Influence of seagrass restoration on nutrient cycling across contrasting estuarine systems in the southern North Sea

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; seagrass restoration is increasingly seen as a nature-based solution for improving nutrient uptake and ecosystem health. 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 (NO₃, NH₄), phosphate (PO₄), and dissolved organic carbon (DOC). In the Jade Bay, DOC increases by approximately 100–170% across seasons, PO₄ decreases by 24–34%, and summer NO₃ 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.