This study was conducted to assess the health of the coastal ecosystem based on bio-physical assessment of the coral reef ecosystem at Palk Bay. A total of 11 sites along the coast from the eastern part of Rameswaram Island to Mandapam were chosen for this study. Good live coral cover (acroporid and non-acroporid corals) of 30.08 ± 6.72% was recorded in the eastern part of Rameswaram Island. Principal component analysis demonstrated that the contribution of live coral cover was closer to the selected study sites of the eastern part of Rameswaram Island. Metric multi-dimensional scaling analysis showed three major clusters with 75% similarity based on life-form categories observed in the reef ecosystem. In this study, there was a significant segregation of study sites of the eastern part of Rameswaram Island in two clusters with 80% similarity because of their richness of live coral cover. Multi-dimensional scaling based on coral species diversity showed three major clusters with 50% similarity. Principal component analysis showed that the major contribution and abundance of coral species were closer to the northern part of Rameswaram Island. Moreover, Shannon–Wiener species diversity index of this region was in the range of 1.592–1.856, which was higher than the other sites. Hence, it was observed that there is no relationship between mean live coral cover and coral species diversity index of the reef ecosystem. The impact of biological indicators such as Halimeda algae and Palythoa sp. (Zoanthids) on the reef ecosystem was also analyzed. About 31 species of live corals belonging to 9 families and 18 genera were recorded during the study period. The outcome of this study helped in the understanding of the health of the ecosystem based on the intensity of mean live coral cover and coral species diversity. It also made it easier to understand the longitudinal variations and distribution pattern of coral reef community categories.
Information on the sessile benthic community of coral reefs in terms of different life-form categories can help to assess the health of a reef ecosystem, in addition to ensuring species richness of the reef community (English et al., 1997). India has a long coastline of 7517 km with four Marine National Parks. Core ecosystems of these Marine National Parks are coral reefs, mangroves and seagrass ecosystems. It is vital to monitor the overall health of the coast, considering the impacts of regional and global climate change on coastal resources.
Coral reefs are one of the key ecosystems as it guards shores as well as supports breeding grounds for diverse groups of associated organisms. In India, the total coral coverage is 2374.9 km2 (Bahuguna et al., 2013). One such distinct area among coral environments of Indian coastal regions is Palk Bay in South India (Figure 1) compromising considerable coverage longitudinally east-west between the southern end of Ramnad district and south of Rameswaram Island (Kumaraguru et al., 2005; Bahuguna et al., 2013).
Palk Bay is located in the southwest part of Bay of Bengal and is largely inhabited by fringing reefs and reef associated fauna and flora (Wilson, 2009). This reef ecosystem extends from Agnitheertham (Rameswaram) in the east to Munaikaud (Mandapam) in the west. The overall extent of the reef ecosystem includes approximately a 25–30 km stretch of coral reef flat area (Kumaraguru et al., 2005). The eastern part of Rameswaram Island is dominated by Acropora spp., and the northern part of Rameswaram Island and Mandapam coastal area are dominated by Porites sp.
Coastal ecosystems are highly vulnerable to certain key physical factors such as sea surface temperature (SST) and salinity. It is estimated that an increase of more than 1°C within a very short period of time may lead to coral bleaching (Reaser et al., 2000; INCOIS, 2011). In recent times, coral reefs in the Palk Bay ecosystem experience increasing natural and anthropogenic interventions (Kumaraguru et al., 2003), which leads to the degradation of the reef ecosystem. One of the prime reasons for the degradation of the reef ecosystem observed in other regions through a process in which excess CO2 reacts with ocean water to form carbonic acid that rapidly dissociates into hydrogen (H+) ions (Jiang et al., 2010). Due to this process, lowering in pH of the coastal water, calcification rate is minimized in the reef formation process of the corals.
Hence, the main objective behind this study was to assess ecosystem health based on available resources such as coral growth forms, its associated life-form categories and richness of coral species diversity.
