van Leeuwen et al. 
observational data gathering, the applied approach, negotiations and 
overall methodology. Therefore their submitted current state results are 
‚ncluded in Appendix F, but are not included in the presented analysis. 
2.5 Current state scenario 
The current state scenario (hereafter, CS) applied the ICG-EMO 
database of European rivers for the riverine nutrient inputs of the 
selected period. This database contains daily values for flow and 
autrients for 368 rivers discharging onto the European Shelf, 
tollowing optimization to daily values from originally sourced 
observational data (Lenhart et al., 2010; ICG-EMO, 2021). For an 
overview of the rivers included see Appendix A (Supplementary 
Materials). The average atmospheric deposition rates for NO, NH3z 
and Total Nitrogen over the North Western Continental Shelf as 
estimated by EMEP (EMEP, 2020) were used for atmospheric input 
of nutrients, with values provided in Table 2. 
For the open sea boundaries, information from CMEMS 
(Copernicus Marine Service, EU, https://marine.copernicus.eu/) was 
used: NORTHWESTSHELF_REANALYSIS_PHY_004_009 (https:// 
doi.org/10.48670/moi-00059) for the physical requirements and 
NORTHWESTSHELF_REANALYSIS_BIO_004_011 (https:// 
doi.org/10.48670/moi-00058) (Ciavatta et al., 2018) for the 
chemical and biological state variables (both 0.067 x 0.111 degree 
spatial resolution with 24 depth levels). The exchange between the 
Baltic Sea and North Sea in the Kattegat and Skagerrak area is very 
complex (deep stratified waters, different layers flowing in different 
directions). To simplify the inflow of nutrient-rich Baltic waters into 
:he North Sea the boundaries of the participating models were 
selected to be at two shallow sills where flow patterns are less 
complicated: Darss sill and Drogden sill. Estimates from simulated 
Baltic Sea discharges by DHI for recent years provided monthly mean 
climatologies for the water flows at the two sills. For nutrient 
concentrations recent observations (2009 —- 2014) near the sills 
were used to provide monthly mean climatologies (flow and 
autrients: Stiig Markager, pers. comm.). For silicate, an annual 
mean estimate of 10.4 uM was used (Mantikci, 2014). 
2.6 Pre-eutrophic scenario 
The pre-eutrophic or historical scenario (hereafter, HS) should 
‚eflect the state of European Shelf marine waters before major 
anthropogenic nutrient inputs occurred. Here, we follow the 
TABLE 2 Average N deposition rates for the current (2009-2014) and 
historic (1890-1900) time periods over the North Western Continental 
Shelf and the respective ratios (Schöpp et al., 2003). 
TOxN 183.57 
NH; 167.49 
TotalN 351.07 
U... 4 
105 7° 
0.63 
„32.2 
0.38 
"rontiers in Marine Science 
10.3389/fmars.2023.1129951 
definition of the project Joint Monitoring Programme of the 
Eutrophication of the North Sea with Satellite data (JMP- 
EUNOSAT, Enserink et al., 2019) which uses a period around the 
year 1900 during the European industrialization but before 
agricultural intensification. In the 19” century there were likely 
already first signs of eutrophication in freshwater systems and 
coastal waters (e.g. Billen & Garnier, 1997, Billen et al., 1999), but 
impacts in coastal waters were probably limited to a more local scale 
(Nixon, 2009). Most importantly, the end of the ya 
centuryprecedes the establishment of the Haber-Bosch process 
that industrialized the production of inorganic nitrogen fertilizers 
(first demonstrated in 1909 with first industrial-level production 
starting in 1914, Kissel, 2014). Furthermore, anecdotal evidence of 
high-water transparency and seagrass coverage (two important 
quality indicators for eutrophication effects, Reise and Kohlus 
(2008)) indicate good water quality status in the coastal waters of 
the German Bight during this period (Brockmann et al., 2002). In 
the closely connected Baltic Sea, the same time period is used as a 
reference (Schernewski and Neumann, 2005), as in the Kattegat and 
the Belt Seas evidence exists of extensive macrophyte fields around 
1900 (Krause-Jensen et al., 2021), which severely declined due to 
disease and eutrophication. Frederiksen et al. (2004) show further 
evidence of eelgrass decline in Danish coastal waters since 1940 
following increasing nutrient pressures. 
The JMP-EUNOSAT project applied pre-eutrophic load 
estimates from a dedicated simulation of the watershed model E- 
HYPE (see https://hypeweb.smhi.se/for the HYPE model suite, with 
E-HYPE the European application), representing conditions 
around the year 1900 (Enserink et al., 2019). These loads, which 
did not include hydrological or morphological changes in river 
basins (e.g. reservoir construction, dams and barriers, etc.), are 
simulated per coastal area and are not necessarily associated with 
actual rivers. Nevertheless, this dataset provides a consistent set of 
pre-eutrophic nutrient loads going into the marine environment on 
the European Shelf. Local, more detailed studies offer additional 
information. Kerimoglu et al. (2018) describe historic riverine loads 
for the German Bight based on simulations of the detailed 
catchment model MONERIS (Venohr et al., 2011; Gadegast & 
Venohr, 2015). They found significantly lower historical DIP 
levels for major German rivers compared to the E-HYPE 
historical scenario. Danish authorities commissioned a similar 
study where two independent water quality models (one Bayesian, 
one mechanistic) simulated undisturbed conditions for Danish 
:ivers (Timmermann et al., 2021). This study found differences in 
historical coastal DIP loads (compared to JMP-EUNOSAT) up to 
„10%. Stegert et al. (2021) used estimates of historical river inputs 
from both MONERIS and E-HYPE and compared their influence 
on the nutrient and chlorophyll-a concentrations in the North Sea. 
They found higher marine nutrient concentrations, particularly in 
coastal zones, if the E-HYPE values were applied, with coastal zone 
DIP differences of 40% in the German Bight. 
An expert group consisting of, amongst others, members from 
LCG-EMO and ICG-Eut (Intersessional Correspondence Group on 
Eutrophication) defined the pre-eutrophic scenario as using the E- 
HYPE historic N load percentages (E-HYPE estimate of percentage 
difference between the historic state and current day loads) for all 
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