THE BENTHIC INVERTEBRATE FAUNA OF THE PEEL-HARVEY ESTUARY OF SOUTH WESTERN AUSTRALIA AFTER COMPLETION OF THE DAWESVILLE CHANNEL

2004·3.4 MB·English

by  Corey S Whisson

RecordsoftheWesternAustralianMuseum22:81-90(2004).
The benthic invertebrate fauna of the Peel-Harvey Estuary of south-
western Australia after completion of the Dawesville Channel
CoreyS.Whisson’‘FredE.Wells^andTomRose^
'SchoolofEnvironmentalBiology,CurtinUniversityofTechnology
GPOBoxU1987,Perth,WesternAustralia6001,Australia
^WesternAustralianMuseum,FrancisStreet
Perth,WesternAustralia6000,Australia
^WaterandRiversCommission,AquaticSciencesBranch,
3PlainStreet,EastPerth,WesternAustralia6004,Australia
'Presentaddress:WesternAustralianMuseum,FrancisStreet,Perth,WesternAustralia6000,Australia
Abstract - During the 1970's the Peel-Har\'ey Estuary experienced severe
symptomsofnutrientenrichment.Therewerelargeinputsofnutrientstothe
system, and only limited oceanic flushing occurred, via the long, narrow
Mandurah Entrance Channel. In 1994 the Dawesville Channel was
constructed to increase oceanic flushing within the Peel-Harvey Estuary;
reduce the occurrence of toxic phytoplankton blooms (eg. blue-green
Nodularia spumigena) and decrease macroalgal growth. The present paper
compares the benthic invertebrate fauna in the Peel-Harv'ey Estuary before
andaftertheDawesvilleChannelwasconstructed.
Onehundred sediment coreswere collected during earlyautumnand late
winter 2000, yielding a total of 17,443 benthic invertebrates, comprising 52
species. Polychaetes, molluscs and crustaceans dominated species richness
and density in both seasons. There was a clear difference in benthic
invertebrate species richness and density between seasons. During early
autumn,benthicinvertebratecommunities in thePeel-Har\'eyEstuaryhad a
low species richness (27species),but thisincreasedsubstantially (46species)
inlatewinter.Totalmean densityquadrupled from6397m'’in earlyautumn
to26,180m‘Hnlatewinter.Theincreasedspeciesrichnesswasduetoincreases
insmallcrustaceanswithashort lifespanand, to alesserextent,chironomid
insectlarvae.Withtheexceptionofonesite,theincreaseddensitywasgeneral
throughout both Peel Inlet and the Harvey Estuary. There was no apparent
correlation between increased densities and proximity to the entrance
channels.
Pre-Dawesville Channel data on benthic invertebrates are few. Molluscs
provided the best comparisons between pre and post Dawesville Channel
benthic invertebrates. In the late 1970's mollusc diversity was low and was
dominatedbytwosmallestuarinespecies:thebivalveArthriticasemenandthe
gastropod Hydrococcus brazieri. A two-year study at one site in Peel Inlet
recorded a maximum density of45,491m’^for A. semen and 19,959m'^ for H.
brazieri. Bythe1980'sthedensityofbothspecieshad declined;H.brazierihad
nearlydisappearedfromthePeel-HarveyEstuary.In2000,themeandensities
ofA. semen and H. brazieri remained low. Several marine species that were
recorded in the post Dawesville Channel samples were not recorded in the
1970's. DatasuggeststhatthePeel-HarveyEstuaryhasbecomemoremarine,
andthatincreasedrecruitmentofplanktoniclarvaethroughtheshort,narrow
DawesvilleChanneland improved waterqualityhas probablyenabled these
speciestoestablishthemselves.Allofthesespecieswererecordedinverylow
densities, and they have not established the dense populations previously
attainedbyH.brazieriandA.semen.
Keywords: Benthicinvertebrates, Peel-HarveyEstuary, DawesvilleChannel,
WesternAustralia.
INTRODUCTION to human impacts (Hodgkin and Lenanton, 1981).
