title: The supply of nutrients to the subsurface chlorophyll maximum in temperate shelf seas
Creators: name: Williams, Charlotte
Publisher: name: University of Liverpool Repository
Year: 2014

description: Shelf seas are believed to play an important role in the oceanic export of carbon (C). The combination of enhanced primary productivity partly induced by highly energetic mixing, together with the euphotic zone being in close proximity to the seabed, make the shelf seas highly efficient region for biological C sequestration. The high productivity observed in shelf seas also has high economical significance by supporting &gt;90% of the global fish catch. A subsurface peak in biomass, termed the subsurface chlorophyll maximum(SCM), at the base of the thermocline occurs as a result of phytoplankton being nitrate limited in the surface layer. The SCM is believed to be responsible for as much primary production as the spring bloom. The turbulent supply of nutrients across the thermocline, driven by the internal tide and wind-driven inertial oscillations, is believed to support growth at the SCM. However, although inertial oscillations are recognised as an important mixing mechanism, the turbulent flux of nitrate that they supply has not yet been estimated. In this thesis the importance of diapycnal fluxes of nutrients, particularly those generated by wind-driven inertial oscillations, is investigated in terms of sustaining productivity in the SCM, using a Lagrangian numerical model, observational and experimental approaches. The results in this study indicate that diapycnal nitrate fluxes limit new production at the SCM, and mixing generated by wind-driven inertial oscillations play a key role in supporting primary production at the SCM. In addition, this study demonstrates that the background nitrate flux, as well as transport of nitrate via dinoflagellate migration, are too low to sustain primary productivity at the SCM. Our observations demonstrate that wind-driven large, short-lived dissipation events increase the daily nitrate flux to the SCM by a factor of at least 17, supplying the SCM with ~33% to 71% of the nitrate required fornew production in shelf seas during summer. Thus it is intermittent mixing events which must supply the required nitrate for new production at the SCM. Using an experimental approach, a wind event was simulated in order to estimate the impact of mixing of water from the SCM and BML on primary and secondary production in the SCM. Nutrients were assimilated rapidly and phytoplankton production increased as expected. However, up to two thirds of the C fixed was exuded as DOC and there was a significant increase in bacterial activity. As well as supplying nutrients to the SCM, we show that diapycnal mixing may also be important in redistribution plankton, especially bacteria, which subsequently impacts both the inorganic and organic nutrient pools. Thus the sensitivity of the SCM, in terms of autotrophy and heterotrophy, to diapycnal mixing is highlighted. This study highlights the importance of short-lived events in supplying nutrients to the SCM. Concluding that these are likely undersampled and also not well represented in shelf sea models. In addition, this thesis demonstrates that there is a need to consider secondary as well as primary production to fully understand the C cycle in the shelf sea.