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Full text: Deriving pre-eutrophic conditions from an ensemblemodel approach for the North-West European seas

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- 
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