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

4. Benthic Primary Producer Habitat SupplementaryMonitoringSupplementary benthic primary producer habitat monitoring occurred at sites adjacent to theGage Roads Offshore Disposal Area during the early summer periods of 2011/2012 and2012/2013. This monitoring project is entitled "Gage Roads Offshore Disposal Area SeagrassRecovery Project" (with details provided in the previous compliance report; Oceanica 2013).The purpose of the supplementary monitoring is to investigate the rate and controllingmechanisms of Posidonia angustifolia (Cambridge and Kuo) seagrass recovery within andsurrounding the Gage Roads Offshore Disposal Area, subsequent to the Inner Harbour andChannel Deepening (IHD) Project.The specific objectives of the surveys, within and immediately surrounding the OffshoreDisposal area, are to:1. Define the spatial extent of seagrass habitat;2. Determine seagrass health, as indicated by P. angustifolia shoot density and percentagecover, within "undisturbed", "transitional" and "disturbed" zone specific sites; and3. Assess the sediment chemistry and physical characteristics of coincident sites, to enablespatial assessment of potential physicochemical controls on seagrass health.Fifteen permanent seagrass monitoring sites were set up in November 2011 surrounding theOffshore Disposal Area; with five sites in each undisturbed, transitional and disturbed "zone".These permanent sites have all been placed in locations where seagrass habitat existed priorto sediment disposal in depths of 18-21 m. Site determination consisted of semi-qualitativemethods of towed video to select undisturbed, transitional and disturbed seagrass "zones",within which monitoring sites were established based on initial survey data of seagrasspercentage cover and shoot density. This determination process created a robust three zoneexperimental design, capable of capturing changes in P. angustifolia cover and shoot density.'Undisturbed’ seagrass sites were located outside, but within 1600 m, of the OffshoreDisposal Area. These undisturbed sites included the use of reference monitoring sites used inthe WQMP (Oceancia 2010c) during the Project works. Transitional sites had seagrass coverwhich appeared impacted by dredging material, but not drastically disturbed. Disturbed siteswere located at the fringe of the transitional areas with low seagrass coverage and adjacentto barren areas with apparent deposition of dredge materials.The initial survey in 2011 collected baseline data and produced:1. an updated benthic habitat map;2. seagrass health (shoot density and percentage cover) data and baseline comparisons;and,3. physico-chemical properties at each site which showed no sediment parameters whichwould complicate seagrass recovery.Findings showed seagrass loss modelling undertaken prior to the dredging and disposalactivities were accurate within the zones of loss, potential loss and potential effect.A second seagrass survey was completed in December 2012 and designed to measure forchanges in percentage cover and shoot density after two years of recovery (Oceanica 2013).Finally, a third seagrass survey is scheduled for the 2014/2015 summer period, to continueto measure these changes in seagrass health, subsequent to five years of recovery.Oceanica: Fremantle Ports: Fremantle Port Inner Harbour and Channel Deepening, Compliance Assessment Report 2013 15