Materials and methods
In this study, 11 sites (Figure 1) were selected to represent the entire coral reef ecosystem of Palk Bay coast from east (Rameswaram) to west (Mandapam). Furthermore, the study sites were classified into three regions such as Rameswaram East (Agnitheertham to Devils point), Rameswaram North (Vadakaud to Thangatchimadam) and Mandapam (Bison House to Munaikaud). Sessile benthic communities were studied using the line intercept transect method (English et al., 1997) from September to December 2014. A 50-m long flexible underwater tape was laid on the reefs roughly parallel to the shore with replicates at each site. Video-transects were also taken for further analysis and the benthos coming under the transition points were recorded using international codes (English et al., 1997; Kumaraguru et al., 2003; Marimuthu et al., 2010, 2013; Kumar et al., 2014; Al-Sofyani et al., 2014; Ravindran et al., 2014; Sawall et al., 2014). The raw data were sorted and assessed using AIMS Reef Monitoring Data Entry System (ARMDES [AIMS reef monitoring data entry system] V1.6).
The percent cover was calculated for different life-forms in the reef ecosystem. Cumulative percentage of live coral cover in-terms of acroporid (ACR) and non-acroporid (NAC) corals, sponges (SP), soft corals (SC), lifeless form of corals in terms of dead coral with algae (DCA), rubble (R) and rock (RCK), algal cover in-terms of algal assemblage (AA) and Halimeda (HA), abiotic forms such as sandy flat (S) and silt (SI) and other associated organisms (OT) such as clams, gastropods, sea stars, and sea cucumbers, were also estimated.
Principal component analysis (PCA; to identify the dominant life-forms and species corresponding with the study sites) and metric dimensional scaling (MDS; for similarity analysis) were carried out using PRIMER 7 Version 7.0.5 (Clarke and Gorley, 2015).
Live coral cover
The live coral cover observed along the Palk Bay coast was classified into two categories, ACR and NAC.
It was observed that the tabular coral forms dominated ACR, whereas massive forms dominated NAC in the Palk Bay. Figure 2 shows the bio-physical status of coral reef life-form categories and distribution of ACR and NAC in Palk Bay. Among 11 study sites, live coral cover (39.75%) was maximum at Olaikudah followed by Sangumal (29.53%) and Agnitheertham (25.65%) (Figure 2). Tabular, branching and digitate forms of ACR were found to dominate the eastern part of Rameswaram Island. Similarly, massive and branching forms of NAC were found to dominate the same part. Healthy live coral cover was observed in the Eastern and the Northern sectors, the former being more dominant. In addition, the average cover of SP, an associated life-form category of the reef ecosystem also observed to be higher in the eastern part of Rameswaram Island. DCA, R and RCK were classified as lifeless form of corals. R and RCK cover contribute extreme space for further attachment of coral spat recruitment. The contribution of DCA was observed to be dominant in the northern part of Rameswaram Island (Figure 2b) followed by Mandapam coast whereas R cover was observed more in Mandapam coast. Hence, Mandapam reef ecosystem was determined to be the most degraded based on these life-form categories.
Among two different types of algal cover observed in this study (AA and HA), AA contributed more in the eastern part of Rameswaram Island followed by Mandapam coast (Figure 2c). The mean cover of HA, which is a positive indicator for the reef ecosystem, dominated the northern part of Rameswaram Island followed by Mandapam coast. S, SI and water cover belong to abiotic forms in the reef ecosystem. The eastern part of Rameswaram Island followed by Mandapam coast showed a higher covering of abiotic forms (Figure 2b). False corals, such as zoanthids, act as competitors for the settlement of young corals and hence they are classified in the category of negative indicators. The mean cover of zoanthids was observed to be more in northern part of Rameswaram Island than the other sites (Figure 2c). The mean coverage of other organisms (OT) was observed much less (0.2 to 0.35%) in a few study sites.
During this study, the available Scleractinian coral fauna checklist with corresponding sites was prepared (Table S1 in the supplemental information). About 31 species of 9 families were identified in this study and verified (Veron, 2000; Venkataraman et al., 2003). The dominant genera recorded all along the Palk Bay coast were Porites sp., Favia sp., Acropora sp., Platygyra sp., Goniastrea sp., Favites sp. and Siderastrea sp. However, Goniopora sp., Gardineroseris sp., Montastrea sp., Hydnophora sp., Cyphastrea sp., Coscinaraea sp., Montipora sp., Symphillia sp., Astreopora sp. and Turbinaria sp. were rarely found in the study sites. Shannon–Wiener diversity index (H') of the northern part of Rameswaram Island (Villunditheertham followed by Thangatchimadam) was greater compared with the other sites (Table S2 in the supplemental information).
