Differences in reproductive effort and sexual recruitment of the seagrass Zostera japonica between two geographic populations in northern China

Coastal seagrass beds are pivotal but threatened marine ecosystems throughout the world. The seagrass Zostera japonica Asch. & Graebn. is an endangered species in its native range along the northwestern Pacific coast. In this study, we used ecological survey methods and microsatellite analysis to evaluate sexual reproduction and its role in recruitment of Z. japonica populations at Swan Lake lagoon (SLL) and Huiquan Bay (HQB) in northern China. Mixed annual and continuous meadows of Z. japonica at SLL produced a high number of seeds (mean +/- SD: 40 244 +/- 18 666 seeds m(-2)) and formed a relatively stable seed bank (1460 +/- 417 seeds m(-2)) in the sediment. About 41 % of the seed bank and 6% of shoots survived over winter, and recruitment from seeds accounted for 41 +/- 24 %. In contrast, perennial and fragmented Z. japonica at HQB had lower seed production (12 501 +/- 5748 seeds m(-2)) and a much smaller seed bank (10 +/- 6 seeds m(-2)). About 66% of shoots survived over winter, but seedling recruitment was rare at HQB. Thus, relatively large differences in genetic and clonal diversity were predicted between SLL and HQB. Results of the microsatellite analysis of samples collected in 2012 and 2015 showed higher clonal (R) and genetic diversity (H-o) at SLL (2015: R = 1; H-o = 0.55) than at HQB (2015: R = 0.40; H-o = 0.42). These results highlight the role of sexual and asexual reproduction in maintenance and evolutionary connectivity of seagrass populations and emphasize the need to understand local recruitment strategies before starting restoration and management projects.