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 deficiency,
occur regularly in the North Sea. A major process regulating
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 primary
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 reveals
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-toyear
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 adjacent
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 characterised
by a cyclonic pattern mainly driven by the southward
Atlantic inflow across the shelf edge defining its northern
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 regional
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 extent
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 riverine
nutrient run-off, often reveal stratified conditions at subseasonal
timescales (e.g., Burt et ah, 2014).
In the 1980s, events of O2 deficiency reaching values below
3mg02L _1 occurred regularly in the stratified southeastern
central North Sea and in the German Bight (Brockmann
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 relation
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 anthropogenic
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, eutrophication
thus causes an enhanced supply of organic matter
sinking into the subsurface layer and reinforces O2 consumption
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 % reduction
of river nutrient loads (inorganic nitrogen and phosphorus)
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 classified
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. Therefore,
low O2 concentrations constitute a major indicator of
eutrophication (category 3 indicator, i.e., “evidence of undesirable
disturbance”; OSPAR-Commission, 2003) and concentrations
lower than 6mg02L _1 result in the classification
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, hypoxia
refers to the negative impact of low O2 concentrations
on organisms. An overview of the impact of hypoxia on marine
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 Maritime
and Hydrographic Agency, Germany). These monitoring
programmes provide daily time series of O2 and related
parameters (e.g., temperature, salinity, chlorophyll) in different
depths and allow for the analysis of the temporal evolu-
