Detecting an environmental impact of dredging on seagrass beds with a BACIR sampling design

The impact of maintenance dredging an access channel to a canal estate in Deception Bay, Australia, on the nearby seagrasses was monitored over 18 months with a Before/After, Control/Impact, Repeated measures (BACIR) sampling design. Three seagrasses were collected in the study area; Zostera capricorni Aschers., Halophila ovalis (R.Br.) Hook. f. and Halophila spinulosa (R.Br.) Aschers. All seagrasses were found less than 700 m offshore. The biomass of Z. capricorni, the numerically dominant seagrass, was significantly lower in the access channel border compared with the control area before dredging, which was attributed to direct or indirect effects associated with the channel. There was no significant effect of maintenance dredging statistically detected for Z. capricorni biomass in the access channel border even though seagrass was absent in the access channel 14 months after dredging. This was due to the high background variability of seagrass biomass in the control area. In contrast the biomass of H. ovalis declined at a significantly higher rate in the control area than in the access channel border but had also disappeared from the access channel border 14 months after dredging. Without a control we may have concluded that the disappearance of seagrass from the access channel border was due to the effects of dredging, whereas with a BACIR sampling program there remained a possibility that the decline in seagrass was due to larger scale changes in the bay