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

The study addresses how temporary hypoxic events associated with algal blooms impact coral reefs, focusing on a Noctiluca scintillans bloom in the Gulf of Mannar (GoM), India. Global and coastal waters are experiencing declining dissolved oxygen due to warming and eutrophication, threatening coral reefs alongside bleaching and acidification. Hypoxia alters coral physiology, behavior, growth, calcification, and can cause mortality when thresholds are exceeded. Literature indicates increased frequency of algal blooms under climate change, including N. scintillans in Indian waters, with green and red tide forms; green tides are considered non-toxic but can depress dissolved oxygen. The GoM hosts important coral reefs that have suffered climate- and human-driven disturbances; a 2019 green tide bloom and associated fish kills prompted investigation into coral impacts. The purpose of this study is to document and quantify coral mortality associated with the 2019 N. scintillans bloom and characterize environmental conditions during and after the event.

Prior work documents global ocean deoxygenation and its ecological impacts on coral reefs, with hypoxia affecting coral productivity, respiration, reproduction, calcification, bleaching, and community processes. Thresholds for lethal hypoxia vary among coral species (approximately 0.5–4 mg O2 l−1), and branching/fast-growing corals (e.g., Acropora, Pocillopora) may be more susceptible than massive forms. Algal blooms, including N. scintillans, have increased in frequency with climate change; in Indian waters, blooms have become more common in recent decades. N. scintillans can occur as red (heterotrophic) or green (with symbiont) tides; red tides are harmful, while green tides are considered harmless but can induce low dissolved oxygen. Few studies have reported coral mortality directly linked to algal bloom-induced hypoxia, though events elsewhere (e.g., Flower Garden Banks; eastern Pacific) implicate localized hypoxia in coral die-offs. Previous GoM observations (2008) noted minimal coral impacts from N. scintillans, motivating reassessment during the 2019 event.

Study area: 21 GoM islands grouped into Mandapam, Keelakarai, and Tuticorin. A bloom and fish kills were reported on 12 September 2019 near Mandapam. Preliminary surveys (14–18 September 2019) used SCUBA to visually inspect corals around Mandapam group islands (Shingle, Krusadai, Pullivasal, Poomarichan, Manoliputti, Manoli, Hare). Impacted areas were delineated via GPS and mapped in QGIS. Quantitative assessments: Belt transects (20 m × 2 m; 40 m² each) were laid within impacted areas at Shingle (10 transects) and Krusadai (5 transects) on 17 and 27 September and 4 October 2019, with ≥10 m spacing. On each date, the same number of transects were deployed, placed arbitrarily within the impacted area. Colonies were counted to determine densities and prevalence of recently dead vs live corals, focusing on affected genera (Acropora, Montipora, Pocillopora) and noting other genera present. Bloom quantification: Surface water samples for N. scintillans density were collected by towing a no. 30 plankton net (60 µm mesh) for 30 minutes; cell concentrations were measured with a Sedgwick-Rafter counting chamber. Environmental measurements: Triplicate water samples were collected just above coral colonies (1–2 m depth) between 08:00–09:00 for dissolved oxygen (DO) by Winkler titration (fixation with manganous chloride and alkaline iodide, acidified with sulfuric acid, titrated with sodium thiosulfate using starch indicator) within two hours of collection. Additional in situ parameters: temperature (digital thermometer), salinity (handheld refractometer), pH and TDS (Hanna HI98129). Depths were shallow (<3 m, coral growth up to 1 m above bottom).

