Water Quality in Experimental Treatments

Experimental treatments of suspended and settling coal particles mimicked five broad pulse intensities (ranging from 0–275 mg coal l−1, Table 1) lasting 28 d. Attenuation of light in coal treatments ranged from 44–99%, relative to control values (Table 1). Coal deposition rates ranged from 11–241 mg cm−2 d−1 in sediment traps and 2–46 mg cm−2 d−1 on flat surfaces (pods) (Table 1, Supplementary Material Fig. S1). Trace metal analysis of experiment treatment water (filtered leachate) sampled at 28 d showed significantly (P < 0.05) higher concentrations of arsenic, cobalt and nickel in certain coal treatments in comparison with control water (Table 2). However, the highest metal concentrations were not always measured in the highest coal treatments. The magnitude of change in dissolved metal concentrations in relation to control seawater was minimal: arsenic varied by 0.3 μg l−1; cadmium 0.1 μg l−1; cobalt 0.2 μg l−1; copper 0.2 μg l−1; lead 0.1 μg l−1; manganese 0.3 μg l−1; molybdenum 0.8 μg l−1; nickel 2.4 μg l−1; zinc 0.9 μg l−1. These findings suggest that metals were not likely contributing to the observed effects.

Table 1 Summary of water quality parameters. Full size table

Table 2 Elemental analysis (μg l−1) from water samples (n = 3) in each treatment (mean ± s.e.m.). Full size table

Although the tanks were moderately turbulent (water flow of 5–10 cm sec−1) due to the presence of pumps, the coal particles attached to many surfaces within the tanks, contributing to lower total suspended coal (TSC) exposures in the latter half of the four week pulse (Supplementary Material Fig. S1). While there is limited evidence documenting the concentrations of suspended coal present in seawater during a spill event, our high coal treatment (275 mg coal l−1) was lower than the concentrations applied to temperate species in other experimental studies (500–13,500 mg coal l−1)16,17. Moreover, the results of the present experiment may be considered conservative in relation to the broader effects of coal during a spill event as we only investigated the effects of fine coal particles (<63 μm) which are likely to remain in suspension for long periods3,18. A large spill scenario at sea would also release larger particles that settle more rapidly3,18, posing further risks of physical damage, including smothering.

Responses of tropical marine organisms to coal exposure

Corals

In all coal treatments, particles settled directly onto coral polyps and connecting tissue (i.e., coenosarc, Fig. 1b,c) and the initial response of corals to coal exposure was the release of fine mucus strands, which trapped coal particles and removed them from the tissue surface, similar to the response of corals exposed to sediments7,19,20. Branching corals, such as A. tenuis, are often considered among the most resistant morphologies to sedimentation due to their vertical growth20, yet despite their active mechanisms for coal removal; some coral tissue died and sloughed off the skeleton within 14 d in all treatments ≥38 mg coal l−1 (Figs 1b,c and 2a). The extent of tissue mortality on coral branches differed significantly among coal treatments (Permanova, Pseudo-F 4,10 = 43.6, P = 0.0001) and over time (Permanova, Pseudo-F 1,10 = 20.5, P = 0.0009) (Fig. 2a, see statistical outputs in Supplementary Material Tables S1 and S2). After 14 d of exposure, control and low (38 mg coal l−1) coal treatments exhibited significantly lower coral mortality than treatments ≥202 mg coal l−1 (Student-t post hoc, Monte Carlo simulation, P < 0.05). After 28 d, mortality in all coal treatments ≥38 mg coal l−1 was significantly higher than the controls (Student-t post hoc, Monte Carlo simulation, P < 0.05). Corals in the control treatment exhibited less than 3% mortality, while 100% tissue mortality occurred in all branches in the three highest coal exposures (Figs 1c and 2a). Corals in the 38 mg coal l−1 treatment exhibited significantly lower mortality than corals in treatments ≥73 mg coal l−1 (Student-t post hoc, Monte Carlo simulation, P < 0.05). Pair-wise comparisons between treatments revealed lowest observed effect concentrations (LOEC) of 202 mg coal l−1 and 38 mg coal l−1 at 14 and 28 d, respectively. Fitting four-parameter sigmoidal curves to the data revealed lethal concentrations (LC 10 and LC 50 ) of 29 mg coal l−1 and 87 mg coal l−1 at 14 d, respectively and 34 mg coal l−1 and 36 mg coal l−1 at 28 d, respectively (Supplementary Material Fig. S2a).

Figure 1 Comparison of three taxa after coal exposure. Stages of coral health degradation after 14 d exposure to 0 mg coal l−1 (a), 73 mg coal l−1 (b) and 275 mg coal l−1 (c). Mucus strands were used to actively remove settled coal (b) and coal deposition that exceeded removal efforts resulted in nubbin mortality (c). Fish from control vs. coal exposed treatments (0 mg coal l−1–275 mg coal l−1) after 28 d exposure (d). Coal settled onto seagrass leaves and substrate (e). Note: No fish were present in the 202 mg coal l−1 treatment. All scale bars = 5 mm. Full size image

Figure 2 Differences in measures of key demographic rates in relation to coal concentration and exposure duration. Differences in the mean (±s.e.m.) survival of corals (A. tenuis) (a), growth rates of fish (A. polyacanthus) (b) and seagrass (H. uninervis) (c) and percentage change in seagrass shoot density (d) at 14 d (closed circle) and 28 d (open circle) exposure. Asterisks depict a significant difference (P < 0.05) between the mean coal treatment and control values. Note: mean change in seagrass shoot density (d) is relative to time 0 values at each treatment level in each replicate seagrass pot. Mean values above 0 suggest growth, while values below 0 suggest mortality. No fish were present in the 202 mg coal l−1 treatment. Full size image

Coral mortality in response to the suspension of fine coal particles may have a number of causes. The accumulation of coal particles on the vertical tissue could have caused anoxia at the coral-coal interface21. Similar surface accumulation of particles was not observed after a month in comparable exposures of Acropora millepora branches to fine carbonate sediments22 and could indicate either greater adhesion by coal or reduced fitness in corals exposed to coal rather than sediments. The energetic costs of removing deposited particles (including mucus production) may be further exacerbated in the presence of coal by the strong attenuation of light over 14 and 28 d, which would reduce primary production rates by the symbiotic dinoflagellates. Although the corals were fed once per week with Artemia nauplii, heterotrophic feeding behaviour may have been altered in smothered sections of coral colonies19.

