Deriving pre-eutrophic conditions from an ensemble model approach for the North-West European seas

Abstract

The pre-eutrophic state of marine waters is generally not well known, complicating target setting for management measures to combat eutrophication. We present results from an OSPAR ICG-EMO model assessment to simulate the pre-eutrophic state of North-East Atlantic marine waters. Using an ecosystem model ensemble combined with an observation-based weighting method we derive sophisticated estimates for key eutrophication indicators. Eight modelling centres applied the same riverine nutrient loads, atmospheric nutrient deposition rates and boundary conditions to their specific model set-up to ensure comparability. The pre-eutrophic state was defined as a historic scenario of estimated nutrient inputs (riverine, atmospheric) at around the year 1900, before the invention and widespread use of industrial fertilizers. The period 2009-2014 was used by all participants to simulate both the current state of eutrophication and the pre-eutrophic scenario, to ensure that differences are solely due to the changes in nutrient inputs between the scenarios. Mean values were reported for winter dissolved inorganic nutrients and total nutrients (nitrogen, phosphorus) and the nitrogen to phosphorus ratio, and for growing season chlorophyll, chlorophyll 90th percentile, near-bed oxygen minimum and net phytoplankton production on the level of the OSPAR assessment areas. Results showed distinctly lower nutrient concentrations and nitrogen to phosphorus ratio’s in coastal areas under pre-eutrophic conditions compared to current conditions (except in the Meuse Plume 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 60% 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 less associated to member states boundaries than to ecosystem boundaries.

Validation of the coupled physical-biogeochemical ocean model NEMO-SCOBI for the North Sea-Baltic Sea system

“…Importantly, a variety of models simulating the biogeochemistry in similar areas should be used together, as they all differ significantly in their biogeochemical complexity and have different performing skills. The use of ensemble mean assessments have been shown to be good or even better than the results from individual models Eilola et al, 2011a;van Leeuwen et al, 2023. We have shown that the NEMO-SCOBI model can be use to derive relevant indicators for HELCOM and OSPAR initiatives and to produce climate projection for the Baltic Sea-North Sea system, for which specific relevant improvements will be applied. Large model bias, especially that of nitrate, are also strongly linked to the applied forcing, which is continuously updated depending of available data and/or down scaling methods.…”

“…Nonetheless, our results are comparable to those of Daewel and Schrum (2013), who show an overall good agreement with observations in the North Sea, but with a southern coast of the and heavily impacted by runoff and nutrient input that results in a high spatio-temporal variability. This remains a challenge for most model to recreate as also highlighted in a recent model ensemble study (van Leeuwen et al, 2023).…”

A first ecological coherent assessment of eutrophication across the North-East Atlantic waters (2015–2020)

“…The coherence was the development of an ensemble model approach used to derive pre-eutrophic conditions (Lenhart et al, 2022;van Leeuwen et al, 2023). Nutrient loads into the NE Atlantic were estimated from rivers under reference conditions, using the European model E-HYPE and observations (Stegert et al, 2021;Lenhart et al, 2022).…”

“…Nutrient loads into the NE Atlantic were estimated from rivers under reference conditions, using the European model E-HYPE and observations (Stegert et al, 2021;Lenhart et al, 2022). In the ensemble model approach, nutrient concentrations in the North-East Atlantic were then estimated under pre-eutrophic conditions simulating a period prior to the Haber Bosch process (i.e., no mineral fertilizers) and including a reasonably modern population density such as cities like London and Hamburg with sewage infrastructure and combining the nutrient loads from E-HYPE with ecosystem models (Stegert et al, 2021;Lenhart et al, 2022;van Leeuwen et al, 2023). The chlorophyll concentrations were estimated corresponding to the estimated nutrient concentrations under reference conditions.…”

“…Assessment seasons are specific for the different assessment parameters. Whilst OSPAR regions span a wide and climatically variable area, agreement was made on a common assessment season for COMP4 based on the modeling work to derive thresholds (van Leeuwen et al, 2023).…”