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Aim: The seagrass Zostera japonica is a dramatically declined endemic species in the Northwestern Pacific from the (sub)tropical to temperate areas, however, it is also an introduced species along the Pacific coast of North America from British Columbia to northern California. Understanding the population's genetic patterns can inform the conservation and management of this species. Location: North Pacific. Methods: We used sequences of the nuclear rDNA internal transcribed spacer (ITS) and chloroplast trnK intron maturase (matK), and 24 microsatellite loci to survey 34 native and nonnative populations (>1000 individuals) of Z. japonica throughout the entire biogeographic range. We analysed the phylogeographic relationship, population genetic structure and genetic diversity of all populations and inferred possible origins and invasion pathways of the nonnative ones. Results: All markers revealed a surprising and significant deep divergence between northern and southern populations of Z. japonica in the native region separated by a well‐established biogeographical boundary. A secondary contact zone was found along the coasts of South Korea and Japan. Nonnative populations were found to originate from the central Pacific coast of Japan with multiple introductions from at least two different source populations, and secondary spread was likely aided by waterfowl. Main Conclusions: The divergence of the two distinct clades was likely due to the combined effects of historical isolation, adaptation to distinct environments and a contemporary physical barrier created by the Yangtze River, and the warm northward Kuroshio Current led to secondary contact after glacial separation. Existing exchanges among the nonnative populations indicate the potential for persistence and further expansion. This study not only helps to understand the underlying evolutionary potential of a widespread seagrass species following global climate change but also provides valuable insights for conservation and restoration. [ABSTRACT FROM AUTHOR]

Zostera japonica populations along the coastline of the northwestern Pacific Ocean are declining, mainly due to anthropogenic and natural disturbances. Although reproductive strategy is an important factor in achieving population persistence, changes in the reproductive strategy of Z. japonica under anthropogenic disturbances and tidal stresses are largely unknown. Thus, the duration and frequency of flowering, reproductive effort, potential seed production, and seed density in sediments were measured at three study stations (undisturbed upper, undisturbed lower, and disturbed stations), which were classified based on the levels of inundation stress and clamming activity, in monospecific meadows of Z. japonica on the southern coast of Korea. The flowering duration was approximately six months in the disturbed station, with disturbance due to clam harvesting, whereas the duration was about five months in the undisturbed lower station, and only three months in the undisturbed upper station. The maximum flowering frequency was 25.5% in the disturbed station, which was approximately 4- and 2-fold higher than in the undisturbed upper (6.1%) and lower (12.3%) stations, respectively. A similar trend in reproductive effort was also found among the three study stations. Potential seed production was 7850, 6220, and 1560 seeds m −2 in the disturbed, undisturbed lower, and undisturbed upper stations, respectively. The annual maximum seed density in sediments was also higher in the disturbed and undisturbed lower stations than in the undisturbed upper station, but the densities were relatively low (ranging from 71 to 254 seeds m −2 ) at all three study stations. It was found that the allocation to sexual reproduction was highest in the disturbed station, followed by the undisturbed lower station, and lowest in the undisturbed upper station, suggesting that sexual reproduction in Z. japonica tends to be enhanced under disturbed and inundated environmental conditions for population persistence. [ABSTRACT FROM AUTHOR]

Given a large quantity of epiphytes and other material attached on eelgrass leaf blades, we explored the relationship between eelgrass sheaths and different-aged leaf blades (1st, 2nd, 3rd, and 4th leaf blade) on nutrient content and their ratios (C, N, P, C/N, C/P, and N/P) to identify whether eelgrass sheaths could be used to instead of leaf blades in terms of nutrient content. In addition, we explored the relationship between eelgrass sheath length and shoot length. Results showed that there were significant relationships between the sheath and leaf blades in terms of N and P content and their ratios. For length analysis, there was a significant relationship between sheath length and shoot length, and shoot length was approximately four to five times (mean 4.4659) longer than sheath length, such that shoot length can be estimated by sheath length. These significant relationships suggest that eelgrass sheath could be used as a suitable predictor of leaf blade in length and nutrient stoichiometry, thus eelgrass sheath could be used as an indicator for further eelgrass nutrient monitoring and research. • Eelgrass shoot length can be estimated by sheath length. • N, P, C/N, C/P, and N/P of eelgrass sheath can reflect those of leaf. • N and P contents of leaf showed high values in winter and low values in summer. [ABSTRACT FROM AUTHOR]

• The sea around the Korean coast had become warmer according to the long-term monitoring result. • The elevated sea surface temperatures threatened the sexual reproduction of Zostera marina. • Increasing water temperatures decreased the intensity of flowering in Z. marina. • Sea warming increases the vulnerability of seagrasses by decreasing their sexual reproduction ability. • Long-term records of flowering performance may be used as an indicator of regional sea warming. The growth and reproduction of the marine seagrass foundation species Zostera marina (eelgrass) are threatened by rising sea surface temperatures (SSTs). After both natural and anthropogenic disturbances, the recovery of Z. marina largely depends on seed germination and seedling establishment. Therefore, understanding the effects of increasing water temperature on the phenology and intensity of flowering is critical for predicting the persistence of Z. marina and its associated ecosystems. The effects of increasing SST on Z. marina flowering were investigated using long-term records (2002–2018) of Z. marina flowering and daily SSTs along the southern coast of Korea. Significant SST increases occurred during the study period. The annual maximum SST increased at a faster rate than the annual mean SST, whereas no warming trend was observed for the minimum winter temperature. Gradual declines in the reproductive intensity of Z. marina were observed during 2011–2018, when annual mean SST increased rapidly and summer marine heatwave (MHW) events became more intense and frequent. Reproductive parameters such as flowering shoot density, flowering frequency, and reproductive allocation showed significant negative relationships with mean SSTs. Thus, increasing heat stress due to sea warming would have led to decreased reproductive output of Z. marina over the study period. The results of this study suggest that warmer water temperature in the future may decrease the reproductive performance of Z. marina and limit this species' capacity for recovery via seedling recruitment. Measuring long-term sexual reproduction in Z. marina may be useful as a bioindicator of sea warming and will improve our understanding of seagrass responses to climate change. [ABSTRACT FROM AUTHOR]