Long-term changes in coral communities under stress from sediment
Introduction
Disturbances of coral reefs are usually caused by a complex combination of several stresses, such as sedimentation, coral bleaching, destructive fishing practices, low water quality, predation and diseases. Many of these stressors are increasingly driving coral reef degradation on a regional and global scale. Thus, maintaining and/or increasing the resilience of coral reefs will be dependent on a reduction in local impacts (Cooper et al., 2009).
There are several scientific reports on how different disturbances affect coral reefs (Fabricius, 2005, Pratchett et al., 2009) however there are still critical gaps in the knowledge of how coral reefs can recover from disturbances and retain a coral-dominated state following various disturbances (Hughes et al., 2010). Some researchers have shown that coral reefs that maintain their structure and function can recover from large-scale disturbances (Halford et al., 2004, Sheppard et al., 2008).
Disturbances to coral reef ecosystems can be chronic or acute (Connell et al., 1997). Sedimentation and nutrient input from coastal development are considered chronic disturbances to coral reefs. They are slow to change and exert continuous stress on the system (McCulloch et al., 2003, Fabricius, 2005, Cooper et al., 2009). An acute disturbance, e.g. a coral disease epidemic or a coral bleaching phenomenon, can change the community into a new dominant component where reinforcing feedback mechanisms can prevent coral recovery (Mumby et al., 2007, Hughes et al., 2010, Scheffer et al., 2001). However coral recovery may occur between acute disturbances and it is necessary to gain insights on how coral-dominated communities can persist in the face of global change (Graham et al., 2011).
Coral recovery trends are influenced by several factors, such as disturbance patterns, reef characteristics, reef connectivity, ecological characteristics and human impacts. High reef connectivity is very important for replenishing marine organisms (Ayre and Hughes, 2004, Jones et al., 2009). The grazing of sea urchins and herbivorous fish and high functional diversity can promote ecosystem functions that will recover more rapidly from disturbances (Mumby et al., 2007, Burkepile and Hay, 2008). Coral reefs in the vicinity of urban areas may exhibit slower recovery times because of increased terrestrial inputs and the exploitation of natural resources (Sandin et al., 2008, Graham et al., 2011).
Recent studies have provided a large database showing that sedimentation, nutrient enrichment and turbidity can degrade coral reefs on a local scale (Fabricius, 2005). Sedimentation is recognized as a severe disturbance for coral reefs by reducing growth and survival in a wide range of coral taxa. Elevated sedimentation rates from coastal erosion and development have severely damaged nearshore coral communities (Rogers, 1990, McClanahan and Obura, 1997). Most sediment is transported into coastal ecosystems via rivers. The majority of the larger sediment grain sizes are deposited within a few kilometers of a river's mouth, whereas fine grain sizes can be carried over longer distances. High sedimentation rates may damage exposed coral tissue within a few days and low sedimentation rates can reduce photosynthesis rates in corals (Riegl and Branch, 1995, Philipp and Fabricius, 2003). Coral degradation is not only dependent on the quantity and duration of sedimentation, but is also strongly dependent on sediment types and environmental conditions. Low sedimentation rates, when combined with transparent exopolymer particles, cause mortality of coral recruits (Fabricius et al., 2003). Exposure to sediments in a few days may cause long-term impacts in coral populations and inhibits coral recovery following a disturbance. Therefore several nearshore coral communities may potentially shift forwards assemblages dominated by sediment-tolerant coral species (Sofonia and Anthony, 2008).
The present study is aimed at assessing the long-term changes of coral communities (2004–2010) by examining sedimentation rates, coral community structures and coral recruitment patterns at the study sites in the Gulf of Thailand.
Section snippets
Study area
The Gulf of Thailand is a semi-enclosed tropical sea, located in the South China Sea and surrounded by four countries, i.e. Cambodia, Malaysia, Thailand and Vietnam. It is relatively shallow, with a mean depth of 45 m and a maximum depth of 80 m (Wattayakorn, 2006). Two coral communities on the south of Samui Island, Thailand were selected for the long-term study, namely Ao Thong Tanote (9°24′57″N, 99°56′18″E) and Ao Thong Krut (9°25′7″N, 99°56′49″E) (Fig.1). Samui Island has been recognized as
Variation of sedimentation rates
The means of the sedimentation rates during the study periods at Ao Thong Tanote ranged from 29.49 to 52.31 mg cm−2 d−1 while those at Ao Thong Krut varied from 45.51 to 59.53 mg cm−2 d−1 (Fig. 2). There were significant statistical differences in sedimentation rates during study periods (Table 1). According to the multiple comparisons, the sedimentation rates between the years 2004–2006, 2004–2008, 2006–2008, and 2008–2010 were significantly different. The variation in sedimentation rates may have
Discussion
The means of the sedimentation rates of 29.49–59.53 mg cm−2 d−1 found in the present study are relatively high (Rogers, 1990, Lane, 1991). The coral communities in this study had accumulated tremendous loads of sediment to the extent that dead corals were found to have mud draped on them. Therefore sediment deposition is considered as a chronic disturbance to coral reefs at Samui Island (Connell et al., 1997). It has caused a decrease of filter feeders and grazers, especially Diadema setosum (
Acknowledgments
This study was funded by a budget for research promotion from the Thai Government awarded to Ramkhamhaeng University in the fiscal years B.E. 2552–2555 and partly supported by the Provincial Electricity Authority (PEA) of Thailand. We acknowledge Professor Dr. Jing Zhang, the leader of the project “Coral Reefs under Climate and Anthropogenic Perturbations” (CorReCAP) under IOC/WESTPAC for his support. The author would like to thank the staff of the Marine Biodiversity Research Group, Department
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