Population dynamics and seed banks of the threatened seagrass Halophila beccarii in Pearl Bay,Guangxi

Halophila beccarii Ascherson is one of two species in the oldest lineage of seagrasses,and belongs to the family Hydrocharitaceae. It is a small submerged creeper,found in the upper intertidal zone,and grows on mud or muddy sand substrates in estuarine and coastal areas. This species has been recorded to be at the coastlines of East Asia,Southeast Asia,Central Asia,Kenya and Tanzania. Global population trends indicate this species is declining,although it is fast growing and may recover quickly from disturbance. However,it also has a narrow,restricted depth range that coincides with the intertidal zone,an area subject to much anthropogenic disturbance. This species was listed as ‘Vulnerable'by the International Union for Conservation of Nature under criterion B2. H. beccarii occupies a very limited area in China and it is threatened by many forms of disturbance. Very little ecological information has been reported from China or subtropical regions globally for this species,and existing ecological studies are restricted to tropical regions in Southeast Asia or South Asia. The lack of basic ecological information on H. beccarii in China affects our understanding of the mechanisms underlying its decline,and thus hinders efforts to conserve and manage this threatened seagrass species.To examine temporal variation in the area,percent cover,above-ground / below-ground biomass,shoot density,reproductive organ density and the sediment seed banks of H. beccarii in Pearl Bay,Guangxi,samples were taken and surveys were carried out from April 2012 to April 2013 at roughly one-month intervals. Our results indicate that the area,percent cover,shoot density,biomass and reproductive organs density of H. beccarii in Pearl Bay vary greatly at different stages of population development over an annual cycle. Generally,all population parameters initially increased from April2012,until they reached a peak value and then decreased. It seems the growth dynamics of the population followed "n"patterns with the peak values for shoot density(21602 / m2),female flower density(5404/m2) and above-ground biomass(42.712 g/m2(DW)) occurring in early August; male flower density(2189/m2) in late August; and area(21.4 hm2),percent cover(55%),below-ground biomass(39. 701 g / m2(DW)) and total biomass(70.583 g/m2(DW)) in late October. We conclude that the flowers of H. beccarii in Pearl Bay are strongly protogynous. The ratio for above-ground to below-ground biomass decreases continuously from 1. 95 to 0. 53 during population development. Mean fruit density(as calculated from October 2012) was 4125 / m2. Analysis of 186 H. beccarii fruits from Pearl Bay(1.7% of fruits had one seed,79.7% had two seeds,13.6% had three seeds and 5.1% had four seeds),revealed that seed counts varied from 1—4with a mean of 2.22 mature seeds per fruit. Seed reserves in the soil varied with sampling location within the meadow,with an average of 5749 / m2in January,5652 / m2in March and 2728 / m2in April. Based on our results for the area of seagrass and soil seed bank information,we estimate that the soil seed reserve of H. beccarii in Pearl Bay in January is about 1.23 ×109. We conclude that the H. beccarii population in Pearl Bay,Guangxi has the highest area,coverage,shoot density and biomass of all the sites where this species has been recorded in China. We suggest that the continuous decrease of the ratio of above-ground to below-ground biomass with population development means that the value of this ratio can be used as indicator for distinguishing between different developmental stages of H. beccarii populations. Despite being characterized by high seed production and fast growth,this species is threatened by high levels of human induced habitat disturbance,high rates of seed loss in sediments and low seed germination rates,which are all negatively affecting the regeneration of H.beccarii populations.