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DIN 
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Longitude [deq. E] 
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Current state 
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“IGURE 7 
Neighted ensemble results for the current state (2009-2014 average), for surface DIN (A), DIP (B) and Chl (C). 
3.4 Differences between current and pre- 
eutrophic state 
The difference between the ensemble mean values for the pre- 
eutrophic state and the current state exhibits up to 50 - 60% less 
dissolved inorganic nutrients in the coastal zones in the pre- 
eutrophic state (Figure 9). Almost no changes are observed in 
oceanic areas, with at most a 1% difference. Note that in general a 
decrease in nutrient input can lead to local increases in some 
dissolved nutrients, as the input reduction of the limiting nutrient 
will decrease primary production, thus reducing nutrient uptake 
and causing a possible local increase in non-limiting nutrients. DIN 
ıevels were up to 62% lower in the pre-eutrophic state than in the 
current state, particularly along the Dutch and German coast. Both 
DIP and Chl concentrations were up to 40% lower in the pre- 
eutrophic state than in the current state. In contrast there is no effect 
of the DIP concentration within most of the Channel area and the 
coastal region of France while the difference for Chl lies around 20% 
in the Channel area and increases at the French coast. Also for the 
Eastern North Sea area, east of the Dogger Bank, the difference 
petween the two simulations is higher for Chl than for DIP, but still 
‚ower than for DIN. 
"rontiers in Marine science 
Other eutrophication effects include N:P ratio changes, 
increased net primary production and low oxygen levels near the 
bed through remineralization of excess organic material by bacteria. 
Figure 10 shows the differences for the unweighted (due to lack of 
observations) ensemble mean differences for these eutrophication 
related phenomena. Both the N:P ratio and net primary production 
were much smaller for the pre-industrial state in the coastal zones 
(maxima of 35% and 30% respectively) than in the current state. 
Note that the net primary production reported here relates 
predominantly to pelagic production, as the models do not 
include macrophytes. Pre-eutrophic conditions are known for 
extensive macrophyte presence (Nienhuis, 1996), and would thus 
be characterized by a larger benthic primary production 
contribution. Near bed oxygen levels were higher in the Southern 
Bight of the North Sea, the English Channel area and the coastal 
parts of the Bay of Biscay under pre-eutrophic conditions. The 
Meuse plume is the only area where near bed O, levels were slightly 
lower (<1%) in the historic state. Oxygen values in the Bay of Biscay 
are dominated by 1 of only 2 contributing models: without 
weighting due to lack of observations outlier values can have 
disproportionate influence. The high value for O, difference in 
the Gironde Plume (GDPM, -50.7%) is deemed artificial, and this 
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