The Dark Side of Artificial Light

: Impacts on Coral Reefs

Abstract

Artificial light at night (ALAN) is a pervasive anthropogenic stressor encroaching on a quarter of the world’s coral reefs. Although historically understudied in marine environments, it is now recognised that ALAN can disrupt natural nocturnal light cues, interfering with the diel cycles critical to the physiological and behavioural processes of many reef organisms. In this thesis, I present the results of two complementary studies investigating the effects of ALAN at the individual and community level. The first study used experiments to evaluate the short-term impact of ALAN on the filter-feeding behaviour of Tridacna maxima clams, a key benthic filter-feeder. Clams exposed to white ALAN exhibited a significant reduction in phytoplankton filtration compared to those under natural dark conditions, whilst individuals introduced to orange (longer-wavelength light) ALAN displayed a non-significant filtration decrease. This suggests that ALAN has a negative impact on these crucial species but that longer wavelengths may reduce effects on the feeding behaviour. The second study used passive acoustic monitoring to investigate long-term ALAN effects on a fringing reef in the northern Red Sea. The soundscape of artificially lit areas exhibited significantly reduced amplitude across both high- and low-frequency bands. At high frequencies, the acoustic complexity index (ACI) was greater and the acoustic diversity index (ADI) was lower in ALAN areas compared to dark control areas, but there were no significant differences at low frequencies. Bioacoustic changes may reflect shifts in community composition, behaviour or activity levels in response to chronic light exposure. In combination, my findings highlight how ALAN can impair individual behavioural processes and disrupt broader ecological dynamics within coral reef ecosystems. Future research must prioritise a deeper understanding of the wavelength-specific nature of these effects to produce targeted light pollution mitigation strategies and support the long-term conservation of reef ecosystem functionality.

Date of Award	30 Sept 2025
Original language	English
Awarding Institution	University of Bristol
Supervisor	Steve Simpson (Supervisor) & Andrew N Radford (Supervisor)