Estimating responses of marine benthic communities to reduced nutrient loading using process‐based and statistical modelling

Abstract Shallow benthic habitats are critically important for safeguarding marine biodiversity but are severely affected by human pressures, including eutrophication. Monitoring programs usually assess biodiversity status after decades of exposure to eutrophication and may therefore miss changes in biota if eutrophication status improves or declines. We combined predictive biogeochemical modelling and statistical species distribution models to estimate how nutrient load reductions could affect distributions of 81 benthic species in the northern Baltic Sea. We used two nutrient abatement scenarios that aim to (i) substantially reduce loading and (ii) achieve good ecological status, using nitrogen as a proxy for eutrophication. To simplify the interpretation of how nutrient load reductions would affect species, we used hierarchical clustering to group species into broader ecological units and assessed species turnover under reduced eutrophication. Under both nutrient abatement scenarios, models predicted considerable increases in suitable habitat for species inhabiting shallow photic substrates. Many habitat‐forming species sensitive to eutrophication, such as perennial fucoid algae and charophytes, gained substantial increases in suitable habitats (21%–29% and 2%–64%, respectively), especially in inner archipelago areas, which historically have suffered most from eutrophication. In contrast, soft‐bottom benthic invertebrates remained relatively unaffected by nutrient abatement. Species turnover under the most intensive nutrient reduction scenario was highest in shallow photic substrates and in the inner archipelago, where biodiversity gains would also be the largest. Synthesis and applications . Our framework links policy targets (achieving good ecological status) and actions (nutrient emission reductions) to realized biodiversity benefits. The approach also supports the optimal placement of monitoring sites under different nutrient abatement scenarios. Furthermore, detailed estimates of the responses of benthic biota to nutrient reductions enable assessments of the cost‐effectiveness of reductions. The approach is applicable to any marine area affected by eutrophication, including estuaries and other semi‐enclosed seas.