Coral Recruitment on Artificial Patch Reefs Deployed in The Marginal Reefs: Effect of Multilevel Substrate on Density of Coral Recruit

A study of coral recruitment on Artificial Patch Reefs (APR) was performed in the marginal reef of Panjang Island, Central Java (Java Sea) to examine whether multilevel substrates of APR affect the density of coral recruits. Long-term and short-term observations were applied in yearly monitoring 2017-2019 and biweekly observations for 3 months in early 2019. Coral recruitment significantly varied among level substrates of APR (F(a,b) .05=3.08; p-value<0.05) and there was a significant difference at the beginning of the year (F(a,b) .05=5.52; p-value<0.05). The density of recruits on the substrates after 4 years post-deployment of APRs was 0.2 to 129.2 m-2 while the recruitment rate within short-term observations was 0.28-1.28 m-2 per month. The highest coral recruitment occurred at the middle to the top level of APR, while the lowest recruitment was found in the lowest level of APR. This is possibly due to high resuspension from the seabed. Oulastrea was dominant in both long- and short-term recruitment periods while Pocillopora was rare due to post-settlement mortality which trigger the overgrown coral-killing sponges. The results indicated that the adaptation of coral recruitment in the marginal environment is determined by the high recruitment of the small-colony coral species which possessed stress-tolerant for turbidity disturbance, such as Oulastrea crispata. This study suggested that the multilevel substrates, Artificial Patch Reefs (APR) are one of the reef rehabilitation methods which can be applied in the marginal environment enhancing coral recruitment.

Differential Symbiodiniaceae Association With Coral and Coral-Eroding Sponge in a Bleaching Impacted Marginal Coral Reef Environment

Marginal reefs are known for severe stress-inducible perturbations such as high sedimentation, eutrophication, ocean warming, and acidification from anthropogenic climate change. The corals striving in such stressful environments develop physiological adaptations induced by differential genomic expressions or association with thermal stress-tolerant algal symbionts (Symbiodiniaceae). Despite such adaptations, corals are threatened by other space competitors such as algae and sponges. Coral-eroding sponges belonging to the Cliona viridis complex are one such space competitors that also associate with Symbiodiniaceae algal photosymbiont. The diversity of Symbiodiniaceae associates with the coral and sponge from the same ecosystems is scarcely known. In the present study, Symbiodiniaceae community structure in the coral Turbinaria mesenterina, a newly described coral-eroding sponge Cliona thomasi, and their surrounding seawater was determined from the nearshore marginal reef along the central west coast of India. The results revealed a significantly higher relative abundance of Durusdinium and Gerakladium than Symbiodinium and Cladocopium in the seawater. Interestingly, both investigated host species showed differential Symbiodiniaceae association with significantly higher abundance of Durusdinium in coral and Gerakladium in sponge. The beta diversity analysis by Permutational multivariate analysis of variance (PERMANOVA) confirmed significant differences in Symbiodiniaceae profiles between sponge and coral. Durusdinium and Gerakladium are thermotolerant genera known to associate with different hosts in suboptimal conditions. Our field surveys suggested the bleaching resistance of the coral T. mesenterina despite the fact that the sea surface temperature reached the coral thermal threshold of 31°C during different periods of the years 2015, 2016, 2017, 2018, and 2019. Therefore, the thermal tolerance of the investigated coral and sponge species may be attributed to their respective thermotolerant photosymbiont associations. Furthermore, the results also indicated the host-specific photosymbiont selection from the local environment. Although these observations provide valuable biological insight, more research is needed to understand the tripartite association of sponge-coral-symbiont together to evaluate the competitive fitness of holobionts.

