Experimental transplants of the large kelp Lessonia nigrescens (Phaeophyceae) in high-energy wave exposed rocky intertidal habitats of northern Chile: experimental, restoration and management applications

Summary

Large kelp Lessonia nigrescens is one of the most ecologically and economically important species dominating the wave-swept intertidal rocky habitats of the eastern southern Pacific. However, L.nigrescens has been decimated along large coastal stretches in northern Chile due to the arrival of subtropical warm waters during El Niño years. Disposal of copper-mine wastes into coastal environments, also in northern Chile, has also resulted in the loss of most of the biota, including L.nigrescens.

This case study describes a simple and inexpensive device for re-planting kelps into the lower intertidal zone of wave-swept rocky habitats. The device allows a wide range of plant sizes to be anchored by the holdfast. Transplanted kelps re-attached to the substratum, overgrew the transplanting device and regenerated.

Transplant experiments of large kelp Lessonia nigrescens were undertaken on the coast of northern Chile. In July 2003, 50 immature L.nigrescens plants (3 to 10 cm in holdfast diameter, 128 cm maximum frond length) were collected at two sites 107 km apart, where natural populations of kelp flourish. These were sites, Guanillo and Zenteno, both located in open coastal areas continuously affected by strong wave action.

Transplant device design: The transplanting device was designed to hold L. nigrescens plants in place at the lower wave-swept rocky intertidal zones, after their detachment from the natural rock substratum. The device (a 'transplant unit') consisted of 20 × 20 cm pieces of 1 cm mesh Vexar net, reinforced along the edges with plastic clips secured with 3 mm plastic cable ties (see original paper for diagram). Two 5 cm long perpendicular cuts intersecting at the net centre were made to allow fronds and stipes of different sizes to slip through, whilst retaining the holdfast. Plastic cable ties passing over the holdfast and stipes were used to secure the kelp to the net. Transplant units were then secured to the rock substratum by four 5.5 cm stainless steel bolts screwed into hammer-drilled holes.

There has been at least one previous attempt at re-planting juvenile L.nigrescens (Vasquez & Tala 1995). This used non-toxic epoxy cement to glue plants to the substratum. However, results were discouraging as 100% of the plants were lost in only two weeks.

Removal of plants for transplanting: Removal of plants was undertaken by detaching the holdfast from the rock (L.nigrescens attaches directly to rock or coralline crusts and not to other living organisms as some other Laminariales do) with iron chisels. After removal, frond length and holdfast diameter were recorded. All plants, identifiable by numbered plastic tags fastened to the Vexar net, were then anchored within the same site of collection using the transplant device. In each site five additional undisturbed plants of similar characteristics to those detached for transplanting were selected and tagged as controls.

Monitoring: Frond length and holdfast diameter were recorded every two to three months. Frond and holdfast size were compared with their respective values at the beginning of the experiment to calculate the average growth of surviving plants. Survivorship was estimated as the number of surviving plants out of the total number transplanted.

Attachement & survival: Transplanted L.nigrescens rapidly developed holdfasts and within 2 months the transplant netting was overgrown by holdfast tissue. In many cases, new holdfast tissue overgrew the plastic frame and colonized the rocky surface beyond the transplant device periphery. At this stage, plants were firmly attached to the substratum by extensive development of the holdfast. After 7–9 months, healthy stands of fully developed L. nigrescens were established. In spite of a constant strong wave action, particularly during winter storms, and commencement of illegal harvesting of natural kelp stands, over 70% of the transplanted plants were alive after three months and 60% after six months. No control plants were lost during the first six months. By April 2004 survivorship of transplanted and control plants was 43% and 60% respectively. Losses in the control group occurred mainly during the maximum period of the illegal harvesting and the experiment was discontinued in July 2004, when all control plants were lost. At this time, 28% of transplanted individuals were still healthy and firmly attached. During the trial 18% and 40% of transplanted and control individuals respectively became reproductive, displaying well developed sori.

Growth rates: Nine months following transplanting, fronds of control and transplanted individuals increased an average of 134.3 cm (± SD 43.5 cm) and 63.4 cm (± SD 69.7 cm) in length respectively. This seemingly large difference between control and transplants was, however, only marginally significant. Changes in holdfast diameter showed a similar trend, after nine months control individuals had a significantly higher diameter growth (4.5 cm, ± SD 0.7 cm) compared to the transplanted individuals (1.8 cm, ± SD 2.7 cm).

Practicability of transplanting: An advantage of this transplant system is that it allows attachment of a relatively large number of transplant units during a single low-tide period, with a maximum of up to 35/low tide achievable by a team of four people.

Conclusions: The use of the transplanting device to move and re-attach L.nigrescens appears to adversely affect growth rate, as indicated by the smaller size of the fronds and holdfasts as compared with control plants. It is thought that a recovery time is required by the plants to adjust to their new conditions and attach to the substratum. An important observation was the high variability in the final size of transplanted kelps, which may be due to the effect of several factors. One of these is the physical damage suffered by fronds and stipes trapped between the plastic frames and rocks when the corners of those frames are not fully fastened against the substratum. Careful manipulation of frame during anchoring has resulted in significant improvements, with kelp survival rates after nine months of 80% or higher. Regardless of their slightly diminished growth performance, transplanted individuals reached maturity. As a restoration technique it could help in the recovery of stands lost to pollution or harvesting.


References
Vasquez J.A. & Tala F. (1995) Repopulation of intertidal areas with Lessonia nigrescens in northern Chile. Journal of Applied Phycology, 7, 347-349.


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