Soft shore WRITE UP
- 1. Single Transect Studies of a Sheltered/Muddy Site and a Exposed/Sandy Site on Austrovenus stutchburyi and Macomona liliana Size and Density J. J. Cole and the class of 2015 in the Marine Biology and Monitoring paper of the Bachelor in Applied Science at the University of Waikato 1.0 INTRODUCTION 1.1 Topic Introduction Two sitesonat TuapiroPointinTauranga Harbour, New Zealand,have beenselectedforthisstudy.One isexposedtowave actionandthe prevailingwindsandthe otherisshelteredfromthe prevailingwinds and has muddysediment. Invertebrates have beencountedusing sample coresandthe dataanalysed. From the bivalve data, Macomona liliana andAustrovenus stutchburyidatawasfurtheranalysedtogive size anddensitystatistics. Hewittet.al.,(1996) has beenchosenasa comparisonstudy. That study alsoanalysedthe size and densitystatisticsof M.liliana and A. stutchburyiandwasconductedinthe Manukau Harbour nextto the Auckland International Airport andalsohadtwo similarsitestothisstudy. The differencestothis study’smethods isthus:Hewittet.al.,(1996) chose midtide sites,whereasthisstudywasconductedat three tidal elevationsand they tookmore samplesoveralargerareaof the Manukau Harbour estuary. Theyalsoperformedmore advancedstatistical analyses. The Hewittet.al. (1996) studybearsmore significance inthatitisbecomingincreasinglyapparentin ecologythatthe distributionof organismsoftenchange overdifferentscalesof study(forexample,Allen and Starr, 1982; Daytonand Tegner,1984; Powell,1989; Legendre,1993; ArdissonandBourget,1992; Horne and Schneider,1994). Althoughthe three differenttidal elevationsinthisstudywillgive awider scope to Hewittet. al.,(1996). 1.2 Literature Review Hewittetal., (1997) had samples1m apart nestedwithineach5 m, whichwere inturn,1 km apart. Thiswas repeated3times. This way,theirdata wasnot as homogeneousasitis inthisstudyand it
- 2. representedalargerscale andtherefore gave aclearerpicture of the ecologyof the entire sandflat. The data collectedinthisstudyonlyrepresentsaverynarrow band of sand flat. Legendre etal.,(1997) foundthat large scale experimentsare neededtoexplainthe spatial structure of M. liliana and A.stutchburyi. It isonlyat these largerscalesthatmakesit possible toobserve physical factors thatinfluence the spatial structure of thesebivalves. Therefore,itisdifficulttoexplainthe distributionof the bivalvesthatwere sampledinthisstudy. More samplesare neededtobe takenas perthe methodusedbyHewittetal.,(1997) to make thispossible. Hewittet.al. (1996) conducted a studyat two midtide sitesinthe ManukauHarbour nextto the AucklandInternational Airport.Theycalledtheirtwosites,muddyandsandy.Thisstudy wasalso conductedat twositeswithone being muddierthanthe other.These twositesare calledthe sheltered (muddy) andexposed(sandy),althoughthisstudysampledatthree tidal elevations.InHewittet.al. (1996), bothA. stutchburyiandM.liliana were more abundantinthe sandy(exposed) site,whichisalso true in thisstudy,especiallyatmidtide. Anothersimilaritybetweenthisandthe Hewittet.al.(1996) studyisthat there were veryfewpredatorspresent. Justsome seabirdsandlow densitiesof crab as indicatedbythe crustaceans countin figure 6. Hewittet.al.(1996) suggeststhatM. liliana are not verymobile asadultsbutA.stutchburyiare. Although Legendre etal.,(1997) foundthat bothadultbivalvesare lessmobile than juveniles.Theyalso foundthat clustersof adultbivalvesoccurmore oftenbecause of avariationof effectsincluding predation,competitionandadvectionetc. Hightide clustersof largerbivalvesindicate the hydrological historyoverseveral years. The distributionof smallerbivalvestendtobe more random due to wide rangingwave action beingsolelyresponsiblefor the depositionof larva. They then,overa periodof some years,congregate inareasthat advectionhasplacedthemandthen fine tune their positionbymoving ontheirown tositesthat serve themwell intermsof foodavailabilityandlackof predators. Indeed,inthisstudy,the presence of the seagrass Zostera marina,increasesthe size and densityof all marine invertebratessince itservesasa predatorhideoutandis hot spotfor feeding(Reed & Hovel,2006). 1.3 Aims and Objectives Thisstudy aimsto provide a widerscope toHewittet.al.,(1996) instudyingthe three differenttidal heights. Thisstudyalso aimsto examine the twocommunitiesof the shelteredandexposedsitesatthree differenttidal elevations(low,midandhigh) withineachsite alongatransect line, todecide whether theirtaxonomiesare statisticallysignificantlydifferent. Special attentionwill be paidtothe density and size of the bivalves. Those beingthe clam A.stutchburyi– a suspensionfeederandthe wedge shell M.liliana – a depositfeeder.