Figure 3a shows the PCA of different life-form categories with their selected study site description. Live coral forms including ACR and NAC were found in higher covers in the eastern part of Rameswaram Island such as Olaikudah, Sangumal, Devils point and Agnitheertham. Strong variability was obtained between Rameswaram Island and Mandapam coast based on the observed live coral cover (PC1: 39.7% variance) which supports the earlier observation (Figure 2). DCA and R covers were observed to be in abundance in the anthropogenic-impacted areas of the northern part of Rameswaram Island and Mandapam coastal regions such as Villunditheertham, Munaikaud, Thonithurai and Shooting spot, where seaweed culture and fishing vessel activities are more intense. PCA based on coral species diverstiy (Figure 3b) shows that the major contribution of coral species is towards the northern part of Rameswaram Island such as Villunditheertham, Thangatchimadam and Vadakaud. Major species found in this region were Favites, Favia, Platygyra, Astreopora, Turbinaria, Siderastrea, Hydnophora, Symphyllia, Montastrea and Syphastrea sp. The recorded Shannon–Wiener species diversity index of the corresponding site was found to be higher than the other sites. Moreover, species diversity gradually reduced towards Rameswaram east and Mandapam coastal area from Rameswaram north (Table S2 in the supplemental information).
Figures 3c and d show the MDS analysis of both life-form categories and coral species with their site description. Based on life-form categories observed in the reef ecosystem, three major clusters were formed with 75% similarity. Among them, one mixed cluster included sites from Agnitheertham to Villunditheertham (eastern and northern sides of Rameswaram Island), second one included sites from Thangatchimadam to Bison House and third one included sites from Thonithurai to Shooting spot (Mandapam coast). Based on coral species diversity, three major clusters were formed with 50% similarity. Among them, one cluster included sites between Villunditheertham and Thangatchimadam in the northern part of Rameswaram Island, second one comprised the eastern and northern parts of Rameswaram Island (Sangumal to Vadakaud), and third one included sites between Thonithurai and Munaikaud.
Among the 11 sites, the maximum mean cover of live corals observed in this study was 39.75% at Olaikudah in the eastern part of Rameswaram Island whereas the minimum cover (1.5%) was recorded at Shooting spot in the Mandapam coast. The reason behind lower cover observed in the Mandapam coastal region might be the impact of natural disturbances such as 2004 tsunami (Kumaraguru et al., 2005; Wilson et al., 2005; Wilson, 2009), catastrophic bleaching events that occurred in 1998 (Arthur, 2000), 2002 (Kumaraguru et al., 2003) and 2010 (Ravindran et al., 2012) and regular movement of silt from Gulf of Mannar during the south-west monsoon. Hence, the cumulative cover of DCA and R was recorded as 63.56 ± 21.19% in this region during the study period.
Although low live coral cover was observed in the sites of northern part of Rameswaram Island, species diversity index of these sites was found to be higher than the other regions. It was noted that large numbers of community people were engaged in seaweed harvest from wild for the agar industry. The reason for high species diversity observed in this part might be the occurrence of less cover competitive organisms such as AA and zoanthids. It was also found that the greater contribution of positive indicator of the reef ecosystem, HA, was observed to be more towards Villunditheertham and Vadakaud (mean cover of 3.45%) of northern part of Rameswaram, followed by Mandapam coast (1.68%).