Estuariesinsouth-westernAustraliaaregenerally Whilethecatchmentsmayhavelargeareas,rainfall
small, and their physical and geomorphological islow andwatervolumesinriverinflowsaresmall
characteristics make them particularly susceptible byworldstandards.Astheriverreachestliecoastal
82 C.S.Whisson,F.E.Wells,T.Rose
plain,manyestuariesbroadenintoshallowlagoons. channel, the Dawesville Channel, opened in April
The lagoons are sedimentary and there are often 1994 andwasdesignedtoincrease oceanicflushing
fewhardsubstratesorrockybottomsfororganisms within the Peel-Harvey Estuary, raise salinities to
to colonise. A narrow entrance channel to the sea create conditions unsuitable for harmful
oftenrestricts tidalflow intoandoutoftheestuary phytoplankfon blooms, and decrease macroalgal
(Hesp, 1984;Hodgkinand Hesp, 1998). Tidesinthe growth.
southwest are microtidal, ranging up to Im o\'er The estuarine food chain typically consists of
the year, but averaging 0.3m or less on each tidal abundant and productive primary producers
cycle (Hodgkin and DiLollo, 1958). Many estuaries (macroalgae, seagrasses, and/or phytoplankton),
have a sand bar across the mouth which restricts which are grazed by primary consumers such as
water exchange from several months through to zooplankton, worms, molluscs, small crustaceans
years, until the bar is broken; some bars have and fish. Benthic species often dominate
become permanent, while others are kept invertebrate abundance and production in an
permanentlyopenbyhumanintervention(Hodgkin estuary (Day, 1981; Hodgkin et al., 1985). While
andHesp, 1998). individual species may be abundant, species
The combination of a low riverine input; a large diversity is usually low (Barnes, 1974; McLusky,
lagoon; narrow channel to the sea, and small tidal 1989). Benthic invertebrates are the food source for
amplitude,all serve to minimiseexchangebetween a wide variety of estuarine fish, large crustaceans
south-westernAustralianestuariesandthesea,thus and waterhirds, which in turn are available to
concentratiirgcontaminants,mainlynutrients,inthe highertrophiclevels(Barnes,1974;Day,1981).
estuaries(HodgkinandHesp, 1998). Prior to the opening of the Dawesvdlle Channel,
Ramfallin thesouthwestisstronglyseasonaland few benthic invertebrate species were able to
Mediterranean, with most occurring from late tolerate the environmental conditions in the Peel-
autumn to early spring (May to September). Harvey Estuary. These were mainly small, short-
Estuarine salinities during winter are low, often lived opportunists that were highly productive
falling to as low as 3%c. over broad portions of the (Rippingale, 1977; Wells et ah, 1980; Chalmer and
lagoons and basins, and remaining at those levels Scott, 1984; Rose, 1994). The opening of the
for weeks. As rivers cease to flow in spring, fidal DawesvilleChannelincreased waterexchangewith
intrusion combined with increased ev'aporation the sea and made the Peel-Harv^ey Estuary a more
fromgreaterdaylengthandairtemperatures,cause marine environment. Tides are now of similar
the estuaries to become more saline. Hypersaline amplitude to those along the coast and salinity is
(>45%o) conditions and high water temperatures less variable. The presentpaper has two purposes:
can occur over most of fhe summer and early to characterise the present benthic invertebrate
autumn(HodgkinandLenanton,1981). fauna of the Peel-Harvey Estuary and to examine
As with several other south-western Australian changes which have occurred in the system as a
estuaries, thePeel-HarveyEstuary(32°S;115°E)has resultoftheopeningoftheDawesvilleChannel.
been severely affectedby nutrientenrichment. The
system consists of the circular Peel Inlet, with an
area of 75kmT arid the elongate Harv'ey Estuary, MATERIALSANDMETHODS
with an area of 61km^ (Hodgkin and Lenanton, SampleswerecollectedfromtensitesinPeelInlet
1981;Figure1).Thetwohavesimilarmeanvolumes and the Harvey Estuary during early autumn
(61 X 10'’m^ and 56 x 10“m’ respectively) and (March) and late winter (August) 2000 (Figure 1).
