van Leeuwen et al.
rivers. For P loads, E-HYPE percentage results were used for most
vers, but for some rivers alternative estimates were used. Due to
‚arge differences between E-HYPE and the finer scale catchment
models, and uncertainty in the E-HYPE P load estimation
‚Donnelly et al., 2013; Stegert et al., 2021), its historic P load
percentages were replaced for rivers where alternative, more
detailed information was available, as follows. The Danish
authorities, basing their estimates on Timmermann et al. (2021)
set their pre-eutrophic P loads at 36% of current day loads for all
Danish rivers. The German authorities opted to use the MONERIS
results for P loads in German rivers. In bilateral negotiations with
‚he Netherlands, Dutch riverine P loads of rivers arriving through
German territory (or severely interlaced with such rivers) were
adjusted to reflect the MONERIS results (Rhine, Meuse, Lake
IJssel). Note that E-HYPE historical nutrient levels were not used,
only the E-HYPE estimate of the percentage change in riverine
nutrients compared to current day loads. E-HYPE coastal areas
were then linked to actual rivers, and the CS and HS riverine loads
were derived from the observation-based ICG-EMO riverine
database for 2006-2014, using 100% and the reduction percentage
estimates, respectively. The reduction percentages are shown in
Figure 4, while Appendix A provides the same information as a
table. No change was applied to rivers with pre-eutrophic loads
higher than current loads (mainly Scottish rivers north of Inverness
where populations have declined), in order to preserve reduction
effects from other rivers. River freshwater discharges were kept at
current day levels and therefore are equal to those of the simulated
period: this choice was made to allow for easier definition of
(achievable) nutrient reductions in the current situation.
Estimates of atmospheric nitrogen deposition rates around 1900
were calculated based on the trends in TOxN and NH; emissions
Historic Scenario N levels
Historic level
3$ „urrent day
100
Y
10.3389/fmars.2023.1129951
estimated by Schöpp et al. (2003) over Europe, including its
marginal seas. These trends were then used to estimate the
spatially resolved nitrogen deposition rate estimates by EMEP
\2020) for the years 1890-1900 following the method of Große
et al. (2016). Table 2 shows the current and pre-eutrophic
atmospheric deposition rates estimated by Schöpp et al. (2003),
and their ratio. These historic/current ratios are applied to current
deposition fields from EMEP to estimate historic atmospheric
deposition rates. Atmospheric phosphorous deposition rates were
deemed negligible, both in the current state and historical scenario.
For the nutrient inputs across model open boundaries, we
assumed that boundaries to the open sea were sufficiently far
away from riverine sources to not be affected by nutrient
reductions, and these were kept the same for CS and HS. For the
Baltic boundary a different approach was taken, as the Baltic is
highly eutrophic. As such, the pre-eutrophic boundary should
reflect the historic nutrient status at the Darss sill and Drogden
sill. Reduction percentages for nutrients at these locations were
derived from a long model simulation (1850 - 2008) with the
ERGOM model provided by Thomas Neumann (IOW, Germany).
The resulting historic percentages (compared to current-day loads)
are given in Table 3.
2.7 Weighted ensemble method
As models have varying skills in different areas, and variables,
we applied the weighted ensemble approach of Almroth and Skogen
2010) to calculate ensemble averages. This method uses
observations to determine a model’s skill in representing a certain
variable in a certain area, and assigns appropriate weights to the
. . . Historic level
Historic Scenario P levels % of current day
65
ar
5}
E
a
1
50
rh
Pe
u
3
U
Longitude [deg. LE
1Ur
P
k
vv
Longitude [deg.
FIGURE 4
?re-eutrophic riverine loads as percentages of current day loads. Left: pre-eutrophic N loads, right: pre-eutrophic P loads.
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