SEASONALITY OF BEACHWRACKAT OAKAJEE IN THE MID-WEST REGION OF WESTERN AUTRALIA

7 Pages·2002·3.4 MB·English

by  Fred E Wells

Recordsofthe WesternAustralianMuseum 21; 269-275 (2002).
Seasonality of beachwrack at Oakajee in the mid-west region
of Western Australia
FredE.Wells
WesternAustralianMuseumPerth,WesternAustralia6000,Australia
Email: [email protected]
Abstract - This study examines seasonal fluctuations in beachwrack
accumulations near Oakajee, Western Australia (28"34'S; 114°34'E). Twelve
monthly samples were made ofbeachwrack concentrations at Drummonds
CoveandCoronationBeachfrom5May1998until10April1999.Fourweekly
samples were made from 23 July to 12 August 1998 to measure short-term
variations inbeachwrack volumes. There was no seasonality in beachwrack
accumulations at Drummonds Cove. At Coronation Beach beachwrack
concentrations were high in late summer and early winter, but declined
sharply from latewinter toearlyspring. There was considerable movement
ofwrackontoandoffthebeaches,Beachwrackaccumulationsmigratedalong
the beach within Drummonds Cove, but there was no evidence of such
longshoremovementontheopencoast. Beachwrackaccumulatesinbaysand
other protected areas and at the tip of peninsulas, such as Point Moore.
Macroalgae and theseagrassgeneraAmphibolis and Posidonia dominated the
wrack.
Keywords:beachwrack,macrophytes.WesternAustralia,seagrass,seaweeds,
macroalgae
INTRODUCTION beachwrack decomposes (Robertson & Hansen,
OceanicwatersoffWesternAustraliaarenutrient 1982). This recycling is important in the low
poor, at levels characteristic of the open ocean. nutrientenvironmentoffWesternAustralia.
Consequently, phytoplankton production is The present study extends our knowledge of
minimal (Rochford, 1980; Pearce 1991; 1997). beachwrackaccumulationstoOakajee,justnorthof
Instead benthic plants, both seagrass and Geraldton. The study tested the hypotheses that
macroalgae, provide most of the primary beachwrack accumulates seasonally, with a
production. The plants are tom from the bottom maximum due to winter storms, and once on the
during storms and then moved about by currents beach, the beachwrack migrates longshore in
andwaves.Aportionistransportedtoshorewhere responsetoprevailingwaveandwinddirections.
it accumulates as beachwrack. The beachwrack is
reworked by wind and waves, accumulating in
THEOAKAJEEMARINEENVIRONMENT
some areas andbeing absentin others. It is mobile,
often alternatingbetween the waterand thebeach. TheOakajeeregion(Figure1)isanopen,exposed
Volumeschangeconsiderablyovershortperiodsof coastline subject to the full force of sea conditions,
time(Hansen, 1984). particularly during storms. It is thus a very high
Beachwrack in the Perth area develops an energy environment. Virtually the entire shoreline
abundant invertebratefaunawhichfeeds on it. The from Geraldton north to the mouth of the Bowes
invertebrates are dominated by amphipods, River is composed of intertidal sand. Beachrock
particularly AUorchestes compressa (Robertson & platforms occuralongtheshorelinethroughoutthe
Lucas, 1983; McLachlan, 1985; Fong, 1999). Oakajee region but are more common north of the
Nearshore the beachwrack and associated Oakabella River. They range from small platforms
invertebratesprovideshelterandfoodforavariety ofafewsquaremetrestoanextensiveplatformjust
of fish species (Lenanton, 1982; Lenanton et al., south ofCoronation Beach. The platforms begin at
1982; Robertson & Lenanton, 1984), including the the low tide level and continue into the subtidal
commercially important cobbler Cnidoglanis region. Large portions may be exposed by winter
macrocephalus (Lenanton & Caputi, 1989), and wave action then recovered by sand the following
abalone (Wells & Keesing, 1989). Nutrients are summer. The platforms at Oakajee have a greatly
released back into the water column as the reduced biota, possibly due to the dynamic wave
270 F.E.Wells
Figure1 MapoftheOakajeecoastlineshowinglocalitiesmentionedinthetext.
action at Oakajee that would physically remove Seaward ofthe reefcrest is a low reliefplatform.
much of the macroalgae and fauna. In addition, While the platform surface is not uniform, it tends
sand moved about by the waves would provide a to have a gradually increasing depth seaward.
