A review of: Potential climate change impacts on microbial distribution and carbon cycling in the Australian Southern Ocean. Evans et al. Deep-Sea Research II, 58, 2001, p.2150-2161.
Climate change effects on ecosystems have the potential to dramatically alter carbon cycling throughout the land, oceans and the atmosphere. It is important to understand how these processes work, especially within oceans, as they have some of the largest processes within the Carbon cycle. Microbial communities promote the transfer of atmospheric carbon to the deep oceans via the biological pump, and it is the effect on this pump that is of concern when discussing climate change. Changes can be easily observed in microbial plankton as they have short generation times and respond easily to temperature changes and ocean currents.
The sub-Antartica Zone (SAZ) of the Southern ocean is characterised by high levels of nitrate but low levels of silicate and chlorophyll. It is of specific importance as it is a major sink of atmospheric C02 due to the composition of the microbial plankton present. If oceanic currents change due to climate change then an influx of tropical waters into the SAZ may occur. The result will be a change in the microbial assemblage. These biogeographical changes have already been documents in other areas making it likely the same effects will be observed here.
There were 5 distinct biogeographical regions identified by the SAZ-Sense survey. The Polar frontal zone, the Sub-Tropical zone and three regions within the SAZ. The microbial communities found in each region corresponded to the assemblage expected from the physio-chemical properties of the regions. Statistical analysis showed that salinity temperature and Nox availability, explained the microbial variation with Fe availability also being important.
Comparing the differences between these regions allows predictions to be made about the future microbial assemblages and any changes that may occur. this information will be a key tool in predicting the effect of climate change on the biological pump in certain areas of the ocean.
Under the assumption that the SAZ was previously homogenous, the contrast between the three regions can be accredited to the intrusion of the East-Australian current (EAC), South-Easterly into the SAZ, resulting in higher inputs of warmer, more saline and Fe rich waters. The Fe arrives via increased Aeolian dust deposition from Australia plains and increased movement of shelf sedimentary Fe, moved by the stronger EAC. This has the effect of changing the microbial community.
Higher primary production was observed in the regions affected by the EAC. This is not only due to the higher availability of nutrients such as Fe but due to the increased stability of the water column. This occurs because the sub-Antarctic water and sub-tropical waters have very different density profiles due to salinity and temperature differences. It has been previously reported that shallow stable surface layers lead to an increase in cyanobacteria and other phytoplankton.
Associated with this higher production were higher levels of bacteria, bacteriophages and viruses. Viruses are believed to account for a much higher rate of bacteria mortality in the Eastern waters affected by the EAC. Up to 40% bacteria mortality is due to viruses. This creates high levels of DOM which then stimulates higher bacteria abundance and activity.
The poleward intrusion of the EAC over the last 62 years has been increasing and predictions suggest that it will continue to get worse. The reason for this is wind-driven and circulation changed caused by human induced climate change. This results in the Southward extension of the ranges of cyanobacteria and therefore the bacteria and viruses associated with the ecosystem.
Due to the low silica present the previously observed dominance of smaller autotrophs over larger diatoms will likely result in higher importance of the microbial loop. The export of particulate carbon to the deep ocean from diatoms will be reduced in favour of higher production of smaller cells that are then subject to predation my microzooplankton. The higher levels of virus lysis will cause higher levels of DOM over particulate, providing nutrients for substrate dwelling heterotrophs therefore making the system more regenerative.
The increased viral lysis will cause higher rates of respiration in the euphotic zone and reduce the transfer of carbon to higher trophic levels, thereby reducing the effect of the biological pump.
The poleward intrusion of the EAC over the last 62 years has been increasing and predictions suggest that it will continue to get worse. The reason for this is wind-driven and circulation changed caused by human induced climate change. This results in the Southward extension of the ranges of cyanobacteria and therefore the bacteria and viruses associated with the ecosystem.
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