HYDROGRAPHIC OBSERVATIONS OF OXYGEN AND RELATED
PHYSICAL VARIABLES IN THE NORTH SEA AND WESTERN
ROSSSEA POLYNYA
investigations using seagliders, historical observations and
numerical modelling
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.
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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
