Our study's findings reveal substantial variations in the coral health assessment in two atoll islands. In Kavaratti reef and Kadamat reef of Lakshadweep. A survey of Kavaratti reef showed 55.04% live coral, 39.4% dead coral and 5.4% other coral, whereas Kadamat reef 52% live coral and 41% dead coral and 7% live coral. Furthermore, the zooxanthellae density increased across three coral species, two islands, and different periods. Notably, the Kavaratti and Kadamat Reef lagoon corals, which are endemic to these islands, have exhibited signs of long-term stress. This stress can be attributed to continuous anthropogenic impacts that have adversely affected coral ecosystems. Both islands are popular tourist destinations, attracting visitors worldwide and playing a significant role in local economies. Previous research has consistently demonstrated that the corals in the lagoon areas of these islands are particularly vulnerable and experience elevated levels of stress (Ranith et al., 2017); similar studies have been conducted in the Malvan Marine Sanctuary region (Kalyan et al., 2020). The variation in the zooxanthellae density in coral reefs is the best characteristic for predicting many health factors in coral reefs worldwide.
We found that Porites cylindrica from both island lagoons had greater zooxanthellae density than did Acropora hyacinthus and Acropora intermedia from the same area. Corals such as Acropora are more likely to bleach than Porites species according to earlier studies (Li et al., 2008; Marshall & Baird, 2000). For instance, in the Lakshadweep archipelago, in Porites, there was less zooxanthellae loss from deteriorated corals (Harithsa et al., 2005). In contrast to this study, we have also shown our results for Porites species, which switch to a higher density of zooxanthellae than other species. It might be that thermotolerance-based clades are more common among Porites species (Lien et al., 2013). The density of zooxanthellae varies significantly among the three coral species, as shown by the highly significant results of the global test for species group comparisons. Our ANOSIM test revealed that (Table 1).
The results of the PERMANOVA (Table 2) shed light on the variables responsible for the observed differences. The findings demonstrated that, with very low P values and a large number of permutations, all factors, including islands, species, and their interactions, were significant. This finding suggested that the density of zooxanthellae in various coral species is significantly influenced by the geographical area of the coral reef. This outcome is in line with the findings of other studies claiming that coral species geographic distribution could have an impact on the number of zooxanthellae present in their tissues (McClanahan et al., 2020; Van Oppen et al., 2005)
The minimum temperature was 28.1°C, which was the lowest temperature recorded during the monitoring period(our recorded temperature data (Fig. 5). In contrast, in March 2021, a peak temperature of 33.5 degrees Celsius was recorded. Over the duration of the observation period, Kavaratti Lagoon experienced a wide range of extremely significant temperature variations. The temperature ranged from a low of 28.1 degrees Celsius during the cooler months to a high of 33.5 degrees Celsius during the hotter months of March and April. Seasonal variations and the impacts of different oceanic currents are also responsible for these variances. Additionally, forecasting future changes and implementing effective conservation measures are crucial. Similar findings were reported for Andaman and Nicobar coral, which were bleached due to temperature fluctuations from 30°C to 34°C (Marimuthu et al., 2013). High temperatures cause the algae to live in their tissues and become vulnerable to disease and death (Brown & Howard, 1985). Without immediate action to mitigate rising temperatures, the long-term survival of coral reefs is at risk.
We also noticed in our PAR data (photosynthetically active radiation) (Fig. 6) that the peaks of average light irradiance and temperature data were similar and moderate between 2019 and March 2022, with a slightly greater peak in 2019. On the other hand, the average light irradiance peaks were found to be higher beginning in April 2021 (209.1 Quantum µmol/m2s) and then decreased slightly until March 2022. Overall, while there are slight variations in temperature between the two islands, the data suggest that they have similar climates, with temperatures typically ranging between 29 and 30°C. Based on the PAR and temperature data, a Phuket, Thailand, report showed that the number of zooxanthellae decreases annually because the temperature is increasing. (Brown et al., 1999)The high-light-intensity photoinhibition of zooxanthellae can be a significant problem.(Hoegh-Guldberg, 1999) Similar results are shown for our three coral species. Similarly, the average zooxanthellae density has shown a decreasing trend for Porites cylindrica and Acropora intermedia and Acropora hyacinthus coral species, which has shown a fluctuating trend.
Corals suffer greatly from elevated temperatures, which increase their vulnerability to damage (Brown, 1997). The difference in light irradiance compared to the seasonal and yearly data also resulted in varying zooxanthellae densities depending on the season(Lesser et al., 1990). Our results showed that the density of zooxanthellae fluctuated, with the premonsoon season experiencing a decrease in density and an increase during the postmonsoon season. A difference in zooxanthellae density between the two seasons was observed on both islands, indicating the impact of these seasons on the marine ecosystem in the study areas. A similar study conducted in Kenya reported a greater density of zooxanthellae in corals during the North monsoon season in the equatorial region (Mwaura et al., 2009).
In addition, sedimentation rates were recorded from 2019 to 2022 in Kavaratti Reefs. The sedimentation rate was shown to be a minimum of 5.3 mg/cm2/day. where the maximum sedimentation was 202 mg/cm2/day. This could ultimately result in the loss of biodiversity and degradation of the coral reef ecosystem (Morgan et al., 2020). Hence, we must monitor and mitigate sedimentation rates to protect and preserve these coral reefs in Kavaratti and Kadamat. Additionally, implementing measures to reduce sediment runoff from nearby land activities can help mitigate the negative impacts on coral reefs and promote their long-term survival.
The RDA triplot (Fig. 7) provided a clear visual representation of the relationship between the density of zooxanthellae and the various environmental factors. The triplot was constructed of points that represented the arrows that indicated the directions and magnitudes of the environmental variables and colour gradients that represented the number of zooxanthellae present in three different coral species that were collected from two different islands. The triplot clearly showed that the density of the zooxanthellae varied among the different environmental variables. It is important to note that the arrows that represent the different environmental factors all point in the same direction, indicating that they are strongly positively related. This finding lends credence to the hypothesis that higher densities of zooxanthellae are associated with higher concentrations of the aforementioned environmental variables.
Previous research has shown that factors such as water temperature, photosynthetically active radiation (PAR), salinity and chlorophyll play important roles in determining the number of zooxanthellae that can be found in coral reef ecosystems. The findings of the RDA are in line with these earlier findings. It makes sense that there would be a positive correlation between the temperature of the water and the number of zooxanthellae present because these microorganisms have a high thermal tolerance and tend to flourish in warmer environments. The positive correlations with PAR (photosynthetically active radiation), salinity and chlorophyll are consistent with the roles that these environmental factors play in preserving the physiological conditions that are most favorable for zooxanthellae.
The pilot study is constrained by its limited sample size and focus on a select few coral species. Nonetheless, it offers preliminary insights into factors impacting temperature anomalies across different coral species. Further investigation is warranted in the Lakshadweep reefs to comprehensively understand coral mortality reduction strategies and variations in zooxanthellae populations. Future studies should adopt a systematic approach, considering variations in coral species thermotolerance and the influence of environmental factors on mortality rates. This study sheds light on the correlation between coral mortality rates, temperature, and photosynthetically active radiation, particularly on how these factors affect zooxanthellae densities in various Scleractinian corals within the Lakshadweep reefs. Our findings reveal significant variations in symbiont algae densities based on geographic location and species distribution, highlighting the crucial role of environmental factors such as light and temperature and sedimention.