‚an Leeuwen et al. 
DIN 
a Bm 
A DIP 
N 
10.3389/fmars.2023.1129951 
HS 
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Longitude [deg. E1 
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Lonaitude [deg. E] 
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“IGURE 9 
difference between the weighted ensemble results for the current state (CS) and the pre-eutrophic state (historic scenario, HS) for DIN (A), DIP 
B) and Chl (C). Green colours indicate areas where the pre-eutrophic levels were lower than those of the current state. Note that the colour bar 
>xtends to -5% only, indicatina areas where pre-eutrophic levels were sliaghtly hiaher than current levels. 
different formulations of ecological processes and exclude the 
influence of the underlying hydrodynamic model we compared 
relative changes in chlorophyll mean concentrations with relative 
changes in nutrient concentrations (Figure 12). Some models, such 
as the Deltares and RBINS models show decreases in Chl 
concentrations almost proportional to decreases in DIN 
concentrations (close to the black line). Other models such as the 
JRC model and Oldenburg model show a much smaller response in 
Chl concentrations to decreasing winter nutrient concentrations. 
Aere we see how a relative reduction in winter nutrient 
concentrations (pre-eutrophic state compared to current state, 
[CS-HS]/CS) induces a relative reduction in mean Chl. The 
relative reductions in winter DIN were in general stronger (up to 
75%) than the reductions in winter DIP (up to -55%) in the 
individual model results. Due to the already achieved P reduction 
measures between peak discharges in the 1980’s and the current 
state period, we observe smaller differences for winter DIP between 
pre-eutrophic and current loads in our model study. The 
corresponding reductions in mean Chl reach 60% at most. In 
many areas, the required reductions in nutrients to reach the pre- 
eutrophic mean Chl seem higher for DIN than for DIP. Although 
the relationship between winter nutrients and mean Chl is non- 
“rontiers in Marıne © zie.17e 
linear and perturbed by other factors like the underwater light 
climate, grazing and regeneration of nutrients, the results suggest 
that certain areas are less sensitive to further DIN reduction than 
others. These are mainly coastal areas where DIP is likely the 
limiting nutrient already, due to the successful reductions in 
riverine P loads (Billen et al., 2011). 
1 
Discussion 
4.1 Eutrophication on the European shelf 
We have presented our ensemble results for the pre-eutrophic 
state of marine waters on the European Shelf. This estimate of pre- 
eutrophic conditions follows the steps taken by OSPAR to move 
towards a harmonized and integrated eutrophication assessment 
across the North-East Atlantic, taking into account the Water 
Framework Directive (WFD; EC, 2000) and Marine Strategy 
Framework Directive (MSFD; EC, 2008), which require the 
definition of a common ‘baseline’ to which the eutrophication 
status of waters can be compared. The approach, to define 
ecologically relevant threshold levels for eutrophication indicators 
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