Life History Strategies Of Eelgrass(Zostera Marina L.) In Swan Lake, Shandong Peninsula, China

In this thesis, life histories, sexual reproductive strategies, seed dispersal pattern, seed bank dynamics and seedling recruitment of eelgrass Zostera marina were investigated in Swan Lake, Shandong Peninsula, China. The studies were conducted in situ by monthly monitoring visits. Artificial seagrass units and permanent quadrats were used and eelgrass cup-sowing was performed. Results might contribute to our understanding of how life history strategies of eelgrass change responding to different natural/anthropogenic disturbance and stress in the lake, which might furthermore help to gain insightful knowledge about the mechanism of eelgrass adaptation and reestablishment in heterogeneous habitats. The results are summarized as follows:1) The sexual recruitment and growth of Z. marina were investigated in Swan Lake from December 2012 to January 2014. Results showed that, in Swan Lake, eelgrass in the intertidal location declined caused by clam digging and low temperature in winter, and then recovered via seed germination and seedling establishment. Seedlings in both intertidal and subtidal areas showed little difference from each other at individual level, but higher abundance and lower survival rates of seedlings were found in the perennial eelgrass bed than in the annual eelgrass bed. Lower shoot densities, lower biomasses and smaller morphological size were observed in the annual eelgrass than in the perennial plants. Flowering season in subtidal area started in May, one month prior to that in the intertidal area and both ended in August. Additionally, the annual eelgrass showed lower reproductive effort (RE) and lower potential seed production than the perennial eelgrass, indicating the poor adaptation to such a habitat. Eelgrass seedlings in the subtidal area could not survive the summer due to the high shoot density, so their life cycle was not completed, while in the intertidal areas, eelgrass survived the summer and autumn, though died out in the winter, but achieved a complete life cycle.2) The sexual reproduction and seed dispersal of annual and perennial Z. marina was investigated using artificial seagrass units in Swan Lake from April to November 2013. Results showed that, Z. marina exhibited different sexual reproduction patterns in the heterogeneous habitats. The RE showed positive responses to intertidal stress, anthropogenic disturbances and natural fragmentation in perennial eelgrass areas but negative response to low tide exposure in annual patches. In addition, seed production showed higher values in disturbed and stressful subtidal areas and lower values in stressed intertidal areas than in undisturbed eelgrass meadow center. Furthermore, dispersed seed density in Swan Lake didn’t reflect the seed production but suggested that a large proportion of seeds were lost from the seagrass beds. These results suggested that, in Swan Lake, further losses of eelgrass habitats in intertidal area would probably occur due to unsuccessful reproduction strategies.3) Seed bank dynamics and seedling recruitment patterns were investigated in Swan Lake, and whilst, eelgrass cup-sowing was conducted in the lake from January to May 2014. Results showed that, seed bank of eelgrass was heterogeneously distributed in Swan Lake, with a density range of 16-368 seeds m-2. As most of eelgrass seeds in the seed bank germinated during the early spring and the remains could hardly survive to the next year, the seed bank was thought to be a transient seed bank. Similar cumulative germination rates of eelgrass seeds were found in intertidal and subtidal areas, while survival rates of seedlings in the establishment month were higher in less vegetated intertidal zone. Results of the cup-sowing showed that, number of seedlings per cup and seedling survival rate exhibited lower values in two bare planting locations than in two planting locations dominated by eelgrass patches. The overall results indicated that although less vegetated locations were more suitable for seedling establishment, strong current in these locations might also destroy and remove eelgrass seedlings before they successfully established.