| Zostera marina L.(eelgrass), a widespread seagrass in North Hemisphere, dominates the temperate regions in North China. Eelgrass meadows in Swan Lake(Rongcheng) and in Huiquan Bay(Qingdao), acting as typical eelgrass meadows in Shandong Peninsula, can propagate by sexual and vegetative reproduction and represent the natural and recovered populations. In this study we focused on the population recruitment under natural circumstances in both two areas. Combining with the observation of clonal growth under the different depth gradient in Ailian Bay(Rongcheng), we aimed to explore eelgrass natural population recruitment, and the main results were as followed:1. A review has been finished on seagrass research. Seagrass beds providing important ecosystem service functions are now facing serious degradation. The current restoration methods including natural recovery, transplantation and seeding, are based on sexual and vegetative reproduction. However, there are few relative studies on the whole recruitment process of eelgrass.2. Eelgrass sexual reproduction ability was depended on potential seed production and seed bank size. We conducted our studies in both eelgrass meadows in Swan Lake and Huiquan Bay from July 2014 to August 2015. We surveyed density, shoot height, spathe number and seed number of per spathe of flowering shoot and seed bank size using random samples in both two areas. The results showed that flowering shoot appeared one month earlier in Huiquan Bay(started from April) than in Swan Lake, but both peaked in June with 517.27 shoots·m-2 in Swan Lake and 995.02 shoots·m-2 in Huiquan Bay. Flowering shoot degraded in early July and disappear in early August in both two areas and the potential seed productions were 37991.49 and 76478.23 seeds·m-2, respectively. Matured seeds, fell off and were buried in sediment from July and ended in October when seed bank peaked in both areas with 472.49 ? 408.29 and 399.93 ? 770.14 seeds· m-2, respectively. Whereas seed banks in both areas declined after October due to natural mortality and seed predation.3. With the increase of water temperature, eelgrass seeds germinated from the mid of March and then seedlings grew. We conducted a survey on seed germination and seedling growth in Swan Lake where we chose three stands, the center stand, the margin stand along eelgrass meadow and the patch stand of eelgrass, to measure the density, shoot height, sheath height, leaf number, and clonal growth of seedlings. The results showed that seed germinated from the mid of March and peaked in April(481.77?303.42 shoots·m-2), ending in May when seedling clonal growth commenced. Seedling grew with the rise of water temperature and could not be distinguished with overwintering vegetative shoots after June.4. Vegetative reproduction is a vital approach for eelgrass propagation. We utilized stable following-up samples in Swan Lake where we chose three strands(THE-1,THE-2 and THE-3) to measure shoot density of seedlings and vegetative shoots. The result showed that clonal growth was slow during March and April, and vegetative shoot density never increased apparently until May when the density peaked, while vegetative shoot density declined slowly after June. The dramatic declines occurred in August and September, with vegetative shoots disappearing in THE-1,2 and seldom remaining in THE-3(21.88?24.20 shoots·m-2). There was no significant difference between the three stands in shoot height, with the decline in same time after August and September due to Chaetomorpha linum Kutz bloom in Swan Lake. Eelgrass at different depth(1m, 2m, 3m, 4m and 6m) in Ailian Bay showed that eelgrass propagated and grew well only at the depth of 1-2m where the relative light intensity was still above 30% of water surface. However, eelgrass at the depth of 4m and 6m where the relative light intensity was under 100 μphotons·m-2·s-1, degenerated gradually.5. Eelgrass rapid growth owing to the rise of water temperature may also be the result of release of nutrient element from sediment in spring. Thus we conducted aan in-situ decomposition experiment of organic sediments in Swan Lake, which included faeces of Cygnus cygnus and detritus of eelgrass. The percentages of Carbon, Nitrogen and Phosphorus in sediments were measured. We found that the organic nutrient content declined rapidly with the rise of water temperature in spring and reached almost the same degree as contracted one in organic nutrient content in May. After that, the percentages of C, N and P turned to stable until August when field survey ended with the mean content were 2.05 ? 0.19, 0.26 ? 0.02, 4.02 ? 0.30%, respectively. Organic nutrients including C, N and P percentage content in different organs and parts of eelgrass can interpret the difference composition of nutrient elements, and were measured in Swan Lake to evaluate eelgrass health and growth there. There were significant differences between above- and below-ground parts with values of three elements all higher in above-ground parts. C content increased with a small decline in May with the minimum 40.07 ? 0.75 % and the same tendency with N minimized in June with 1.60 ? 0.23 %. P content declined straightly and minimized with 0.07 ? 0.06 % in September. C/N, C/P and N/P in plants can response the utilization of the N, P elements. C/N and C/P in eelgrass different organs showed an obvious seasonal change with higher value in below-ground parts than above-ground parts, indicating that the above-ground parts utilized more N and P in spring when eelgrass started grow again.6. Eelgrass propagation including sexual and vegetative reproduction which play an importance role in its recruitment involved 4 processes, seed germination and seedling growth; vegetative shoot clonal growth; occurrence of flowering shoots and seed burial; senescence of shoots to overwinter. The difference between two areas was the seed germination time and flowering shoot commencing time, but the contribution of seedling to population recruitment both ranged from 20-30 %. |
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