Accounting for environmental uncertainty in the management of dredging impacts using probabilistic dose–response relationships and thresholds

Abstract

Dredging and related activities are common across nearshore marine environments, potentially threatening nearby ecosystems. Regulatory frameworks are essential for minimizing environmental impacts, yet rely heavily on a sound understanding of how ecosystems will respond to environmental stressors, and the thresholds that delineate benign and harmful conditions. Here, we use novel statistical approaches and mathematical tools to account for uncertainty in deriving in situ dose–response relationships and thresholds for the environmental management of dredging, based on estimates of the probability of non‐zero mortality of corals during a dredging campaign at Barrow Island, Western Australia. Using modified receiver operating characteristic curves, we derive thresholds with explicit Type I and Type II errors rates, across the full range of primary stress pathways and exposure dimensions (intensity, frequency and duration). Monitoring coral health and mortality can be expensive and water quality indicators are often used to supplement direct receptor monitoring during dredging management. We found strong relationships between coral mortality and a range of water quality exposure metrics, lending support to the use of water quality metrics as management tools for protecting corals during dredging. Metrics based on sediment deposition were more statistically powerful than those based on either light or turbidity, but may be more difficult to implement in practice. Thresholds reflecting aversion to a false sense of security in environmental protection or aversion to the costs of false alarms varied substantially for all exposure metrics examined. Synthesis and applications. Strategies for managing environmental harm under uncertainty are critical to achieving an informed risk‐weighted balance between environmental protection outcomes and development costs. Our study demonstrates the complexities of how communities respond to variable environmental exposure across a range of pressures in time and space, and the value of integrating probabilistic approaches in environmental management to account for that complexity and its associated uncertainty. Coral mortality was strongly related to a range of water quality exposure metrics associated with dredging activities, and regulatory thresholds based on water quality can provide a solid and cost‐effective foundation for protecting corals during dredging. The probabilistic dose–response relationships and thresholds presented here are the first to be derived from in situ data using dredging related coral mortality and represent a step forward in integrating formal decision science approaches into environmental management.

Synergistic and antagonistic impacts of suspended sediments and thermal stress on corals

“…Benthic light was measured using a 2 Π quantum sensor, with 10 min readings modelled using generalised additive models (GAM) for each day separately, with predicted values used to determine the sum of the per second quantum flux measurements 46,42 as the daily light integral (DLI) calculated as mol photons m -2 d -1 . The instrument platform also included a sediment accumulation sensor 47,48 which, while unable to provide absolute units of sedimentation, provides a relative sedimentation index 49 , and was scaled to between 0 and 1. Water temperature (degrees Celsius, °C) was recorded by the in situ loggers.…”

Sediment deposition and coral smothering

“…The material dredged was predominantly unconsolidated, undisturbed carbonate sediments forming a thin veneer (0.5–3 m deep) overlying limestone pavements, ranging from rubble to typically gravelly sand mixed with fine silts and clays [26]. A full description of the dredging program (type and nature of the dredging and substrates) as well as data collection and methods and analyses of the water quality monitoring data can be found in [23–25].…”

“…Temporal and spatial patterns in turbidity and light levels during this and other capital dredging projects have been discussed in detail in [23, 24]. Patterns of coral mortality and dose response relationships have also been derived for the coral communities, with a focus on the uncertainty when combinations of stressors are involved[25]. The focus in this study is the settling of sediment and smothering of corals, the significance of sediment deposition as a cause-effect pathway and the size of the area where sediment smothering occurs compared to the long-distance movement of sediments in plumes.…”

Mathematical Modeling Framework of Physical Effects Induced by Sediments Handling Operations in Marine and Coastal Areas

“…The evaluation of the significance of effects must necessarily consider different aspects of the induced perturbations to the environmental effects, such as intensity, duration and frequency of events of SSC and DEP increase (e.g., [10,26,[78][79][80][81]). The relationship between intensity, duration of perturbation and the related environmental effects on the specific receptor can be derived on the basis of site-specific data, on literature data or by expert judgment.…”