DETECTING AN ENVIRONMENTAL-IMPACT OF DREDGING ON SEAGRASS BEDS WITH ABACIR SAMPLING DESIGN

The impact of maintenance dredging an access channel to a canal estate in Deception Bay, Australia, on the nearby seagrasses was monitored o ver 18 months with a Before/After, Control/Impact, Repeated measures ( BACIR) sampling design. Three seagrasses were collected in the study a rea; Zostera capricorni Aschers., Halophila ovalis (R.Br.) Hook. f. an d Halophila spinulosa (R.Br.) Aschers. All seagrasses were found less than 700 m offshore. The biomass of Z. capricorni, the numerically dom inant seagrass, was significantly lower in the access channel border c ompared 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 seagras s was absent in the access channel 14 months after dredging. This was due to the high background variability of seagrass biomass in the cont rol area. In contrast the biomass of H. ovalis declined at a significa ntly higher rate in the control area than in the access channel border but had also disappeared from the access channel border 14 months aft er dredging. Without a control we may have concluded that the disappea rance of seagrass from the access channel border was due to the effect s of dredging, whereas with a BACIR sampling program there remained a possibility that the decline in seagrass was due to larger scale chang es in the bay.