Distribution and community structure of subtidal seagrasses,mangroves and Spartina alterniflora on the coastal wetlands of Weizhou Island, Guangxi

Distribution and community characteristics of the subtidal seagrasses, mangroves and exotic cordgrass Spartina alterniflora in Weizhou Island, Guangxi, were systematically studied. The results from 2019 to 2020 show that seagrasses, growing in subtidal zone and covering an area of 4 246 m², were close to the coral reef area in the west side of Nanwan Bay, Weizhou Island. The subtidal seagrass bed in the Nanwan Bay was dominated by Halophila ovalis. Six true mangrove species (Rhizophora stylosa, Excoecaria agallocha, Avicennia marina, Aegiceras corniculatum,Acrostichum aureurm and the exotic species Sonneratia apetela) in Weizhou Island were identified, accounting for 50% of total true mangroves species in Guangxi. They were distributing in Xijiaogou (northwest of the island), Niujiaokenggou (north of the island) and Wucaitan (southeast of the island) with areas of 450 m². There were 6 semimangrove plants found in the island, accounting for 75% of the total in Guangxi. The exotic cordgrass, Spartina alterniflora, was rapidly spreading, covering an area of 70 m² which were 3 times of the original area in 9 months at Xijiaogou, the northwest of the island. Although the area of mangroves and seagrass beds in Weizhou Island, Guangxi is relatively limited, it is one of a few areas where the three typical coastal ecosystems (seagrass bed, mangrove forest and coral reef) coexist in China, showing high values for conservation and scientific purpose. Thus, the management of the coastal wetlands in Weizhou Island should be strengthened for the conservation of the native mangrove plants. A close attention should be paid to monitoring the exotic species S. apetala and S. alterniflora and they should be removed when necessary. Besides, proactive measures should be taken to prevent and control the harmful algal blooms in order to avoid their negative impact on the seagrass bed and even the entire marine ecosystem of Weizhou Island.