F. Große et al.: Looking beyond stratification
2513
www.biogeosciences.net/13/2511/2016/
Biogeosciences, 13, 2511-2535, 2016
Figure 1. Extent of observed Ot concentrations < 6mg02L _1 in
the German Bight area from 1980 to 2010. Dotted lines indicate
geographical limits of the individual surveys. Light grey line marks
German Maritime Area (from Topcu and Brockmann, 2015).
tion of stratification and CD concentrations at the location of
observation.
Greenwood et al. (2010) published the first data from con
tinuous measurements of bottom CD concentrations for two
sites (“North Dogger” and “Oyster Grounds”) in a European
shelf sea. Using these measurements, the dynamic interac
tion between stratification and the evolution towards low bot
tom Ot concentrations can be observed, as well as the rapid
recovery to saturated Ot conditions after the breakdown of
stratification due to mixing in autumn. However, even these
continuous measurements did not provide sufficient infor
mation to fully understand the processes which caused the
observed O2 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 “plau
sible mechanisms” like vertical mixing or advection when
the measurements could not be explained in detail. In con
sequence, Greenwood et al. (2010) stated that the data pro
vided insight into the processes affecting the O2 dynamics
but models are required to further elucidate the significance
of the seasonal drivers.
Ecosystem models produce a temporally and spatially con
sistent picture on O2 and can therefore provide insight into
the balance between the physical and biological factors and
processes governing the evolution of the bottom O2 concen
trations. Thus, they can help understand and interpret mea
surements of O2 and related parameters and can further de
scribe the history of events of low O2 conditions.
In this study we use the three-dimensional physical-
biogeochemical model system HAMSOM-ECOHAM to pro
vide a detailed description of the current state of the North
Sea in terms of its O2 conditions, and the processes leading
to low bottom O2 concentrations. The interpretation of the
model results will enable the following questions to be an
swered: what are the main drivers for the O2 dynamics in the
various subregions of the North Sea? Why are certain North
Sea regions more susceptible to low O2 conditions than oth
ers despite similar stratification patterns?
For this purpose, we first validate the simulated bottom O2
concentrations with respect to their temporal evolution and
spatial distribution in order to show that the model captures
the main features. Subsequently, we present a regional char
acterisation of the parameters controlling the bottom O2 dy
namics and propose a simple O2 deficiency index which ex
tends this characterisation to the entire North Sea. Finally, we
attribute the individual contributions of the governing pro
cesses to the temporal and spatial variability of the overall
O2 evolution, under particular consideration of the continu
ous O2 measurements at North Dogger (Greenwood et ah,
2010).
2 Material and methods
2.1 The ECOHAM model
Our study is based on the coupled physical-biogeochemical
model system HAMSOM-ECOHAM. The physical model
HAMSOM (HAMburg Shelf Ocean Model; Backhaus, 1985)
is a baroclinie primitive equation model using the hydrostatic
and Boussinesq approximation (Pohlmann, 1991). HAM
SOM provides the temperature (T) and salinity (S) distri
bution, in addition to the advective flow fields and the ver
tical turbulent mixing coefficient, which are used as forc
ing for the biogeochemical model ECOHAM (ECOsystem
model HAMburg). For a detailed description of HAMSOM
the reader is referred to Pohlmann (1991). Further informa
tion on the application of HAMSOM can be found in Back
haus and Hainbucher (1987) and Pohlmann (1996).
The biogeochemical model ECOHAM (Lorkowski et ah,
2012; Patsch and Klihn, 2008) represents the pelagic and
benthic cycles of carbon (C), nitrogen (N), phosphorus (P),
silicon (Si) and O2. The O2 module within the ECOHAM
model incorporates physical and biogeochemical processes
determining the pelagic O2 concentrations (Patsch and Ktihn,
2008).
The air-sea exchange of O2 at the sea surface constitutes
an important physical process besides the effects of advec
tive transport and vertical diffusion in the interior water col
umn. The air-sea flux of O2 in the present application is pa-
rameterised according to Wanninkhof (1992). In relation to
the biology, the O2 cycle is linked to the C cycle by pho
tosynthesis, zooplankton respiration and bacterial remineral
isation. While photosynthesis is a source of O2, zooplank
ton respiration and bacterial remineralisation act as O2 sinks.
