Numerous mitigation transplants of the eelgrass Zostera marina in southern California shuffle genetic diversity and may promote hybridization with Zostera pacifica

Intensive human pressures along the southern California coast have led to >50 mitigation transplants of eelgrass over the past 30 years. We analyzed diversity and population structure of Zostera marina and Zostera pacifica at 36 locations to identify potential management units and further develop transplant guidelines. Normalized allelic diversity of Z. marina was uniformly moderate to high (4.78; 3.48-6.44) and nearly twofold higher than mainland Z. pacifica (2.70; 1.74-4.89). More than half of the Z. marina populations exhibited strongly significant inbreeding coefficients coupled with strong linkage disequilibrium attributable to transplant effects; neither attribute was found in Z. pacifica. Both species were characterized by high genotypic diversity and an absence of large clones. A Bayesian analysis of population structure suggested 6 potential management units for Z. marina and 3 for Z. pacifica; some units included disjunct locations associated with transplants. Hybridization between Z. marina and Z. pacifica Was documented at Newport Bay Entrance Channel and south San Diego Bay. The presence of two species requires management plans for each, as well as avoidance of potential transplant-induced hybridization. Although transplant admixtures elevate diversity, shuffling among locations may potentially reduce the genetic potential necessary to ensure rapid adaptation, even though overall transplant success has been successful. Given that transplants will continue (from both plants and seeds), we recommend that the current requirement for two additional distinct donor sites be restricted to within a management unit for small, routine mitigations and expanded to among-management units for wholesale de novo restorations. (C) 2014 Elsevier Ltd. All rights reserved.