Factors Influencing Seedling Establishment Rates in Zostera marina and Their Implications for Seagrass Restoration

Abstract

Selection of strategies to efficiently utilize limited seed supplies in efforts to restore the seagrass Zostera marina (eelgrass) requires a better understanding of the processes that limit seedling establishment at potential restoration sites. We investigated the effect of seed distribution timing on seedling establishment and tested for interactive effects of seed burial and distribution timing. We also investigated the effect of habitat type on seedling establishment by distributing Z. marina seeds inside and outside of established Ruppia maritima (widgeongrass) patches and examined mechanisms causing habitat differences by manipulating seed position (buried or unburied) and vulnerability to seed predators (unprotected or protected in packets). Seeds distributed on the sediment surface in the summer (July or August) produced fewer seedlings than seeds distributed in fall (October) in five of six trials over 3 years.Seed burial increased success rates for seeds distributed in summer at one of two sites tested, eliminating the effect of season, but reduced success at the other site. Seeds placed in R. maritima generally produced fewer seedlings than seeds in bare sand, and although seed burial and protection in packets increased success in bare sand at three of four sites, the effect was less consistent in R. maritima. We conclude that seed predation and physical interactions were influential in reducing seedling establishment in R. maritima, contrary to hypotheses positing a nursery role for existing vegetation. Efficient restoration efforts with Z. marina seeds should target unvegetated areas after summertime sources of mortality have diminished. Direct seed burial may enhance seedling establishment rates.

Use of Posidonia oceanica seedlings from beach-cast fruits for seagrass planting

“…The use of seedlings for this purpose (Lewis 1987, Balestri et al 1998, Kirkman 1999 and to evaluate different planting techniques (Marion andOrth 2010 , Zarranz et al 2010 ) has been satisfactory. In the case of P. oceanica , the only previous attempts to use seedlings in restoration were those of Balestri et al (1998) .…”

Seedling establishment in eelgrass: seed burial effects on winter losses of developing seedlings

“…Seagrass canopies decrease wave energy reaching the sediment−water interface, and consequently reduce sediment resuspension (Koch et al 2006, Hansen & Reidenbach 2012. Higher initial success of seedlings might therefore be expected within established canopies, although experiments in Ruppia maritima canopies found no enhancement for surface seeds (Marion & Orth 2010b), and seedlings establishing within Z. marina canopies subsequently face light competition from adult plants (Olesen 1999). We suggest that Z. marina is more likely than R. maritima to reduce wave and current energy within the meadow, enhancing seedling establishment within sand gaps and creating a positive feedback cycle for meadow persistence.…”

“…Preliminary trials during the development of this technique showed reliable planting in the target 2 to 3 cm depth range. All seeds used were individually assessed as viable seeds exhibiting an intact seed coat, high density, and rapid fall velocity in seawater (Marion & Orth 2010b). Plots were placed in 2 rows along a 210 m × 10 m constant-depth region parallel to the shoreline at each site, and were divided into 4 blocks for sampling.…”

Seed addition facilitates eelgrass recovery in a coastal bay system

“…Seeded plots were separated by at least 60 to 100 m of unvegetated bottom, though patterns varied among bays and years. Additional small-scale seed experi- ments covering 1 to 4 m 2 were conducted in all 4 bays between 1999 and 2008, but these represented <1% of the total seeds broadcast over the 11 yr time period (Orth et al 2003, Marion & Orth 2010b. Plot sizes, numbers, and seeding densities used each year were generally determined by seed availability and the design of individual experiments conducted as part of the restoration effort (e.g.…”

“…For large plots (>100 m 2 ), seedlings were counted in 1 m line transects along the diagonals of each plot, and initial seedling establishment rates were calculated. Seedling establishment was assessed in smaller-scale experimental plots by complete counts of seedlings (Orth et al 2003, Marion & Orth 2010b. Data from both scales was used to assess seedling success for each bay.…”