A compilation of existing literature on the characteristics of Southern Ocean diatom species.

Collection of processed BGC-Argo float profiles, used to calculate phytoplankton phenology from chlorophyll, phytoplankton carbon and nitrate.

We compare the formulation and emergent dynamics of 11 CMIP6 IPCC marine biogeochemical models. We find that the largest source of uncertainty across model simulations of marine carbon cycling is grazing pressure (i.e. the phytoplankton specific loss rate to grazing). Variability in grazing pressure is driven by large differences in zooplankton specific grazing rates, which are not sufficiently compensated for by offsetting differences in zooplankton specific mortality rates. Models instead must tune the turnover rate of the phytoplankton population to balance large differences in top-down grazing pressure and constrain net primary production. We then run a controlled sensitivity experiment in a global, coupled ocean-biogeochemistry model to test the sensitivity of marine carbon cycling to this uncertainty and find that even when tuned to identical net primary production, export and secondary production remain extremely sensitive to grazing, likely biasing predictions of future climate states and food security.

Antarctic krill (Euphausia superba) are a keystone species in the Southern Ocean, but little is known about how they will respond to climate change. Ocean acidification, caused by sequestration of carbon dioxide into ocean surface waters (pCO2), is known to alter the lipid biochemistry of some organisms. This can have cascading effects up the food chain. In a year-long laboratory experiment adult krill were exposed to ambient seawater pCO2 levels (400 μatm), elevated pCO2 levels that mimicked near-future ocean acidification (1000, 1500 and 2000 μatm) and an extreme pCO2 level (4000 μatm). The laboratory light regime mimicked the seasonal Southern Ocean photoperiod and krill received a constant food supply. Total lipid mass (mg g -1 DM) of adult krill was unaffected by near-future levels of seawater pCO2. Fatty acid composition (%) and fatty acid ratios associated with immune responses and cell membrane fluidity were also unaffected by near-future pCO2, apart from an increase in 18:3n-3/18:2n-6 ratios in krill in 1500 μatm pCO2 in winter and spring. Extreme pCO2 had no effect on krill lipid biochemistry during summer. During winter and spring, krill in extreme pCO2 had elevated levels of omega-6 fatty acids (up to 1.2% increase in 18:2n-6, up to 0.8% increase in 20:4n-6 and lower 18:3n-3/18:2n-6 and 20:5n-3/20:4n-6 ratios), and showed evidence of increased membrane fluidity (up to three-fold increase in phospholipid/sterol ratios). These results indicate that the lipid biochemistry of adult krill is robust to near-future ocean acidification.

Antarctic krill (Euphausia superba) have a keystone role in the Southern Ocean, as the primary prey of Antarctic predators. Any decreases in krill abundance could result in a major ecological regime shift, but there is currently limited information on how climate change may affect krill. Increasing anthropogenic carbon dioxide (CO2) emissions are causing ocean acidification, as absorption of atmospheric CO2 in seawater alters ocean chemistry. Ocean acidification increases mortality and negatively affects physiological functioning in some marine invertebrates, and is predicted to occur most rapidly at high latitudes. Here we show that, in the laboratory, adult krill are able to survive, grow, store fat, mature, and maintain respiration rates when exposed to near-future ocean acidification (1000 – 2000 μatm pCO2) for one year. Despite differences in seawater pCO2 incubation conditions, adult krill are able to actively maintain the acid-base balance of their body fluids in near-future pCO2, which enhances their resilience to ocean acidification.

These data were collected on Southern Surveyor transit voyage SS2013_T01 from Sydney to Hobart in February 2013. The voyage was a teaching voyage as part of KSA724. Masters students participated in the collection of standard oceanographic data, focusing on eddies of the East Australian Current. This dataset includes for reference the nutrient and hydrography bottle data as produced by the Marine National Facility, as well as the fluorometrically determined extracted chlorophyll concentration

Data collected from Southern Ocean phytoplankton laboratory culture experiments to examine the effect of iron limitation on the Chlorophyll fluorescence (F) to chlorophyll (Chl) ratio. Irradiance levels at which cultures were grown are indicated by the photon flux density (PFD). Growth rates of Fe limited cultures (-Fe) relative to Fe replete cultures (+Fe) are referred to as μ / μmax (unitless).