Nitrogen threshold for successful eelgrass (Zostera marina) restoration in Danish estuaries

Eutrophication is a major cause of seagrass decline in coastal waters. Limited natural recovery is evident in areas that have experienced a state change, despite reductions in nutrient discharge, due to non-linear recovery trajectories. Assisted recovery can be successful in some locations, but require thorough site-selection, because multiple stressors can hinder restoration. Site-selection based on habitat suitability models and in-situ stressor monitoring, requires knowledge about specific stressor thresholds, that are often country or region specific. Here, we provide a framework for in-situ determination of stressor thresholds that can feedback into habitat suitability models, and we exemplify it with determination of a water column dissolved inorganic nitrogen (DIN) threshold in a Scandinavian context. The aim of this study was to provide a framework for determining seagrass stressor thresholds in-situ and to identify the existence of a water column DIN threshold for eelgrass restoration to improve future site-selection for large-scale eelgrass restorations. Shoot production of small-scale eelgrass transplantation trials along eutrophication gradients in Danish estuaries were correlated with site specific DIN concentrations. Positive eelgrass growth was observed at DIN concentrations <3.7 μM (range 1.0-12.4 μM, 95% CI). Furthermore, it was evident that, the inner parts of Danish estuaries are still suboptimal for eelgrass restoration due to eutrophication. The threshold should not be used in isolation, other stressors such as hydrodynamics, salinity, sediment quality, light limitation or bioturbation may also constrain eelgrass establishment. However, the threshold can be used to calibrate local habitat suitability models, while the scientific framework for determining the threshold can be applied elsewhere. The threshold can be used to guide management efforts locally, by informing decisionmakers if they should focus on reducing nutrient discharge before attempting restoration. A comprehensive evaluation of multiple stressors is essential for a complete understanding of restoration potential, making this framework for assessing stressor thresholds an essential tool to improve habitat suitability models that can assess multiple stressors.