Aquatic botany ELSEVIER Aquatic Botany 59 (1997) 1-15 Restoring eelgrass, Zostera marina L., habitat using a new transplanting technique: The horizontal rhizome method Ryan C. Davis *, Frederick T. Short Department of Natural Resources, Jackson Estuarine Laboratory, UniuersiO, of New Hampshire, 85 Adams Point Road, Durham, NH 03824, USA Accepted 19May 1997 Abstract Using a technique we call the 'horizontal rhizome method', we recently transplanted 2.52 ha of eelgrass in the Great Bay Estuary, New Hampshire, to mitigate for port expansion impacts to an existing eelgrass population. The project represents the largest, most northerly eelgrass transplant- ing ever attempted on the east coast of the United States. For our revised method, we created a planting unit (PU) by overlapping the rhizomes of two eelgrass shoots in opposite directions and securing them horizontally into the sediment with a bamboo staple. A variation of the bare-root technique, the horizontal rhizome method reduces the number of plants required by up to 80%, has less impact on the donor site, and provides survival rates that equal or exceed that of other methods. One-year survival rates at three subtidal transplant sites were 75-95% for 1993 transplants, and were 98-99% at four of the five subtidal sites planted in 1994. One-year survival rates varied tremendously; low percent survivals resulted from ice damage at all intertidal sites and animal disturbance at some subtidal sites. We found that intertidal transplanting in this tidally dynamic estuary was not successful (sites had 0-15% survival). Sea ice damage, in conjunction with tidal action, caused this lack of success, although natural intertidal eelgrass beds occur both up- and down-estuary of our transplant sites. At the less successful subtidal sites, low survival rates of 1-5% resulted from disturbance of newly transplanted shoots by crabs and clam worms. Overall survival rates for subtidally transplanted eelgrass using the horizontal rhizome method equalled or exceeded those reported for other methods. Shoot density at one transplant site (234 shoots m 2 _+ SE 52.0) exceeded that of the control site (162 shoots m -2 ___20.6) within 1 yr of transplanting. Shoot density at all subtidal transplant sites surpassed that of the control site (100 shoots m 2 _+ 11.6) within 2 yr. The horizontal rhizome method is a successful, minimum-impact * Corresponding author. 0304-3770/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. Pll S0304-3770(97)00034-X