- 3. 2.0 METHODS 2.1 Study site Location and Description Figure 1. Map of the studysitesat TuapiroPoint inTauranga Harbour inthe NorthIslandof New Zealand. The whelteredsite tothe westiswell protected,whilethe exposedsitetothe east,while still protectedbyMatakana Island,ismore exposed. Thismap isthe same one as usedby Lelieveld, Pilditch,&Green,(2004), butthe shelteredandexposedsiteshave beenaddedinaswell asthe transect lines.
- 4. Figure 2. Low tide at the shelteredsite showingmangrove Avicennia marina var.resinifera seedlings (Sveda,G.,2015b).
- 5. Figure 3. Photoof the exposedsite atlow tide takenfromhightide mark. Thisshowsthe Z. marina seagrassmeadow(Sveda,G.,2015a) 2.2 Description of Methods A semi-systematicsamplingschemewasusedtoquantitatively analyse community arraysacrossthe shore (Fig.1). In thissampling scheme, positions were distributed systematicallyacrossthe transect line,atlow,midandhightidal elevations.Ateachposition, 21replicate core,(13 cm diameterby15 cm deep),samples randomly selectedwerecollected. The cores were thensieved usinga1 mmmesh,and invertebrates were assignedtothe taxonomic groupsof gastropods,polychaetes,crustaceansand bivalves.Eachgroupwas then counted. Within the bivalves, A.stutchburyiandM.liliana were identified andmeasurementsof the maximumshell lengthof twocommonspecieswere taken tothe nearestmm. The double headedarrowsinfig. 4and fig.5 showexactly whatshell lengthwasmeasuredandcare wastakento ensure everyonewastaking the same measurements. All data wasthencollatedandenteredintoaspreadsheetforstatistical analysis.
- 6. 2.3 Predictions The firstpredictionisthatshell size mayvarywithtidal elevationbecauseof adecrease infeeding opportunities. The second,that M.liliana will be more denselypopulatedinthe shelteredsite. Figure 5. A.stutchburyi(Bould,G., 2008a) Figure 4. M.liliana (Bould,G., 2008b)
- 7. 3.0 RESULTS 3.1 Community Composition 3.1.1 Total Mean Densities Figure 6. Total mean density of invertebrates(Polychaetes,Crustaceans,GastropodsandBivalves) inthe exposed(blackbars) andsheltered(whitebars) sitesathigh,midandlow tide. Standarddeviationerror bars are,on the whole,relativelyshort. The highestdensityof invertebratesare atthe mid tide exposedsite. Thiswoulde because of sightings of the seagrass, Z.marina. Also,the densitiesare relativelyhighatbothlow tide sites. Apart fromthat midtide exposedsite,asthe tidal heightincreases,thereisaproportionate decrease in invertebrate meandensities. Figure 7. Close upphotoof Zostera marina inthe midtidal zone of the exposedsite 0.00 10.00 20.00 30.00 40.00 50.00 60.00 High tide Mid tide Low tide High tide Mid tide Low tide EXPOSED SHELTERED Totalmeandensity(freq.percore) Survey Sites
- 8. 3.1.2 Proportional Density of all Invertebrates Figure 8. The proportionof densitythateachinvertebrate groupexistsin. Bivalvesare mostdominate atmidand low tide onthe shelteredsite.Gastropodsare mostdominantat the midtide inthe exposedsiteandhightide atthe shelteredsite.Crustaceansare notdominantatall but occur at the more exposedtidal areasandlessinthe more shelteredsites.Polychaetesare inhigher densitiesinhighertidal sites. The exceptionisthe midtide,exposedsite,where theyare inverylow numbersand seemtobe replacedbyGastropods because of the observed presence of aseagrass,Z. marina. 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% High tide Mid tide Low tide High tide Mid tide Low tide EXPOSED SHELTERED ProportionalDensity Survey Sites Polychaetes Crustaceans Gastropods Bivalves
