Using a coupled model and GIS tool in assessing the effect on primary production after nutrient reduction to a Danish estuary

• GIS tool shows local effects of nutrient reductions along eutrophication gradient. • Nutrient cuts improve clarity but sediments slow full recovery. • Nutrient reduction and assisted restoration are vital to meet WFD targets. • Tool links nutrient reductions to local gains, aiding targeted restoration. • Tool Supports targeted coastal management beyond WFD methods. This study presents a GIS-based tool using high-resolution spatial data from a validated mechanistic biogeochemical model, as management tool to quantify ecological benefits of nutrient reduction in a Danish estuary. Three nutrient reduction scenarios were simulated, assessing the derived effects on primary production, eelgrass stress parameters, and restoration potential. Results showed a clear eutrophication gradient, with dissolved inorganic nitrogen (DIN) concentrations exponentially decreasing from the main river outlet towards the outer fjord. The GIS tool effectively captured the responses to nutrient reductions, indicating a substantial decrease in phytoplankton production and a shift from pelagic to benthic-controlled primary production. Reduced phytoplankton concentrations improved benthic light conditions, creating extensive areas suitable for natural and/or assisted eelgrass restoration. However, within the relatively short simulation period, this restoration potential was limited, with only slight increases in eelgrass biomass and production observed. This limitation was attributed to high sediment organic content, along with low initial biomass of eelgrass, exceeding thresholds necessary for eelgrass growth. Consequently, our tool shows that nutrient reductions significantly enhanced water clarity and accelerated the ecological regime shift toward benthic-dominated systems, demonstrating that nutrient reductions are essential for achieving good ecological status (GES). However, the tool also shows that nutrient reduction alone cannot fully achieve GES since sediment characteristics respond slower. Therefore, assisted marine restoration measures are vital for accelerating the recovery process. Finally, through analysis along eutrophication gradients, the tool generates compressible knowledge that can directly be used on local marine planning and management.