Ex-situ mariculture can support the restoration of the endangered seagrass Zostera capensis

Seagrass meadows face ongoing declines and are increasingly targeted for restoration. Traditional in-situ restoration techniques involve trade-offs between restoration success and donor meadow impacts. To overcome these challenges, we provide the first assessment of ex-situ mariculture of the endangered seagrass Zostera capensis to support its restoration in South Africa. Seagrass cores with diameters of 5 cm and 10 cm, including their sediment, were harvested and grown in mariculture pools for 195 days. Changes in seagrass leaf length (cm) and shoot density (cm2) were monitored and the effect of core size on these morphometrics was investigated. Core size had a significant effect on seagrass shoot density and leaf length, as smaller cores had lower shoot densities and leaf lengths over time, suggesting that larger cores might be more effective to maximise seagrass cover during ex-situ mariculture. Overall, shoot densities saw limited increases for the first nine days, followed by a large percentage increase between days 9 and 24, before growth remained steady until peak shoot density was reached between days 100 and 124. Leaf lengths gradually increased until peaking between days 100 and 124. This study successfully demonstrates the proof of concept that ex-situ mariculture can sustainably upscale Z. capensis restoration by increasing the amount of plant material available for re-transplantation. To build on the work presented here, we provide a framework, incorporating guidance from published literature, to advise future seagrass restoration trials.