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 C. +i^ COCOO LoOOON ^o^ON capitata od cNj aif C. o o 7>+^1^0iCr4O% C0O 0CCNO semen A. +1-' OCO LTj LCTNj dulongi 04 O O Tt CN T. ^ ^ CN CO capitata +1 Os'C NO CtOo aff C. capitnta aff C. ^+^1 COmO 0CN0OO CNCNOO minor C. o O ^+! C^t-Nh oNOOn oCT-OH ^SS: limnicola ™ c '<?r VC) TcO B. 2 E ^(Z oC CSu-Co +1 ^CCNO ^COO LCOONN minor C. 'u £ C > c c^ ^ t- Q oj ^Uj -g c 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. REFERENCES Wells, F. E., and Threlfall, T. J. (1980). A comparison of Barnes, R. S. K. (1974). Estuarine Biology. Institute of molluscan communities on intertidal sand flats in Biology, Edward Arnold Publishers, London. Studies Oyster Harbour and Peel Inlet, Western Australia. inBiology,No.49;1-76. JournalofMolluscanStudies,46:300-311. Chalmer, P. N., and Scott, J. K. (1984). Fish and benthic Wells, F. E., and Threlfall, T. J. (1981). The molluscs of faunal surveys of the Leschenault and Peel-Harvey Peel Inlet, WesternAustralia, and a comparisonwith estuarine systems of south-western Australia in other south-western estuaries. Journal of the December 1984. Western Australian Department of MalacologicalSocietyofAustralia,5:100-111. ConservationandEnvironment,Bulletin149. Wells, F. E., and Threlfall, T. J. (1982a). Salinity and Day, ]. H. (1981). Estuarine ecology with particular temperature tolerance of Hydrococcus brazieri (T. referencetoSouthAfrica.A.A.Balkema,CapeTown. Woods,1876)andArthriticasemen(Menke,1843)from Hale,]., and Paling, E. (1999). WaterQuality ofthe Peel thePeel-Harveyestuarinesystem.WesternAustralia. Harvey Estuary: Comparisons before and after the Journal oftheMalacological Society ofAustralia, 5: 151- openingoftheDawesvilleChannel(June1985toJuly 156. 1999). Marineand Freshwater Research Laboratory, 99/4 Wells, F. E., and Threlfall, T. J. (1982b). Reproductive (WaterandRiversCommission). strategies ofHydrococcus brazieri and Arthritica semen Hesp, P. A. (1984). Aspects of the geomorphology of in Peel Inlet, Western Australia. Journal of the south-western Australian estuaries (pp. 61-83). In: MalacologicalSocietyofAustralia,5:157-166. Hodgkin, E.P (ed.) Estuarine environments of the Wells, F. E., and Threlfall, T. J. (1982c). Density Southern Hemisphere. Western Australian Department fluctuations, growth and production of Hydrococcus ofConservationandEnvironment,Bulletin161. brazieri and Arthritica semen in the Peel-Harvey Hodgkin, E. P., and DiLollo, V. (1958). Tides of south- estuarine system, Western Australia. Journal of western Australia. Journal of the Royal Society of MolluscanStudies,48:310-320. WesternAustralia,41:42-51. Hodgkin, E. P., and Lenanton, R. C. J. (1981). Estuaries and coastal lagoons of south-western Australia (pp. Manuscriptreceived7April2003;accepted24October2003 307-321). In: Neilson, B.J. and Cronin, L.E. (eds.) Estuaries and Nutrients. Humana Press, Clifton, New Jersey.