A numerical modelling investigation of the impact of mesoscale heterogeneity on oceanic primary productivity

• Main Author: Burren, Claire Louise
• Format: Thesis
• Language: English
• Published: University of Southampton 1993
• Subjects: North Atlantic
• Online Access: https://eprints.soton.ac.uk/462637/

Phytoplankton have a profound effect on the carbon budget of the oceans through the impact of the biological pump. Marine plankton are also known to aggregate over a wide range of spatial and temporal scales. Knowledge of the processes that control heterogeneity in planktonic distributions in the upper ocean is thus required for studies of the global carbon cycle. The impact of the underlying mesoscale eddy field on the spatial and temporal scales of biological production, and overall rates of primary productivity are investigated. This is achieved through the development of a coupled quasigeostrophic-mixed layer-ecosystem model (QGECO). The model exhibits similar temporal trends in the biological and physical fields to those observed in the North Atlantic; i.e the mixed layer shallows in spring causing a rapid increase in phytoplankton concentrations and a corresponding decline in nutrient levels. Heterogeneity is produced in the mixed layer through Ekman pumping velocities resulting from the interaction of windstress and surface currents. This variability impacts on biological production in two ways. Firstly, spatial variations in the depth of the mixed layer affect the photosynthetically active radiation (PAR) availability and hence production rates, and secondly, eddy enhanced exchange between the surface water and those at depth bring additional nutrients into the euphotic zone. These processes result in significant spatial and temporal heterogeneity in the ecosystem distributions. Investigation of the spatial heterogeneity of the biological system finds variability to be significantly greater than that of the mixed layer. The relationship between the eddy field and the ecosystem is investigated. The structure and correlation of the biogeochemical fields changes with time. The biological fields are found to have a shorter horizontal scale, but whiter spectrum than the underlying eddy field.