Impact of Sonneratia caseolaris, an exotic mangrove species, on macrobenthic community
Student thesis: Master's Thesis
An exotic mangrove plant, Sonneratia caseolaris (L.) Engl, was discovered in the foreshore mudflat and mangrove areas in Deep Bay, Hong Kong SAR in early 2000. Its presence is mainly due to the dispersal of fruits and seeds from the nearby Futian Mangrove Nature Reserve, Shenzhen, China, where Sonneratia had been introduced for afforestation in 1993. Owing to its fast growing and proliferating properties, Sonneratia has become invasive, and frequent removal works have been conducted by government officials to prevent its spreading. Many studies were conducted in the last decade to evaluate the effect of Sonneratia on nutrient dynamics and productivity, as well as its competition with native mangroves. However, the effect of Sonneratia on the macrobenthic community, which is an important link to the trophic level and nutrient recycling in a mangrove ecosystem, is not fully understood. The present study aims to (i) examine the possible impact of Sonneratia; (ii) the effect of tidal levels within a Sonneratia stand and (iii) the effect of deforestation of Sonneratia on the macrobenthic community in Futian Mangrove Nature Reserve. Core sediment samples were collected seasonally from August 2008 to December 2009 in five regions within the Reserve dominated by different mangrove plants, namely Kandelia obovata, Avicennia marina, S. caseolaris, mixed forest and open mudflat. In early 2009, a small patch of Sonneratia in the Reserve was chopped down and core sediment samples were also collected to study the deforestation effect. Each sediment sample was sieved and macrobenthic animals were identified and quantified. The physico-chemical properties of the sediment, including redox potential, particle size, concentrations of total organic matter (TOM), total Kjeldhal nitrogen (TKN), total phosphorus (TP), polycyclic aromatic hydrocarbons (PAHs) and heavy metals were analyzed. The relationships between the physico-chemical properties and macrobenthic community structure were determined by multivariate analysis. A total of 41 macrobenthic species, mainly composed of polychaetes, followed by gastropods, was recorded. Oligochaete, Limnodriloides sp. was the most abundant species in the Reserve, followed by polychaetes, Capitella capitata. These opportunistic species contributed 76.9% of the total number of individuals. The dominance of these saprobic species was related to the continuous discharge of raw municipal sewage from the surrounding premises, which make the mangrove sediments anoxic and rich in organic matter and heavy metals, especially cadmium. Multivariate analyses revealed that the species composition in the Sonneratia stand was significantly different from that in the Kandelia, Avicennia stands and the natural forest, with more deposit and filter feeders, such as Ampharete arctica, Polydora sp., Potamilla acuminata and Tharyx sp. The benthic animal community structure in the open mudflat was also different from that in the Sonneratia stand where fewer aforementioned deposit and filter feeders were observed. The presence of Neanthes glandicincta differentiated the community structure in the mudflat from that in the vegetated area. After deforestation, N. glandicincta could colonize this area and the community structure could be restored to the status quo (i.e. open mudflat). Seasonal changes in species richness and species abundance were observed. Many polychaete species, such as Ampharete arctica, Capitella capitata, Neanthes glandicincta, Polydora sp. and Potamilla acuminata showed a clear seasonal variation pattern. A decreasing trend of both species abundance and richness was found from February 2009 to September 2009, partly attributed to the change of salinity which was positively correlated with the biomass and species richness. The presence of Sonneratia and spatial variation did not significantly affect the physico-chemical properties of mangrove sediments but these properties were subject to temporal changes, which would trigger an alteration of community structure. The temporal variation of toxic pollutants, especially cadmium, was remarkable, which reached an alarming level in August 2008 and dropped afterward. The Reserve was found to be less polluted in terms of metallic and organic pollutions from August 2008 onwards. BIOENV analysis indicated that habitat type and tidal level were the paramount factors influencing the community structure, whereas sediment physico-chemical properties were supposed to play a secondary role. Nonetheless, toxic pollutants including cadmium and PAHs were found to be negatively correlated with biomass and species richness. The present study revealed that the invasion of Sonneratia could modify the macrobenthic community by increasing habitat heterogeneity and providing a more stable habitat for the macrobenthos. A new community structure with more species was therefore created. However, Neanthes glandicincta, an important food source for waterfowl, became a rare species in the Sonneratia stand, suggesting that bird community could be jeopardized by a Sonneratia invasion. Regular monitoring works are essential, and a precautionary approach should be adopted to control its spreading.
- Sonneratiaceae, Mangrove plants, China, Hong Kong, Ecology, Mangrove ecology, Benthic animals