Recreating pulsed turbidity events to determine coral–sediment thresholds for active management

Abstract

Active management of anthropogenically driven sediment resuspension events near coral reefs relies on an accurate assessment of coral thresholds to both suspended and deposited sediments. Yet the range of coral responses to sediments both within and amongst species has limited our ability to determine representative threshold values. This study reviews information available on coral physiological responses to a range of sediment loads at varying time frames and provides a novel approach to assess coral thresholds to suspended and deposited sediments. The new approach replicates natural turbidity regimes by creating pulsed turbidity events at two environmentally realistic levels (moderate = ~ 50 mg l− 1, peaks at 100 mg l− 1; severe = ~ 100 mg l− 1, peaks at 250 mg l− 1). Corals (Merulina ampliata, Pachyseris speciosa, Platygyra sinensis) were subjected to two exposure regimes: pulsed turbidity events for four weeks followed by two months of recovery (constant regime) or pulsed turbidity events every other week followed by one month of recovery (periodic regime). Coral thresholds were greater than commonly used estimates with little to no effect on corals at moderate sediment levels. At extreme sediment levels, species morphological differences were potentially key determinants of coral survival. The periodic exposure regime was less detrimental to all coral species than the constant exposure regime as demonstrated by elevated yields and lower tissue morality rates. To improve knowledge on coral–sediment threshold values, research needs to expand to incorporate a broader range of species and exposure regimes. Realistic threshold values combined with modelling efforts would improve prediction of reef health and enable managers to react to declines in health before coral mortality occurs.