Eelgrass (Zostera marina) can tolerate salinity fluctuations

Seagrass meadows are rapidly declining worldwide, and restoration is challenging in altered ecosystems impacted by human and climate-driven changes. Predicting suitable habitats for restoration is particularly difficult in dynamic systems with fluctuating stressors. We examined the ecophysiological limits of eelgrass (Zostera marina) from the Limfjord (Denmark) using a 12-week controlled multi-stressor mesocosm experiment. We tested three mean salinities (20, 28, 31), a salinity regime factor (constant vs. fluctuating ±6, applied only to the 20 and 28 treatments, with 1.25 fluctuation cycles), and two light levels (ambient and 65% reduced), based on the Dutch Wadden Sea, where ∼150 km² of eelgrass has been lost. Salinity fluctuations did not directly impact eelgrass morphology but did influence physiological traits and biomass investment patterns, leading to thicker leaves, heavier shoots, and increased rhizome sucrose. The 65% light reduction was the dominant stressor, causing substantial declines in shoot numbers (−44%), leaf numbers (−50%), and aboveground biomass (−34%). Constant salinity levels affected leaf area per aboveground weight, shoot mass, and leaf C:N ratios. Medium salinity (28) supported the highest leaf surface area, belowground biomass, and leaf numbers under ambient light. Our findings suggest that eelgrass can tolerate multi-week-scale salinity fluctuations ex situ, but light availability is important for survival and strongly influences the morphology. In the Dutch Wadden Sea, for the highest eelgrass growth, we recommend targeting sites with moderate salinity (28) and light levels ≥200 μmol photons m⁻² s⁻¹. These results can refine knowledge-based site selection for eelgrass restoration.