shallow water depths, averaging 0.8m and 1.0m To allow direct comparisons, the sample sites and
respectively (McComb and Lukatelich, 1995). sampling regime were the same as those used in a
Historically, tidal exchange with the Indian Ocean study of molluscs in the late 1970's (Wells et al,
was restricted by the long, narrow Mandurah 1980). Site 10 (Dawesville) waschosen to assess the
Entrance Channel in the north of Peel Inlet, immediate area of the Dawesville Channel. Most
resulting in poor nutrient flushing from the Peel- sites were 10-15m from shore in water depths of
Harvey Estuary. As agriculture expanded in the approximafely 0.5m. Site 1 (Coodanup), Site 4
catchment and fertiliser usage increased, nutrients (Robert Bay) and Site 6 (Caddadup) were 100m
progressively accumulated in the Peel-Harvey fromshoreasthewaterwasextremelyshallow.Site
Estuary. This resulted firstly in large increases in 5 (located in the mid-region of Peel Inlet) was
benthic macroalgae followed later by seasonal approximately 2km south of Site 1 and at a water
bloomsofharmfulphytoplankton (toxicblue-green depthoflessthan1.5m.
Nodularia spiimigeua). As a result of extensive Samples were collected using a 10.3cm diameter
environmental studies (eg. Rippingale, 1977; Wells PVC tube which was driven into the sediment to a
efai, 1980;Hodgkinetai,1985;Rose,1994),ashort, depth of 20cm. Six replicates were taken at each
narrow channel was constructed through the site.Samplesweresievedthrougha0.5mmmeshin
coastal dunes in the northern Harvey Estuary. This the field. They were labelled and placed in 10%
BenthicinvertebratesinthePeel-HarveyEstuary 83
Figure1 MapofsamplinglocationsinthePeel-HarveyEstuary.
formalin for two weeks beforebeing transferred to salinities were measured using an Atago S/Mill-E
75%alcohol. In thelaboratory,samplesweresorted refractometer. Sediment samples were also
under a dissecting microscope. Any invertebrates collected for analysis of physical and chemical
that were alive when collected were removed, characteristics. The upper 10cm of sediment was
identified,countedandretainedin75%alcohol. removed using clean PVC tubing and partitioned
Surface and bottom water temperatures and intotwo5cmhalvesbeforebeingstoredonice.One
dissolved oxygen concentrations were measured portion from each core was treated with 6%
using an analog YSI 51A oxygen meter, and hydrogen peroxide for 48 hours to remove any
84 C.S.Whisson,F.E.Wells,T.Rose
organic matter, washed overnight, and dried at
60°C for 24 hours. Sediment grain size and
frequency were then determined using a standard
set of geological sieves and the Wentworth scale.
Particulate organicmatterin the remainingportion
was determinedby removing any macrophyte and
animal material from the sediment, drying the
sediment at 60"’C for 24 hours and burning on
ignition at530°Cfor 16hours, thenweighing to the
nearest Img (Holme and McIntyre, 1984). Five
replicate cores using clean PVC tubing were taken
to a depth of 10cm at each site to determine redox
depthasdescribedbyRose(1994).
Dendograms were calculated using the IBM
program PC Ord. This program groups sites by
cluster analysis using average linkage based on
Sorenson's Index of Similarity. Species diversity
(H'), evenness (E) and dominance (D) were
calculated using the Shannon Weiner formula
(Krebs,1989).
RESULTS
PhysicalandChemicalData
Salinity and temperature in the water column
were variable between sampling periods (Table 1).