considerablescouringforce. Small crevices and depressions occur throughout
In the nearshore environment the substrate is the surface of the limestone. The platform is
limestone that may or may not be covered with channelled with a very small spur and groove
sand. While considerable local variation in depth formation in some areas, where the undulations
occurs, there is a steady progression of habitats maybe 30 cm orless in height. The surfaces of the
from the intertidalsand beach outto the 20m line. crestareexposedlimestone.Thechamrelsareeither
A shallow lagoon up to 5 m deep occurs near the exposed limestoneorarecoveredby athin layerof
shore. The lagoon is more distinct south of the sand. Sandaccumulatesinthedepressions,someof
Oakajee River. North of the Oakajee, the lagoon is which are large, tens ofmetres across. In some low
less distinct and has numerous bombies, many of energy areas of the bottom the entire limestone
which nearly reach the sea surface. An indistinct platform is covered by a layer of sand several
limestonereefoccursto thewestofthelagoon.The centimetres deep. In deeper waters, where depths
reefrunsnorth tosouthparalleltotheshore,andis arebetween 15and20m,theplatformgiveswayto
absent only off the mouth of the Oakajee River. In a level bottom that is predominantly covered with
mostareas the reefshoals toadepthof2m orless, sand,butthereareexposedareasofflatlimestone.
butinsomelocalitiesitislessdistinct. Thebottomhabitatscanchangeconsiderablyover
BeachwrackatOakajee 271
a space ofa few metres, particularly in theshallow steppedoffandcomparedwiththenumberofsteps
inshorewatersandthehighreliefreef.Overaspace required to cover a known distance. Width and
of100to200mgoingseaward,thebottomcanvary depth were measured atconsistent intervals with a
from high relief reef with sand patches in the tape measure marked to the nearest 1 cm. The
depressions, to a low relief reef dominated by the intervals were calculated to provide at least 6
seagrass Amphibolis antarctica, to areas co- measurements at each of the two localities; usually
dominatedbyA.antarcticaand macroalgae,toareas 8-10 measurements were made. Width of
whereonlymacroalgaearepresent. Theremayalso beachwrack was measured to the nearest 10 cm
besandybottoms whereA. antarctica formsadense from the seaward to the landward edge of the
seagrassmeadow. Theability ofA. antarctica tolive beachwrack. Depth was normally measured to the
on either sandy or rocky bottoms is well nearest 1 cm at three locations; top of the seaward
demonstratedatOakajee. beachwrackcrest, middle of the accumulation, and
Shallow vertical columns, which occur near the the landward margin; only two measurements of
shore south of the Oakajee River, are one of the depth were made in small accumulations where
mostdistinctivefeaturesofthemarineenvironment there was little beachwrack present. Total volume
in theOakajee area. Thecolumns, whichmay have ofbeachwrackoneachbeachwasthencalculated.
a diameter of 5-10 m or more, emerge vertically To measure the composition ofbeachwrack, four
fromsandatadepthofup to5mtonearthelower samples were collected each at Drummonds Cove
intertidal level. This is an area of breaking waves and at Coronation Beach on 12 June 1998 and 16
during even moderate seas. The upper surfaces of January 1999, placed in labelled plastic bags and
the columns are largely devoid of macroalgae, but frozen until analysis. Samples were collected from
may be colonised by small patches of Sargassum freshly deposited areas of wrack to minimise the
which are kept to short lengths by the continuous proportion of unidentifiable detritus. The samples
waveaction.TheSargassumislongeronthesidesof werewashedinfreshwaterthensortedtofivemajor
the columns than on the upper surfaces. On the plant types: macroalgae, the seagrass genera
leeward side of the colurrms is a mixture of low Amphibolis,PosidoniaandHalophila,andunidentified
encrusting species of macroalgae, including plantmaterial. Individualcomponents wereplaced
Caulerpa. The base of the columns is at a depth of on aluminium foil and dried for 48 hours in a
approximately 6 m and is surrounded by sand drying oven at 80°C. They were then weighed to
mixed with some Amphibolis. There may be small thenearest1mgonaSartoriuselectronicbalance.
