An experimental evaluation of different methods of restoring Phyllospadix torreyi (surfgrass)

Summary

Surfgrass Phyllospadix torreyi is an abundant seagrass found on rocky shores of the North American Pacific coast. The ecological importance of seagrasses and their susceptibility to damage resulting from human activities has led to interest in developing methods for seagrass bed restoration. However, surfgrasses such as P.torreyi pose special restoration challenges as transplanting techniques developed for other seagrasses are not applicable, as, unlike most seagrasses wihich grow is soft benthic sediment, surfgrasses grow on exposed rocky coasts. Adventitious roots from the rhizome secure the plant to the hard substrate. This study evaluated three techniques for surfgrass restoration in intertidal and subtidal habitats in southern California where surfgrass is adversely affected by a range of natural events and anthropogenic activities (e.g. increases in nutrient loading, polluted waste from sewage and industrial discharges, and boating and fishing).

Study sites: Two study sites near Santa Barbara, California, USA were selected. Intertidal studies were conducted during low tide at More Mesa Reef, a broad, gently sloping reef interspersed with sand channels. Surfgrass Phyllospadix torreyi formed dense beds on most of the emergent reef in the low intertidal zone to the shallow subtidal zone. Seedlings, sprigs, and plugs were transplanted to cleared areas on the adjacent reef. The sand channels broaden with depth, resulting in little rocky habitat in the subtidal zone. Consequently subtidal studies were carried out at nearby Mohawk Reef, where surfgrass forms large, patchy beds on a gently sloping reef with stands of coralline algae. Seedlings, sprigs, and plugs were transplanted to depths of 2.5–4 m below MLLW in plots cleared of coralline algae and other biota that were located next to established surfgrass beds.

Transplant techniques: Transplant experiments were initiated in the late summer and autumn, the time of year when most surfgrass seeds are released and germinate in California. Transplant survival and growth months, and recovery of the donor population from which transplants were collected was evaluated after 6 months. The percentage change in the number of leaves was used to assess the condition of surviving transplants. This was measured as opposed to the more traditional method of counting shoots as it was difficult to accurately count shoots in the wave-swept surf zone where shifting sand frequently buried shoots above the sheath. The three vegetative sources and transplant techniques used in the surfgrass restoration experiements to these reefs were:

Seedlings: In the laboratory, each seedling was attached to a 7 cm long piece of braided nylon line (1 mm diameter) by inserting one of the arms of the fruit into an opening in the line made by untwisting the braids. Tiny bristles on the fruit hooked onto one of the braids, locking the seedling in place. Attached seedlings were transported in seawater-filled plastic bags in insulated containers. They were transplanted to 12 experimental 30 × 30 cm plots at each of the study sites in November 2000 by fastening the ends of each nylon line to the reef using Z-Spar A788 marine epoxy putty. Eighteen seedlings were transplanted in a grid to each plot at both sites. All plots were adjacent to healthy surfgrass stands and were cleared of algae and sand before transplanting to facilitate attachment. Branches of coralline algae growing adjacent to the plots were trimmed to reduce seedling abrasion and dislodgement.

Sprigs: Forty-two sprigs were gathered from both sites (an unbranched terminal end of an actively growing rhizome was carefully removed from the perimeter of a bed with a knife. The rhizome of each sprig was 5 cm long and had several lateral shoots and a terminal shoot) in November 2000. Sprigs were immediately transplanted just outside (within 2 m) the surfgrass bed to a 15 × 15 cm area cleared of other biota. The cut end of the rhizome was attached using marine epoxy. Sprig leaves were trimmed to 20 cm in length before attachment to reduce the chance of dislodgement. Pilot studies determined that reducing the drag by trimming leaves was a necessary for effective attachment. The average number of leaves per sprig was 11.9 for subtidal and 26.6 for intertidal transplants. The collection of sprigs resulted in a small loss in surfgrass from the donor bed and to measure recovery, a marker was glued to the reef next to the cut end of each donor rhizome and the coverage of new rhizome that grew from the cut end of the donor rhizome estimated.

Plugs: Cohesive clumps of mature surfgrass were collected and transplanted beginning in August 1999. Square plugs of intertwined rhizomes and shoots were harvested from the middle of a surfgrass bed using a wide-bladed putty knife and transplanted outside (within 2 m) the surfgrass bed to areas cleared of other biota. Clearings were made larger than the plugs to provide a 5 cm wide buffer. Plugs were attached by pulling the leaves through a square of 2.5 cm diameter stretch mesh nylon net cut to slightly larger in size than the square plug. The net was pulled over the plug and secured to the reef at the edges with marine epoxy. Leaves were trimmed to 20 cm in length to reduce drag. Three different plug sizes (small, 5 × 5 cm; medium, 10 × 10 cm; and large, 20 × 20 cm), were collected, six of each transplanted in each site. Plug collection resulted in loss of surfgrass in the donor beds. Donor patch recovery was monitored to assess the extent of in-growth from the edges of the bare patches by neighboring rhizomes.

The effort (i.e. person hours) involved in all steps of each transplant technique, was calculated.

Seedlings: Transplanted seedlings survived poorly with substantial mortality within the first few days of transplanting at both sites, and < 3% of the 432 seedlings surviving for 6 months. Survivorship was equally poor at both sites. Despite minimal rhizome growth those few surviving individuals had on average, a 275% increase in leaf number.

Sprigs: Overall, sprig survival was 59.5%. Sprigs transplanted to the subtidal zone had higher survivorship (71 versus 48%) and a greater increase in aerial coverage of rhizome (86 versus 42%) than those transplanted to the intertidal.

Plugs: Survivorship of transplanted plugs was high in both sites (100% at both sites for all plug sizes, except the large six plugs transplanted in the intertidal zone at More Mesa Reef intertidal where only one survived (16.7%) to 6 months However, the physical disturbances to the donor populations when the plugs were extracted, yielded an overall substantial net loss in surfgrass.

Conclusions: Of the three techniques, transplanted sprigs had the greatest overall increase in aerial coverage per unit effort, suggesting that this method may be the most effective approach for restoring surfgrass given that other habitat factors are suitable. Despite poor seedling survival, those few that survived grew very well. This suggests that there is potential in using surfgrass seedlings for restoration, especially when considering the small impact on beds caused by seed collection.


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