Understanding impacts of dredging on the light climate – insights for impact prediction and threshold development in seagrass habitat

Management of seagrass habitat requires understanding of the pressures that they are exposed to as well as their tolerance and resilience to these pressures. One pressure that is of crucial importance to the survival and... [ view full abstract ]

Management of seagrass habitat requires understanding of the pressures that they are exposed to as well as their tolerance and resilience to these pressures. One pressure that is of crucial importance to the survival and health of seagrass habitat is benthic light. Seagrasses have high light requirements, so any reductions in light have direct impacts, and many human activities result in reduced light such as dredging and eutrophication, often leading to loss and a reduction in condition. In highly variable and turbid environments the effects of human induced changes in light are poorly understood. Recent large-scale dredging operations in NW WA have collected extensive data on the light environment prior to (~ 12 months) and during (~ 18 months) dredging events. This study used the industry data to provide insight into the environmental drivers of benthic daily environment and how dredging effects the magnitude, duration and frequency of light reduction. We found that in the NW of WA the benthic daily light was highly variable over daily, weekly and monthly cycles. Benthic daily light was greatest from September to June, and lowest from June to August. The main environmental drivers of light were day-length, wind speed and the interaction between wind speed and wind direction and day-length and wind direction. Dredging significantly reduced daily benthic light and the effect declined with distance from dredge. Within 1 km of the dredge light on average reduced by 65% compared to background conditions, and this occurred for up to a maximum of 185 consecutive days. At 10 km from the dredge light reduction was reduced by 29% relative to background conditions, and this occurred for up to a maximum of 78 consecutive days. There was also a temporal aspect to the magnitude of the decline, with greater reductions from October – December. The duration and magnitude of duration is of a level that is likely to impact seagrass habitat, even 10 km from the dredge site.