- 9. 3.2 Bivalve Mean Size Figure 9. Mean size of Austrovenusstutchburyi (blackbars) andMacamona liliana(whitebars) ateach tidal zone inthe exposedandshelteredsites. The standard deviationerrorbarsare longin mostcases, especiallyforM.liliana. M. liliana are a largerspeciesthan A.stutchburyi.The size of eachspeciesdoesnotchange verymuch overeach site. However, M.liliana are largestin the sheltered,hightide siteandsmallestatthe exposedmidtide andshelteredlowtide sites. A. stutchburyiare notablylargestinthe exposedlow tide site butit doesnothave a notable site atwhichtheyare at theirsmallest. 0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00 High tide Mid tide Low tide High tide Mid tide Low tide EXPOSED SHELTERED MeanSize(mm)&StDev Survey Site A. stutchburyi M. liliana
- 10. 3.3 Bivalve Density Figure 10. Mean densityof A.stutchburyi(blackbars) and M. liliana (white bars) ateach tidal zone withinthe exposedandsheltered sites. Standarddeviationerrorbarsare extremelylongindicating that there isa verywide variabilityof densities inthe core samples ateachsite. A. stutchburyi(686 total individuals)ismore denselypopulatedthan M.liliana (266). A.stutchburyiis mostdenselypopulatedinthe exposedmidtide site.Itisalsodenselypopulatedinbothlow tide sites. There isa spike of bothspeciesatmidtide onthe exposedtransectbecause of the presence of the seagrass, Z.marina. Apart fromthat site,itisapparentthat increasesintidal heightrepresent proportionate decreasesinbothspeciesmeandensities.This isasimilarpatterntofigure 6. M. liliana it ismostlypopulatedatthe shelteredlow tide site. It isalso denselypopulatedinthe exposedmid tide site. Overall,thisgraphisa similarshape tofigure 6. The maindifferencesisthe lengthof the errorbars, whichisbecause there ismare data infigure 6, whichmakesitmore precise. Table 1. Total countsof A. stutchburyi andM lilianaateachsite Exposed Sheltered A. stutchburyi 431 255 M. liliana 112 154 There are significantlymore A.stutchburyiinthe exposedsite samplecoresthaninthe shelteredsite’s cores. There doesnotlooklike thatthere ismuch difference betweenthe M.liliana countsfor each site. 0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 High tide Mid tide Low tide High tide Mid tide Low tide EXPOSED SHELTERED MeanDensity(freq.percore) &StDev Survey Site A. stutchburyi M. liliana
- 11. 3.4 Statistical Analysis of Bivalve Size and Abundance Table 2. ANOVA pvaluesof the data setswithineachsite. All pvaluesindicate verysignificant differencesinthe valuesof all sites. Data sets Sites p values Density Exposed 0.00 Sheltered 0.00 Sheltered Exposed 0.01 Size 0.00 Thisindicatesthatmore analysisneedstobe performedtofindoutexactlywherethe significant differencesliewithinthe sites. Table 3. DensityT-testpvaluesbetweenshelteredandexposedsiteswithineachspecies. A. stutchburyi 0.00 significant difference M. liliana 0.11 no significant difference There isa verysignificantdifference inthe densitiesof A.stutchburyibetweenthe exposedand shelteredsites. Thisis notthe case for M.liliana. Table 4. Size T-testp valuesbetweenshelteredandexposedsites withineachspecies A. stutchburyi 0.04 significant difference M. liliana 0.67 no significant difference There isa statistically significantsize differenceof A.stutchburyibetweenthe exposedandsheltered sitesandno significance forM.liliana. Both data setsare lesssignificantlydifferentthanthe density data sets. Table 5. T-testp valuesforoverall densitiesatall combinationsof tidal heightsateachsite. Non- significantdifferencesare heavilyshaded. Exposed Sheltered Tidal heights A. stutchburyi M. liliana A. stutchburyi M. liliana High/Mid 0.00 0.00 0.02 0.00 High/Low 0.01 0.01 0.00 0.00 Mid/Low 0.00 0.22 0.06 0.00 All tidal combinations(exceptforbetweenmidandlow tidesfor M.liliana onthe exposedtransectand for the same tidal combinationfor A.stutchburyionthe shelteredtransect –heavilyshaded)have significantlydifferentdensitiesbetweeneachtidal combination.
