Enhancing seagrass restoration success: Detecting and quantifying mechanisms of wave-induced dislodgement.

Abstract

Seagrass meadows are one of the most productive ecosystems of the world. Seagrass enhances biodiversity, sequesters CO₂ and functions as a coastal protection measure by mitigating waves and enhancing sedimentation. However, populations are declining in many regions and natural recolonization of bare sediment beds is protracted and unlikely. The widely used single shoot transplantation method for seagrass restoration is time-consuming and expensive, thus it is important that chances of survival are high. Dislodgement due to wave action poses a particular high risk during the first days after transplantation. This study replicates the transplantation method with a total of 224 harvested shoots (Zostera marina) planted in a wave flume under real sea state conditions. After varying rooting periods in cultivation tanks with low hydrodynamic exposure, the shoots together with their surrounding soil were installed inside the flume and exposed to increasing sea state in intermediate water depth (near-bottom maximum orbital velocity MOV = 0.25-0.59 m/s) for 250 min (≈5000 waves). Half the plants were protected by a willow fence, serving as a restoration facilitator. Our results show that dislodgement is not driven by singular exceptional large waves, but by the wave-induced stress from long-term cyclic loads (fatigue). Furthermore, we found that shoots with a rooting period <12 days are especially vulnerable. We also detected that dislodgement is critically impacted by belowground biomass and leaf surface. The deployed restoration facilitator enhances shoot survival by 22.4 % and mitigates the effect of the rooting period. The findings indicate that wave exposure and shoot morphometrics are crucial to shoot survival in the first 12 days after transplantation. Considering morphometrics in shoot selection for transplantation may thus reduce the need for restoration facilitation. In conclusion, our research facilitates planning of seagrass restoration including the identification of suitable weather windows, restoration facilitator necessity, and shoot traits.