Fremantle Port Inner Harbour and Channel ... - Fremantle Ports

1. Introduction1.1 BackgroundThe Gage Roads Offshore Disposal Area is located approximately 6 km from the entrance ofFremantle Harbour in Western Australia (Figure 1.1). The Offshore Disposal Area was utilisedduring 2010 to dispose of approximately 1.5 million m 3 of dredge material associated withthe Inner Harbour and Channel Deepening (IHD) Project. The site had not previously beenused as a disposal site, but it is used as an anchoring ground for commercial vessels awaitingcargo at the Fremantle Port facilities; and therefore has a history of seabed disturbance fromanchors.Prior to the IHD Project, it was estimated that approximately 15.4 ha of seagrass habitat waspresent within the Offshore Disposal Area (Figure 1.2) (Oceanica 2010a). Immediatelysurrounding the disposal area, it was estimated that a further 7.9 ha of seagrass habitat waspresent within a predicted zone of potential loss, plus 48.7 ha within a predicted zone ofpotential effect (from sediment deposition and light deprivation associated with the offshoredisposal plume from the IHD Project) (Oceanica 2010a).To facilitate an estimate of seagrass loss within the Gage Roads region post-completion of theIHD Project, a benthic habitat map was compiled. However, the actual loss within theOffshore Disposal Area or adjacent area to the south-west could not be confirmed (due to thepresence of an apparent algal bloom preventing visual penetration to the benthicenvironment and consequent inability to map habitats) (Oceanica 2010a).Seagrass habitat survey within and surrounding the Offshore Disposal Area subsequent to theIHD Project is being undertaken to facilitate seagrass impact assessment particularly toinform future disposal campaigns and complete a benthic habitat map for reference.Fremantle Ports seeks to investigate the rate and controlling mechanisms of recovery of theseagrass Posidonia angustifolia (Cambridge and Kuo) at the Offshore Disposal Area andsurrounding areas following disturbance (i.e. from burial, smothering and light deprivation).This study will aid Fremantle Ports' development of a long-term dredge material disposal planthrough a greater understanding of impacts upon seagrass after disposal of materials andthrough an updated benthic habitat map for the Gage Roads area.1.2 Scope of this documentThis document describes the initial (Phase I) survey of seagrass health, coverage andcoincident sediment physicochemical data, comprising a 'baseline' for a medium term (up to3 years) investigation into seagrass recovery within and around the Gage Roads OffshoreDisposal Area. This document collates relevant historical data and outlines methods used forthe Phase I survey, also intended for subsequent surveys, to ensure consistent standardoperating procedures in the assessment of seagrass recovery.Oceanica: Fremantle Ports: FP – Gage Roads Offshore Disposal Area Seagrass Recovery Project, Phase I – Initial Survey 1