Sediment deposition and coral smothering.

Ross Jones,Rebecca Fisher,Pia Bessell-Browne,Judi Hewitt

doi:10.1371/journal.pone.0216248

Abstract

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.

Highlights

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 [1]
Predicting what may occur before a dredging program at the environmental impact assessment (EIA) stage, and managing dredging when underway through environmental monitoring, is predicated upon establishing a relationship between these dredging pressures and biological responses in underlying communities i.e. developing thresholds or guidelines
These predictions are important at the EIA stage of a project and in monitoring programs associated with the dredging phase, where proponents need to make sure they are compliant with the EIA predictions

Summary

Introduction

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 [1]. In Australia, these are used in zonation schemes [6, 7] and proponents (i.e. those proposing to undertake the dredging) need to define, beforehand, zones of high and moderate impact, and a zone of influence, which is where the plumes are likely to be seen but where there are no biological consequences. These predictions are important at the EIA stage of a project (where forecasts are made of possible environmental effects) and in monitoring programs associated with the dredging phase, where proponents need to make sure they are compliant with the EIA predictions

Methods

Results

Conclusion