The scale and intensity of marine cage culture have increased in the Asian-Pacific region, particularly in oligotrophic waters where coral reef organisms flourish. In this study, the influence of marine cage culture on subtropical coral communities in turbid waters was evaluated by measuring environmental parameters and benthic community compositions at Magongwan in the Penghu Islands, Taiwan. A canonical discriminant analysis of environmental parameters revealed that elevated levels of ammonium, nitrite, and chlorophyll a (chl a) released from the cages were the main pollution indicators that, in addition to sedimentation and turbidity, distinguished Impact Zone 1 (cage-culture zone) from the other 2 zones -Impact Zone 2 (800 m away from the cages) and the reference zone -in these turbid waters. Results of the canonical correlation analysis indicated that the coverage extents of macroalgae, sponges, and zoanthids were strongly correlated with levels of ammonium, nitrite, phosphate, chl a, and dissolved oxygen. Coral communities in Impact Zone 1 were mostly composed of stress-tolerant massive and submassive corals, but were lacking branching Acropora corals. In contrast, coral communities in the other zones, with high habitat complexity and species richness, were dominated by coral species with diverse morphologies, including branching Acropora coral communities. These results suggest that marine cage culture has been causing chronic nutrient enrichment in the surrounding waters at Magongwan, which may have resulted in a deterioration of suitable habitats for coral reef organisms. Nevertheless, intermediate levels of nutrients and particulate organic matter relative to the other zones might have been caused by the adjacent cage culture, resulting in the high coral coverage and diversity in the Impact Zone 2 in particular.

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Fine sediments, which account for the majority of total fluvial sediment flux, have been suggested to degrade coral reefs on a global scale. Furthermore, sediment impacts can be exacerbated by extreme rainfall events associated with global climate change and anthropogenic nutrient enrichment. We report the findings from a series of mesocosm experiments exploring the effects of short-term sedimentation and nutrient enrichment on the interactions between the hard coral Acropora muricata, the sea anemone Mesactinia ganesis, and the green macroalga Codium edule. Mesocosms were manipulated to simulate either unimpacted reefs or reefs exposed to elevated levels of fine sediments for 10 or 14 days to simulate the effects of heavy rainfall. The first and second experiments were aimed to examine the effects of inorganic and organic sediments, respectively. The third experiment was designed to examine the interactive effects of nutrient enrichment and elevated sediment loads. Neither inorganic nor organic sediment loadings significantly affected the physiological performance of the coral, but, importantly, did reduce its ability to compete with other organisms. Photosynthetic efficiencies of both the green macroalga and the sea anemone increased in response to both sediment loadings when they were simultaneously exposed to nutrient enrichment. While organic sediment loading increased the nitrogen content of the green macroalga in the first experiment, inorganic sediment loading increased its phosphorus content in the second experiment. The coral mortality due to sea anemones attack was significantly greater upon exposure to enriched levels of organic sediments and nutrients. Our findings suggest that the combined effects of short-term sedimentation and nutrient enrichment could cause replacement of corals by sea anemones on certain coral reefs.

Background: Tidal cycle is an important factor which regularly changes coastal fish assemblages in shallow waters. However, the variations in fish assemblage and trophic structure across tidal cycles in tropical seagrass beds are rarely discussed. We used underwater visual censuses to quantify fish abundance and diversity from the shallow intertidal to the subtidal zone during both flood and ebb tides in seagrass beds surrounding the Dongsha Island, South China Sea. We also recorded fish feeding habits by analyzing stomach contents.

Very little is known about the characteristics of herbivory and selection by various grazers in tropical, multi-species seagrass beds. We used an in situ shoot tethering method to quantify grazing on 3 dominant seagrass species (Thalassia hemprichii, Cymodocea rotundata, and C. serrulata) that co-inhabit extensive seagrass beds surrounding Dongsha Island in the South China Sea. We measured the amount of seagrass grazing as well as leaf production in different habitats and seasons. The dominant seagrass grazers in Dongsha Island were parrotfish, followed by meso-invertebrates, suggesting that herbivory by small invertebrates may also be critical in tropical seagrass ecosystems. Our results revealed significant spatial and seasonal differences in leaf biomass losses. The level of grazing was 3.6× higher in the subtidal than in the intertidal zones, and 2.6 × higher in the warm than in the cool season. Leaf biomass losses were positively associated with parrotfish density, water depth, leaf production, and starch content. However, the nitrogen and total sugar contents did not affect the preferences of the grazers. The 2 Cymodocea species were preferred by most grazers. Collectively, all grazers typically removed <10% of the leaf production in Dongsha Island, which indicates that most seagrass biomass was not directly used by herbivores and flows into detrital pathways.