Chronic Exposure of Corals to Fine Sediments: Lethal and Sub-Lethal Impacts

Florita Flores,Mia O Hoogenboom,Andrew P Negri,Timothy F Cooper,David Abrego,Luke D Smith,Brian Gratwicke

doi:10.1371/journal.pone.0037795

Abstract

Understanding the sedimentation and turbidity thresholds for corals is critical in assessing the potential impacts of dredging projects in tropical marine systems. In this study, we exposed two species of coral sampled from offshore locations to six levels of total suspended solids (TSS) for 16 weeks in the laboratory, including a 4 week recovery period. Dose-response relationships were developed to quantify the lethal and sub-lethal thresholds of sedimentation and turbidity for the corals. The sediment treatments affected the horizontal foliaceous species (Montipora aequituberculata) more than the upright branching species (Acropora millepora). The lowest sediment treatments that caused full colony mortality were 30 mg l−1 TSS (25 mg cm−2 day−1) for M. aequituberculata and 100 mg l−1 TSS (83 mg cm−2 day−1) for A. millepora after 12 weeks. Coral mortality generally took longer than 4 weeks and was closely related to sediment accumulation on the surface of the corals. While measurements of damage to photosystem II in the symbionts and reductions in lipid content and growth indicated sub-lethal responses in surviving corals, the most reliable predictor of coral mortality in this experiment was long-term sediment accumulation on coral tissue.

Highlights

Dredging for the development and maintenance of ports and harbours is becoming increasingly regulated due to the need to balance economic benefit with the principles of environmentally sustainable development
Environmental Conditions in Tanks The daily measured total suspended solids (TSS) concentrations were stable over the 12 week sediment exposure period and the mean measured TSS over this time was very close to the target for each treatment (Table 1, Fig. S1)
In the two highest exposure treatments, the sediment deposition rates overwhelmed the ability of M. aequituberculata to remove particles and heavy sediment accumulation on the horizontal surface of the colonies was observed (Fig. 2A & D), with sediment layers reaching a thickness of 4 to 7 mm after 12 weeks (Fig. 2D)

Summary

Introduction

Dredging for the development and maintenance of ports and harbours is becoming increasingly regulated due to the need to balance economic benefit with the principles of environmentally sustainable development. The severity, duration and scale of impacts vary widely among techniques, and are dependent on the depth and geological nature of the sea floor along with the hydrodynamic conditions of the area [1]. Models that predict the extent and severity of turbidity and sediment deposition and incorporate thresholds for organism health are increasingly being used as tools in the impact assessment process [3]. The usefulness of any model prediction hinges on robust inputs on the biological response to the environmental stressor in the receptor organism of interest

Methods

Results

Conclusion