Arctic Ocean productivity, its ice-free future and application to ice-free oceans

• Main Author: Lawrence, Jonathan
• Format: Thesis
• Language: English
• Published: University of Southampton 2019
• Subjects: Arctic; Arctic Ocean; Central Basin; Phytoplankton; Sea ice
• Online Access: https://eprints.soton.ac.uk/432096/; https://eprints.soton.ac.uk/432096/1/Lawrence_Jonathan_deposited_thesis.pdf

This thesis examines productivity in the Arctic Ocean and its response to a future ice free Arctic. Phytoplankton produce atmospheric oxygen, regulate atmospheric carbon dioxide and underpin ocean ecosystems. Production dynamics and distributions in the Arctic are poorly understood — especially the extent of growth under ice and in prevalent subsurface chlorophyll maxima is unknown — and therefore the perturbation under anthropogenic sea ice retreat is poorly understood. We relate the vertical distribution of production to the ratio of nitrate limitation to light limitation limitation across the Arctic. Depth-integrated production in each water column is then easily related to the vertical distribution because light-governed production rates decrease exponentially with depth. The scaling elucidates under ice and subsurface production magnitudes, and works equally well across the diverse hydrographic (shelves, inflows, central basin) and biogeochemical provinces of the Arctic. Further, the scaling is shown to elucidate biogeochemical transformations of water masses as they transit the Arctic and to be time-invariant. The latter fact is used to predict perturbations to plankton dynamics under anthropogenic ice retreat. Further, unique boundary conditions make the Arctic a powerful place to study general (global) plankton responses to environmental perturbations. We explore this by deriving oceanic photosynthesis across the globe from the theory developed in the Arctic Ocean. The major implication of our results are that oceanic photosynthesis is explicable in terms of a coherent dependence on ocean nutrient and light conditions, and this entails minor productivity increases in an ice-free Arctic Ocean and in the global ocean over the coming century.