No description available.

Shelf seas occupy only 7% in area and less than 0.5% in volume of the entire ocean, but they play an important role in the carbon cycle by taking about 20% - 50% of all the CO2 absorbed by the ocean. Diapycnal mixing is a key process in transporting nutrients, carbon, water mass etc. between the surface and the lower mixed layers in a seasonally stratified shelf sea. The identification and quantification of the processes responsible for driving diapycnal mixing in seasonally stratified seas are the subjects worth study. Early researchers have examined the correlation between enhanced bulk shear and the wind. The bulk shear is defined as the average of the shear in two defined layers which are either side of the thermocline. However the contribution from the barotropic tide has generally been neglected. This study examines two stages of the evolution of water column stratification: the spring development stage and the autumn break down stage. Rotary spectral analysis shows that the shear across thermocline corresponds to different drivers when the water stratification is different. At the spring development stage, the shear across the thermocline corresponds to near-inertial oscillations, which are related to wind. Whilst at the autumn break down stage, the shear across thermocline relates to both the near-inertial oscillations and the barotropic tide. Thus, in contraction to earlier research, our research suggests that the barotropic tide is another dominant driver in the generation of shear. However not all observations can be explained by the wind or barotropic tide. The additional consideration of the baroclinic tide helps explain the signal of an odd shear spike observed in the northern North Sea, which occurred during a period of weak shear production by the wind and barotropic tide. A 1D two-layer vertical dynamic numerical model and a 1D turbulence closure numerical model were applied to investigate the impact of wind and barotropic tide on shear, respectively. In addition, the impacts of hydrographic conditions on the driver of shear were considered. Coherence analysis was applied to examine the similarity of constituents (in frequency domain) between the modelled shear production and the observations. The model sensitivity analysis demonstrates that the switch of driver of shear is highly related to the depth ratio, which is the ratio of thermocline depth over water depth.

To facilitate the process of setting up small scale environmental models in coastal and offshore areas a wave model and a 2D current model for the Baltic Sea have been set up with MIKE 21. The ambition is that the Baltic Sea model can serve as a source of boundary conditions for local models. The main focus in the project has been to determine which input data to use to get the best results and then to calibrate and validate the model with the best data sets available. The wave model has been tested with three different sources of wind forcing and the results with the different sources are evaluated. The wave model has been calibrated and validated against five wave buoys and the current model against three current observation stations. The comparison shows that the wave model gives good estimations of wave height, period and direction. The output of the current model coincides with observations where distinct current patterns exist. The wave model is considered a good source to extract wave statistics from for the entire Baltic Sea. The current model can be used for scenarios or areas that do not suffer from the limitations of a 2D model.

Shelf seas represent an important source of iron (Fe) to the open ocean. Additionally, shelf seas are highly productive environments which contribute to atmospheric carbon dioxide drawdown and support large fisheries. The work presented in this thesis describes the seasonal cycle of Fe in the Celtic and Hebridean Shelf Seas, and determines the physico-chemical speciation of Fe supplied from oxic margins. The results from repeated field surveys of the central Celtic Sea showed a nutrient type seasonal cycling of dissolved Fe (< 0.2 µm; dFe), which is surprising in a particle rich shelf system, suggesting a balance of scavenging and remineralisation processes. Coincident drawdown of dFe and nitrate (NO3-) was observed during the phytoplankton spring bloom. During the bloom, preferential drawdown of soluble Fe (< 0.02 µm; sFe) over colloidal Fe (0.02-0.2 µm; cFe) indicated greater bioavailability of the soluble fraction. Throughout summer stratification, it is known that NO3- is drawn down to < 0.02 µM in surface waters. This study revealed that both dFe and labile particulate Fe (LpFe) were also seasonally drawn down to < 0.2 nM. Consequently, it is hypothesised that the availability of Fe seasonally co-limits primary production in this region. At depth both dFe and NO3- concentrations increased from spring to autumn, indicating that remineralisation is an important process governing the seasonal cycling of dFe in the central Celtic Sea. In spring, summer and autumn, distinctive intermediate nepheloid layers (INL) were observed emanating from the Celtic Sea shelf slope. The INLs were associated with elevated concentrations of dFe (up to 3.25 ± 0.16 nM) and particulate Fe (up to 315 ± 1.8 nM) indicating that they are a persistent conduit for the supply of Fe to the open ocean. Typically > 15% of particulate Fe was labile and 60-90% of dFe was in the colloidal fraction. Despite being < 50 km from the 200 m isobath, the concentration of dFe was < 0.1 nM in surface waters at several stations. Broadly, the concentration of nutrients in surface waters described an oligotrophic environment where co-limitation between multiple nutrients, including Fe, appears likely. Over the Hebridean shelf break, residual surface NO3- concentrations (5.27 ± 0.79 µM) and very low concentrations of dFe (0.09 ± 0.04 nM) were observed during autumn, implying seasonal Fe limitation. The dFe:NO3- ratio observed is attributed to sub-optimal vertical supply of Fe relative to NO3- from sub-surface waters. In contrast to the shelf break, surface water in coastal regions contained elevated dFe concentrations (1.73 ± 1.16 nM) alongside low NO3-. Seasonal Fe limitation is known to occur in the Irminger and Iceland Basins; therefore, the Hebridean shelf break likely represents the eastern extent of sub-Arctic Atlantic seasonal Fe limitation, thus indicating that the associated weakening of the biological carbon pump exists over a wider region of the sub-Arctic Atlantic than previously recognised. These key findings demonstrate that the availability of Fe to phytoplankton may seasonally reach limiting levels in temperate shelf waters and that oxic margins persistently supply Fe dominated by colloidal and particulate fractions to the ocean.