patchesofSargassum inshore.Invertebratediversity
onthetopsofthecolumnsislow. RESULTS
Thegreatestaccumulationsfoundduringthefirst
MATERIALSANDMETHODS survey on 5 May 1998 were at Drummonds Cove,
Twelveaerialflightsweremadeatapproximately where 7559 m^ of beachwrack occurred on the
monthly intervals over the coastline at Oakajee shoreline, and at Coronation Beach, where there
(28°34'S; 114°34'E) from 5 May 1998 until 10 April was 2477 m^ (Figure 2). Smaller accumulations
1999 using a Cessna 172 aircraft. Most flights were occurred in other areas. Accumulations at
madeat140-165kmh' atanaltitudeof100mfrom Drummonds Cove were consistently the largest
south of the Port ofGeraldton to the mouth of the alongtheshorelinebetweenDrummondsCoveand
BowesRiver,adistanceofapproximately40km. A theBowesRiver,some25kmofcoastline.
videorecordingwasmadeoftheentireshorelineon
the flight north. A still camera was used on the Seasonalvariation
flight south to photograph major beachwrack The predicted pattern of seasonality was not
accumulations.Fourweeklyflightsweremadefrom clearlydemonstrated atDrummondsCoveduring
23 July to 12 August 1998 to measure short-term the study (Figure 2). The accumulation at
variationsinbeachwrackvolumes. Drummonds Cove on 7 May 1998 was spread
The only major accumulations ofbeachwrack on over an estimated 1786 m of beach. As expected,
the first flight were at Drummonds Cove and at the beachwrack increased to 8510 m^on 12 June,
Coronation Beach. As vehicle access to the shore but instead of continuing to increase during the
was possible, the two beaches were chosen for winter,wrackvolumesdeclined sharply, reaching
ground truthing. On most occasions the two sites a low of 817 m^ on 6 August. Wrack
were the locations with the greatest beachwrack accumulations at Drummonds Cove increased to
concentrations. Methods used by Hansen (1984) 3154 m^ one week later on 13 August. They
wereusedtodeterminebeachwrackvolumesonthe continued to increase until 9 October, when a
shoreline. Length and width of beachwrack peak of 7436 m^ was reached. Significant
accumulations at Drummonds Cove and at accumulations remained for the remainder of the
Coronation Beach were measured. Length was study, varying from 3196 m^ to 5780 m^ over
272 F.E.Wells
Drummonds Cove
Coronation Beach
Figure2 Seasonal patterns of accumulation of beachwrack at Drummonds Cove and Coronation Beach from May
1998toApril1999.Meansandstandarderrorsareshown.
summer, declining to 2221 m^ on the final month In contrast to Drummonds, the four week
of the survey (10April1999). intensive survey atCoronation Beach showed little
In contrast, the pattern at Coronation Beach was v'ariation in beachwrack concentrations (Figure 3).
seasonal(Figure2). Accumulationstotalled2477m^ During the first three weeks (23July, 30July and 6
on7May 1998 andwerealong629 mofbeach. The August) there was essentially no beachwrack
beachwrackincreasedsignificantlyto7043m^on 12 present. An increase to 1221 m^occurred by 13
June. During the period of late winter to early August.
summer beachwrack at Coronation Beach declined
to low levels, ranging from 58 m*on 23July to443 Longshoremovement
m^ on 11 September. Beachwrack volumes There was no clear evidence of longshore
increased sharply to 3647 m^ on 20 February (late movement of beachwrack along the Oakajee
summer) and remained relatively high for the coastline, but the rapidly changing volumes,
durationofthestudy. particularly during the short term study at
Drummonds Cove, showed that there is
Short-termvariation considerably flux in wrack between the shore and
Considerable short-term variation was found theoffshorewaters.
during the four week intensive survey at At the beginning of the study, beachwrack
Drummonds Cove (Figure 3). Concentrations at occupied much of the bay at Drummonds Cove. It
Drummondstotalled4372m^on23July.Twoweeks moved severalhundred metres to thenorth during
later they had dropped to 817 m^. Beachwrack the winter and early spring, but remained in
increasedduringthenextweekto3154m^. Drummonds Cove. In spring fresh build-ups
BeachwrackatOakajee 273
Drummonds Cove
Dates
Coronation Beach
Figure3 Short-term variations in accumulation ofbeachwrack at Drummonds Cove and Coronation Beach in July
andAugust1998.Meansandstandarderrorsareshown.
Table1 CompositionofbeachwrackattwolocationsalongtheOakajeecoastlineinwinter1998andsummer1999.
