2530
F. Große et al.: Looking beyond stratification
Biogeosciences, 13, 2511-2535, 2016
www.biogeosciences.net/13/2511/2016/
This study demonstrated that ecosystem models are capa
ble of describing the key features of the O2 dynamics as an
integral part of the North Sea ecosystem. This, in combina
tion with the provision of a spatially and temporally consis
tent picture is useful for the detection of regions susceptible
to low O2 conditions which therefore require enhanced man
agement. Additionally, this is of importance for monitoring
authorities as our model showed that bottom O2 measure
ments taken in late summer provide a synoptic picture of the
North Sea O2 conditions.
This study provides a general characterisation and
process-based analysis of the North Sea O2 dynamics in its
present state, including the actual eutrophication status in the
“continental coastal region” as defined for the OSPAR as
sessment (Claussen et ah, 2009). The question on the anthro
pogenic contribution to the O2 deficiency problem in rela
tion to elevated nutrient supply is beyond the scope of this
study. However, the capability of three-dimensional models
to describe the O2 dynamics in the context of the natural
variability of the ecosystem can be related to changes in an
thropogenic drivers, such as increased atmospheric deposi
tion (Troost et ah, 2013) or riverine nutrient input (Lenhart
et al., 2010).
Similar model studies are essential for the assessment
within the Water Framework Directive (WFD), in which dis
solved O2 is used as a key parameter (Best et ah, 2007). As
the WFD assessment depends strongly on the description of
pristine conditions, related to natural nutrient levels (Topcu
et al., 2009), ecosystem models can provide a consistent pic
ture of the North Sea O2 dynamics under these pristine con
ditions and of the effects of WFD reductions (Schemewski
et al., 2015). As river load reductions within the WFD reg
ulation affect the entire North Sea ecosystem, also in terms
of bottom O2 conditions, these scenarios can also be inter
preted within the frame of the Marine Framework Directive
(MSFD), which involves O2 as one of the main descriptors
for the definition of the “Good Environmental Status”.
Recent observational studies explicitly highlight the im
portance of organic nutrient loads on the O2 dynamics in
the context of nutrient reductions (Kemp et al., 2009; Topcu
and Brockmann, 2015). Thus, future modelling studies on
the effects of nutrient reductions on the marine environment
should differentiate between the effects of organic and in
organic nutrient inputs in order to optimise measures in the
catchment area with respect to cost efficiency.
Diaz and Rosenberg (2008) report an exponential expan
sion of global O2 deficiency (and hypoxia) since the 1960s
and argue that future changes in O2 conditions will strongly
depend on the effects of climate change on stratification and
riverine nutrient supply. For the North Sea, several model
studies predict a rise in water temperature (e.g., Lowe et al.,
2009; Meire et al., 2013; Mathis and Pohlmann, 2014) which
will reduce the O2 solubility (Weston et al., 2008). Stratifica
tion intensity may either increase (Lowe et al., 2009; Meire
et al., 2013) or even decrease (Mathis and Pohlmann, 2014),
implying opposed effects on bottom O2. Primary production
could increase due to enhanced nutrient supply caused by
changes in weather conditions (Rabalais et al., 2010) or due
to a temperature-driven increase in metabolic rates (van der
Molen et al., 2013), which could eventually aggravate the
O2 conditions (Justic et al., 2003). In contrast, Groger et al.
(2013) predicted a North Sea wide reduction in primary pro
duction by about 30 % due to reduced winter nutrient import
from the Atlantic. As these potential changes in the O2 condi
tions will also affect the biocoenosis of the North Sea (Emeis
et al., 2015), it is important to foster the analysis of potential
impacts of climate change and changes in nutrient loads on
the O2 dynamics.
Data availability
The time series data from the Cefas station North Dogger can
be accessed via the Cefas Data Hub (https://www.cefas.co.
uk/cefas-data-hub/) according to Greenwood et al. (2016).
The time series data of MARNET station Ems can be re
trieved from the Deutsches Ozeanographisches Datenzen-
trum (DOD Data Centre) via email query to dod@bsh.de.
The spatially resolved data from the North Sea cruises in
2001 and 2005 have been released in the framework of the
EU-FP6 project CARBOOCEAN. These data can be ac
cessed via the CARBOOCEAN data portal (http://dataportal.
carboocean.org/) according to Thomas et al. (2012) and
Thomas and Borges (2012), respectively. The data of the
North Sea cruise 2008 (R/V Pelagia 64PE294; Zemmelink,
2008) have not been published, yet, but can be requested via
the CODIS data portal (http://www.nioz.nl/portals-en; regis
tration required).
