Climate change and ocean acidification impacts on lower trophic levels and the export of organic carbon to the deep ocean

摘要

Most future projections forecast significant and ongoing climate changeduring the 21st century, but with the severity of impacts dependent onefforts to restrain or reorganise human activity to limit carbon dioxide(CO₂) emissions. A major sink for atmospheric CO₂, and a key source ofbiological resources, the World Ocean is widely anticipated to undergoprofound physical and – via ocean acidification – chemical changes asdirect and indirect results of these emissions. Given strong biophysicalcoupling, the marine biota is also expected to experience strong changes inresponse to this anthropogenic forcing. Here we examine the large-scaleresponse of ocean biogeochemistry to climate and acidification impacts duringthe 21st century for Representative Concentration Pathways (RCPs) 2.6 and 8.5using an intermediate complexity global ecosystem model, MEDUSA-2.0. Theprimary impact of future change lies in stratification-led declines in theavailability of key nutrients in surface waters, which in turn leads to aglobal decrease (1990s vs. 2090s) in ocean productivity (?6.3%). Thisimpact has knock-on consequences for the abundance of the low trophic levelbiogeochemical actors modelled by MEDUSA-2.0 (?5.8%), and these wouldbe expected to similarly impact higher trophic level elements such asfisheries. Related impacts are found in the flux of organic material toseafloor communities (?40.7% at 1000 m), and in the volume of oceansuboxic zones (+12.5%). A sensitivity analysis removing an acidificationfeedback on calcification finds that change in this process significantlyimpacts benthic communities, suggesting that a~better understanding of theOA-sensitivity of calcifying organisms, and their role in ballasting sinkingorganic carbon, may significantly improve forecasting of these ecosystems.For all processes, there is geographical variability in change – forinstance, productivity declines ?21% in the Atlantic and increases+59% in the Arctic – and changes are much more pronounced under RCP 8.5than the RCP 2.6 scenario.