Comparisons of benthic filter feeder communities before and after a large-scale capital dredging program

Drill cuttings and drilling fluids (muds) transport, fate and effects near a coral reef mesophotic zone

The study was conducted to improve knowledge and provide guidance on reducing uncertainty with impact predictions when drilling near sensitive environments. Near/Far-field hindcast modelling of cuttings/drilling fluid (mud) discharges from a floating platform was conducted, based on measured discharge amounts and durations and validated by ROV-based plume and seabed sampling. The high volume, concentration, and discharge rate water-based drilling mud discharges (mud pit dumps) were identified as the most significant dispersal risk, but longer-range movement was limited by the generation of jet-like plumes on release, which rapidly delivered muds to the seabed (80 m). Effects to the sparse benthic filter feeder communities close to the wells were observed, but no effects were seen on the epibenthic or demersal fish assemblages across the nearby mesophotic reef. For future drilling near sensitive environments, the study emphasized the need to better characterise drilling fluid discharges (volumes/discharge rates) to reduce uncertainty in modelling outputs.

Early recovery dynamics of turbid coral reefs after recurring bleaching events

The worlds' coral reefs are declining due to the combined effects of natural disturbances and anthropogenic pressures including thermal coral bleaching associated with global climate change. Nearshore corals are receiving increased anthropogenic stress from coastal development and nutrient run-off. Considering forecast increases in global temperatures, greater understanding of drivers of recovery on nearshore coral reefs following widespread bleaching events is required to inform management of local stressors. The west Pilbara coral reefs, with cross-shelf turbidity gradients coupled with a large nearby dredging program and recent history of repeated coral bleaching due to heat stress, represent an opportune location to study recovery from multiple disturbances. Mean coral cover at west Pilbara reefs was monitored from 2009 to 2018 and declined from 45% in 2009 to 5% in 2014 following three heat waves. Recruitment and juvenile abundance of corals were monitored from 2014 to 2018 and were combined with biological and physical data to identify which variables enhanced or hindered early-stage coral recovery of all hard corals and separately for the acroporids, the genera principally responsible for recovery in the short-term (<7 years). From 2014 to 2018, coral cover increased from 5 to 10% but recovery varied widely among sites (0-13%). Hard coral cover typically recovered most at shallower sites that had higher abundance of herbivorous fish, less macroalgae, and lower turbidity. Similarly, acroporid corals recovered most at sites with lower turbidity and macroalgal cover. Juvenile acroporid densities were a good indicator of recovery at least two years after they were recorded. However, recruitment to settlement tiles was not a good predictor of total coral or acroporid recovery. This study shows that coral recovery can be slower in areas of high turbidity and the rate may be reduced by local pressures, such as dredging. Management should focus on improving or maintaining local water quality to increase the likelihood of coral recovery under climate stress. Further, in turbid environments, juvenile coral density predicts early coral recovery better than recruits on tiles and may be a more cost-effective technique for monitoring recovery potential.

Responses of corals to chronic turbidity

Dredging increases suspended sediment concentrations (SSCs), causing elevated water turbidity (cloudiness) and light attenuation. Close to dredging, low light periods can extend over many days, affecting phototrophic epibenthic organisms like corals. To improve the ability to predict and manage dredging impacts, we tested the response of corals to an extended period of elevated turbidity using an automated sediment dosing system that precisely controlled SSCs and adjusted light availability accordingly. Replicates of four common species of corals encompassing different morphologies were exposed to turbidity treatments of 0-100 mg L-1 SSC, corresponding to daily light integrals of 12.6 to 0 mol quanta m-2 d-1, over a period of ∼7 weeks. Symbiotic dinoflagellate density and algal pigment concentration, photosynthetic yields, lipid concentrations and ratios and growth varied among the turbidity treatments, with corals exhibiting photoacclimation within low turbidity treatments. A range of physiological responses were observed within the high turbidity treatments (low light), including bleaching and changes in lipid levels and ratios. Most corals, except P. damicornis, were capable of adjusting to a turbidity treatment involving a mean light level of 2.3 mol photons m-2 d-1 in conjunction with a SSC of 10 mg L-1 over the 7 week period.

