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drift using O2 concentrations determined from discrete wa 
ter samples to give an accuracy of 0.5 % (Greenwood et al., 
2010). For validation purposes the O2 data derived from the 
sensor at 85 m depth were used. North Dogger data are pub 
lished and can be accessed according to Greenwood et al. 
(2016). 
The BSH operates a continuous monitoring station at 
54° 10' N, 6°21 / E (see Fig. 2, region 1; hereafter referred to 
as station “Ems”). The O2 saturation is measured hourly us 
ing opto-chemical sensors (optodes). Sensors are located in 
6 and 30 m depth, respectively, and the bottom depth is 33 m. 
The applied sensors have a resolution of 0.03 mg O2 F _1 and 
an accuracy better than 0.26 mg 02 F -1 . Before deploying 
the sensors a 0-100 % calibration is conducted, and they are 
re-calibrated after operation to quantify any drift. In addition, 
a regular on-site validation takes place using a calibrated fast 
optode (accuracy of ±2 %) or by applying the Winkler titra 
tion (accuracy better than ±1 %). 
2.3.2 Spatially resolved “snapshot” data - the North 
Sea programme 
During the North Sea programme, carried out by the Royal 
Netherlands Institute for Sea Research (NIOZ) with support 
from the Dutch Science Foundation (NWO) and the Euro 
pean Union, the North Sea was sampled from 18 August 
to 13 September 2001, and from 17 August to 5 Septem 
ber 2005 and 2008. The North Sea was covered by an 
approximate 1° x 1° grid, sampling approximately 90 sta 
tions in each of the years (Bozec et ah, 2005, 2006; Salt 
et ah, 2013). During each cruise, a total of 750 water sam 
ples were collected for dissolved O2. In 2001, the O2 con 
centrations were determined by the Winkler titration using 
a potentiometric end-point determination with an accuracy 
of ±2pmol02kg _1 (less than ±0.07 mg 02 F -1 depending 
on T and S). In 2005 and 2008, the O2 concentrations 
were obtained applying the spectrophotometric Winkler ap 
proach with a precision of less than 0.03 mg 02 F -1 . A de 
tailed description of the measurement system used is given 
in Reinthaler et al. (2006). The data for the years 2001 and 
2005 have been published and can be accessed according to 
Thomas et al. (2012) and Thomas and Borges (2012), respec 
tively. 
The data available were gridded to the model grid (Fig. 2). 
In the case of multiple measurements for the same model grid 
cell and date, the average of these measurements was used 
for validation. To compare our model results to these data, 
we calculated the averages and standard deviations of our 
simulation over the observation period of the corresponding 
year. 
2.4 Deriving a regional O2 characterisation of the 
North Sea 
2.4.1 Identification of the key parameters 
For the development of a regional O2 characteristic, potential 
controlling factors were analysed in relation to bottom O2. 
Besides stratification, eutrophication is considered as a ma 
jor driver for developing low O2 conditions (e.g., Diaz and 
Rosenberg, 2008; Kemp et ah, 2009). Thus, primary produc 
tion within the mixed layer and the resulting organic matter 
export into the layers below the MFD must be considered to 
be the main source for degradable organic matter. In addition, 
organic matter can be advected from surrounding waters in 
the form of phyto- or zooplankton and detritus, subsequently 
sinking out of the mixed layer. 
Another important criterion is the water volume below the 
thermocline (Druon et al., 2004). A smaller volume separated 
from the surface due to stratification holds a lower initial in 
ventory of O2 than a larger volume even though concentra 
tions can be similar or even higher in the smaller volume. 
Thus, our set of 02-related characteristics consists of mixed 
layer primary production (PP m id), horizontal advection of or 
ganic matter into and out of the mixed layer (ADH or g m and 
ADH 0 |g 0U i; including phyto-/zooplankton and detritus), ver 
tical organic matter export below the MFD (EXP org ; only de 
tritus) and mixing of O2 below the MFD (MIXo 2 ), and the 
sub-MFD volume V su b. 
To detect regional characteristics within the North Sea 
area, we defined four different sub-domains encompassing 
4x4 model water columns each (see Fig. 2, red boxes): 
(A) southern North Sea (SNS) under strong tidal influence, 
(B) southern central North Sea (SCNS) with high year-to- 
year variability in stratification, (C) northern central North 
Sea (NCNS) with a dominant summer stratification each 
year, and (D) northern North Sea (NNS) with a dominant 
summer stratification each year and a strong influence of 
the Atlantic. For all these regions, the parameters described 
above were calculated for the years 2000-2012 relative to 
a reference depth D re f, which is defined as the bottom depth 
of the model layer directly below the annual maximum MFD 
among all four regions. We decided to use a D re f > MFD to 
ensure that for the different regions all parameters were de 
termined on a comparable level. This implies that the values 
for PPmid. ADHoig in and ADH org oU | are integrated from the 
surface to D re f, whereas EXP org and MIXo 2 are the vertical 
fluxes through D re f. The same D îe f was applied to all regions, 
but year-to-year variations were allowed. 
To determine the annual maximum MFD, we first calcu 
lated the stratification period for the 4x4 regions B-D using 
Eq. (1). Region A was excluded from this calculation as no 
persistent MFD developed due to tidal mixing. In this con 
text, SstratM of a region is only 1 if /»stratCU y,t) — 1 for all 
16 water columns within a 4 x 4 region. The daily MFD for 
each water column within a region was calculated by apply-