Climate Change Induced Thermal Stress Caused Recurrent Coral Bleaching over Gulf of Kachchh and Malvan Marine Sanctuary, West Coast of India

Coral reefs are one of the most sensitive, productive, and invaluable biological resources on the earth. However, coral reefs are facing unprecedented stress due to ongoing climate changes and intensified anthropogenic disturbances globally. Elevated Sea Surface Temperature (SST) has emerged as the most imminent threat to the thermos-sensitive reef-building corals. The 2010–2014-2016 El Niño Southern Oscillation (ENSO) caused prolonged marine heat waves (MHWs) that led to the most widespread coral bleaching and mortality in the tropical Indi-Pacific regions. Coral bleaching prediction is vital for the management of the reef biodiversity, ecosystem functioning, and services. Recent decades, satellite remote sensing has emerged as a convenient tool for large-scale coral reef monitoring programs. As thermal stress is a critical physical attribute for coral bleaching hence, the present study examines the effectiveness of the elevated SSTs as a proxy to predict coral bleaching in shallow water marginal reefs. Advanced Very High-Resolution Radiometer (AVHRR) satellite data from the NOAA Coral Reef Watch’s (CRW) platform has been used for this study. Coral bleaching indices like Bleaching Threshold (BT), Positive SST Anomaly (PA), and Degree Heating Weeks (DHW) are computed to analyze the thermal stress on the coral reefs. The computed thermal stress from satellite-derived SST data over regions concurrence with the mass coral bleaching (MCB) events. This study concludes that in the last decades (2010 to 2019) the coral cover around these regions has dramatically declined due to higher SST, which indicates that the thermal stress induced recurrent bleaching events attributed to the coral loss.

Decadal (2006-2018) dynamics of Southwestern Atlantic’s largest turbid zone reefs

Tropical reefs are declining rapidly due to climate changes and local stressors such as water quality deterioration and overfishing. The so-called marginal reefs sustain significant coral cover and growth but are dominated by fewer species adapted to suboptimal conditions to most coral species. However, the dynamics of marginal systems may diverge from that of the archetypical oligotrophic tropical reefs, and it is unclear whether they are more or less susceptible to anthropogenic stress. Here, we present the largest (100 fixed quadrats at five reefs) and longest time series (13 years) of benthic cover data for Southwestern Atlantic turbid zone reefs, covering sites under contrasting anthropogenic and oceanographic forcing. Specifically, we addressed how benthic cover changed among habitats and sites, and possible dominance-shift trends. We found less temporal variation in offshore pinnacles’ tops than on nearshore ones and, conversely, higher temporal fluctuation on offshore pinnacles’ walls than on nearshore ones. In general, the Abrolhos reefs sustained a stable coral cover and we did not record regional-level dominance shifts favoring other organisms. However, coral decline was evidenced in one reef near a dredging disposal site. Relative abundances of longer-lived reef builders showed a high level of synchrony, which indicates that their dynamics fluctuate under similar drivers. Therefore, changes on those drivers could threaten the stability of these reefs. With the intensification of thermal anomalies and land-based stressors, it is unclear whether the Abrolhos reefs will keep providing key ecosystem services. It is paramount to restrain local stressors that contributed to coral reef deterioration in the last decades, once reversal and restoration tend to become increasingly difficult as coral reefs degrade further and climate changes escalate.

The abiotic environment

Coral reefs are largely restricted to shallow tropical seas, where water is warm, nutrient poor and well illuminated for photosynthesis and where sufficient calcium carbonate (aragonite) exists in seawater for the precipitation of coral skeletons (i.e. calcification). Extreme temperatures and salinities cause thermal and osmotic stress, while large amounts of sediment smother corals and block light. High concentrations of nutrients encourage algal growth at the expense of corals, while low seawater aragonite concentrations prevent net accretion of the reef framework. At local scales, the hydrodynamic regime influences reef growth, as corals are damaged by storms and wave surge. The typical abiotic environment in which reefs are found, and which determines reef distribution, is defined. The chapter also discusses marginal reefs, where corals live at the margins of their survival, for example in the warm, salty seas of the Persian Gulf and the relatively cold waters of Australia’s Lord Howe Island.