Responses of a common New Zealand coastal sponge to elevated suspended sediments: Indications of resilience

Suspended sediments can affect the health of marine benthic suspension feeders, with concomitant effects on community diversity, abundance and ecosystem function. Suspended sediment loads can become elevated through trawling and dredging, and via resuspension of bottom sediments and/or direct input from land during storms. We assessed the functioning (survival, respiration, morphology) of a common New Zealand cushion sponge, Crella incrustans (Carter, 1885), during four weeks of exposure to a gradient of suspended sediment concentrations (SSC). Survival was high, and oxygen consumption was not affected. Sponges did, however, develop apical fistules, a phenomenon never-before observed in this species. Although sediments accumulated internally within the sponges, around a third had cleared these sediments two weeks after the elevated SSCs were removed. The environments these sponges inhabit may predispose them to coping with high SSCs. Such experiments are useful for defining SSC tolerances, which may influence how such impacts can be managed.

Effects of sediment resuspension on the larval stage of the model sponge Carteriospongia foliascens

Sponges are important components of many marine communities and perform key functional roles. Little is known on the processes that drive larval dispersal and habitat selection in sponges, and in particular under stress scenarios. The increase in sediment in the marine environment is a growing concern for the health of ecosystems, but scarce information exists on the effects of sediment on sponge larvae. This study assessed the effects of suspended and deposited sediment on the larva of Carteriospongia foliascens. A suspended sediment concentration (SSC) of 100 mg L^-1 caused homogenisation of the natural pattern of phototactic responses, leading to 100% of photonegative behaviours and a reduction of swim speeds by 27%. After 24 h exposure to suspended sediments, fine particles were found attached to larval cilia, causing abnormal swimming behaviours. Larvae did not have the ability to remove the attached sediment that led to a transformation of the larval body into a cocoon-like morphology and death. Mortality tripled from 3 mg L^-1 (9%) to 300 mg L^-1 (30%) and the relative SSC EC₁₀ and EC₅₀ values corresponded to 2.6 mg L^-1 and 17.6 mg L^-1 respectively. Survival, as determined by live swimming larvae, exceeded 50% even in the highest SSC of 300 mg L^-1, however settlement success decreased by ~20%. Larvae were able to settle onto substrate having deposited sediment levels (DSLs) up to 3 mg cm^-2 (~24%), but recorded a 25 × chance of dislodgement compared to settlers on substrate with DSL of 0.3 mg cm^-2. Larvae avoided settling onto substrates with DSLs >10 mg cm^-2 and preferentially settled onto alternative vertical substrate that were free of sediment. While C. foliascens larvae have some ability to survive and settle through conditions of elevated sediment, detrimental effects are also clear.

Sediment deposition and coral smothering

Dredging in the marine environment to create and maintain safe, navigable shipping channels, and subsequent disposal of the material at sea in dredge material placement sites (spoil grounds) can generate large quantities of suspended sediment that can impact upon epibenthic marine communities. For sensitive taxa such as hard corals, understanding the mechanisms of mortality and the spatial scale over which these occur is critically important for impact prediction purposes, management of dredging using zonation schemes, and also public perception. We describe the sediment deposition field from suspended sediment falling back out of suspension created around a large (7.6 Mm3) 1.5-year capital dredging project on a reef, using data from 2 weekly repeat observations of >500 individually tagged corals at multiple locations from 0.2-25 km from the dredging. The observations were supported by concurrent in situ measurements of proxy suspended sediment concentrations, underwater light, and sediment deposition (using optical backscatter sensors), and before and after surveys of seabed particle size distributions (PSDs). The distance at which 90% of the effect (from maximum to minimum) had dissipated (ED10) was 20 km away from the dredging for suspended sediment concentrations (estimated via nephelometry), and underwater light (measured using PAR sensors) associated with turbid plumes, 14 km for sediment deposition (measured using optical backscatter sensors) and 4.6 km for changes seabed clay and silt content (PSD analysis). The ED10 for smothering of corals (the build-up of pools of loose sediment on the surface that could not be removed by self-cleaning) occurred much closer still at 3-3.3 km or (0.5-0.6 km for an ED50). Smothering was common on encrusting and foliose forms where sediments accumulated in hollows and massive hemispherical forms where surface undulations (bumps) allowed sediments to pool. Smothering was never observed on branching species, even under extreme levels of sedimentation. Sediment smothering resulted in tissue bleaching and partial mortality (lesion formation), but if sediments were removed (by currents) bleached areas regained pigmentation over weeks and there was regrowth/reparation of lesions over weeks and months even before the dredging was completed. Overall sedimentation tolerance was highly related to coral morphology and surface inclination and the ability to avoid smothering by having uninterrupted downhill pathways for sediment transport across the colony.

