Interactions of organic carbon and nitrogen cycles with phytoplankton in a changing Southern Ocean

Abstract

Marine phytoplankton are the foundation of the Southern Ocean ecosystem and carbon cycle. Carbon fixation during summertime photosynthesis produces organic matter and drives the Southern Ocean towards being a seasonal carbon sink. Consequently, the Southern Ocean sequesters a disproportionately large amount of anthropogenic carbon and heat relative to its size. However, increased heat uptake is having widespread environmental impacts on the Southern Ocean, including reduced sea ice concentrations, increased sea surface temperatures, and increased variability in nutrient supply resulting from changes to stratification. These multiple stressors feedback on marine phytoplankton and can influence rates of growth and nutrient uptake, alongside potential ecological changes in community composition and size class.

This thesis takes a multi-methods approach, combining model evaluation, laboratory experiments and in situ sampling to further our understanding of how Southern Ocean phytoplankton will respond to the projected effects of climate change, and the subsequent impacts for carbon and nitrogen cycling. Earth system model outputs from the coupled model intercomparison project (CMIP6) were analysed for properties of Southern Ocean phytoplankton, physics and biogeochemistry to examine the extent of projected change under a high emission, low mitigation pathway (SSP5-8.5). Productivity across the Southern Ocean was shown to increase by 30% over the 21st century, against a backdrop of global decreases in net primary productivity. Since most CMIP6 models have simplistic representations of phytoplankton, there is widespread inter-model variability in the extent to which productivity will increase across the different latitudinal zones of the Southern Ocean. However, ensemble means suggest that diatoms and picophytoplankton (2-20 μm) will increase at roughly similar rates across much of the Subantarctic and Transitional zones, while growth of picophytoplankton is expected to exceed that of diatoms in the coastal Antarctic zones. The greatest source of variability was identified as being from light concentrations (>15000 (μE m-2 s-1)2), particularly close to ice shelves. Uncertainties in light supply leads to variability in phytoplankton growth in this iron and light limited system. Projected changes in phytoplankton growth were analysed from the perspective of the Southern Ocean Observing System to identify spatial and temporal frames whereby targeting of additional sampling may be most beneficial in resolving uncertainties for future generations of model development.

To understand how potential ecological changes in the composition of phytoplankton communities will impact the carbon cycle, sites were sampled along the West Antarctic Peninsula (WAP) to analyse the molecular composition of dissolved organic matter (DOM) associated with different phytoplankton types. DOM was analysed by ultrahigh resolution FT-ICR-MS to identify possible molecular fingerprints of phytoplankton types. Analysis of the stable carbon isotope (δ13C) and stoichiometric data allowed for the identification of different DOM sources and transformation processes. Smaller classes of phytoplankton (<20 μm), including cryptophytes and flagellates, were most abundant in the warmer northern peninsula region and were associated with fresher forms of DOM. In comparison, larger diatoms were associated with aged DOM, which had a higher degree of degradation and was depleted in carbon and nitrogen compared to the bulk pool. All forms of DOM were rapidly transformed below the euphotic zone, resulting in a recalcitrant background pool of DOM at depth. If future Southern Ocean phytoplankton communities shift towards a smaller size class, a greater amount of organic matter is likely to be partitioned to the dissolved phase, potentially weakening the organic carbon sink.

Since nutrient requirements differ between phytoplankton types, one consequence of shifts in community composition could be changes to nutrient utilisation. Utilisation of nitrogenous nutrients are of particular interest because the Southern Ocean exports nutrients across the global ocean, ~75% of which is limited by nitrogen outside of high nutrient zones. An experiment was conducted whereby different Southern Ocean phytoplankton species in culture were incubated with isotopic tracers (15N) to measure the uptake of nitrate, ammonium and urea. This showed that at realistic nutrient concentrations for the WAP during summer, all species have a preference for ammonium, accounting for >50% of total nitrogen uptake. The greatest amount of variability in nitrogen uptake existed at the species level between the pennate diatoms Fragilariopsis cylindrus and Pseudo-nitzschia subcurvata, perhaps due to growth limitation in one species. This study further investigated variability in the application of the 15N tracer method by adapting an existing chemical method from the freshwater literature to work in seawater. This was successfully applied to show that a titanium mediated conversion is effective for determining the isotopic composition of 15N enriched NH4+in seawater, allowing for the correction of ammonium uptake rates for ammonium regeneration. Accounting for ammonium regeneration increased ammonium uptake rates by 52% (± 16%), suggesting the importance of ammonium for phytoplankton may be greater than previously thought.

Taken together, the results of this thesis demonstrate that physical change in the Southern Ocean resulting from climate driven environmental change, will increase overall primary productivity and variability in the abundance of different phytoplankton size fractions across the latitudinal zones of the Southern Ocean. Biogeochemical implications of this ecological shift could manifest through increased DOM partitioning reducing export efficiency and more variability in nitrogenous nutrient uptake. Improving consistency in nutrient uptake methodologies and targeting additional observations to regions of existing uncertainties could help to resolve knowledge gaps in our understanding of phytoplankton-biogeochemical coupling in this large and climatically sensitive region.