In this study, uneven distribution of NAC was observed between the selected regions in the Palk Bay coast. A gradual increase in the ACR cover from Mandapam to Olaikudah has also been observed and an ACR domination in the eastern part of Rameswaram Island. Overall, the NAC cover contributed more to the live coral cover than ACR. Therefore, the overall live coral cover also increased from Mandapam to Rameswaram east (Figure 3a). Indicator organisms can be used to assess the health of the reef ecosystem (Al-Sofyani et al., 2014). In this study, HA was the commonly observed indicator in the northern part of Rameswaram Island and Mandapam. However, their contribution was more in the northern part of Rameswaram Island. Hence, northern part of Rameswaram Island was determined to be a healthy ecosystem in terms of abundance of coral species diversity (H’, 1.592 to 1.864) and mean cover of HA (3.45 ± 0.09%).The relative abundance of Halimeda are believed to be a good indicator and vital for reef builders (Mohammed and Nasser, 2005; Youle, 2010). Moreover, the loss of coral cover of northern part of Rameswaram Island from bleaching event 2002 (Kumaraguru et al., 2003) and Tsunami event 2004 (Kumaraguru et al., 2005) were reported as 11.25% and 6.4%, respectively. These losses were observed comparatively less when compared to Rameswaram Eastern part (cumulative loss of 26.1%) and Mandapam coast (cumulative loss of 18.6%). Hence, it might be the reason for the highest diversity index of coral communities in this region. Bahuguna et al. (2008) also reported tsunami mediated reef degradation in the Andaman and Nicobar Islands as 22–29% cover by using Advance Wide Field Sensor data of Indian satellite RESOURCESAT. A large array of organisms competes for occupying any unoccupied surface in the marine environment. Among them, zoanthids (a member of the order, Zoanthidea) play an important role in colonizing some of the substrates (Nylund and Pavia, 2003) of the reef ecosystem. Zoanthids resemble corals (false coral or pseudo coral), but lack hard skeleton. Moreover, their polyps are embedded in their mesogloea (Allen and Steene, 2007).
It is believed that the sediment incorporation ability of zoanthids from the bottom helps in their growth as well as the protection of their polyp (Allen and Steene, 2007). In this study, Palythoa sp. (Figure S1 in the online supplemental information) was found to be distributed all along the coast of Rameswaram Island, but the intensity (2.18% cover) was observed to be higher in Vadakaud (northern part of Rameswaram Island). In addition, the live coral cover observed in this site was less when compared with the eastern part. As a tertiary fouler, algae can also compete with the coral recruits (McCook, 2001; Birrell et al., 2005, 2008) on the dead coral surface for perpetuation (Marimuthu and Joshi, 2014). It is noted that an uneven distribution in the cover of AA all along the coast. Considerably, less cover of AA was observed in the northern part which coincided with the activity of the community people such as seaweed harvest from the wild for agar industry.
Sewage discharges, aquaculture and fish vessel activities are the major threats observed in the Mandapam coast (Figure S1 in the online supplemental information). Moreover, Mandapam coastal area of Palk Bay was also reported as microbial polluted region which plays a vital role in occurrence of coral diseases. Thinesh et al. (2011) interpreted this coastal region with the discharge of sewage from fish processing and fish landing activities which leads to pathogenic microbial pollution (with the decline of beneficial bacteria) and also reported six types of coral diseases in this region. There were significant variations in Dissolved oxygen content (2.33 to 2.96 ml l−1) reported nearer to Thonithurai site of Palk Bay compared to Gulf of Mannar in the year 2003 (Sulochanan and Muniyandi, 2005) and southwest monsoon period of 2006 and 2007 (Sulocahanan et al., 2011).
This assessment provided a baseline data set of the present scenario of Palk Bay corals. Moreover, it helped in the evaluation of the coastal health status, which is needed for outlining suitable management strategies. Furthermore, these results will provide support to select suitable recipient sites for reef restoration. The NAC were observed to be more common than the ACR and dominated all the study sites. The overall live coral cover significantly decreased towards the west (Mandapam coast). This study will help to conserve vulnerable coral species based on the results derived from species richness. Moreover, awareness of marine pollution on coastal habitats and the importance of conservation strategies play a major role in the improvement of these habitats. Increased implementation of Marine Protected Areas is crucial for the preservation of available coral resources in Palk Bay.
The authors acknowledge with thanks the Tamil Nadu Forest Department for their constant encouragement and field support. We are grateful for the reviewer's critical comments, which improved the manuscript substantially.
Supplemental data for this article can be accessed on the publisher's website.