• Spatial extent: Impacted reef areas were confined to shoreward, nearshore zones—approximately 8.1 ha at Shingle Island (79°14′14.38″E, 9°14′44.23″N; 1–3 m depth) and 2.1 ha at Krusadai Island (79°13′20.78″E, 9°15′00.88″N; 1–2 m depth). Other nearby islands and non-impacted parts of these reefs appeared healthy.
• Bloom progression: Fishermen observed bioluminescent blooms offshore on 6 September 2019, moving shoreward by 12 September with extensive fish kills. Settling N. scintillans cells accumulated on benthic organisms, later washing ashore.
• Environmental conditions: • 14 Sep 2019: Extremely high N. scintillans densities and hypoxia—Shingle: 43.4×10^5 cells l−1; DO 1.48 mg l−1; Temp 29.9°C. Krusadai: 27.3×10^5 cells l−1; DO 2.02 mg l−1; Temp 29.8°C. Elevated pH/TDS relative to later dates. • 17 Sep 2019: Bloom waning—Shingle: 1.63×10^3 cells l−1; DO 3.78 mg l−1. Krusadai: 0.88×10^3 cells l−1; DO 3.39–4.02 mg l−1 (text notes 4.02 mg l−1). Water clearing. • 27 Sep 2019: Bloom ended—N. scintillans absent; DO recovered to 6.02 mg l−1 (Shingle) and 5.73 mg l−1 (Krusadai). • 4 Oct 2019: DO ~7.1–7.2 mg l−1; N. scintillans absent.
• Coral mortality and densities: • Shingle Island: – 17 Sep: Overall coral density 134.25±3.28 no. 100 m−2 (n=537; 10 transects), dominated by Acropora (64%) and Montipora (15%). Recently dead colonies: 33.52% (n=180). Dead densities: Acropora 34.5±1.05 no. 100 m−2 (n=138); Montipora 7.75±0.75 (n=31); Pocillopora 2.75±0.35 (n=11). Many other genera (e.g., Dipsastraea, Favites, Porites, Hydnophora, Goniastrea, Echinopora, Turbinaria, Platygyra, Goniopora, Symphyllia) remained alive, often producing excess mucus. – 27 Sep: Overall density 135.75±2.82 no. 100 m−2 (n=543); 70.35% (n=382) of Acropora, Montipora, Pocillopora colonies dead. Secondary algae started colonizing dead colonies. – 4 Oct: Overall density 138±2.08 no. 100 m−2 (n=552); 71.23% (n=393) of Acropora, Montipora, Pocillopora dead; dead colonies overgrown by algae, reef appearing green. • Krusadai Island: – 17 Sep: Overall density 66±2.54 no. 100 m−2 (n=132; 5 transects), dominated by Acropora. Recently dead Acropora: 6±1.03 no. 100 m−2; no recent mortality recorded for Montipora, Pocillopora, or massive genera. – 27 Sep: Overall density 65.5±1.83 no. 100 m−2 (n=131); 9.09% (n=12) Acropora dead. No further mortality by 4 Oct.
• Rapid mortality signature: Coral tissues on dead colonies appeared intact and colored; gentle wafting caused tissue to slough, exposing skeleton, consistent with acute hypoxia-related death.
• Taxon-specific susceptibility: Fast-growing branching genera (Acropora, Montipora, Pocillopora) suffered the vast majority of mortality; massive and other genera largely persisted.
• Physical context: Impact likely exacerbated nearshore by current patterns trapping bloom water between mainland and islands.

Findings demonstrate that a transient Noctiluca scintillans bloom induced severe, spatially restricted hypoxia that rapidly killed large proportions of fast-growing branching corals, especially at Shingle Island. DO values fell well below commonly cited detrimental thresholds for corals (e.g., <4 mg l−1), reaching ~1.5–2.0 mg l−1 during peak bloom, aligning with literature on hypoxia-driven coral mortality and greater vulnerability of branching taxa. Recovery of DO and disappearance of N. scintillans coincided with cessation of new mortality, supporting hypoxia as the proximate driver. Temperature during the event (28.4–29.9°C) was below typical local bleaching thresholds (~30°C), but near them, suggesting mild heat stress may have compounded hypoxia effects. The rapid onset of death and tissue sloughing indicates acute physiological collapse under low oxygen. Community-level consequences were evident as secondary algae rapidly colonized dead skeletons, potentially impeding coral recruitment and favoring phase shifts. The event underscores how localized bloom dynamics and nearshore hydrodynamics can concentrate stressors and cause patchy but intense reef mortality. With climate change projected to increase frequency and magnitude of algal blooms and deoxygenation, episodic hypoxia may become a more common and significant driver of coral decline in shallow reefs like those of GoM.

A green tide bloom of Noctiluca scintillans in September–October 2019 caused acute hypoxia and rapid, substantial mortality of fast-growing branching corals in the Gulf of Mannar, with up to 71.23% mortality of Acropora, Montipora, and Pocillopora in the most affected area. The study documents the environmental conditions, spatial extent, and taxon-specific impacts, highlighting algal bloom-induced hypoxia as an underappreciated driver of coral mortality. Given expected increases in algal blooms with climate change, shallow coral reefs are likely to face more frequent episodic hypoxia. Future research should elucidate mechanistic pathways of mortality under bloom conditions, quantify species- and growth form-specific hypoxia thresholds, assess longer-term community shifts and recovery potential, and evaluate management strategies to reduce eutrophication and mitigate bloom impacts.

• The origin (time/place) and drivers of the bloom were not directly determined; causes are presumed (high temperature, nutrients, low tidal amplitude, weak currents).
• Causality is inferred from temporal association between bloom, hypoxia, and mortality; no experimental manipulations were conducted to isolate mechanisms.
• Environmental measurements (e.g., dissolved oxygen) were taken in the morning at shallow depths and within two hours of collection; diel and depth variability were not characterized.
• Transect placements within impacted areas were arbitrary on each date; unaffected areas and other islands were assessed qualitatively during the rapid survey phase.
• Potential confounding stressors (e.g., mild heat stress) cannot be entirely excluded.
• The study focuses on short-term impacts; longer-term recovery, recruitment, and community shifts were not monitored beyond early October 2019.