All sites werehypersalineinearly autumn. Salinity
increased with distance from the Dawesville and
Mandurah channels. During late winter, salinities
were highest at sites located close to the charmels,
and in the eastern region of Peel Inlet. Salinities
were low (<16%o) in the southern regions of Peel
InletandtheHarveyEstuary.Inlatewinter,bottom
salinitiesweregenerally thesame(<l%o difference)
as at the surface except at Site 6, Site 4 and Site 9
(Herron Point), where they greatly exceeded those
at the surface (up to 9%o difference). The surface
and bottom water temperatures at all sites varied
seasonally, ranging from23.5“C in early autumn to
14.0°C in latewinter (Table 1). Therewasvery little
difference (<0.5"C) in bottom and surface water
temperatures.
Fauna
One hundred cores were taken during early
autumn and late winter 2000, yielding 17,443
benthic invertebrates and a total of 52 species.
During early autumn, species richness of benthic
invertebrates was low, consisting primarily of
polychaetes, molluscs and crustaceans (Table 2).
Together the three taxa accounted for 25 of the 27
species recorded andwere also themost abundant.
Fiv'especiescomprised88.4% oftotalmeandensity
(Table 3); thebivalveArthriticasemen (2457m’); the
polychaetes CapitelJn aff capitata (1657m'^),
Ceratonereis aequisetis (404m‘-), and Leitoscoloplos
normalis (298m"^), and the amphipod Corophiuni
minor(840m'^).
)
BenthicinvertebratesinthePeel-HarveyEstuary 85
Table2 Numberofspecies,meandensityandpercentagecontributionofmajortaxonomicgroupsinthePeel-Harvey
Estuaryduringearlyautumnandlatewinter2000.
EarlyAutumn LateWinter
Taxon Number MeanDensitym- Contribution Number MeanDensitym'^ Contribution
1
ofSpecies (X±SE) (%) ofSpecies (X±SE) (%)
Polychaeta 9 2777±272 43.4 10 11,232±890 42.9
Mollusca 9 2587±711 40.5 11 2545±444 9.7
Crustacea 7 1028±237 16.1 18 8465±1621 32.3
Insecta 1 2±2 >0.0 3 3916±1107 15.0
Coelenterata 1 2±2 >0.0 2 10±5 >0.0
Priapulida 0 0±0 >0.0 1 CO1+GO >0.0
Echinodermata 0 0±0 >0.0 1 2±2 >0.0
Nemertea 0 0±0 >0.0 1 2±2 >0.0
Total 27 6397±1224 100.0 46 26,180±4079 100.0
Species richness was substantially greater (46 meandensityvariationsintheHarveyEstuarywere
species) in late winter. As in autumn, polychaetes, greater in early autumn (from 1060m^at Site 10 to
molluscs and crustaceans dominated, with an 24,690m'^atSite 9) than in Peel Inlet (20m’^atSite4
increase of 11 crustacean species. In addition, a to 9824m'^atSite 5). Totalmean densities increased
single species each of priapulid, echinoderm and in latewinteratallsitesexceptSite9intheHarvey
nermetean were formd in late winter; none were Estuary, where densitydecreased from24,690m’^to
recorded in early autumn. Total mean invertebrate 13,705m'^. MeaninvertebratedensityatSite8inthe
density quadrupled to 26,180m'^ in late winter Harvey Estuary increased by 26-fold, from 2201m'^
comparedto6397m'^inearlyautumn.Molluscshad in early autumn to 58,083m'^ in late winter.
approximately the same density in early autumn DensitiesatSites3and 6inPeelInletalsoincreased
and late wdnter, but polychaetes and crustaceans by over an order of magnitude. The greatest
increased substantially, by 8455m‘^ and 7437m'^ percentage increase occurred at Site4 in Peel Inlet,
respectively. In contrast to their near absence in wheredensities increasedbynearly three orders of
early autumn, the density of insects in late winter magnitude, from 20m'^to 17,287m‘^. There was no
was 3416m'^. In late winter the fivemost abundant clearrelationshipbetweeninvertebratedensity and
species comprised only 73.5% of the total mean proximity to the DawesvilleCharmel. Inparticular.
density. They were the polychaete Capitella aff Site 6near thenorthernend of the Harvey Estuary
capitata (7015m*), the amphipods Corophium minor had atotalmeandensityof53,942m'^inlatewinter,
(4304m"^) and Paracorophium excavatiim (2129m'^), a while the nearby Site 10 at the entrance to the
chironomid larva (3902m-) and the bivalve Dawesville Channel had a total mean density of
Arthriticasemen(1895m’^). only8403m'^.
