Productivity diagram
Marine plants that photosynthesize in the world’s ocean generate primary production. Primary producers include a diverse range of organisms from microscopic phytoplankton to seagrasses to giant kelp. Primary productivity throughout the world’s ocean is expected to shift dramatically due to ocean warming, changes in ocean circulation and ocean acidification. Upwelling, a process that brings nutrient-rich cold waters to the surface, occurs along the eastern margins of ocean basins (or the west coast of continents). The delivery of nutrients drives primary production, which is expected to decrease as upwelling circulation is altered by climate change.1,2 In addition, increased sea-surface temperature results in a more highly stratified water column which decreases the ability of surface, deep water and nutrients to mix. Both altered upwelling and reduced vertical mixing are expected to decrease overall primary productivity.
Ocean acidification may also impact primary production if ocean pH falls below the point where carbonate dissolution occurs. Many primary producers that have a calcified shell, including diatoms, which play a major role in coastal food webs, will be removed from the pool of producers. This removal will have a cascading effect, altering the species composition and abundance of phytoplankton in the world’s ocean.3 Altering primary production, especially in coastal upwelling regions, could further influence the global climate cycle. Phytoplankton remove a large amount of CO2 from the earth’s atmosphere when they photosynthesize – turning sunlight into organic matter. Because of their small size, they are able to reproduce and grow rapidly. This process results in CO2 being taken out of (sequestered from) the atmosphere. Decreases in primary productivity are likely to further exacerbate rising CO2 levels in the earth’s atmosphere by creating a positive feedback loop (see figure). If productivity is reduced, less CO2 will be removed from the atmosphere, resulting in an increase in atmospheric CO2 concentration. This increase will enhance ocean warming and acidification, both of which may lead to further reductions in primary productivity.
