Please use this identifier to cite or link to this item: https://scholarbank.nus.edu.sg/handle/10635/118571
Title: THE SIGNIFICANCE OF SARGASSUM ON SINGAPORE'S REEFS
Authors: LOW KIM YEW JEFFREY
Keywords: Sargassum, seaweeds, algae, Singapore, coral reefs
Issue Date: 24-Jul-2014
Citation: LOW KIM YEW JEFFREY (2014-07-24). THE SIGNIFICANCE OF SARGASSUM ON SINGAPORE'S REEFS. ScholarBank@NUS Repository.
Abstract: This thesis presents the first quantitative studies on the macroalgae Sargassum, which has long been known to comprise a major component on Singapore's coral reefs, but was never fully studied. It comprises chapters that reviewed its classification, quantified its distribution and seasonal variation, and investigated the effects of herbivory within Singapore's reef system. Forty-one valid species were documented from both the Singapore Herbarium (at the Singapore Botanic Gardens) and the Herbarium, Lee Kong Chian Natural History Museum. Morphological and DNA analyses of fresh specimens, however, confirmed the presence of only five species in the field, namely, S. ilicifolium, S. granuliferum, S. aquifolium and S. swartzii, which are subtidal, while the fifth species, S. polycystum, was intertidal. The occurrence and distribution of the other 36 species are currently unknown. Sargassum was found to be distributed throughout the islands south of Singapore, with long term reef monitoring data documenting its occurrence for over two and a half decades. Sargassum was persistent on the reefs (the basal portions are perennial), with higher densities on the reef flat than the reef crest. The densities on the reef crest ranged between 15 ? 11.9 m-2 and 30.2 ? 9.4 m-2, and on the reef flat, between 57.4 ? 20.5 m-2 and 93.8 ? 28.4 m-2. All species showed cyclical growth and die-back, which coincided with the Northeast Monsoon (between August and March) and the Southwest Monsoon (between April and July) respectively. Peak average length and biomass for S. ilicifolium were 219 ? 35 cm and 18.5 kg/m2 wet weight (WW), while for S. aquifolium were 88 ? 14 cm and 465.6 g/m2 WW. The biomass for S. ilicifolium was 5-40 times that reported elsewhere. Not enough data could be collected for the other two subtidal species. Herbivory pressure was measured in terms of bite rates using underwater video, and was many magnitudes lower than reported elsewhere, at just 0.24 bites/h. Consumption rates using macroalgal bioassays were also low, estimated at 0.125 cm/h. An indirect measure of herbivory, using bite marks on leaves, showed marginally higher fish bites at the outer reefs. This result was supported by fish surveys which showed non-signifcant differences between sites for herbivorous fish, which were more abundant in the outer islands. The macro-herivores on local reefs comprised two fish genera, Scarus and Siganus, and the sea urchin, Diadema setosum. Both their abundances were low, with maximum densities of 0.203 individuals/m2 and 0.205 individuals/m2 respectively. Similarly, herbivory by epifauna was low, despite showing similar community structure with studies elsewhere. Only about 30% of the fauna observed were herbivores, and occurred in abundances of no more than 7 individual/g WW of algae. This was 3 to 32 times lower than reported for substantial effects on macroalgal cover to be observed. No spatial variation could be seen in epifaunal communities on Sargassum, nor on Hypnea and Bryopsis which were used for comparative studies. There were, however, distinct differences in the epifaunal communities between Hypnea and Bryopsis, with the assemblage from Sargassum overlapping both.
URI: http://scholarbank.nus.edu.sg/handle/10635/118571
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