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

2.3.3 Sediment redistributionRates of sediment erosion or deposition (redistribution) on the new seabed surface createdby the dredge materials may directly affect the recovery of seagrass within and surroundingthe Offshore Disposal Area. Depths of the Offshore Disposal Area will be assessed fordifferences in depth from preceding surveys and the permanent site locations will be overlaidto qualitatively determine if individual sites are being affected by erosion or deposition. Highrates of erosion or deposition (>0.1-0.2 m) may directly affect recovery or recolonisation ofseagrass. The necessary bathymetry data was collected via multi-beam sonar surveys duringDecember 2009, December 2010 and December 2011 (Fremantle Ports).2.4 Statistical analysis2.4.1 Overview of data presentation and statistical analysesSeagrass shoot density data are presented in graphs as means (n=15) encompassed withintwo standard errors (se). Percentage cover of seagrass and sediment PSD are presented asaccumulative bar graphs in which each portion of the bar illustrates the mean (n=50,percentage cover; n=5, sediment PSD). Sediment chemical data are presented in graphs asmeans (n=5) encompassed within two standard errors.Statistical analyses were performed using (PERMANOVA; Anderson et al 2008) and post hoctests were performed using (PRIMER, v. 6.0; Clarke and Gorley 2006).2.4.2 Hypothesis - Phase I surveyFor the Phase I survey, data were collected to produce a "baseline" for further surveys. Assuch the hypothesis for this survey was "sites within zones are not similar".The assessment of seagrass recovery is designed to be undertaken in three phases (seeSection 5). Future survey data will be tested against previous survey data and thus futuresurvey hypothesis testing will be that "sites are not similar between zones and betweenyears".2.4.3 Experimental designBenthic habitat mapsTo assess spatial extent of seagrass recovery, benthic habitat maps will be produced annuallyfrom satellite imagery and ground-truthed habitat data collected from video tows. Todetermine comparable coverage from survey to survey, towed video transects will start at thetarget location and follow the same trajectory and same distance, to the extent possible. Arepeat of approximately 70 towed video transects will be completed during each phase.Seagrass densityDuring this initial survey, shoot counts were analysed by zone, with sites being nested(n=15). Analysis was a three factor design: zone (undisturbed, transition or disturbed), site(1-5) and transects (1-5).In subsequent surveys, shoot counts are designed as fixed permanent quadrats which will beanalysed individually by repeated measures methods.Percentage cover seagrassPercentage cover measurements were derived from independent random 'quadrats' per site(n=50). Each of the ten random frames within the five random videos were consideredindependent and were used for the calculation of the mean and variation of each site. Forthis initial survey, analysis was a three factor design: zone (undisturbed, transition ordisturbed), site (1-5) and frame (1-10).In subsequent surveys, percentage cover will be analysed as a three factor design: time(year 1, 2 etc), zone (undisturbed, transition or disturbed) and site (1-5).16 Oceanica: Fremantle Ports: FP – Gage Roads Offshore Disposal Area Seagrass Recovery Project, Phase I – Initial Survey