Transparent Exopolymer Particles in the Surface Arctic Ocean by Ocean Biogeochemistry Modeling

In light of the Arctic Amplification of global warming, it is fundamental to enhance our comprehension of ecosystem dynamics in the Arctic Ocean. This will facilitate predictions about how alterations in phytoplankton and broader ecological processes may evolve under future warming scenarios. The primary production of the Arctic Ocean is principally based on phytoplankton building up organic carbon. The growth and distribution of phytoplankton are strongly shaped by the seasonality of polar night and day, sea ice cover and nutrient availability. Focusing on one essential component of the organic carbon cycle, I simulate transparent exopolymer particles (TEP) in the upper ocean. As in situ observations are scarce, modeling can extend our knowledge on their spatial and temporal occurrence patterns and trends. Additionally, these particles have recently been reported to act themselves as biogenic aerosol precursors, or as precursor for other organic compounds. These may be an important source of primary marine organic aerosols in the Arctic atmosphere.
In the first part of my dissertation, I present a coupled ocean sea-ice biogeochemistry model where I integrate dissolved acidic polysaccharides (PCHO) and TEP. Phytoplankton exude organic carbon into the surrounding ocean, particularly under nutrient-depleted conditions. PCHO are defined as one part of the exuded organic carbon, which can then aggregate to form larger particles, such as TEP. There is a strong seasonal cycle of TEP in the upper ocean, as the occurrence of TEP follows the phytoplankton blooms both temporally and spatially. The simulation provides an initial estimate of TEP concentration with the highest levels reaching 200-400 µg C/L in the upper ocean (0-30 m depth) simulated in the Fram Strait and on the continental shelves under conditions of nutrient depletion for June to August. In the central basins, TEP concentration range from 10 to 50 µg C/L. When considered in the context of observation datasets, this simulation performs well in terms of Total Chlorophyll a (TChla) and particulate organic carbon. There is reasonable agreement for TEP compared with the few in situ datasets available. It would be recommended to gather more observational data on TEP in conjunction with data on TChla and other relevant biogeochemical parameters. This would allow deeper insights into the ecosystem dynamics and time series of TEP in the Arctic Ocean. As a consequence of the simulation analysis, the regions of interest for in situ measurements should be the marginal ice zones, and especially the high Arctic due to the seasonal variations of sea ice and its overall declining trend.
Moreover, the simulation for the period 1990 to 2019 indicates a significant negative trend of TEP concentration in summer in regions affected by the inflow of Atlantic water, such as the eastern Fram Strait, the Barents Sea, and parts of the Eurasian Basin. Regions of the Arctic Ocean influenced by Pacific water exhibit a significant positive trend in TEP concentration, including the Amerasian Basin, the Canadian Arctic Archipelago and the Kara Sea.
In the second part of my thesis, I re-analyze the simulated environmental variables in order to ascertain their role as TEP drivers in three exemplary regions. The analysis demonstrates that TChla is an important predictor for TEP occurrence in general, but also physical factors such as photosynthetically active radiation and sea ice concentration exert a significant influence on the distribution of TEP in the Laptev Sea, while nutrient availability plays an important role in shaping the time series observed in the Fram Strait.
In the third part, I asses the long-term trend of TEP in the Arctic Ocean following a high-emission scenario proposed by the Intergovernmental Panel on Climate Change. The Arctic-wide TEP concentration is projected to increase significantly from 80 to 105 µg C/L in the upper ocean until 2100. This increase is driven mostly by the retreat of the sea ice cover, which triggers increases in phytoplankton carbon concentration and a subsequent increase in phytoplankton nutrient limitations. However, there are regional differences as, e.g., in the western Fram Strait, the trends seem to be shaped by the sea ice retreat and light availability, whereas in the eastern Fram Strait, nutrient availability and remineralization of TEP are important factors.
The final part of my dissertation revisits the role of biogenic aerosol precursors in the upper ocean. I briefly discuss one application of the knowledge gained about the organic carbon cycle in the upper Arctic Ocean. By serving as an ocean-atmosphere boundary condition, the simulation enables the determination of primary marine organic aerosol emissions in an aerosol-climate model. I conclude with a discussion on two perspectives for future research, namely the simulation of organic matter enrichment at the ocean surface and the distribution of TEP in the water column, especially with respect to sinking and degradation processes.