Life-stage transitions of annual Zostera marina under varying seeding densities and water depths: Potential applications to seed-based restoration

초록

Zostera marina is generally considered a perennial seagrass; however, annual or mixed-annual populations of this species have been documented in several regions worldwide, typically occurring in environmentally harsh or highly disturbed habitats where perennial populations struggle to persist. Adapted to unfavorable deep-water conditions, the annual population in Jindong Bay on the southern coast of Korea may provide useful ecological insight and, in some cases, a locally relevant seed source for restoration in the region. However, the demographic mechanisms through which seed density and local depth-associated environmental factors regulate the persistence of annual populations and influence their restoration performance remain poorly understood. We conducted a field seeding experiment in Jindong Bay, using seeds from an annual Z. marina population inhabiting deeper waters adjacent to a shallow perennial meadow, and monitored successive life-stage transitions (emergence, establishment, and reproduction) and reproductive output in experimental plots established in deep and shallow waters at 4 m and 2.5 m MLLW, respectively. Four seeding densities (400, 100, 25, and 4 seeds per 0.25-m2 plot) were tested. In deep water, seedlings successfully completed all life-stage transitions across all seeding densities. In contrast, all seedlings emerging in shallow water failed to establish or reproduce, regardless of seeding density. Emergence probability increased with seeding density and was higher in deep (0.898) than in shallow (0.719) water. In deep water, emergence peaked at the highest density, whereas establishment and reproduction probabilities were highest at the intermediate density (100 seeds plot-1), resulting in the greatest cumulative transition probability. Seedling morphology and reproductive traits were also strongly influenced by seeding density and water depth. Intermediate seedling density produced taller seedlings at both depths and, in deep water, larger reproductive shoots and higher per-shoot seed production. Consequently, potential seed production was maximized at the intermediate seeding density, reaching approximately 1.5 times that observed at the highest density. These findings provide new insights into the mechanisms underlying the persistence and deep-water confinement of annual Z. marina populations under local conditions, as well as their potential relevance for regional seed-based restoration.

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