Sediment characteristics influence the fertilisation success of the corals Acropora tenuis and Acropora millepora

Elevated suspended sediment concentrations (SSCs) often impact coral fertilisation success, but sediment composition can influence effect thresholds, which is problematic for accurately predicting risk. Here, we derived concentration-response thresholds and cause-effect pathways for SSCs comprising a range of realistic mineral and organic compositions on coral fertilisation success. Effect concentration thresholds (EC₁₀: 10% fertilisation inhibition) varied markedly, with fertilisation highly sensitive to inshore organic-clay rich sediments and bentonite clay at <5 mg L^-1. Mineral clays and organic matter within these sediments likely promoted flocculation of the coral sperm, which in turn reduced fertilisation. In contrast, sediments lacking these properties bound less sperm, leading to higher SSC thresholds for coral fertilisation (EC₁₀ > 40 mg L^-1). The effect thresholds for relevant sediment types were combined with in situ turbidity data from locations near dredging operations to assess the risks posed by dredging to coral fertilisation at these locations.

Sediment tolerance mechanisms identified in sponges using advanced imaging techniques

Terrestrial runoff, resuspension events and dredging can affect filter-feeding sponges by elevating the concentration of suspended sediments, reducing light intensity, and smothering sponges with sediments. To investigate how sponges respond to pressures associated with increased sediment loads, the abundant and widely distributed Indo-Pacific species Ianthella basta was exposed to elevated suspended sediment concentrations, sediment deposition, and light attenuation for 48 h (acute exposure) and 4 weeks (chronic exposure). In order to visualise the response mechanisms, sponge tissue was examined by 3D X-ray microscopy and scanning electron microscopy (SEM). Acute exposures resulted in sediment rapidly accumulating in the aquiferous system of I. basta, although this sediment was fully removed within three days. Sediment removal took longer (>2 weeks) following chronic exposures, and I. basta also exhibited tissue regression and a smaller aquiferous system. The application of advanced imaging approaches revealed that I. basta employs a multilevel system for sediment rejection and elimination, containing both active and passive components. Sponges responded to sediment stress through (i) mucus production, (ii) exclusion of particles by incurrent pores, (iii) closure of oscula and pumping cessation, (iv) expulsion of particles from the aquiferous system, and (v) tissue regression to reduce the volume of the aquiferous system, thereby entering a dormant state. These mechanisms would result in tolerance and resilience to exposure to variable and high sediment loads associated with both anthropogenic impacts like dredging programs and natural pressures like flood events.

Effects of combined dredging-related stressors on sponges: a laboratory approach using realistic scenarios

Dredging can cause increased suspended sediment concentrations (SSCs), light attenuation and sedimentation in marine communities. In order to determine the combined effects of dredging-related pressures on adult sponges, three species spanning different nutritional modes and morphologies were exposed to 5 treatment levels representing realistic dredging scenarios. Most sponges survived under low to moderate turbidity scenarios (SSCs of ≤ 33 mg L⁻¹, and a daily light integral of ≥0.5 mol photons m⁻² d⁻¹) for up to 28 d. However, under the highest turbidity scenario (76 mg L⁻¹, 0.1 mol photons m⁻² d⁻¹) there was 20% and 90% mortality of the phototrophic sponges Cliona orientalis and Carteriospongia foliascens respectively, and tissue regression in the heterotrophic Ianthella basta. All three sponge species exhibited mechanisms to effectively tolerate dredging-related pressures in the short term (e.g. oscula closure, mucus production and tissue regression), although reduced lipids and deterioration of sponge health suggest that longer term exposure to similar conditions is likely to result in higher mortality. These results suggest that the combination of high SSCs and low light availability can accelerate mortality, increasing the probability of biological effects, although there is considerable interspecies variability in how adult sponges respond to dredging pressures.