2512 
F. Große et al.: Looking beyond stratification 
Biogeosciences, 13, 2511-2535, 2016 
www.biogeosciences.net/13/2511/2016/ 
1 Introduction 
Low oxygen (O2) conditions (concentrations < 6 mg O2 L _ 1 ; 
OSPAR-Commission, 2003), often referred to as O2 defi 
ciency, occur regularly in the North Sea. A major process reg 
ulating the seasonal dynamics of bottom O2 is the occurrence 
and duration of thermal stratification (e.g., Greenwood et ah, 
2010; O’Boyle and Nolan, 2010), which limits the vertical 
exchange of O2 between the oxygenated surface layer and the 
deeper layers. In combination with events of enhanced pri 
mary production, and the subsequent degradation of organic 
matter, this favours the evolution of O2 deficiency (e.g., Diaz 
and Rosenberg, 2008). Although the northern North Sea re 
veals strongest stratification, lowest O2 concentrations occur 
in the central North Sea, which is shallower and where the 
duration of stratification is shorter and shows highest year-to- 
year variability. Thus, one can argue that stratification is an 
important prerequisite for O2 deficiency, but its severity and 
duration is controlled by the complex interaction between the 
hydrodynamical condition and the biogeochemical processes 
involved. 
The North Sea is a temperate, semi-enclosed shelf sea ad 
jacent to the northeastern Atlantic ocean. It has an average 
depth of about 90 m (Ducrotoy et ah, 2000) with northward 
increasing bottom depth. The North Sea circulation is char 
acterised by a cyclonic pattern mainly driven by the south 
ward Atlantic inflow across the shelf edge defining its north 
ern boundary. Lenhart and Pohlmann (1997) showed that 
about 85 % of the incoming Atlantic water is recirculated 
north of the Dogger Bank, a shallow area with water depth 
less than 40 m and 300 km of zonal extent at about 55° N, 
2° E (Kroncke and Knust, 1995). The circulation south of the 
Dogger Bank is governed by the inflow through the English 
Channel and follows the continental coast. At the southern 
tip of Norway it joins the Norwegian coastal current leaving 
the North Sea at its northern boundary. 
Stratification in the North Sea reveals some substantial re 
gional differences. While the shallower southern parts are 
permanently well-mixed due to the strong influence of the 
M2 tidal component (Otto et ah, 1990), the deeper parts north 
of 54° N reveal seasonal, mostly thermal stratification (e.g., 
Burt et ah, 2014; Pingree et ah, 1978; van Leeuwen et ah, 
2015). Seasonal haline stratification occurs to a lesser ex 
tent along the Norwegian coast. The transition between these 
permanently mixed and seasonally stratified regions occurs 
gradually (Pingree et ah, 1978). In consequence, even areas 
relatively near to the coast, which are affected by high river 
ine nutrient run-off, often reveal stratified conditions at sub- 
seasonal timescales (e.g., Burt et ah, 2014). 
In the 1980s, events of O2 deficiency reaching values be 
low 3mg02L _1 occurred regularly in the stratified south 
eastern central North Sea and in the German Bight (Brock- 
mann and Eberlein, 1986; Brockmann et ah, 1990; Rachor 
and Albrecht, 1983). During that time, the problem of low O2 
conditions in the North Sea reached public awareness in rela 
tion to eutrophication as demersal animals died across a large 
area due to these low O2 concentrations (von Westernhagen 
et ah, 1986). Eutrophication, or in other words, high anthro 
pogenic nutrient loads mainly supplied by rivers (Brockmann 
et ah, 1988; Jickells, 1998; Rabalais et ah, 2010), may raise 
the ambient nutrient concentrations followed by an increase 
in biomass production. Under given physical conditions, eu 
trophication thus causes an enhanced supply of organic mat 
ter sinking into the subsurface layer and reinforces O2 con 
sumption near the sea floor due to bacterial remineralisation. 
Even though the second International Conference on the 
Protection of the North Sea (INSC-2) prescribed a 50 % re 
duction of river nutrient loads (inorganic nitrogen and phos 
phorus) in order to mitigate the effects of eutrophication 
(de Jong, 2006), Fig. 1 shows that O2 deficiency remains 
a persistent problem in the North Sea up to the present day. 
According to Kemp et al. (2009) these events can be classi 
fied as “persistent seasonal”. Low bottom O2 concentrations 
may cause death of benthic organisms or fish eggs as well 
as avoidance of the affected areas by benthic species. There 
fore, low O2 concentrations constitute a major indicator of 
eutrophication (category 3 indicator, i.e., “evidence of unde 
sirable disturbance”; OSPAR-Commission, 2003) and con 
centrations lower than 6mg02L _1 result in the classifica 
tion as “problem area” in terms of eutrophication within the 
OSPAR assessment (Claussen et ah, 2009). In the present 
study the term “oxygen deficiency” is used in this OSPAR 
context rather than “hypoxia”. While O2 deficiency is clearly 
defined within OSPAR by the 6mg02L _1 threshold, hy 
poxia refers to the negative impact of low O2 concentrations 
on organisms. An overview of the impact of hypoxia on ma 
rine biodiversity can be found in Vaquer-Sunyer and Duarte 
(2008). Further descriptions on the ecological disturbance of 
different levels of low O2 concentrations are summarised by 
Friedrich et al. (2014) and Topcu et al. (2009). 
Despite the relevance of the bottom O2 concentrations for 
the assessment of the ecological status of an ecosystem, O2 
measurements are sparse and either temporally or spatially 
limited. In addition, it is difficult to place the measurement at 
the right time and location to obtain a comprehensive picture 
of the duration and spatial extent of summer O2 deficiency 
(Friedrich et ah, 2014). One way to address this problem is to 
analyse the representativeness of available data with respect 
to eutrophication assessment (Brockmann and Topcu, 2014). 
Only in recent years continuous measurements for the 
North Sea have become available by, e.g., the SmartBuoy 
programme of Cefas (Centre for Environment, Fisheries and 
Aquaculture Science, UK; Greenwood et ah, 2010) or the 
MARNET programme (MARine Monitoring NETwork in 
the North Sea and Baltic Sea) of the BSH (Federal Mar 
itime and Hydrographic Agency, Germany). These monitor 
ing programmes provide daily time series of O2 and related 
parameters (e.g., temperature, salinity, chlorophyll) in differ 
ent depths and allow for the analysis of the temporal evolu-