Distribution ofspecies intheestuarywasuneven Mostofthe increase in totalmean density in late
(Table 4; Figure 1). In early autumn thenumber of winter was due to five species: the polychaete
species varied from one at Site 4 and two at Site 3 Capitella affcapitata increased inmeandensityfrom
(South Yunderup) to 15 at Site 2 (Mandurah 1657m‘^in early autumn to 7015m‘^in late winter;
EntranceCharmel);allofthesesitesareinPeelInlet. the amphipods Corophium minor from 840m’^ to
Species were more evenly distributed throughout 4304m'^and Paracorophium excavatum from Om’^to
all sites in the Harvey Estuary,rangingfrom seven 2129m anunidentifiedchironomid larvafromOm'^
species at Site 7 (Mealup Point) to nine species at to 3902m'^; and the isopod Tanais dulo7rgifrom Om’^
Site 10. With the exception of Site 9, all other sites to 998m'^. Except for the chironomid larva, all of
had more species in late winter. There was also an these are estuarine species. The estuarine bivalve
increasedvariabilityinspeciesdistributions.InPeel Arthritica semen decreased in density from 2457m'^
Inlet, the number of species ranged from eight at inearlyautumnto1895m‘^inlatewinter.
Site 4 to 26 at Site 6. The range in the Harvey Cluster analysis indicated there were no clear
Estuary was from six species atSite 9 to 23 species community differences in the invertebrate com-
atSite8(PointRepose). munity in different parts of the estuary (Figure 2).
Similarly, there were considerable variations in In early autumn there was some separation of
thetotalinvertebratedensitybetweensites(Table4; invertebrate communities in the Peel Inlet and
Figure 1). The minimum total mean density Harvey Estuary between Sites 6 and 10, near the
recordedinearlyautumnwas20m^atSite4inPeel entrance to the Dawesville Charmel. Site 4 in Peel
Inlet; the maximum was 24,690m'^at Site 9 in the Inlet was the exception, due to the occurrence of
HarveyEstuary.Incontrasttospeciesrichness,total only one species at this site, Anthicidae sp., a
86 C.S.Whisson,F.E.Wells,T.Rose
Table3 Mean densityofbenthic invertebratespeciescollected in the Peel-Harvey Estuary duringearly autumn and
latewinter2000.
Taxon MeanDensitym-2(X±SE)
EarlyAutumn LateWinter
POLYCHAETA
AustralonereischlersiAugener,1913 170±56 812±200
BoccardiellalimnicolaBlake&Woodwick,1976 108±30 326±93
Capitellaa{{capifntaFabricius,1780 1657±224 7015±839
CeratonereisaeqidsetisAuginer,1913 404±93 856±133
Hetcromastusfiliformis(Claparede,1864) 52±19 22±8
Laonomesp. 66±14 1056±214
LcitoscoloplosnormaUsDay,1977 298±69 812±112
MarphysasanguineaMontague,1815 12±7 6±4
NephtysgravieriAuginer,1913 10±5 0±0
NephtyslongpipesStimpson,1856 0±0 4±3
Spirorbidaesp. 0±0 322±109
TotalPolychaetes 2777±272 11,232±890
MOLLUSCA
Bivalvia
Arthriticasemen(Menke,1843) 2457±711 1895±421
