Spatial extent and historical context of North Sea oxygen depletion in August 2010

Abstract

Prompted by recent observations of seasonal low dissolved oxygen from two moorings in the North Sea, a hydrographic survey in August 2010 mapped the spatial extent of summer oxygen depletion. Typical near-bed dissolved oxygen saturations in the stratified regions of the North Sea were 75-80 % while the well-mixed regions of the southern North Sea reached 90 %. Two regions of strong thermal stratification, the area between the Dooley and Central North Sea Currents and the area known as the Oyster Grounds, had oxygen saturations as low as 65 and 70 % (200 and 180 lmol dm -3 ) respectively. 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. Historical data over the last century from the International Council for the Exploration of the Sea oceanographic database highlight an increase in seasonal oxygen depletion and a warming over the past 20 years. The 2010 survey is consistent with, and reinforces, the signal of recent depleted oxygen at key locations seen in the (albeit sparse) historical data.

Looking beyond stratification: a model-based analysis of the biological drivers of oxygen deficiency in the North Sea

“…This suggests that region 5 is unlikely to be affected by low bottom O 2 concentrations. However, Queste et al (2013) found bottom O 2 concentrations of about 6 mg O 2 L −1 near this area in 2010, which indicates that this area can be affected by O 2 deficiency.…”

“…Queste et al (2013) also observed lower bottom O 2 concen- trations north of the Dogger Bank in August 2010, however, they found the minimum concentrations a bit further north around 57 • N.…”

“…However, even these continuous measurements did not provide sufficient information to fully understand the processes which caused the observed O 2 evolution. Greenwood et al (2010) and Queste et al (2013), who extended the locally confined findings by Greenwood et al (2010) to the spatial scale using survey data from August 2010 and ICES historical data, refer to "plausible mechanisms" like vertical mixing or advection when the measurements could not be explained in detail. In consequence, Greenwood et al (2010) stated that the data provided insight into the processes affecting the O 2 dynamics but models are required to further elucidate the significance of the seasonal drivers.…”

Megacities and Large Urban Agglomerations in the Coastal Zone: Interactions Between Atmosphere, Land, and Marine Ecosystems

“…However, it should be noted that the impact of enhanced nutrient inputs on a particular coastal sea is highly dependent on the physical environment, as illustrated by contrasting the Baltic and North Sea, adjacent large coastal seas receiving large atmospheric and fluvial nutrient inputs from large urban zones of Northern Europe. The Baltic is subject to chronic hypoxia (Conley et al 2009) while the North Sea has until recently shown little evidence of hypoxia (Queste et al 2012), with the difference attributable in large part to the short residence time of waters within the North Sea relative to flushing with open North Atlantic waters, compared to the Baltic. The estimated residence time in the Baltic is more than 20 years with slow vertical mixing (Meier et al 2006), whereas in the North Sea it is about 1 year with seasonal or shorter time scale vertical mixing of the water column (Jickells 1998).…”

Marine Ecosystem Connections: essential indicators of healthy, productive and biologically diverse seas

“…Seasonal cruises allowed more in-depth studies of these key parameters, as well as of sediment resuspension (Couceiro et al 2012), hypoxia and nutrient cycling (Neubacher et al 2011(Neubacher et al , 2012, structure and functioning of seabed communities (Birchenough 2012a, b), and the trophic dynamics of zooplankton (Kürten et al 2011) and benthic and pelagic functional feeding groups (Kürten et al 2012). Broader sampling contributed further understanding of particulate nutrient sources in coastal waters (Bristow et al 2012), temporal and/or spatial variability in primary production (Fernand et al (2013);van Leeuwen et al 2012), pelagic microbial communities (Brandsma et al 2012) and oxygen depletion in bottom waters (Queste et al 2012). …”