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

Table 2.1Seagrass sites, zone designation and distance to Offshore Disposal Area boundarySite Name Depth (m) Treatment12BTD 21 Disturbed 12837D 21 Disturbed 15474D 21 Disturbed 8836TD 21 Disturbed 510D 20 Disturbed 15674T 21 Transition 7610T 20 Transition 18211T 20 Transition 4912DT 20 Transition 15436DT 21 Transition 910H 20 Undisturbed 23636H 21 Undisturbed 49SH32 18 Undisturbed 1590SH50 20 Undisturbed 520SH51 20 Undisturbed 12202.2.2 Seagrass shoot densityNumber of quadrats per siteDistance to OffshoreDisposal Areaboundary (m)Baseline seagrass shoot density data from November 2010 was pooled from the IHD seagrasshealth monitoring sites (SH32, SH50 and SH51), and the minimum detectable difference(MDD) was statistically calculated. Baseline data (n=42, mean=599, se=51.4) showed thatwhen sampling 15 quadrats per site, the natural variability resulted in being able tostatistically determine a difference when a site had a mean which deviated from thepopulation mean by at least 32%. This was determined as an adequate sample size based onoperational constraints (diver bottom time) weighted against the diminishing returns (inMDD) of increasing the sample size.Shoot densityTransects were purposefully placed over patches of seagrass to ensure measurements ofseagrass shoot density. Along three permanent transects, five fixed quadrat positions (at1.5, 3.0, 4.5, 6.0, 7.5 and 9.0 m) were set for determination of seagrass shoot counts, i.e.totalling 15 shoot density quadrats per site. Divers took two photos of each quadrat (QA/QCpurposes) and recorded total shoot density per quadrat by vigorously waving their hand overthe quadrat to 'wash out' surface sediment (~0.002-0.003 m), thereby exposing shootswhich aids in separating out shoots from individual leaves. Shoot density quadrats haddimensions of 0.2 x 0.2 m for a 0.04 m 2 coverage area. Shoot densities were extrapolated toa per m 2 area by multiplying the shoot density by 25.Future seagrass shoot counts will be collected in the month of November, if logisticallypossible, following annual seed production to minimise the influence of seasonal variation.2.2.3 Seagrass coverTo estimate percentage cover of seagrass per site, five random 20 m video transects wererecorded at each site and analysed by 'TransectMeasure' software (SeaGIS, Bacchus MarshVictoria). Video transect starting points were allocated to permanent transect posts fromwhich the diver recorded 20 m of video by swimming out at a predetermined compassbearing. Compass bearings were random, but vetted to ensure that video transects did notintersect. Videos were recorded at approximately 0.84 m from the seabed to capture a 0.5 mwide transect belt. Panasonic Lumix cameras with underwater housings collected video at 25frames per second.Analysis of videos using 'TransectMeasure' consisted of a marine scientist removing beginningand end portions of the video that contained collection "noise" (i.e. divers recording slatelabels and retrieving cameras). Once the video was prepared for analysis, ten randomframes were selected by the computer with the only limiting parameter being that any twoOceanica: Fremantle Ports: FP – Gage Roads Offshore Disposal Area Seagrass Recovery Project, Phase I – Initial Survey 13