DonaxcoliimbellaLamarck,1818 0±0 12+6
Sangiiinolariabiradiata(Wood,1815) 2±2 2±2
SpisulaIrigoneUa(Lamarck,1818) 4±3 344±104
TellinadeltoidalisLamarck,1818 2±2 26±10
Gastropoda
Acteocinasp. 80±43 72±29
Assimineasp. 8±4 26±8
Hydrococcusbrazieri(T.Woods,1876) 30±15 134±40
Cephalaspideansp. 0±0 10+4
Nassariusburchardi(Philippi,1849) 2±2 16±7
Hydrobiabuccinoides(Quoy&Gaimard,1834) 0±0 8±5
PatellaperoniiBlainville,1825 2±2 0±0
TotalMolluscs 2587±711 2545±444
CRUSTACEA
Isopoda
Cirolanasp. 0±0 2±2
CruranthurasimpliciaThomson,1946 2±2 80±26
Gastrosaccussp. 4±3 10+5
Mimnabrcvicornis(Thomson,1946) 2±2 0+0
TanaisdulongiThomson,1944 0±0 998±375
Copepoda
Harpacticoidasp. 0±0 26±12
Amphipoda
AUorchestescf.compressaDana,1852 0+0 10±4
CaprellascauraTempleton,1836 42±19 0±0
Corophiumminor(Thomson,1946) 840±205 4304±1226
cf.Erichthoniussp. 0±0 12+12
Grandidierellasp. 110+34 638±119
Lysianassidaesp. 0±0 2+2
MelitamatildaJ.L.Barnard,1972 0+0 2+2
MelitazeylanicaJ.L. Barnard,1972 28±13 174±47
Paracorophiiimexcavatum(Thomson,1884) 0±0 2129±785
Parelasmopussp. 0+0 64±64
TethygeniaelanoraJ.L.Barnard,1972 0±0 4±3
Decapoda
Galatlieasp. 0±0 2+2
cf.MetapenacusdalliRacek,1957 0±0 2+2
cf.PenaeuslatisulcatusKishinouye,1896 0±0 6+6
TotalCrustaceans 1028±237 8465±1621
BenthicinvertebratesinthePeel-HarveyEstuary 87
Taxon MeanDensitym-^(X ±SE)
EarlyAutumn LateWinter
INSECTA
Anthicidaesp. 2±2 0±0
Chironomidaesp.(larva) 0+0 3902±1106
Dipterasp.l (lan'a) 0±0 10±7
Dipterasp.2(lan'a) 0±0 4±4
TotalInsects 2±2 3916±1107
COELENTERATA
Haliplanellahiciae(Verrill,1898) 2+2 8±5
Pennatulaceansp. 0±0 2±2
PRIAPULIDA
cf.Pr2iapulussp. 0±0 8±8
ECHINODERMATA
Astropectensp. 0±0 2+2
NEMERTEA
Nemerteansp. 0±0 2±2
OverallTotal 6397±1224 26,180±4079
8
AUTUMN
Similarity (%)
Site 100 75 50 25 0
1
5 -
3
8
9
6
10
4
WINTER
Similarity (%)
Site 100 75 50 25 0
1
3
7
9
5
4
6
—
2
—
10
Figure2 Dendogramofsamplingsitesduringearlyautumnandlatewinter2000.
•
88 C.S.Whisson,F.E.Wells,T.Rose
sp.
On t—
O+1 ^(\NNC. O'sC OCrOo 55 Gmndidierella
+1 CC^^OJ LOTT-NjH Tr0f-0 capitata
aff
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od cNj aif
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7>+^1^0iCr4O% C0O 0CCNO semen
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+1 Os'C NO CtOo
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aff
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^+^1 COmO 0CN0OO CNCNOO minor
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^+! C^t-Nh oNOOn oCT-OH ^SS: limnicola
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5 +1 Species
2 S'5Si PJ Q D
BenthicinvertebratesinthePeel-HarveyEstuary 89
speciesthatdidnotoccurelsewhere.Theseparation 8105m'^) for A. semen and 19,959m'^(mean 9487m'^)
between sites in Peel Inlet and in the Harvey forH. brazieri.
