Harmful microalgal blooms such as red-tide or brown-tide events lead to abrupt light reductions and consequently cause immediate damage to seagrass beds. Because red tide algal blooms usually occur unexpectedly, seagrass responses to the microalgal blooms have rarely been documented. A red tide caused by a dense bloom of Heterosigma akashiwo, a noxious red-tide-causing alga of temperate and subtropical waters, occurred at a study site on the south coast of Korea in late May 2002. Because the red-tide event occurred on an eelgrass bed where seagrass monitoring was being conducted, pre-event conditions were well documented. Nearly all eelgrass shoots disappeared rapidly because of the reduction in light caused by the algal bloom. Additionally, a thick layer of mucilaginous material secreted from algal cysts suffocated eelgrass plants for weeks, directly causing eelgrass death. Eelgrass seedlings were found in the die-off area from December 2002; <1 yr after its destruction, the site was completely re-established by seedling recruitment via germination from the seed bank. Seedling mortality was very low. Seedlings grew exponentially during the spring, and their fast growth also contributed to rapid eelgrass recolonization. During the second year of recolonization, asexual reproduction through lateral shoot production by rhizome elongation and branching played the main role in the persistence and growth of the eelgrass bed. Seed density in the seed bank varied seasonally, increasing to a maximum after seed release and decreasing to nearly zero after seed germination. Many more seedlings were found and nearly all seedlings established successfully in the first year after the bloom, when no adult eelgrass shoots were observed, suggesting significant effects of shoot density on rates of seed germination and seedling establishment. This was a unique opportunity to examine eelgrass responses to dense microalgal blooms, which provided valuable information on the die-off process caused by red tide and the natural recolonization of seagrass after its destruction.KEY WORDS: Recolonization · Red tide algal bloom · Eelgrass · Zostera marina · Die-off · Seed bank · Seedling Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 342: [105][106][107][108][109][110][111][112][113][114][115] 2007 been reported (Onuf 1996). Red tide algal blooms usually occur unexpectedly; accordingly, seagrass conditions both before and after the occurrence of blooms have seldom been documented. On the coasts of Korea, red tide algal blooms occur frequently during the summer period; the frequency of bloom events has been increasing from 5 to 40 times yr -1 during the 1980s to 30 to 120 times yr -1 during the 1990s (Kim et al. 2000). At a study site on the south coast of Korea, where monitoring of seagrass beds was in progress, a red-tide event caused by a very dense bloom of Heterosigma akashiwo occurred in late May 2002 and lasted for about 2 wk. The seagrass bed and abiotic factors, such as un...

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