Biogeosciences, 13, 2511-2535, 2016
www.biogeosciences.net/13/2511/2016/
doi: 10.5194/bg-13-2511-2016
© Author(s) 2016. CC Attribution 3.0 License.
Biogeosciences
Looking beyond stratification: a model-based analysis of the
biological drivers of oxygen deficiency in the North Sea
Fabian Große 1 , Naomi Greenwood 2 3 , Markus Kreus 45 , Hermann-Josef Lenhart 1 , Detlev Machoczek 6 ,
Johannes Pätsch 5 , Lesley Salt 7 , and Helmuth Thomas 8
University of Hamburg, Department of Informatics, Scientific Computing, Bundesstraße 45a, 20146 Hamburg, Germany
2 Centre for Environment, Fisheries and Aquaculture Science (Cefas), Lowestoft, Suffolk, NR33 0HT, UK
3 University of East Anglia, School of Environmental Sciences, Norwich, NR4 7TJ, UK
4 University of Hamburg, Institute for Hydrobiology and Fisheries Science, Olbersweg 24, 22767 Hamburg, Germany
5 University of Hamburg, CEN, Institute of Oceanography, Bundesstraße 53, 20146 Hamburg, Germany
6 Federal Maritime and Hydrographic Agency, Bemhard-Nocht-Straße 78, 20359 Hamburg, Germany
7 CNRS, UMR 7144, Equipe Chimie Marine, Station Biologique de Roscoff, Place Georges Teissier, 29680, Roscoff, France
8 Dalhousie University, Department of Oceanography, 1355 Oxford Street, Halifax, Canada
Correspondence to: Fabian Große (fabian.grosse@uni-hamburg.de)
Received: 30 June 2015 - Published in Biogeosciences Discuss.: 10 August 2015
Revised: 14 March 2016 - Accepted: 8 April 2016 - Published: 28 April 2016
Abstract. Low oxygen conditions, often referred to as oxy
gen deficiency, occur regularly in the North Sea, a temperate
European shelf sea. Stratification represents a major process
regulating the seasonal dynamics of bottom oxygen, yet, low
est oxygen conditions in the North Sea do not occur in the
regions of strongest stratification. This suggests that stratifi
cation is an important prerequisite for oxygen deficiency, but
that the complex interaction between hydrodynamics and the
biological processes drives its evolution.
In this study we use the ecosystem model HAMSOM-
ECOHAM to provide a general characterisation of the dif
ferent zones of the North Sea with respect to oxygen, and to
quantify the impact of the different physical and biological
factors driving the oxygen dynamics inside the entire sub-
thermocline volume and directly above the bottom.
With respect to oxygen dynamics, the North Sea can be
subdivided into three different zones: (1) a highly produc
tive, non-stratihed coastal zone, (2) a productive, season
ally stratified zone with a small sub-thermocline volume, and
(3) a productive, seasonally stratified zone with a large sub-
thermocline volume. Type 2 reveals the highest susceptibility
to oxygen deficiency due to sufficiently long stratification pe
riods ( > 60 days) accompanied by high surface productivity
resulting in high biological consumption, and a small sub-
thermocline volume implying both a small initial oxygen in
ventory and a strong influence of the biological consumption
on the oxygen concentration.
Year-to-year variations in the oxygen conditions are
caused by variations in primary production, while spatial dif
ferences can be attributed to differences in stratification and
water depth. The large sub-thermocline volume dominates
the oxygen dynamics in the northern central and northern
North Sea and makes this region insusceptible to oxygen de
ficiency. In the southern North Sea the strong tidal mixing
inhibits the development of seasonal stratification which pro
tects this area from the evolution of low oxygen conditions.
In contrast, the southern central North Sea is highly suscep
tible to low oxygen conditions (type 2).
We furthermore show that benthic diagenetic processes
represent the main oxygen consumers in the bottom layer,
consistently accounting for more than 50 % of the overall
consumption. Thus, primary production followed by rem
ineralisation of organic matter under stratified conditions
constitutes the main driver for the evolution of oxygen defi
ciency in the southern central North Sea. By providing these
valuable insights, we show that ecosystem models can be
a useful tool for the interpretation of observations and the es
timation of the impact of anthropogenic drivers on the North
Sea oxygen conditions.
Published by Copernicus Publications on behalf of the European Geosciences Union.