Estuary from those near the Dawesville Channel The middle regions of the Peel-Harvey Estuarine
did not occurin late winter (Figure 2). There was a basins were sampled by Rose (1994) in 1986-87,
slightseparationbetweenSite 10, at theentrance to before the DawesvilleChannelwasconstructed. He
the Dawesville Channel, and Site 2 at the entrance sampled two sites each in the Peel Inlet and
to the Mandurah Channel from those further into northern Harvey Estuary and recorded 37 species,
theestuaries,butSite6 (nearSite 10)wasnotinthis with macrobenthic densities being highest in
grouping. autumn and lowest in summer. The benthic fauna
in 1986-87 was dominated by small, short-lived
opportunist species with successful reproductive
DISCUSSION
strategies suitable for estuarine conditions, eg. the
The benthic invertebrate assemblage in the Peel- polychaete Capitella aff capitata, amphipod
Harvey Estuary had low species richness (27 Corophium minor and bivalve Arthritica semeyt.
species) in early autumn. Species richness was Hydrococcus brazieri was absent in the middle
substantially greater (46 species) in late winter. regions of Peel Inlet and, compared to previous
Total mean density quadrupled from 6397m’^ in sampling(WellsandThrellfall,1982a;1982b;1982c),
early autumn to 26,180m‘in late winter and there the density ofA. semen had decreased, particularly
werenocleardifferencesincommunitystructurein in Peel Inlet. An extensive search failed to reveal
the system in either season. Polychaetes, molluscs any live H. brazieri. These low densities were
and crustaceans dominated species numbers and attributed to the deleterious effects of macroalgal
density in both seasons. The increased species andNodulariaspumigenadecomposition.
richness in late winter was primarily due to an The samples takenin 2000 recorded system-wide
increase in diversity of small crustaceans with a mean densities of 30m‘^ for H. brazieri in early
short life-span (isopods and amphipods) and, to a autumn and 134m'^inlatewinter,comparedwitha
lesser extent, a chironomid larvae. Thus there is a similar survey undertaken in January (122m'^) and
clear difference of benthic invertebrate species August 1978 (835m before the Dawesville
‘)
richness and density between the autumn and Channel was opened. The system-wide mean
winter seasons. With the exception ofSite 9 (at the densities of A. semen in early autumn and late
southernend oftheHarvey Estuary) theincreasein winter 2000 were 2457m‘’and 1895m*^respectively;
density was consistent throughout both Peel Inlet compared with January (7175m‘^) and August
and the Harvey Estuary; there was no apparent (1937m^) 1978. Whilst the present-day distribution
correlation between increased densities and ofbothspeciesresemblesthatrecordedinthe1970's
proximitytotheentrancechannels. (Wells and Threllfall, 1982c) densities have
Comparison of the present results with benthic remained low since the Dawesville Channel was
invertebrate populations prior to the construction opened. The reappearance of both species may be
of the Dawesville Channel is difficult as there is attributed to the absence of Nodularia spumigena
little published data available. The best data are blooms since the opening of the Dawesville
those of a three-year study of molluscs conducted Channel and reduced decomposition and
during the late 1970's (Wells et ah, 1980), at a time smotheringby macroalgae, whichbegan to decline
when massive amounts of the green macroalga intheearly1990's(HaleandPaling, 1999).
Cladophora were present in the system. Mollusc Thepresentstudyrecordedatotalof52speciesof
diversity was low (Wells et ah, 1980; Wells and benthic invertebrates; considerably higher than
Threlfall, 1980; 1981), and was dominated by two previousstudiesundertakenpriortotheopeningof
smallestuarine species; thebivalveArthritica se^nen the Dawesville Channel. Chalmer and Scott (1984)
and the gastropod Hydrococcus brazieri, which and Rose (1994) found 25 and 37 species
together accounted for almost all of total mollusc respectively. Several marine species were recorded
density on sand in the Peel-Harvey Estuary. Both in 2000 thatwerenot common orrecorded prior to
were found to be small, short-lived species with the opening of the Dawesville Channel, including
successful reproductive strategies suitable for the molluscs Donax columbella, Nassarius burchardi,
estuarine conditions (i.e. fast maturation, Patella peronii and a cephalaspidean species. All of
continuouseggproductionanddirectdevelopment these species were recorded in very low densities,
enabling release of juveniles during optimum and they have not established the dense
estuarine conditions). The populations were also populationspreviouslyattainedbyH. brazieri.
