Effects of sediment exposure on corals: a systematic review of experimental studies

Abstract

Background Management actions that address local-scale stressors on coral reefs can rapidly improve water quality and reef ecosystem condition. In response to reef managers who need actionable thresholds for coastal runoff and dredging, we conducted a systematic review and meta-analysis of experimental studies that explore the effects of sediment on corals. We identified exposure levels that ‘adversely’ affect corals while accounting for sediment bearing (deposited vs. suspended), coral life-history stage, and species, thus providing empirically based estimates of stressor thresholds on vulnerable coral reefs. Methods We searched online databases and grey literature to obtain a list of potential studies, assess their eligibility, and critically appraise them for validity and risk of bias. Data were extracted from eligible studies and grouped by sediment bearing and coral response to identify thresholds in terms of the lowest exposure levels that induced an adverse physiological and/or lethal effect. Meta-regression estimated the dose–response relationship between exposure level and the magnitude of a coral’s response, with random-effects structures to estimate the proportion of variance explained by factors such as study and coral species. Review findings After critical appraisal of over 15,000 records, our systematic review of corals’ responses to sediment identified 86 studies to be included in meta-analyses (45 studies for deposited sediment and 42 studies for suspended sediment). The lowest sediment exposure levels that caused adverse effects in corals were well below the levels previously described as ‘normal’ on reefs: for deposited sediment, adverse effects occurred as low as 1 mg/cm2/day for larvae (limited settlement rates) and 4.9 mg/cm2/day for adults (tissue mortality); for suspended sediment, adverse effects occurred as low as 10 mg/L for juveniles (reduced growth rates) and 3.2 mg/L for adults (bleaching and tissue mortality). Corals take at least 10 times longer to experience tissue mortality from exposure to suspended sediment than to comparable concentrations of deposited sediment, though physiological changes manifest 10 times faster in response to suspended sediment than to deposited sediment. Threshold estimates derived from continuous response variables (magnitude of adverse effect) largely matched the lowest-observed adverse-effect levels from a summary of studies, or otherwise helped us to identify research gaps that should be addressed to better quantify the dose–response relationship between sediment exposure and coral health. Conclusions We compiled a global dataset that spans three oceans, over 140 coral species, decades of research, and a range of field- and lab-based approaches. Our review and meta-analysis inform the no-observed and lowest-observed adverse-effect levels (NOAEL, LOAEL) that are used in management consultations by U.S. federal agencies. In the absence of more location- or species-specific data to inform decisions, our results provide the best available information to protect vulnerable reef-building corals from sediment stress. Based on gaps and limitations identified by our review, we make recommendations to improve future studies and recommend future synthesis to disentangle the potentially synergistic effects of multiple coral-reef stressors.

Sediment source and dose influence the larval performance of the threatened coralOrbicella faveolata

“…Both of our sediment sources significantly impacted the survival of O. faveolata larvae, however, exposure to both the low- and high-dose Port sediment treatments reduced survival proportions to 86% and 81%, respectively (compared to the control treatment), while the high-dose Reef sediment treatment only reduced the survival proportion to 90% (Fig 2A). Suspended sediments may reduce larval survival through physical abrasion [38] and reduced light over prolonged periods can also lead to decreased photosynthetic efficiency of larval symbionts and subsequent larval mortality from starvation [23]. However, in our study, decreased light was unlikely the driver of our results, given the short exposure to sediments (24 h) and the use of aposymbiotic (symbiont-free) larvae.…”

“…Except for the low-dose Reef sediment treatment, we showed significant effects of both sediment types (Port and Reef) after one week of sediment exposure, resulting in approximately 50-60% reduction in settlement in O. faveolata larvae compared to the control treatment (Figs 2B and S1). Reduced larval settlement has been documented for a few Pacific coral species in the presence of suspended sediments (in concentrations less than 10 mg/L, [23]), presumably due to changes in light quality and quantity (as a result of increased light attenuation). Sediment-covered surfaces have also been shown to decrease larval settlement in various coral species such as Pocillopora damicornis [64], Acropora digitifera [38], Acropora millepora [38], and O. faveolata [31], likely due to limited space to settle (preventing larvae from accessing and attaching to suitable substrate), higher risk of abrasion, smothering/burial by deposited sediments, and masking of appropriate chemical cues for settlement [23].…”

“…and S1). Reduced larval settlement has been documented for a few Pacific coral species in the presence of suspended sediments (in concentrations less than 10 mg/L, [23]), presumably due to changes in light quality and quantity (as a result of increased light attenuation). Sediment-covered surfaces have also been shown to decrease larval settlement in various coral species such as…”

“…Sediments may interfere with the ability of coral larvae to find a suitable substrate on which to settle, which is key to successful recruitment and survival [31, 35, 36]. Even a thin layer of sediments not harmful to adult corals can be detrimental to coral larvae [23, 28, 31, 37–39], both by physically obstructing settlement surfaces [27, 30], and impairing biotic settlement cues such as those from crustose coralline algae (CCA) [30]. However, the mechanisms and cues by which sediments can affect settlement and recruitment success, or benthic habitat quality (e.g., [31]) are poorly understood.…”

“…Sediments can negatively affect corals throughout their life cycle, suppressing coral health, condition, and survival via multiple mechanisms (reviewed in [7,12,13,23]). Reported effects on early life stages of corals include reductions in the fecundity of parent colonies [24], fertilization [25,26], larval settlement [27][28][29][30][31], and recruit survival [5,27,32,33].…”