Climate-induced changes in carbon and nitrogen cycling in the rapidly warming Antarctic coastal ocean

The western Antarctic Peninsula (WAP) is a hotspot of climatic and oceanographic change, with a 6°C rise in winter atmospheric temperatures and >1°C warming of the surface ocean since the 1950s. These trends are having a profound impact on the physical environment at the WAP, with widespread glacial retreat, a 40% decline in sea ice coverage and intensification of deep water upwelling. The main objective of this study is to assess the response of phytoplankton productivity to these changes, and implications for the marine carbon and nitrogen cycles in the WAP coastal zone. An extensive suite of biogeochemical and physical oceanographic data was collected over five austral summer growing seasons in northern Marguerite Bay between 2004 and 2010. Concentrations and isotopic compositions (δ¹⁵N, δ¹³C, ¹⁴C) of dissolved nitrate, dissolved inorganic carbon species, particulate nitrogen, organic carbon and chlorophyll a are used in the context of a substantial ancillary dataset to investigate nutrient supply, phytoplankton productivity and nutrient uptake, export flux and the fate of organic material, and the factors underpinning pronounced seasonal and interannual variability. High-resolution biogeochemical time-series data for surface and underlying seawater, sea ice brine, sediment trap material and coretop sediments allow detailed examination of carbon and nitrogen cycle processes under contrasting oceanographic conditions and the interaction between these marine processes and air-sea exchange of climate-relevant CO₂. This study shows that the WAP marine environment is currently a summertime sink for atmospheric CO₂ in most years due to high productivity and biological carbon uptake sufficient to offset the CO₂ supply from circumpolar deep waters, which act as a persistent source of heat, nutrients and CO₂ across the shelf. For the first time, CO₂ sink/source behaviour is parameterised in terms of nitrate utilisation, by exploiting the relationship between CO₂ and nitrate concentrations, and deriving the nitrate depletion at which surface ocean CO₂ is undersaturated relative to atmosphere and carbon sink behaviour is achieved. This could have vast utility in examining CO₂ sink/source dynamics over greater spatial and temporal scales than by direct CO₂ measurements, of which availability is more limited. This study documents abrupt changes in phytoplankton productivity, nitrate utilisation and biological CO₂ uptake during a period of rapid sea ice decline. In fact, nitrate utilisation, particulate organic matter production and biological CO₂ uptake all decrease by at least 50 % between a sea ice-influenced, high productivity season and one of low sea ice and low productivity. The key driver of interannual variability in production and export of organic material is found to be upper ocean stratification and its regulation of light availability to phytoplankton. Productivity, CO₂ uptake and export are maximal when stratification is sufficient to provide a stable well-lit surface environment for phytoplankton growth, but with some degree of mixing to promote export of suspended organic matter. Strong stratification causes intense initial production, but retention of suspended organic particles in the surface ocean induces a self-shading effect, and overall productivity, CO₂ uptake and export fluxes are low. When stratification is weak, mixing of phytoplankton over a larger depth range exposes cells to a wider range of light levels and reduces photosynthetic efficiency, thus total productivity and CO₂ uptake. A conceptual model is developed here, which attempts to describe the mechanism by which sea ice dynamics exert the principal control on stratification and therefore productivity and CO₂ uptake at the WAP, with potential application to other regions of the Antarctic continental shelf. Although meteoric waters (glacial melt and precipitation) are more prevalent in surface waters throughout the study, sea ice meltwater variability is driven by large and rapid spring/early summer pulses, which stabilise the upper ocean and initiate phytoplankton growth. The timing and magnitude of these sea ice melt pulses then exert the key control on stratification and seasonal productivity. In a low sea ice year of this study, the sea ice trigger mechanism was absent and productivity was low. This strongly suggests that ongoing sea ice decline at the WAP and greater frequency of such low sea ice years is likely to drive a dramatic reduction in productivity and export, which would substantially reduce the capacity of the summertime CO₂ sink in this region. Ongoing warming and ecosystem change are thus likely to have severe impacts on net CO₂ sink/source behaviour at the WAP over the annual cycle, and the role of the Southern Ocean in regulating atmospheric CO₂ and global climate. Finally, factors influencing the stable isotopic signature of particulate organic carbon ( 13CPOC), a common paleo-proxy, are assessed. δ¹³Cₚₒ꜀ is greatly influenced by seasonal shifts in diatom assemblages and isotopically heavy sea ice material, so cannot be used as a robust proxy for ambient CO₂ in the coastal Southern Ocean.