AbstractDissolved oxygen concentrations in the coastal ocean are controlled by a combination of physical and biogeochemical processes. Physical processes include turbulence, advection, and diffusion, biogeochemical processes include respiration, and photosynthesis. However, the influence of these processes on oxygen dynamics within coastal shelf sea environments are not fully understood. The main objective of this research was to develop new methodologies to enable measurements of in situ oxygen dynamics using the Eddy Correlation (EC) method in a dynamic shelf sea environment.
EC data were collected to determine oxygen flux within two study sites L4 and Cawsands within the Western Channel Observatory (WCO). The first study provided further validation of the EC technique within a challenging environment by comparing EC measurements with the well-established microprofiler technique measurements. This study also provided an initial assessment of flux drivers within a seagrass habitat. The second study deployed the EC over a five-month field campaign. During this campaign other parameters were also collected such as: acoustic velocity data to estimate the contribution of the tide, conductivity, temperature and depth sensors, sediment composition and nutrient analysis. These complementary parameters aided assessment and understanding of drivers in relation to oxygen flux within the L4 study findings.
A comparison of the relatively unstudied EC technique was made against a microprofiler, which measures diffusive point flux. Oxygen flux data between microprofiler and EC correlated during dark periods however did not during light periods. The EC captured photosynthetic drivers of oxygen flux during light periods. This validated the EC in a seagrass environment and demonstrated the microprofiler did not measure some key processes as it cannot capture all fluxes during photosynthetic periods.
Previous studies have improved EC processing techniques and have reported the magnitudes of the fluxes at varying sites. However, little is known about benthic oxygen flux drivers, especially within coastal shelf sea environments, yet they have substantial ecological and economic importance. This study (Chapter 4) demonstrated that it is possible to gain insight into the controlling oxygen dynamics at the seabed by analysing the various tidal influences. Contribution of surface versus internal tides was assessed in relation to fluxes. While surface tides were shown to be the primary driver, there was a significant contribution driven by internal tides. During the weak stratification period there was a contribution from internal tides and it is suspected that as the summer progresses the internal tidal contribution would increase, becoming a stronger driver of flux.
Building on this preliminary study, oxygen fluxes over five months were assessed by evaluating all major drivers of benthic oxygen flux at the L4 study site. The EC technique, which measures total turbulent oxygen flux, along with accompanying instruments was used to document and assess drivers of benthic oxygen uptake over the five-month field campaign at L4. Drivers such as nutrients, temperature, the dumping of dredged spoils and chlorophyll concentration were assessed. Flux correlated weakly with flow velocity over the five-month sampling campaign. Furthermore, without sediment analysis and settling chambers at the specific location of where the EC fluxes were measured, the contribution of dredge material, plankton bloom and nutrients have on benthic oxygen flux cannot be conclusively quantified.
A lack of data capturing nutrients within the sediment and ADCP data throughout the five-month sampling campaign at L4 meant that a complete quantification of the drivers of oxygen flux at this site was not achievable. A more complete picture could be obtained if a microprofiler (measuring diffusive flux) and benthic chambers (measuring sediment oxygen uptake) were deployed alongside the EC as well as settling chambers and sediment nutrient analysis. Nonetheless, this study has made advances in EC processing and has provided a first insight into tidal and oxygen dynamics at a historical research station. The work presented has provided further validation of the EC technique and processing methods within a challenging environment by comparing EC and microprofiler measurements. Furthermore, this study has provided the first insight into water column oxygen budgets, with a particular focus on the influence of benthic oxygen flux drivers, in this shelf sea environment.
|Date of Award||8 Apr 2022|
|Supervisor||Chris Blenkinsopp (Supervisor) & Lee Bryant (Supervisor)|