highly productive, but densities varied greatly The DawesvilleChannel appears to allow greater
(Wells and Threllfall, 1982a; 1982b; 1982c). A two- access for planktonic larvae to enter the system (as
year study undertaken at Coodanup in Peel Inlet opposedtothenarrowshallowMandurahEntrance
(near Site 1) from March 1977 to February 1979 Channel) and combined with the increase in
recorded a maximum density of 45,491m’^(mean salinities during winter, allows marine species, at
90 C.S.Whisson,F.E.Wells,T.Rose
least initially, to settle and colonise areas in the Hodgkin, E. P., and Hesp, P. A. (1998). Estuaries to salt
estuary in late winter. Seasonal density patterns of lakes: Holocene transformation of the estuarine
benthicinvertebratesinthis studyweredifferentto ecosytems of south-western Australia. Marine and
previous studies by Rose (1994), suggesting the FreshwaterResearch,49:183-201.
effects of eutrophication are less severe since the Hodgkin, E.P., Birch, P. B., Black, R. E., and Hillman, K.
Dawesville Channel was constructed. Whether (1985). The Peel-Harv'ey Estuarine System: proposals
these new marine "non-estuarine" species can for management. Western Australia Department of
survive throughout the year has not yet been ConservationandEnvironment,Bulletin14:1-54.
determined, although autumn data presented in Holme,N. A., and McIntyre,A. D. (1984).Methodsforthe
study of marine benthos. Blackwell Scientific
this study suggests that few of these species
Publications,London.
maintained their earlier populations. Other factors
such as biological interactions eg. fish predation, KrebRso,w,C.PuJb.li(s1h98e9r)s..NEceolwogYiocralk.methodology. Harper and
asenadsobneanlthdiecnsictoymppeattitteironsn., may now also influence McCHoamrvb',eyA.EstJ.u,arainnde SLuyksatteeml,icWh,esRt.erJn. (A1u9s9t5r)a.liTahe(ppP.ee5l--
17). In: McComb, A.J. (ed.) Eutrophicshallowestuaries
andlagoons.CRCPress,London.
ACKNOWLEDGEMENTS
McLusky, D. S. (1989). The estuarine ecosystem. Second
This paper is modified from a BSc (Hons) thesis edition. Blackie,USA.ChapmanandHall,NewYork.
undertaken at Curtin University of Technology by Rippingale, R. J. (1977). An introductory study of -water
the senior author. We are pleased to acknowledge exchangeinthePeelInletfromDecember1976toFebruary
the considerable assistance of the internal 1977. DepartmentofConserv’ationand Environment.
supervisoratCurtin, Dr Rob Rippingale. Drs Anne Report to the Estuarine and Marine Advisory
BrearleyofTheUniversityofWesternAustraliaand Committee,70pp.
Karen Hillman of DAL Science & Engineering Rose, T. H. (1994). Comparisons of the benthic and
kindly commented on a draft of the manuscript. zooplanktoncommunitiesintheeutrophicPeel-Flarveyand
TAlahnbegoesrleaantiooSrrcyhauuwmtohaocrrhkewria,snhdewshptorooeavxsitsdeisnetddedhissiungprpfaoitreilttdudaaenntddo MUnenuaprrudbboylcihSswhUanenidveEPrsshtiDutayr,Tih2ee6ss7iispn,p.sEonuvtih-riovnemsetenrtnalAuSsctireanlicae.,
encouragement. Wells, F. E., Threlfall,T.J., andWilson, B. R. (1980). The
Peel-Harvey estuarine system study (1976-1980).
BiologyofMolluscs. Western Australian Departmentof
ConservationandEnvironment,Bulletin97:1-106.
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