poorly characterized impacts on marine nutrient biogeochemistry. This is because the various microbes that mediate critical nutrient cycling pathways have marked physiological sensitivities to changes in pH, pCO₂, temperature, and light. Current models and experimental evidence suggest that these environmental shifts could cause major global increases in both marine N₂ fixation and denitrification, by up to 50% over the next century. On the other hand, lower seawater pH could substantially lower rates of oceanic nitrification (ammonia oxidation), producing a potential “bottleneck” in the nitrogen cycle. Large direct effects on the marine biogeochemistry of phosphorus and silicon seem less likely, but these nutrients may react indirectly to acidification through changes in biological community structure. Global change-driven alterations in the C:N:P ratios of phytoplankton are also possible, and would also have large effects on overall marine biogeochemistry. However, culture and natural plankton community experiments that have examined the effects of increased CO₂ on biological elemental ratios show sometimes contradictory and conflicting results. Despite these uncertainties, it is now becoming clear that ocean acidification has the potential to cause fundamental changes in the present day marine biogeochemical cycles of carbon and nutrients.