- 12. Table 6. T-testp valuesforoverall sizesatall combinationsof tidal heightsateachsite. Non-significant differencesare heavilyshaded. Sites Exposed Sheltered Tidal heights A. stutchburyi M. liliana A. stutchburyi M. liliana High/Mid 0.00 0.03 0.01 0.01 High/Low 0.04 0.19 0.01 0.17 Mid/Low 0.06 0.57 0.94 0.05 There are significantsizedifferencesforbothspeciesbetweenthe highandmidtidesatbothexposed and shelteredsites. M.liliana hasno othersignificantlydifferentsizesinanyothertidal comparisonat eithersite. The size of A.stutchburyiare significantlydifferentbetweenhighandlow tidesatboth sites.There are no significant size differencesateithersite,foreither specieswhencomparingmidand lowtides. 3.5 Size Frequencies It seemsthatat all sitesthat the sizesof the bivalvesare verysimilarateach tidal elevation. 3.5.1 A. stutchburyi Exposed site A. stutchburyiare very infrequentonthe exposedhightide site,very frequentatmidtide (because of the presence of the seagrass, Z.marina) andsomewhere inthe middleatlow tide. The trend lines showanormal distributionathighandmid tides. Atlow tide,the size isveryscattered, but the bars still show avery slightnormal distributioncurve. The true meangraduallydecreasesasthe tidal elevationdecreases. The total frequency of A.stutchburyiinthe exposedsite is431 individuals. 0 5 10 15 20 25 30 35 40 45 50 0-1 6-7 12-13 18-19 24-25 30-31 36-37 Freq.onexposedhightidesites A. stutchburyi sizeclasses (mm) 0 5 10 15 20 25 30 35 40 45 50 0-1 5-6 10-11 15-16 20-21 25-26 30-31 35-36 Freq.onexposedmidtitdesites A. stutchburyi sizeclasses (mm) 0 5 10 15 20 25 30 35 40 45 50 0-1 5-6 10-11 15-16 20-21 25-26 30-31 35-36 Freq.inexposedlowtidesites A. stutchburyi sizeclasses (mm) Figure 11. Size frequencyof A.stutchburyionthe exposedsite. Hightide (left),midtide (centre), and lowtide (right). Trendline showsthe movingaverage foreveryfifthdataentry.
- 13. 3.5.2 A. stutchburyi Sheltered site There are higherfrequenciesof eachsize classasthe tidal elevationdecreases. All graphs do showa normal distribution trendline.Althoughatlow tide,there seemstobe two cohorts. One smallercohortwitha meancentring12 – 13 mmas perthe exposedsite. The other largercohort meancentringon 13 – 14 mm. The overall frequencyof A.stutchburyiislowerinthe shelteredsite with255individuals. 3.5.3 M. liliana Exposed site Figure 12. Size frequencyof A.stutchburyionthe shelteredsite. Hightide (left),midtide(centre), and lowtide (right). Trendline showsthe movingaverage foreveryfifthdataentry. 0 3 6 9 12 15 18 21 0-1 5-6 10-11 15-16 20-21 25-26 30-31 35-36 Freq.inshelteredhightidesites A. stutchburyi sizeclasses (mm) 0 3 6 9 12 15 18 21 0-1 5-6 10-11 15-16 20-21 25-26 30-31 35-36 Freq.inshelteredmidtidesites A. stutchburyi sizeclasses (mm) 0 3 6 9 12 15 18 21 0-1 5-6 10-11 15-16 20-21 25-26 30-31 35-36 Freq.inshelteredlowtidesites A. stutchburyi sizeclasses (mm) 0 1 2 3 4 5 6 7 8 9 10 0-1 4-5 8-9 12-13 16-17 20-21 24-25 28-29 32-33 36-37 Freq.inexposedhightidesites M. liliana sizeclasses (mm) 0 1 2 3 4 5 6 7 8 9 10 0-1 5-6 10-11 15-16 20-21 25-26 30-31 35-36 Freq.inexposedmidtidesites M. liliana sizeclasses (mm) 0 1 2 3 4 5 6 7 8 9 10 0-1 5-6 10-11 15-16 20-21 25-26 30-31 35-36 Freq.inexposedlowtidesties M. liliana sizeclasses (mm) Figure 13. Size frequencyof M.liliana on the exposedsite.Hightide (left),midtide (centre),andlow tide (right). Trendline showsthe movingaverage foreveryfifthdataentry. As withA.stutchburyi,there are higherfrequenciesof individualsinthe midtide elevation. The size classesrepresentedatall tidal elevationsare more scatteredthan A.stutchburyi. There were alsosightingsof the seagrass, Zostera marina atmidtide Cohortsseemstofeature more inthe M. liliana data. It isnot clearwhere the true meanslie athigh or lowtides,butat midtide,there seemstobe one true meanat 6 – 8 mm and anotherat 24 – 25 mm. The total frequencyof M.liliana at the exposedsite is112 individuals.
