Annual and perennial life history strategies of Zostera marina populations under different light regimes

Abstract

The seagrass Zostera marina is geneti- cally considered to be perennial, but annual popula- tions of this species exist as well and are under severe environmental stress in many geographical locations. We investigated why Z. marina in Jindong Bay (Korea) exhibited a typical perennial life history at a shallow site but an annual life history at a deep site. We hy- pothesized that Z. marina shoots in the deep popula- tion die off after fall due to severe light reductions, and that the re-establishment of this population is completely dependent on sexual reproduction. Life histories of these populations were examined by con- structing life tables using monthly measurements of shoot recruitment and mortality. In the deep popu - lation, new shoot recruitment was only achieved by seedlings, and approximately 95% of the surviving seedlings flowered. No shoots overwintered in the deep population. In contrast, shoot recruitment in the shallow population was primarily achieved by asexual reproduction. High summer water temperatures and subsequent severe underwater light reduction during summer and fall probably led to the high mortality in Z. marina shoots in the deep population, resulting in no shoot survival after fall. The deep population likely exhibits an annual life history strategy to achieve pop- ulation persistence under repetitive and acute unfa- vorable environmental conditions. From an evolution- ary perspective, the life history traits of Z. marina would define this species as an intermediate between annual and perennial.