A Tale of Four Seasons: Investigating the seasonality of physical structure and its biogeochemical responses in a temperate continental shelf sea

Abstract

Due to their high biological productivity, continental shelf seas are significant sinks of anthropogenic carbon. To better understand the cycling of carbon within them, an accurate prediction of their vertical density stratification is required, as this is a critical control on the carbon drawdown. While the dominant controls on density structure are boundary driven mixing and seasonal heating, internal waves have been shown to play a small but critical role in both open ocean and shelf sea physical and biogeochemical cycles. Current knowledge on the spatial and temporal variability in internal mixing is however still severely limited. The aim of the work in this thesis was to develop new insight into the seasonal variability of physical controls on the vertical density structure and examine its biogeochemical responses in a temperate shelf sea. This thesis presents and examines new results that test the impact of boundary layer and internal wave forcing on stratification and vertical density structure in continental shelves. A new series of continuous measurements of full depth density and velocity structure, meteorological and wave forcing, surface nitrate and surface chlorophyll a spanning 17 months (March’14 − July’15) provide unprecedented coverage over a full seasonal cycle at a station 120 km north-east from the continental shelf break. Work within this thesis showed that the controls on vertical density structure at the mooring site were largely analogous to that of open-ocean environments with tidal mixing only playing a minor role. This result contrasts with the well-known tidally controlled frontal systems described by Simpson and Hunter (1974). Since a large proportion of continental shelf regions are away from tidal mixing fronts this result suggest the requirement for an adjusted third regime that bridges the gap between open-ocean environments and frontal regions, to accurately predict the vertical density structure within them. The long-term observations presented in this thesis reveal a seasonality within the internal wave energy, which suggest internal mixing varying relative to the seasonal cycle of stratification, represented by N². By investigating the representation of this seasonality by three commonly used internal wave parameterisations it was shown that each predicted a seasonality that directly contradicted that observed within the internal wave energy. It was suggested that the reason for this was most likely due to their failure to introduce the enhanced S² that is attributable to internal waves, which have been shown in this work to have a strong seasonal cycle with maximum energy levels during the summer. In an attempt to provide realistic scaling of turbulence an adjusted iteration of the MacKinnon and Gregg (2003a) scaling of turbulence was employed using an observed close relationship between N² and S² to introduce a state of marginal stability in the pycnocline, thus introducing a seasonally varying level of internal mixing that follows the observed seasonality in internal wave energy. Examining the biogeochemical response to the seasonal change in vertical density struc- ture also highlighted the importance of the autumn phytoplankton bloom within the annual cycle of primary production. By putting the autumn phytoplankton bloom within the context of the seasonal cycle it was shown that it has the potential to be as productive as the well-studied spring phytoplankton bloom and the summer sub-surface chlorophyll maximum and thus has the capacity to significantly contribute to the drawdown of atmospheric carbon dioxide.