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Title: Phosphorus dynamics in shelf seas
Author: Davis, Clare Elizabeth
ISNI:       0000 0004 2734 1411
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2012
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Shelf seas are highly productive regions of the world's ocean. Contributing 16 - 30 % to global ocean carbon fixation while representing a mere 7 % of the area, their importance in the carbon cycle is disproportionate to their size. This high productivity has econonimical significance, supporting over 90 % of global fishery yields (Pauly et al., 2002). Shelf seas are also physically dynamic regions. On an annual cycle, the water column in shelf seas is fully mixed in winter and thermally stratified in summer in deeper regions, with enhanced mixing along the shelf-edge. In addition, shelf seas act as a buffer between the land and coastal seas, and the open ocean. However, the processes that transform nutrients within the shelf sea are poorly understood. More so, the role of physical processes in both transporting nutrients, as well as their interaction with the biological processes that govern nutrient concentrations and partitioning are currently unresolved. Here, nutrient dynamics, with a specific focus on phosphorus, were assessed within the Celtic Sea, part of the northwest European shelf. The distribution and partitioning of phosphorus between the particulate (PPhos) and dissolved organic (DOP) and inorganic (DIP) phases was determined over various temporal and spatial scales. The concentration of labile DOP (phosphomonoesters, PME) and its rate of turnover were determined using enzyme rate assays. The microbes involved in PME hydrolysis were identified using enzyme labelled fluorescence techniques. The influence of physical processes, including spring-neap tidal cycle, mixing over topography and storm enhanced mixing on the vertical distribution of phosphorus in the water column were also assessed. DOP accumulated in the surface layer of the thermally stratified shelf waters and at the shelf edge. DOP production was enhanced at the shelf edge and during storm events due to enhanced mixing and fluxes of phosphate, which were rapidly shunted into the DOP pool through enhanced primary production. However, during mixing there was a counteracting downward flux of DOP exported to the bottom waters. Profiles of PME concentrations revealed a relatively labile component to bottom layer DOP, which had turnover times in the order of days. Comparison of observations in the Celtic Sea and Porcupine Bank region demonstrated that the accumulation of DOP on the shelf and at the shelf edge relative to the adjacent slope and oceanic regions is a persistent feature of the western European continental margin. Through a number of shelf-edge exchange processes, including wind mixing on short time scales, tidal mixing on fortnightly timescales and seasonal winter mixing, the Celtic Sea was identified as a potential source of DOP to the North Atlantic, where production is thought to be phosphorus limited (Mather et al., 2008).
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral