Abstract Seagrass meadows play an important role in coastal water quality by regulating nutrient availability, reducing eutrophication pressure and stabilizing sediments. Their decline in many European coastal zones has intensified interest in restoration as a nature-based measure. However, the quantitative influence of seagrass on seasonal nutrient dynamics at the scale of whole estuarine systems remains insufficiently understood. In the Wadden Sea, excessive nutrient load and turbidity are persistent challenges. This study provides a novel, spatially explicit assessment of seagrass impacts on nutrient cycling across an entire annual cycle in two hydrodynamically contrasting regions of the southern North Sea. The study is based on a validated three-dimensional hydrodynamic–biogeochemical modelling framework that reproduces observed water levels, temperature, salinity, waves, and nutrient concentrations across the study area. Paired simulations with and without seagrass were used to quantify changes in dissolved inorganic nitrogen (NO ₃, NH ₄), phosphate (PO ₄), and dissolved organic carbon (DOC). In the Jade Bay, DOC increases by approximately 100–170% across seasons, PO ₄ decreases by 24–34%, and summer NO ₃ is reduced by up to 70%. In the Weser Estuary, the strength of vegetation effects is constrained by high riverine inputs and rapid flushing. Although dissolved organic carbon increases by up to 17% and phosphate decreases by 3–10%, nitrogen responses are smaller and are significantly influenced by river discharge and mixing. Overall, the results show that seagrass restoration can substantially modify local nutrient cycling, but that its effectiveness strongly depends on hydrodynamic conditions and external nutrient load. The study shows that restoration provides ecological benefits in semi-enclosed, moderately flushed systems like the Jade Bay, where biological processes can influence local water quality. In river-dominated estuaries, the effect of seagrass remains more limited because external inputs and rapid transport constrain its influence, unless accompanied by broader catchment-scale measures. The results highlight the potential of seagrass as a targeted nature-based measure for enhancing local water quality in suitable coastal settings, rather than as a stand-alone remedy for eutrophication at the estuarine scale. Graphic Abstract This study assesses the impact of seagrass restoration as a nature-based solution on the seasonal dynamics of nutrients and carbon in two estuarine systems with contrasting hydrodynamics in the southern North Sea. The Jade Bay is a semi-enclosed tidal basin with long residence times and moderate external nutrient inputs. In contrast, the Weser Estuary is river-dominated, with high nutrient loads, strong advection, and rapid flushing. The analysis is based on a coupled three-dimensional hydrodynamic–biogeochemical modeling framework (SCHISM–ICM). Experiments were conducted for one full annual cycle, including a vegetation (VEG) simulation and one without (NO-VEG). Model outputs were analyzed using Hovmöller diagrams and seasonal surface maps for dissolved organic carbon (DOC), ammonium (NH ₄), nitrate (NO ₃), and phosphate (PO ₄). Seagrass increases DOC concentrations in the summer and fall, indicating higher production and retention of organic matter in the water column. This accumulation stimulates sediment remineralization and increases NH ₄ levels. Meanwhile, NO ₃ declines during the productive season due to intensified biological uptake and microbial nitrogen transformations. PO ₄ decreases in all seasons, suggesting efficient phosphorus retention within the system. In contrast, the effects of vegetation in the Weser Estuary are weaker. Slight DOC enhancement and phosphate reduction are detectable, but nitrogen patterns show little response and are mainly controlled by river discharge and transport along the estuary. Overall, the study highlights seagrass restoration’s ability to change nutrient cycling and organic carbon dynamics in areas with long residence times and moderate hydrodynamic forcing. These results indicate that vegetation-based measures are most effective when combined with upstream nutrient management.
Castrillón et al. (Wed,) studied this question.