Environmental impacts of dredging and other sediment disturbances on corals: A review

Abstract

A review of published literature on the sensitivity of corals to turbidity and sedimentation is presented, with an emphasis on the effects of dredging. The risks and severity of impact from dredging (and other sediment disturbances) on corals are primarily related to the intensity, duration and frequency of exposure to increased turbidity and sedimentation. The sensitivity of a coral reef to dredging impacts and its ability to recover depend on the antecedent ecological conditions of the reef, its resilience and the ambient conditions normally experienced. Effects of sediment stress have so far been investigated in 89 coral species (~10% of all known reef-building corals). Results of these investigations have provided a generic understanding of tolerance levels, response mechanisms, adaptations and threshold levels of corals to the effects of natural and anthropogenic sediment disturbances. Coral polyps undergo stress from high suspended-sediment concentrations and the subsequent effects on light attenuation which affect their algal symbionts. Minimum light requirements of corals range from <1% to as much as 60% of surface irradiance. Reported tolerance limits of coral reef systems for chronic suspended-sediment concentrations range from <10 mg L(-1) in pristine offshore reef areas to >100 mg L(-1) in marginal nearshore reefs. Some individual coral species can tolerate short-term exposure (days) to suspended-sediment concentrations as high as 1000 mg L(-1) while others show mortality after exposure (weeks) to concentrations as low as 30 mg L(-1). The duration that corals can survive high turbidities ranges from several days (sensitive species) to at least 5-6 weeks (tolerant species). Increased sedimentation can cause smothering and burial of coral polyps, shading, tissue necrosis and population explosions of bacteria in coral mucus. Fine sediments tend to have greater effects on corals than coarse sediments. Turbidity and sedimentation also reduce the recruitment, survival and settlement of coral larvae. Maximum sedimentation rates that can be tolerated by different corals range from <10 mg cm(-2) d(-1) to >400 mg cm(-2) d(-1). The durations that corals can survive high sedimentation rates range from <24 h for sensitive species to a few weeks (>4 weeks of high sedimentation or >14 days complete burial) for very tolerant species. Hypotheses to explain substantial differences in sensitivity between different coral species include the growth form of coral colonies and the size of the coral polyp or calyx. The validity of these hypotheses was tested on the basis of 77 published studies on the effects of turbidity and sedimentation on 89 coral species. The results of this analysis reveal a significant relationship of coral sensitivity to turbidity and sedimentation with growth form, but not with calyx size. Some of the variation in sensitivities reported in the literature may have been caused by differences in the type and particle size of sediments applied in experiments. The ability of many corals (in varying degre...

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“…High fluxes of both terrigenous and autochthonous sediments are widely identified to have both direct and indirect inhibitory effects on coral reef growth (Larcombe et al, 2001;Erftemeijer et al, 2012;Sanders and Baron-Szabo, 2005;Salles et al, 2018a). For instance, elevated turbidity on midouter platform reefs caused by the suspension of sediment on the Pleistocene reef substrate during initial flooding ∼ 9 ka is hypothesised to be responsible for a delayed initiation of coral growth in the southern Great Barrier Reef (GBR) (Dechnik et al, 2015;Salles et al, 2018b).…”

“…Decades of experimentation carried out on the sensitivity of particular species to sediment have informed a generic understanding of the threshold levels of corals to the effect of natural sedimentation (Hubbard, 1986;Rogers, 1983;Stafford-Smith, 1993); however, these thresholds have only been partially quantified in the literature, and tolerance at the assemblage level is difficult to constrain due to site and within-species variations (Erftemeijer et al, 2012). It has been shown that even under uniform sediment input regimes, inter and intra-site variations in sedimentationresuspension regimes occur depending on water depth and exposure to wave energy (Wolanski et al, 2005).…”

“…Early measurements supported that sedimentation rates exceeding 50 mg cm 2 day −1 produced lethal effects (Rogers, 1990). Yet each coral species has its own tolerance threshold to sediment stress, beyond which sedimentation produces sublethal to lethal effects (Erftemeijer et al, 2012).…”

Coral community changes in response to a high sedimentation event: A case study in southern Hainan Island

“…Dredging is an extreme event causing high sedimentation and is associated with construction of hotels, runways, roads, harbors, and military installations, as well as with beach replenishment [17,18]. It has been considered as a high-risk event because of its likely deleterious effects to the marine environment and is one of the biggest potential sources of reef degradation resulting from human activities, being an especially severe risk in developing countries [17,19].…”

“…Live coral cover decreased by 40% and partial coral death significantly increased during a dredging operation in Hong Kong [23]. Although the detrimental effects of dredging on coral reefs have been reviewed [17,18,22,24], the impact of dredging on corals and coral reef ecosystems is complex and far from fully understood [18]. Few studies have presented detailed and quantitative data on coral community changes before and after a dredging event [22].…”