Marine organisms capture and store inorganic carbon, through two main pathways: (i) the carbon in the bodies of organisms themselves, and (ii) sequestration through burial of organic carbon at the seabed. Whilst the contribution of polar biological processes to global carbon budgets is relatively small, these sinks are increasing in size and represent a rare negative feedback to climate change.
In parts of BAT over the past 25 years, stores of carbon on the seabed and in zooplankton biomass have increased. The Scotia Sea spring phytoplankton blooms may also result in significant carbon storage through direct sinking and burial at the seabed. At SGSSI and BAT, high biomass species such as Antarctic krill and longer-lived species such as baleen whales, barrel sponges and corals also represent a considerable store of biological carbon.
The physical storage of dissolved carbon in Southern Ocean waters represents an important contribution to global carbon budgets, and these cold waters can store carbon at higher levels than in warmer seas.
Medium evidence, high agreement
Analysis of carbon storage in benthic invertebrates is based on extrapolating from sampling limited areas and a subset of species. Carbon budget stored in the biomass of the food web is difficult to quantify with complex annual, interannual and long-term variability.
Certain species are more effective at carbon drawn down than others, for example, those with calcifying structures, or those that are very long-lived are more effective at locking away carbon. Regime shifts in biological communities will affect the rate and amount of carbon taken up and how fast it is transported to the seabed. For example, increases of up to 1°C may enhance zooplankton growth and carbon storage potential. Conversely, a potential decrease in krill biomass with increasing temperature will negatively affect carbon storage and carbon transport to the seabed.
Initially, biological carbon storage on the seabed is likely to increase due to declining sea ice, increasing temperatures and increased phytoplankton availability. However, this effect will taper off as sea ice disappears completely and seasonal light availability acts as a limiting factor on phytoplankton blooms.
Continued warming and acidification may start to have detrimental physiological effects on organisms, reducing the efficiency of carbon storage, the rate of sinking and sequestration over the coming decades.
Medium evidence, medium agreement
Uncertainty in future sea ice projections and physiological responses to environmental change means it is difficult to predict trends in future biological carbon pathways. There are also regional changes in plankton and krill distributions across the Southern Ocean to consider.