Global disruption of coral broadcast spawning associated with artificial light at night

Abstract

Coral broadcast spawning events - in which gametes are released on certain nights predictably in relation to lunar cycles - are critical to the maintenance and recovery of coral reefs following mass mortality. Artificial light at night (ALAN) from coastal and offshore developments threatens coral reef health by masking natural light:dark cycles that synchronize broadcast spawning. Using a recently published atlas of underwater light pollution, we analyze a global dataset of 2135 spawning observations from the 21^st century. For the majority of genera, corals exposed to light pollution are spawning between one and three days closer to the full moon compared to those on unlit reefs. ALAN possibly advances the trigger for spawning by creating a perceived period of minimum illuminance between sunset and moonrise on nights following the full moon. Advancing the timing of mass spawning could decrease the probability of gamete fertilization and survival, with clear implications for ecological processes involved in the resilience of reef systems.

Fig. 1. 21st-century coral broadcast spawning observation locations in relation to underwater light pollution from coastal developments.

Light exposure is defined as the average depth of biologically important ALAN throughout the year 2020 (Methods). A The Red Sea and the Gulf of Eliat; B The Persian Gulf; C The Gulf of Thailand and the South China Sea; D The East China Sea; E The Sulu, Celebes, Banda, and Java Seas. F The Great Barrier Reef. Scale bars in units of degrees. Observations are also included in the analysis from the East African coast and the Caribbean. Red circles indicate the locations of spawning observations used in the analysis and the extent to which ALAN, SST, and Kd₄₉₀ were averaged for each location. Scaling is different between panels, as indicated by coordinates given in white. Coordinates of spawning locations are provided in a Source Data file.

Fig. 2. Underwater ALAN disrupts the timing of broadcast spawning by scleractinian corals.

a Frequency histograms of the days spawning is observed relative to full moon (zero on the x-axis) in corals exposed to underwater light pollution (pink) and not exposed (blue). Superimposed points are modeled mean responses ± 95% confidence intervals (Supplementary Table 3). Results are summarized in Supplementary Table 2. The distribution of observations across species within each genus is given in Supplementary Table 4. b Corals found at higher latitudes also spawn closer to the full moon. The gray region represents the 95% confidence intervals of the relationship (dashed line). c Corals spawned at different times in relation to the full moon in different Ecoregions. Bars are modeled means ± 95% confidence intervals. Colors indicate the number of lit and unlit observations within each ecoregion as a proportion of bar height. Pairwise contrasts between Ecoregions are given in Supplementary Table 5. Ecoregion names are given in Supplementary Table 6. Random effects modeled due to spatial autocorrelation were removed from the prediction of means and confidence intervals in all panels. Raw data is provided as a Source Data file.

Fig. 3. ALAN advances the period of minimum light intensity between sunset and the rise of the full moon that provides the trigger for broadcast spawning.

Natural regimes of darkness (dark gray) at the sea surface are modeled, accounting for solar altitude, and lunar phase and altitude, for a mid-latitude (Eilat Coral Nature Reserve) and a low-latitude (Pulau Bara Lompo) coral reef (Methods). The ALAN regime (red) is assumed to trigger lights on at dusk and off at dawn, with the maximum sea surface irradiance derived from ref. . Periods of natural darkness that are masked by ALAN are depicted in light gray. Green arrows indicate the first two nights following the full moon, where a period of natural minimum irradiance (y-axis = 0) between sunset and moonrise is detectable to corals. Red arrows indicate the first nights following the full moon, where ALAN advances the period of minimum light intensity between sunset and moonrise. Raw data is provided as a Source Data file.