Fish

The health of coal-exposed fish was compromised in all coal treatments and differences in fish size and colour were observed over the course of the experiment (Fig. 1d). Fish growth rates varied significantly between coal treatments and controls (Permanova, Pseudo-F 3,8.1 = 21.7, P = 0.01) and over time (Permanova, Pseudo-F 1,8.5 = 141.4, P = 0.0002) (Fig. 2b, see statistical outputs in Supplementary Material Tables S1 and S2). Significant differences in growth occurred within the first 14 d of the experiment, with fish exposed to coal levels ≥38 mg coal l−1 showing significantly lower growth rates than control fish, irrespective of coal treatment levels (Student-t post hoc, Monte Carlo simulation, P < 0.05, Fig. 2b). Growth inhibition, relative to control fish, at 14 d ranged from 36.1 ± 4.6%–40.2 ± 4.5% and 42.2 ± 6.0%–52.3 ± 4.3% at 28 d (Supplementary Material Fig. S2b). The LOEC on fish growth rates was 38 mg coal l−1 at both time points and linear interpolation of the growth data revealed inhibition concentration (IC 10 ) estimates of 11 and 9 mg coal l−1 at 14 and 28 d, respectively and IC 50 estimates of 73 mg coal l−1 at 28 d. IC 50 estimates were not possible at 14 d because growth inhibition was below 50%.

The negative impact of coal on fish growth is consistent with the response of marine fish to increased suspended sediments which is thought to be caused by visual impairment leading to reduced prey capture success and increased foraging time and energy expenditure23,24. A preliminary post mortem investigation on the coal-exposed fish in this experiment revealed coal in the alimentary tracts, which was mistakenly ingested and could have physically blocked normal feeding and digestion contributing to starvation and debilitation. In addition, it is possible that suspended coal affected fish respiration25,26, an effect that may have been consistent across all coal treatments.

Despite the considerable effects on fish growth, all coal-exposed fish survived except for two individuals that were exposed to the highest coal treatment of 275 mg coal l−1. The lethal effects of suspended sediments on fish are dependent on the particle size, angularity, exposure duration and are typically observed when concentrations reach ≥hundreds of mg l−1 23,27,28. The survival of fish in the current study, along with the very high LC 50 (7000 mg coal l−1) reported for 8 d coal exposures of juvenile coho salmon17, support the notion that coal spills are not likely to cause direct mortality in fish under most coal spill scenarios. However, suspended sediment can prolong reef fish larvae development29, negatively influence gill morphology and increase pathogenic bacterial communities on larval gills30, suggesting further studies are required to investigate the vulnerability of early life stages of fish to suspended coal. Although mortality was low, certain post-settlement processes are size dependent for reef fishes31, suggesting that lower growth rates in situ may have later implications on individual survivorship23. Moreover, as fecundity of fish is size dependent31,32, suppressed growth can lower lifetime reproductive output.

Seagrass

Coal particles were observed to attach to the seagrass leaves less than 24 h after exposure commenced and many leaves were completely coated in a film of coal throughout the experiment (Fig. 1e). Coal also accumulated onto the sediment surface in seagrass pots where new shoots develop (Fig. 1e). Significant differences were measured for leaf elongation (Permanova, Pseudo-F 4,10 = 35.9, P = 0.0002) and shoot density (Permanova, Pseudo-F 4,10 = 9.8, P = 0.0002) between experimental treatments. Leaf elongation rates differed significantly over time (Permanova, Pseudo-F 1,10 = 67.5, P = 0.0001) (see statistical outputs in Supplementary Material Tables S1 and S2). Leaf elongation was more sensitive than shoot density and was significantly affected (Student-t post hoc, Monte Carlo simulation, P < 0.05) in treatments ≥73 mg coal l−1 (LOEC) at both 14 d and 28 d (Fig. 2c). The magnitude of the effect of coal exposure on leaf elongation rates was large, with overall growth inhibited by 6.7 ± 5.0%–45.2 ± 3.8% and 31.1 ± 4.5%–49.5 ± 3.1% relative to controls at 14 and 28 d, respectively (Supplementary Material Fig. S2c). The estimated threshold for impact for this parameter (IC 10 ) was 42 mg coal l−1 at 14 d and 12 mg coal l−1 at 28 d, while the IC 50 was 275 mg coal l−1 at 28 d. IC 50 estimates were not possible at 14 d because growth inhibition was below 50%. Shoot density continued to increase in control and 38 mg coal l−1 treatments throughout the experiment duration, however, was significantly reduced (Student-t post hoc, Monte Carlo simulation, P < 0.05) at 28 d in coal treatments ≥73 mg coal l−1 (28 d LOEC), with a mean net loss of 1.3 ± 3.4%–4.6 ± 1.4% of shoots at this time point (Fig. 2d).