Reduced genetic diversity in eelgrass transplantations affects both population growth and individual fitness

The transplantation of eelgrass (Zostera marina) for mitigation results in reduced genetic diversity among individuals and populations in southern California, the Chesapeake Bay, and New Hampshire. Although genetic variation determines the potential for eelgrass to adapt to the rapidly changing environment in its coastal and estuarine habitats, genetic considerations are not currently included in mitigation and restoration policy. I investigated where and how genetic diversity is lost during eelgrass transplantation. I then explored associations between genetic diversity and both vegetative propagation and sexual reproduction to evaluate the importance of genetic diversity for short-term population growth. Eelgrass beds used as donor populations vary in genetic diversity, and some have little or no detectable genetic diversity. Genetic diversity is reduced upon transplantation because donor plants are collected from small areas, leading to random sampling errors in selecting stock. This loss can be minimized by using information from regional surveys of genetic diversity and structure in potential donor populations and by revising donor stock collection. There were significant positive associations between genetic diversity and the sexual reproduction of eelgrass, with a similar trend for vegetative propagation. Individuals heterozygous for glucose- phosphate isomerase (GPI) developed flowering shoots more than did homozygotes. More seeds germinated from a genetically diverse, untransplanted population than from a transplanted population with low genetic diversity. A field transplantation of known multilocus genotypes revealed that leaf shoot density in high-diversity eelgrass increased almost twice as fast as in low-diversity eelgrass over 22 mo. In a mesocosm experiment under heat stress. eelgrass heterozygous for either GPI or malate dehydrogenase (MDH) produced almost twice as many leaf shoots as homozygotes. The difference between treatments in all experiments increased over time. Together, these results imply that there could be economic incentives to planting genetically diverse eelgrass, and that genetic diversity contributes to eelgrass population viability even over the short term.