Effects of Nutrient Management Scenarios on Marine Eutrophication Indicators: A Pan-European, Multi-Model Assessment in Support of the Marine Strategy Framework Directive

Abstract

A novel pan-European marine model ensemble was established, covering nearly all seas under the regulation of the Marine Strategy Framework Directive (MSFD), with the aim of providing a consistent assessment of the potential impacts of riverine nutrient reduction scenarios on marine eutrophication indicators. For each sea region, up to five coupled biogeochemical models from institutes all over Europe were brought together for the first time. All model systems followed a harmonised scenario approach and ran two simulations, which varied only in the riverine nutrient inputs. The load reductions were evaluated with the catchment model GREEN and represented the impacts due to improved management of agriculture and wastewater treatment in all European river systems. The model ensemble, comprising 15 members, was used to assess changes to the core eutrophication indicators as defined within MSFD Descriptor 5. In nearly all marine regions, riverine load reductions led to reduced nutrient concentrations in the marine environment. However, regionally the nutrient input reductions led to an increase in the non-limiting nutrient in the water, especially in the case of phosphate concentrations in the Black Sea. Further core eutrophication indicators, such as chlorophyll-a, bottom oxygen and the Trophic Index TRIX, improved nearly everywhere, but the changes were less pronounced than for the inorganic nutrients. The model ensemble displayed strong consistency and robustness, as most if not all models indicated improvements in the same areas. There were substantial differences between the individual seas in the speed of response to the reduced nutrient loads. In the North Sea ensemble, a stable plateau was reached after only three years, while the simulation period of eight years was too short to obtain steady model results in the Baltic Sea. The ensemble exercise confirmed the importance of improved management of agriculture and wastewater treatments in the river catchments to reduce marine eutrophication. Several shortcomings were identified, the outcome of different approaches to compute the mean change was estimated and potential improvements are discussed to enhance policy support. Applying a model ensemble enabled us to obtain highly robust and consistent model results, substantially decreasing uncertainties in the scenario outcome.

Effects of Nutrient Management Scenarios on Marine Food Webs: A Pan-European Assessment in Support of the Marine Strategy Framework Directive

“…For species diversity (D4C1), no clear responses were observed at a regional or sub-regional scale, except in the Baltic Sea with a slight increase and in the North-East Adriatic Sea with a decrease in diversity. The Baltic Proper was the only area that showed a slight increase in the two food-web criteria compared to the reference scenario; this might be related to the highly eutrophic nature of this ecosystem and the fact that a reduction of nutrient inputs might lead to an improvement of the marine environment e.g., better bottom oxygen levels, as observed in Saraiva et al (2019) and Friedland et al (2021), and thus better spawning conditions, leading to an increase in the Eastern Baltic cod stock. This response was not observed in other ecosystems that are also considered to be eutrophic such as the North-East Adriatic Sea, the Black Sea and the whole Adriatic Sea (from the JRC model) where nutrients reduction, mainly from the Po and Danube rivers, would reduce the assessed D3 and D4 criteria (Figure 2).…”

“…Thus, limiting the simulations to 8 years could have impeded a clear cause-effect relationship. This limitation was also highlighted in the results of LTL modules (Friedland et al, 2021;Grizzetti et al, 2021) which showed that 8 years of simulations were not enough to reach a new equilibrium. The internal nutrient dynamics and long residence times hampered the effect of the nutrient input reductions in the assessed ecosystems.…”

“…In addition, an example of the spatial output scenarios produced by the hydrological and hydrodynamicbiogeochemical models for the Mediterranean Sea, and integrated for the assessed period (2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012), is presented in Supplementary Figure S2. The full Pan-European assessment for the hydrological and LTL modules can be found in Grizzetti et al (2021) and Friedland et al (2021), respectively.…”

“…Table S1). Relative changes (%) between the two scenarios (MTFR and REF) of riverine total nitrogen (TN) and total phosphorus (TP) loads estimated by the hydrological model (Grizzetti et al, 2021), and total nitrate (DIN), phosphate (DIP) and primary production (PP) at sea estimated by the hydrodynamic-biogeochemical models (Friedland et al, 2021)…”

“…In addition, the stronger reduction in total nitrogen and phosphorus in the land-sea interface rather than in the dissolved nutrients at sea, [as shown in Table 1 and in more details in Friedland et al (2021)], indicate that marine biogeochemical systems depend not only on total nitrogen and phosphorus from riverine inputs, but also on the internal nutrient dynamics, e.g., mixing and stratification of DIN and DIP from deeper layers. Therefore, it is expected that reducing one input source (river) does not relate to a 1:1 reduction of marine (dissolved) nutrients at sea.…”