Algae Seagrasses Detritus Total
Amphibolis Posidonia Halophila
DRUMMONDSCOVE
Winter1998
Mean(±SE)(gm) 10.5+1.5 3.9±0.3 2.7±0.8 0±0 3.8±1.3 20.9±3.1
Percentage 50.2 18.8 12.9 0.0 18.1 100.0
Summer1999
Mean(±SE)(gm) 11.5±1.7 8.8±1.4 0.7±0.2 0±0 2.3±0.7 23.3±0.9
Percentage 49.4 37.8 3.2 0.0 9.7 100.1
CORONATIONBEACH
Winter1998
Mean(±SE)(gm) 12.5±1.2 16.4±1.1 1.0±0.3 0±0 3.4±0.4 33.3±2.2
Percentage 37.6 49.1 3.0 0.0 10.2 99.9
Summer1999
Mean(±SE)(gm) 20.4+3.1 17.3±3.9 3.6±0.4 0±0 3.2±1.4 44.5±5.9
Percentage 45.9 38.8 8.1 0.0 7.2 100.0
274 F.E.Wells
developed along the southern shoreline of localconditionsandmayvaryconsiderablyacrossa
Drummonds Cove and remained there for the range of time scales (days/months/years) at a
durationofthestudy- singlelocation.
At Coronation Beach two accumulations during Itwas thoughtthatonceon thebeachatOakajee,
May 1998 were on thenorthern and southern ends thebeachwrackwouldmigratenorthorsouthalong
of the beach. The southern accumulation theshore in response toprevailing wave and wind
disappeared by June. The beachwrack to the north directions.ThisdidoccurwithinDrummondsCove,
increased in June, but then decreased over time. but there was no evidence of such longshore
Beachwrackaccumulationsincreasedoversummer, movement on the open coast. Instead, the
cmd by February the entire margin of the bay was beachwrack accumulates in bays and other
fringedwithbeachwrack. protectedareasand atthetipofpeninsulas,suchas
There was no longshore movement of other Point Moore. Working on the northern Perth
beachwrack accumulations or drift macroalgae beaches, Hansen (1984) obtained mixed results.
viewed during the aerial surveys. In particular, a Some sites had predictable, large quantities of
major accumulation of beachwrack south of the beachwrack, which remained in the same area.
mouth of the Chapman River remained at Concentrations at other sites were less predictable
consistently high levels and maintained a constant and the beachwrack moved along the shoreline.
position. The aerial surveys found beachwrack Clear patterns of longshore movement occur in
accumulationsonthesouthernshorelineoftheport GeographeBay. Beachwrackreachestheshorefrom
of Geraldton, but they were not substantial. The Dunsborough to Busselton and moves northward
aerial examination and several shoreline searches along the shoreline until it reaches Port Geographe
demonstrated that beachwrack also accumulates (P. Collins, pers. comm.). A major accumulation of
along the Geraldton beachesjust north of the port. 70-80,000 m^ occurred along the southern side of
The mouth of the Chapman River consistently had thedevelopmentattheendofwinterin1998;itwas
the largest accumulations south of Drummonds preventedfrommovingnorthwardsbythegroyne.
Cove. These were measured on 5 December 1998, Composition of beachwrack is variable,
andhadanestimatedvolumeof4907m^,somewhat dependingonthematerialavailable intheoffshore
smaller than the accumulation of 5780 m^ at environment. Beachwrack at Oakajee is a
Drummonds Cove on the same day. The western combination of macroalgae and the seagrass
tipoftheportofGeraldtonhasasmallbaynearthe Atnphibolis. Hansen(1984) foundbeachwrackin the
southern tip of the peninsula; this area regularly northern portion of the Perth area tobe composed
trappedbeachwrack. of a variety of constituents, including the kelp
Ecklonia radiata, seagrasses and numerous small
macroalgae. In Hansen's (1984) study of the Perth
DISCUSSION area, £. radiata was dominant over a full year,
The pattern ofbeachwrack accumulations varied followed by other macroalgae, then seagrass.
between Drummonds Cove and Coronation Beach. Sargassuni was seasonally important during
There was no seasonality at Drummonds Cove. In summer.
