van Leeuwen et al.
10.3389/fmars.2023.1129951
autrophic conditions compared to current conditions (except in the Meuse
Alume and Seine Plume areas). Chlorophyll concentrations were estimated to
be as much as „40% lower in some areas, as were dissolved inorganic
phosphorus levels. Dissolved inorganic nitrogen levels were found to be up to
650% lower in certain assessment areas. The weighted average approach reduced
model disparities, and delivered pre-eutrophic concentrations in each
assessment area. Our results open the possibility to establish reference values
for indicators of eutrophication across marine regions. The use of the new
assessment areas ensures local ecosystem functioning is better represented
while political boundaries are largely ignored. As such, the reference values are
‚Ess associated to member states boundaries than to ecosystem boundaries.
<EYWORDS
autrophication, North Sea, OSPAR ICG-EMO, DIN, DIP, nutrients, chlorophyll
3cosystem modelling
ı Introduction
Nutrient inputs into the marine environment predominantly
come from riverine inputs, direct discharges and atmospheric
deposition. Elevated nutrient concentrations may lead to
ındesirable increases in primary production, and subsequent
degradation of the sinking organic matter can lead to oxygen
deficits near the seafloor (Diaz and Rosenberg, 2008; Greenwood
et al., 2010; Große et al., 2016). This process, called eutrophication,
is related to an increase in nutrient loads from anthropogenic
sources (Jickells, 1998; Nixon, 2009). Additional symptoms of
marine eutrophication include harmful algae blooms (Schoemann
et al., 2005; Riegman et al., 1992) and loss of seagrasses (Burkholder
et al, 2007), resulting in qualitative changes in the local marine food
web. The smelly foam on beaches left in the wake of Phaeocystis
blooms are well known to the general public and tourist’s industries,
but toxins released by some algae blooms also directly threaten
numan economic interests and human life, usually via
(consumption of) affected marine resources (Berdalet et al., 2016).
Eutrophication effects became increasingly evident in the North
Sea around 1980, and it was broadly recognized that this phenomenon
was related to anthropogenic sources. The regional sea convention for
:he North-East Atlantic OSPAR (www.ospar.org) defined
eutrophication as “the enrichment of water by nutrients causing an
accelerated growth of algae and higher forms of plant life to produce an
undesirable disturbance to the balance of organisms present in the
water and to the quality of the water concerned, and therefore refers to
he undesirable effects resulting from anthropogenic enrichment by
nutrients” (OSPAR, 1998, p. 53), confirming the cause-effect
zelationship with anthropogenic sources. Following the early
evidence of eutrophication, OSPAR applied a source-oriented
approach since 1988, through limiting inputs of nutrients and
organic matter to levels that do not give rise to adverse effects on the
marine environment. The proposed reduction was very successful for
phosphorus (which is caused mainly by point-sources, e.g. sewage) but
less so for nitrogen (caused mainly by diffuse sources, e.g. agriculture)
"rontiers in Marıne _
öl» 8
‚Claussen et al, 2009; Conley et al., 2009). As a result, though
eutrophication effects have declined since their 1980’s peak, they are
even now persistent in many western European coastal areas. The latest
application of OSPAR’s Common Procedure (COMP3, OSPAR, 2017)
still identified large parts of the southern North Sea along the Belgian,
Dutch, German and Danish coasts as so-called “problem areas” or
“potential problem areas” with respect to eutrophication, with smaller
areas along the French and British coasts also characterized as such.
The Kattegat was also defined as a “problem area”, as were smaller
parts along the Swedish and Norwegian coasts.
Recovery from a eutrophic state can be a lengthy process (>
decades, McCrackin et al., 2017), and does not always lead to the
ecological state observed before eutrophication occurred (Duarte
et al., 2008; Oguz and Velikova, 2010). It is therefore critical to
establish appropriate restoration goals for eutrophied areas
(McCrackin et al., 2017). The objective of the presented work is
to quantify the pre-eutrophic state of the Northwest European Shelf,
based on an ecosystem modelling ensemble approach applied by
OSPAR’s ICG-EMO (Intersessional Correspondence Group on
Ecosystem Modelling). Here the pre-eutrophic state is defined as
the situation around the year 1900, and is by no means a pristine or
anthropogenically undisturbed state. To account for regional
differences in performance between the models, the ensemble
mean applies a weighting method (Almroth & Skogen, 2010)
based on the level of agreement between current model
simulations and current observations. These weights are then
applied to construct the ensemble-simulated pre-eutrophic state,
thus providing a sophisticated estimate of the mean pre-eutrophic
concentrations, which can serve as baseline for eutrophication
assessments. This paper presents the harmonized modelling
approach, the applied ensemble weighting method, the underlying
assumptions, as well as the resulting estimates of pre-eutrophic
nutrient and phytoplankton concentrations. These values can
support the elaboration of policy thresholds for eutrophication
that are coherent across national boundaries. We demonstrate
that an ensemble modelling approach can help to define pre-
frontiersin.org