Sensitivity of Corals to Turbidity and Sedimentation from Dredging: A Review

A review of published literature on sensitivity of corals to turbidity and sedimentation is presented with an emphasis on effects of dredging. Risks and severity of impact from dredging on corals are related to both intensity and duration of exposure to increased turbidity and sedimentation. Sensitivity of a coral reef from dredging impacts and its ability to recover depends on the ecological condition of the reef, its resilience and typical ambient conditions normally experienced. Corals experience stress from high suspended sediment concentrations (polyp) and their effects on light attenuation (algal symbiont). Minimum light requirements of corals range from <1% to as much as 60% of surface irradiance. Tolerance limits of corals for suspended sediment concentrations range from <10 mg l ^-1 in pristine reef areas to 40 or even 165 mg l ^-1 in marginal nearshore reefs. Enhanced sedimentation causes smothering and burial of coral polyps, shading, tissue necrosis and population explosions of bacteria in coral mucus. Fine sediments have greater effects on corals than coarse sediments. Sedimentation also reduces the recruitment, survival and settlement of coral larvae. Maximum allowable sedimentation rates for corals range from <10mg cm ^-2 day ^-1 to >300 mg cm ^-2 day ^-1 . The duration that corals can survive high sedimentation rates range from <24 hours for sensitive species to >14 days (complete burial) for very tolerant species. Hypotheses to explain substantial differences in sensitivity between different coral species include the growth form and orientation of coral colonies, the ability to actively reject sediment (polyp inflation, mucus production, ciliary and tentacular action) and the size of the coral polyp/calice. Validity of these hypotheses was tested on the basis of 53 published studies on effects of turbidity and sedimentation on 85 coral species, the results of which will be presented.