Use this URL to cite or link to this record in EThOS:
Title: Hydrographic observations of oxygen and related physical variables in the North Sea and western Ross Sea polynya : investigations using seagliders, historical observations and numerical modelling
Author: Queste, Bastien
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 2013
Availability of Full Text:
Access from EThOS:
Access from Institution:
Shelf seas are one of the most ecologically and economically important ecosystems of the planet. Dissolved oxygen in particular is of critical importance to maintaining a healthy and stable biological community. This work investigates the physical, chemical and biological drivers of summer oxygen variability in the North Sea (Europe) and Ross Sea polynya (Antarctica). In particular, this work also focuses on the use of new autonomous underwater vehicles, Seagliders, for oceanographic observations of fine scale (a few metres) to basin-wide features (hundreds of kilometres). Two hydrographic surveys in 2010 and 2011 and an analysis of historical data dating back to 1902 revealed low dissolved oxygen in the bottom mixed layer of the central North Sea. We deployed a Seaglider in a region of known low oxygen during August 2011 to investigate the processes regulating supply and consumption of dissolved oxygen below the pycnocline. Historical data highlighted an increase in seasonal oxygen depletion and a warming over the past 20 years. Regions showing sub-saturation oxygen concentrations were identified in the central and northern North Sea post-1990 where previously no depletion was identified. Low dissolved oxygen was apparent in regions characterised by low advection, high stratification, elevated organic matter production from the spring bloom and a deep chlorophyll maximum. The constant consumption of oxygen for the remineralisation of the matter exported below the thermocline exceeded the supply from horizontal advection or vertical diffusion. The Seaglider identified cross-pycnocline mixing features responsible for reoxygenation of the bottom mixed layer not currently resolved by models of the North Sea. Using the data, we were also able to constrain the relative importance of different sources of organic matter leading to oxygen consumption. iii From November 2010 to February 2011, two Seagliders were deployed in the Ross polynya to observe the initiation and evolution of the spring bloom. Seagliders were a novel and effective tool to bypass the sampling difficulties caused by the presence of ice and the remoteness of the region, in particular they were able to obtain data in the polynya before access was possible by oceanographic vessels. Seagliders were able to survey the region at a fraction of the cost and inconvenience of traditional ship surveys and moorings. We present observations of a large phytoplankton bloom in the Ross Sea polynya, export of organic matter and related fluctuations in dissolved oxygen concentrations. The bloom was found to be widespread and unrelated to the presence of Ross Bank. Increased fluorescence was identified through the use of satellite ocean colour data and is likely related to the intrusion of modified circumpolar deep water. In parallel, changes in dissolved oxygen concentration are quantified and highlight the importance of a deep chlorophyll maximum as a driver of primary production in the Ross Sea polynya. Both the variability of the biological features and the inherent difficulties in observing these features using other means are highlighted by the analysis of Seaglider data. The Seaglider proved to be an excellent tool for monitoring shelf sea processes despite challenges to Seaglider deployments posed by the ice presence, high tidal velocities, shallow bathymetry and lack of accurate means of calibration. Data collected show great potential for improving biogeochemical models by providing means to obtain novel oceanographic observations along and across a range of scales.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available