Date Published: June 19, 2019
Publisher: Public Library of Science
Author(s): Ross Jones, Rebecca Fisher, Pia Bessell-Browne, Judi Hewitt.
Dredging in the marine environment to create and maintain safe, navigable shipping channels, and subsequent disposal of the material at sea in dredge material placement sites (spoil grounds) can generate large quantities of suspended sediment that can impact upon epibenthic marine communities. For sensitive taxa such as hard corals, understanding the mechanisms of mortality and the spatial scale over which these occur is critically important for impact prediction purposes, management of dredging using zonation schemes, and also public perception. We describe the sediment deposition field from suspended sediment falling back out of suspension created around a large (7.6 Mm3) 1.5-year capital dredging project on a reef, using data from 2 weekly repeat observations of >500 individually tagged corals at multiple locations from 0.2–25 km from the dredging. The observations were supported by concurrent in situ measurements of proxy suspended sediment concentrations, underwater light, and sediment deposition (using optical backscatter sensors), and before and after surveys of seabed particle size distributions (PSDs). The distance at which 90% of the effect (from maximum to minimum) had dissipated (ED10) was 20 km away from the dredging for suspended sediment concentrations (estimated via nephelometry), and underwater light (measured using PAR sensors) associated with turbid plumes, 14 km for sediment deposition (measured using optical backscatter sensors) and 4.6 km for changes seabed clay and silt content (PSD analysis). The ED10 for smothering of corals (the build-up of pools of loose sediment on the surface that could not be removed by self-cleaning) occurred much closer still at 3–3.3 km or (0.5–0.6 km for an ED50). Smothering was common on encrusting and foliose forms where sediments accumulated in hollows and massive hemispherical forms where surface undulations (bumps) allowed sediments to pool. Smothering was never observed on branching species, even under extreme levels of sedimentation. Sediment smothering resulted in tissue bleaching and partial mortality (lesion formation), but if sediments were removed (by currents) bleached areas regained pigmentation over weeks and there was regrowth/reparation of lesions over weeks and months even before the dredging was completed. Overall sedimentation tolerance was highly related to coral morphology and surface inclination and the ability to avoid smothering by having uninterrupted downhill pathways for sediment transport across the colony.
Dredging in the marine environment to create and maintain navigable shipping channels and allow safe ship access, is a usual component of many large marine infrastructure developments . Dredging involves the removal of sediment and/or rock from the seabed  and the excavation and subsequent disposal at sea in dredge material placements sites (spoil grounds) can generate large quantities of suspended sediment that can impact upon epibenthic marine communities [1, 3–5].
Sediment deposition resulting in smothering of corals leading to mortality is a significant cause-effect pathway associated with dredging and turbidity generating activities near coral reefs. In comparing the spatial patterns of turbidity and light attenuation, seabed particle size distribution and sediment deposition, with biological responses including sediment smothering and mucous sheet formation in corals, the sediment deposition field was approximately an order of magnitude less than the distances travelled by the plumes. Coral mortality occurred within a few kilometres south of the dredging and within the area defined by the deposition field. Establishing evidence-based footprints of the scale of potential impacts associated with sediment smothering as compared to elevated turbidity and dredging related plumes is important for dredging management, and also for perception by the public and regulators of potential environmental effects . Equating risk based solely on the extent of visible plumes could be very misleading as to the overall spatial effects of dredging projects.