- 14. 3.5.4 M. liliana Sheltered site As the tidal elevationdecreases,the frequenciesof M.liliana increases.Thisisasimilarpatternto A. stutchburyiatthe shelteredsite althoughthere are lessoverall M.liliana individuals. The trend lines doshow aresemblance of normal distributioncurves. The midtide trendline seemsto showat leasttwopossible cohortsalthoughitisnotpossible tofindthe true meanof the smaller cohorts.The largestone seemstohave a true meansize of 26 – 37 mm. There alsoseemstobe a small cohortof smallerindividualsatlow tide withatrue meancentringon9 – 10 mm. The larger cohort ismore obviouswithatrue meanof 22 – 23 mm. The total frequencyof M.liliana inthe shelteredsite is154 individuals. 0 2 4 6 8 10 12 14 16 0-1 5-6 10-11 15-16 20-21 25-26 30-31 35-36 Freq.inshelteredhightidesites M. lilliana sizeclasses (mm) 0 2 4 6 8 10 12 14 16 0-1 5-6 10-11 15-16 20-21 25-26 30-31 35-36 Freq.inshelteredmidtidesites M. lilliana sizeclasses (mm) 0 2 4 6 8 10 12 14 16 0-1 5-6 10-11 15-16 20-21 25-26 30-31 35-36 Freq.inshelteredlowtidesites M. lilliana sizeclasses (mm) Figure 14 .Size frequencyof M.liliana on the shelteredsite.Hightide (left),midtide (centre),andlow tide (right). Trendline showsthe movingaverage foreveryfifthdataentry.
- 15. 4.0 DISCUSSION 4.1 Predictions The firstpredictionthatshell sizesof A.stutchburyiandM.liliana wouldvarywithtidal elevationis partlyjustified. Table 6 showsthatthere are significantsizedifferencesforbothspeciesbetweenthe highand midtidesat bothexposedandshelteredsites. M.liliana hasno othersignificantlydifferent sizesinanyothertidal comparisonat eithersite. The size of A.stutchburyiare significantlydifferent betweenhighandlowtidesatbothsites.There are nosignificantsize differencesateithersite,for eitherspecieswhencomparingmidandlow tides. A.stutchburyi,butnotM.liliana has significant differencesin shellsizesbetweenexposedandshelteredsites(Table 4). M.liliana shell size hasbeen explainedby Hewittet.al.,(1996), who foundthatincrease insedimentgrainsize waspositively correlatedtoincreasesinshell sizesfor M.liliana, butnot A. stutchburyi. The second predictionmade that M.liliana wouldbe more populatedinthe shelteredsite isdisproven since the densityT-testpvalue inTable 3 examiningsignificantdifference betweenthe shelteredand exposedsitesisinsignificant. 4.2 Species diversity The Hewittet. al. (1996) studyonlyfoundbivalvesandatwo speciesof polychaetes,whereasthisstudy founda much more speciesdiverse site (fig.8). It isinterestingtonote thatwhere the Z.marina meadow islocatedat midtide onthe exposedsite that polychaete abundance hasfallendramaticallyandoutof sequence. The normal sequence for polychaetesisfortheirnumberstofall asthe tidal heightdecreases butinthiscase,the midtide atthe exposedsite,itisat itslowestproportional abundance. Theyseemtohave beenreplacedby gastropods,whichare predictedtobe feedingof Z.marina. The opposite istrue at hightide onthe exposedsitewhere polychaete numbersare muchhigherand gastropodsare much lowerwhere there islessvegetationandthe onlysustenance forherbivoresisin the phytoplanktonthatisdeliveredbyhightides. Predationfromthe highestproportionof crustacean (crabs) numbers,isalsolikelytobe contributing tothe lowerproportion of bivalvesandgastropods. On the shelteredside,there are alsolowernumbersof bivalvesandgastropodsthathave probablybeen predatedonby birds. Anotherfactorcouldbe that the sandat hightide ismore compactedbyhuman recreational andscientificactivity. It is harderfor these organismstoburrow intomore compacted sand as describedby Lelieveld,Pilditch&Green,(2004). In the shelteredsite,highernumbersof gastropodsathightide isassumedtobe because of the presence of A.marina var.marina,whichtheyare feedingfrom. 4.3 Bivalve size and density Table 4 andfigure 9 combine toshowthat the difference betweenthe meansizesof bothspeciesof bivalves,especially M.liliana is notall that significant. The p value of the T-testcomparingbothsite’s sizesof A.stutchburyiis0.04, whichisonlyjustsignificantlydifferent. Factorsthat contribute to differentsizesof M.liliana is grainsize (Hewitt et.al., 1996). This wouldtendtosuggestthatthe difference ingrainsize betweenthe exposedandshelteredsitedoesnottendtovaryverymuch. A personal observationof these twositesisthatthe shelteredsite didseemtobe muddierthanthe exposedside,whichwassandier.
