Linking benthic community structure to terrestrial runoff and upwelling in the coral reefs of northeastern Hainan Island
Introduction
Nutrient pollution has caused major damage to many coastal coral reefs around the world (Szmant, 2002, Fabricius, 2011, Huang et al., 2011). For example, nutrient enrichment, which is defined as an increase in nutrient concentrations (especially nitrogen or phosphorus) in a water body (Fabricius, 2011), inhibits coral fecundity, fertilization, and embryo and larval development (Fabricius, 2005) and increases coral's susceptibility to climate change (Wiedenmann et al., 2012). Moreover, nutrient enrichment induces particulate organic matter (POM) enrichment in the water column and thus reduces light availability, which decreases coral's photosynthesis and calcification (Fabricius, 2011). In areas of nutrient upwelling or in heavily polluted locations (e.g. river mouth and aquaculture effluents), chronically elevated levels of nutrients may alter the coral's physiology and calcification, causing noticeable changes in coral communities (Birkeland, 1997). In addition, as macroalgae benefit from nutrient enrichment more than corals, nutrient enrichment stimulates macroalgal overgrowth, enabling macroalgae to rapidly compete for habitat, and cover and smother living coral in areas where grazing by herbivorous fishes or invertebrates is low (Lapointe, 1997, McCook, 1999, McClanahan et al., 2002) and light is not limited (Fabricius, 2011).
Seasonal upwelling is an important factor affecting not only the physiology of corals, but also the ecology of coral communities in some regions. A minimum temperature of 14.7 °C was recorded on coral reefs in the upwelling season of 2001 in the Gulf of Panama, with severe coral mortality occurring (Glynn and Fong, 2006). Strong upwelling has been shown to decrease coral's growth rate (Glynn, 1977), select for species-tolerant to nutrient-rich and cold water (Brown, 1997, Schmidt et al., 2012), reduce coral recruitment (Rodriguez et al., 2009), limit reef development (Mate, 2003, Schmidt et al., 2012) and stimulate macroalgal growth, which will promote coral-algal competition for space (Szmant, 2002). Thus, coastal coral reefs in upwelling regions appear to be more susceptible to nutrient pollution, where overfishing also commonly exists. However, to date, studies have either focused on the upwelling processes or on coral reefs, with less attention to relating the upwelling processes to coral reef condition. The mechanisms affecting coral reef ecosystems in upwelling regions (e.g. benthic community interactions) are not well studied.
Hainan Island is located in the tropical northern periphery of the Indo-Pacific Ocean in the South China Sea. Eastern Hainan Island is characterized by a tropical monsoon climate, with the dry season from November to April and the rainy season from May to October. It is affected by northeast wind and waves in the winter, and southerly wind and waves in the summer (Zhang et al., 2006). In the summer, the coast of eastern Hainan Island is also affected by the seasonal QDU (Jing et al., 2009). The Wenchang area is located in northeastern Hainan Island (Fig. 1). Shrimp and fishponds have expanded quickly since the 1960s at the expense of natural wetlands on the northeastern coast of Hainan Island (Herbeck et al., 2013). Historically, coral reefs were abundantly distributed in shallow waters (<10 m) in the coastal areas of Wenchang (Zhou, 2004). However, since the 1970s, at least 50% of the fringing reefs of Hainan Island have been destroyed by human activities, including overfishing, land clearing, coral mining, aquaculture expansion and pollution (Zhang et al., 2006, Hughes et al., 2013, Li et al., 2013). Unlike the well-studied coral communities in southern Hainan Island (e.g. Sanya) (Hutchings and Wu, 1987, Zhang et al., 2006, Li et al., 2013), there is currently a lack of knowledge regarding the coral species composition and spatial distribution in eastern Hainan Island. Wang et al. (2013) reported that wave energy was highly correlated with coral assemblages off Changqi Harbor in Wenchang and concluded that light intensity and wave energy are important factors regulating the coral community along a depth gradient. However, it is still unclear how terrestrial runoff (sewage or aquaculture effluents) and upwelling influence the coastal coral reef ecosystem in Wenchang (Zhang et al., 2013).
The aim of this study was to test the hypothesis that the structure of benthic communities is significantly affected by terrestrial runoff and upwelling in the coral reefs of northeastern Hainan Island. To test this, the relationships between the benthic communities and environmental parameters were examined, and a conceptual model linking terrestrial runoff and upwelling to the coral reefs was proposed. This study will contribute to our understanding of how terrestrial runoff, especially aquaculture effluents, influences coral reefs in upwelling regions, where they are considered as potential refuges in a future of rapid climate change (Riegl and Piller, 2003).
Section snippets
Study site
The study sites (1S–6D) were located on the northeastern coast of Hainan Island (Fig. 1). The Wenchang and Wenjiao rivers enter Bamen Bay and then discharge into the coastal South China Sea. The study sites were about 2.5–10.1 km away from the outlet of Qinlan Harbor. The nutrients in Bamen Bay are mostly loaded from riverine inputs, groundwater discharges and aquaculture effluents (Liu et al., 2011). Pollutants from Bamen Bay and the bypassing of vessels in Qinlan Channel contribute to the
Environmental parameters
The environmental parameter data are presented in Table A1. Mean values were 1.7 °C higher for seawater temperature, 0.2 lower for salinity and 0.31 mg l−1 higher for DO at back-reef stations 3S–6S than at fore-reef stations 3D–6D in August 2011. Along with the depth profile at stations 3D–6D, seawater temperature and DO decreased while salinity increased at 2–3-m depths (Fig. 4). The in-situ long-term seawater temperature and salinity showed that, in 2011, the QDU, with its low temperature and
Upwelling on the coral reef water
This study is the first to report in-situ long-term seawater temperature and salinity in the coral reefs of eastern Hainan Island during the upwelling season. The results suggest that the QDU influences the reef waters in Wenchang, with this influence particularly evident at deep-water stations and in the summer of 2011. At deep-water stations, we detected a lower temperature (1.7 °C), DO (0.3 mg l−1) and higher salinity (0.2) during the upwelling. Seawater temperature was significantly
Conclusions
In this study, terrestrial runoff (river discharges and aquaculture effluents) and seasonal upwelling have influenced the coastal reef waters and caused nutrient enrichment. “Top-down” (i.e. overfishing) and “bottom-up” (i.e. nutrient and cold water) processes are likely to have resulted in the overgrowth of corals by macroalgae and a phase shift from coral-dominance to macroalgae-dominance at most stations there.
Besides the ecological functions of the coral reefs for fisheries, tourism and
Acknowledgments
This work was supported by the National Natural Science Foundation of China (40830850, 41106141, 41476134, 40931160437 and U1301232), the National Key Technology R&D Program (2012BAC19B08), the Ocean Public Welfare Scientific Research Project (201005012-6), the Postdoctoral Science Foundation of China (2013M540347 and 2014T70406) and the Open Research Fund of State Key Laboratory of Estuarine and Coastal Research (SKLEC-KF201307).
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