Low oxygen levels caused by Noctiluca scintillans bloom kills corals in Gulf of Mannar, India

Abstract

Coral reefs around the world are undergoing severe decline in the past few decades. Mass coral mortalities have predominantly been reported to be caused by coral bleaching or disease outbreaks. Temporary hypoxic conditions caused by algal blooms can trigger mass coral mortalities though are reported rarely. In this study in Gulf of Mannar (GoM), southeast India, we report a significant coral mortality caused by a bloom of the ciguatoxic dinoflagellate Noctiluca scintillans during September–October 2019. Dissolved oxygen levels declined below 2 mg l−1 during the bloom causing temporary hypoxia and mortality (up to 71.23%) in the fast growing coral genera Acropora, Montipora and Pocillopora. Due to global climate change, more frequent and larger algal blooms are likely in the future. Hence, it is likely that shallow water coral reefs will be affected more frequently by episodic hypoxic conditions driven by algal blooms. More studies are, however, required to understand the mechanism of coral mortality due to algal blooms, impacts on community composition and the potential for subsequent recovery.

Effects of Hypoxia on Coral Photobiology and Oxidative Stress

“…Additionally, the oceans’ buffering capacity is diminishing due to increasing CO 2 absorption and excess heat produced from anthropogenic activities [ 5 , 6 , 7 , 8 ]. The consequent ocean warming causes a reduction in oxygen solubility in seawater, which results in higher physiological oxygen demands of many marine organisms [ 3 , 9 ], with potential cascading effects (e.g., alteration of behavioural responses, migrations, reductions in growth rates and fecundity, and increased mortality rates) [ 3 , 10 , 11 , 12 , 13 ]. Oxygen concentrations in the range of 2 to 3.5 mg O 2 /L or below are considered as the hypoxic threshold for marine organisms [ 14 , 15 , 16 , 17 ].…”

“…Coastal ecosystems are specially affected by increased warming waters and nutrient intrusion with more frequent marine heat waves and eutrophication. Therefore, the expansion of oxygen deprived coastal waters, specifically lethal acute hypoxic events, creates “dead zones” that lead to the occurrence of mass-mortalities of residing organisms, including corals [ 10 , 11 , 23 , 24 , 25 , 26 , 27 ].…”

Differential susceptibility of reef-building corals to deoxygenation reveals remarkable hypoxia tolerance

“…Episodes of hypoxia related to deoxygenation associated with stratification, eutrophication, coral spawn slicks, or algal blooms have been implicated in bleaching, mass mortality, and formation of dead zones on coral reefs across the globe, from the Caribbean 13 , 16 and Gulf of Mexico 17 , 18 to the Great Barrier Reef 19 and Indian Ocean 20 . Of the events that have been documented, the organismal impacts of hypoxia scale up to ecosystem-wide consequences that include loss of benthic biodiversity and live habitat 13 , 21 , as well as shifts in reef-associated microbial assemblages 17 , 21 .…”

Comparison of macro‐molluscan assemblages in a protected and a non‐protected tropical seagrass ecosystem

“…A low abundance of herbivorous fishes was observed in this region (Purvaja et al, 2018), indicating that the grazing molluscan community (mesograzers), as seen in the present study, could enhance the low epiphytic load on seagrasses. Both GoMMNP and PB have been linked to recent algal blooms and green tides, which have been related to nutrient level increases (Raj et al, 2020; Ramesh et al, 2020). Enhancement of nutrient levels in the water column was found to increase algal loads on seagrasses, which had a detrimental impact on them by reducing photosynthesis, reducing CO 2 diffusion over the seagrass blade surface, and increasing leaf loss (Nelson, 2017).…”