- 16. The densityof these speciesateachsite givesaclearerpicture of the ecologyof these species. The T- testp value intable 3 and the total countsof eachspeciesintable 1 combine toshowsthatthere isa significantlyhigherdensityandcountof A. stutchburyiinthe exposedsitethan M.liliana. 4.4 Size frequencies Figures11 – 14 showthe spreadof sizesthatoccur for each speciesof bivalve ineachsite. In the exposedsite, A.stutchburyisizes(figure 11) donot tendto vary muchexceptat low tide,where each size classispresent. Thisshowsthat recruitmentisoccurringhere atlow tide. Theythenmove up to higherelevationstofeedon Z.marina at midtide,where there isahighabundance. In figure 12, the lowerthe tidal elevation,the higherthe abundance of A.stutchburyi. The true meansto don’tseemtovary much across the tidal elevations. The spreadof sizesare relativelytightmeaning that the individualsare all aroundthe same age and size. Figure 13 showsthatthe sizesof M.liliana in the exposedsite are muchmore spreadoutindicatinga highvarietyof differentlifestagesof thisbivalve. The recruitmentof thisspeciesistherefore spread out across all tidal elevations. Thistendstoagree withTable 4,whichillustratesthatthe pvaluesof T- testson M.liliana sizesare notsignificant. Figure 14 alsoshowsa wide spreadof M.liliana sizesinthe shelteredsite. 4.5 Size of this study Thisis a verysmall scale study,whichpresentsmanyproblems.We onlysurveyedasmall sectionof TuapiroPointand as such,it isunwise toassume thatthe patternsdiscoveredhere are typical of TuapiroPoint,or anyothersand flat. It is therefore recommendedthatthe methodsusedinthisstudy to be replicatedasperthe methodsusedinHewitt et.al. (1997). Furthermore,numerousstudiesalso indicate the importance forlarge scale toenable the analysisof importantnatural processes,abioticand biotic(forexample,AllenandStarr,1982; Daytonand Tegner,1984; Powell,1989; Legendre,1993; ArdissonandBourget,1992; Horne andSchneider,1994). The spike indensityandsize forall invertebratesincluding M.liliana andA. stutchburyirelatesto sightingsof the seagrass, Z. marina (Fig.7 and Fig.3). ReedandHovel (2006) foundthata certain thresholdof Z.marina densitiespositivelycorrelatedtoincreasesindensitiesof all epi-benthic communities. They alsofoundthat Z. marina tendstooccur in areasthat are disturbedbyhumans. Thiscoincideswithsightingsof horsesanda sledatmid tide onthe exposedsite. The Hewittet. al., (1996) studydoesnotmentionthe presence of anyvegetation. Figure 10 hasverylongstandard deviationerrorbars,which againindicate thatmore datais reqiuired by utilisingthe surveydesignbyHewittetal.,(1997). Thistime,toget a betterrepresentationof the meandensities. The reletivelyinsignificantsize differencesinbargraphin figure 9 gives more weight to the reasonwhythere neededtobe a biggerstudy. With the small amountof time available,this was as muchas couldbe done. Anyfuture studyshouldtake intoaccountthe studydesignbyHewett et. al (1997) and if possible,toallowmore time formore datacollectionalongmore transectlines.
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