contrast, at Coronation Beach beachwrack Onlyasmallproportionofdetachedmacrophytes
concentrations werehigh in latesummerand early ends up on the beach. For example, Kirkman &
winter, but declined sharply from late winter to Kendrick (1997) released 5000 tagged Ecklonia
early spring. Experience in other areas of Western radiata in the Marmion Lagoon in winter 1985. The
Australia has been similarly mixed. Some areas taggedplantstook 15 to23days toreachshore,but
havewintermaxima,butothersdonot. Beachwrack only 53 (1%) were recovered on beaches. In her
accumulates seasonally during winter at the detailed study of beachwrack in the Perth
southerngroyneatPortGeographein Busselton (P. metropolitan area, Hansen (1984) found
Collins, pers. comm). Kirkman (1984) reported considerable variation in the amount of
considerable interannual variation in beachwrack beachwrack, from 1.3 to 45.3 kg dry weight per
quantitiesintheMarmionlagoon.Differentpatterns metre of coastline. On average there was 74 to 80
of seasonality occurred during the years studied. tonnes of carbon per kilometre of coastline,
Hansen (1984) examined beachwrack along 46 km equivalent to about 18% of the nearshore
of coast from Triggs to Two Rocks, and found macrophyte production. These figures emphasise
similar variability between sites, seasonally and that the material in the beachwrack is only a
between years. A series of photographs published fraction of total macrophyte production in
by Kirkman & Kendrick (1997) also demonstrates nearshorewaters.
high short-term variability in beachwrack
concentrations. The results of all these studies
ACKNOWLEDGEMENTS
demonstrate that the timing and magnitude of
beachwrackaccumulationsarehighlydependenton Caroline Bodenham of the Central West College
BeachwrackatOakajee 275
of Technical and Further Education provided Lenanton, R.C.J. (1982). Alternativ< non-estuarine
considerable enthusiastic assistance in the field at nursery habitats for some commercially and
Geraldton. In addition, Greg Burrows, Barbara recreationallyimportantfishspeciesofsouthWestern
Larkin, Priscilla Rodd, and David Wells each Australia. Australian journal ofMarine and Freshwater
assisted on one trip. Contact with TAPE was Research33:881-900.
through StevenWebster. Chris Shine and his pilots Lenanton, R.C.J. and Caputi,N.(1989). Therolesoffood
andstaffatShineAviationServicesprovidedskilful supplyandshelterintherelationshipbetweenfishes,
inparticularCnidoglanismacroceplwlus(Valenciennes),
services during the low level overflights of the
and detached macrophytes in thesurfzoneofsandy
beachesatOakajee.Inadditionthefollowingpeople beaches, journal of Experimental Marine Biology and
are thanked for their assistance in providing Ecology128: 165-176.
iLenPfroorvmoastti,onLoenProthveosptrojDeactm:esPet&erMCooolrlein;sDarndGaIrayn LenaNnetaorns,hoR.rCe.Ja.c,cRuombuelrattsioonn,sAo.fI.daetnadchHeadnsmeanc,rJo.pAh.y(t1e9s82a).s
Kendrick, Department of Botany, University of nurseryareasforfish.MarineEcologyProgressSeries9:
Western Australia; and Kim Nardi and Randall 51-57.
Owens of Fisheries Western Australia, Geraldton. McLachlan, A. (1985). The biomass of macro- and
Dr Gary Kendrick and Dr Anne Brearley kindly interstitial fauna on clean and beachwrack-covered
commented on the manuscript, whichsubstantially beaches in Western Australia. Estuarine Coastal and
improvedthepaper. SlwlfScience21:597-599.
This informationwas originally generated aspart Pearce, A.F. (1991). Eastern boundary currents of the
ofastudyofthepotentialeffectsoftheconstruction southern hemisphere. Pp. 35-45. In Pearce, A.F. and
of a new deepwater port at Oakajee. It was done Walker, D.I. (Editors.) The Leeuwin Current: An
under a contract between the Western Australian Influence on the Coastal Climate and Marine Life of
Department of Resources Development and Western Australia. Journal of the Royal Society of
LeProvost Dames & Moore (now part of URS WesternAustralia 74.
Australia). I am grateful to DRD for permission to PearHcoeu,tAm.aFn. A(b1r9o9l7)h.osTIhselanLdse.euPpw.in11-C4u6r.reInntWelalnsd, Ft.hEe.
pDuabvliidshMetheehainnfoartmDatRiDonfaorndthteoirPceotnesriGdeoroadballelhaenldp A(Ebdriotlohro)s.ITslhaendMs,arWienseteFrlnorAaustarnadliaF.auWneastoefrtnheAuHsotruatlmiaann
andencouragementduringtheproject. Museum,Perth.Volume1.
Robertson, A.I. and Hansen, J.A. (1982). Decomposing
seaweed: a nuisance or a vital link in coastal food
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