'IF 0 t ti J:1..J. ~1"1

13A. Zooplankton of the Murray-Darling system

R. J. Shiel

Introduction

The first studies of the zooplankton of the Murray-Darling system are quiterecent. Walker & Hillman (1977) listed the zooplankton of Lake Hume (areservoir) and Lake Mulwala (a shallow diversion weir), and Powling (1980)described the zooplankton of several impoundments. Most of the availableinformation, however, is from a catchment-wide survey that included samplesfrom 23 reservoirs and 150 sites on Murray tributaries (ShieI1981). The surveyshowed that regulation of Murray flows has favoured a limnoplankton domi-nated by Cladocera and Copepoda, whereas the Darling River, unimpeded formore than 2000 km, retains a typical potamoplankton dominated by Rotifera.The composition and seasonal dynamics ofpotamoplankton in the lower Mur-ray were described by Shiel et al. (1982).

In this chapter the systematic status, ecology and zoo geographic affinities ofthe major zooplankton groups are summarised. The plankton assemblages ofthe Murray and Darling are contrasted, and changes caused by impoundmentare illustrated by events during filling of Dartmouth Reservoir, constructed in1972-77 on the Mitta Mitta River in NE Victoria. The limnoplankton of thisreservoir is compared with that of two longer-established reservoirs, Hume onthe Murray and Eildon on the Goulburn. Attention is given to floodplainhabitats associated with the Murray, as these seasonally provide a substantialproportion of the downstream river plankton.

Systematics and ecology

The poor status of systematic knowledge is an obstacle to studies of zooplank-ton, particularly when the principal taxonomic references available in Australiahave been, until recently, European or North American. As a consequence therehas been a proliferation of "Northern Hemisphere" names for species only

The Ecology of River Systems, edited by B. R. Davies & K. F. Walker~ 1986, Dr W. Junk Publishers, Dordrecht, The Netherlands 661

superficially like the types (cf. Frey 1982). Of the four major groups representedin the zooplankton of the Murray-Darling system, the Protozoa are leastknown. The Rotifera are under review (Koste & Shie11986; in prep.) followingKoste (1978). Systematic problems in the Cladocera were resolved in part bySmirnov & Timms (1983), but further work is necessary. Of the Copepoda,Cyclopoida and Harpacticoida are under revision (D. W. Morton, MonashUniv., pers. commun.; Dr R. Hamond, Univ. Melbourne, pers. commun.); onlythe Calanoida are well known (cf. Bayly 1964).

Of more than 400 taxa identified from the Murray-Darling zooplankton byShiel (1981) (260 Rotifera, 75 Cladocera, 40 Copepoda, 32 Ostracoda), aboutone-third are recorded only from the vegetated littoral regions of billabongs.Limnetic representatives are listed by habitat in Table 1.

Protozoa

With the exception of incidental reports of summer blooms of a heterotrichciliate (cf. Climacostomum) in Lakes Hume and Eildon (Walker & Hillman1977;Powling 1980), and mention of a diverse assemblage of ciliates, rhizopodsand dinoflagellates in the Darling River (Shiel 1985), little is known of proto-zoans in Murray-Darling waters. Preliminary studies of Rhizopoda from the1981 survey suggest that the diversity is comparable to that reported for otherrivers (cf. Green 1963). Billabongs have the most diverse assemblages, withmarked differences in species composition between habitats. A small proportion

. of taxa persists in downstream flows, particularly in floods. Most commonrhizopods in the lower Murray are species of Arcella, Centropyxis and Difflugia,with some cosmopolitan taxa (e.g. Difflugia corona, D. urceolata) and othersapparently endemic (Shiel unpublished).

Ecological information is limited to an intensive study of Naegleria fowleri,an amoeba found in rural water supplies brought by long-distance overlandpumping from the Murray in South Australia. Deaths from amoebic meningitishave been attributed to this organism. Heavy dosing with chlorine has provenonly partly effective in control (Garman 1983).

Rotifera

Rotifers are often the most abundant zooplankters in Murray-Darling waters,and an important link between the nannoplankton (bacteria and algae less than60 flm) and the carnivorous zooplankton. Most are herbivores and detritivores,although there are specialised carnivores and parasites.

Brachionids are the most widespread family, and have most morphologicalvariants and endemic species. Brachionus (44 species and subspecies; cf. Shiel

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Table 1. Common zooplankters (excluding Protozoa) of rivers, billabongs and reservoirs of theMurray-Darling system.

TaxonReservoirsRivers Billabongs

RotiferaBrachionidae

Euchlanidae

Trichotridae

Lecanidae

Trichocercidae

Synchaetidae

Asplanchnidae

Testudinellidae

Conochilidae

Hexarthridae

Filinidae

Clat/oceraSididae

Chydoridae

Macrothricidae

Moinidae

Bosminidae

Daphniidae

CopepodaCentropagidae

Cyc10pidae

B. angularis

B. budapestinensis

B. calyciflorusB. diversicornis

B. falcatusB. un'eolaris

K. australis

K. tropica

P. quadricornisB. dichotomus

B. falcatus

B. patulus

B. quadridentatusK slacki

K procurva

B. angularis

B. falcatusK cochlearis

K. australis

K procurva

L. lunaT. simi/isT. stylataS. longipesS. oblongaS. pectinataP. dolichoptera

P. vulgarisA. brightwelliA. sieboldi

P. comp/anta

c. unicornis

H. mira

F. longisetaF. terminalis

D. unguiculatum

C. sphaericus

M. tenuicornisM. micrura

B. meridionalis

D. lumholtzi

C. cornuta

C. quadrangula

B. triarticulata

B. minuta

C. ampullaC. lucasi

Mesocyc/ops

Australocyclops

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L. luna

T. simi/isT. stylata

S. longipesS. oblongaS. stylataP. dolichopteraP. vulgaris

A. brightwelliA. sieboldi

E. dUatata

T. tetractis

L. lunaris

T. simi/is

T. stylata

S. pectinataS. stylataP. vulgaris

C. dossuarius

A. priodontaA. sieboldi

P. complanta

C. unicornisC. dossuarius

H. intermedia

F. australiensis

F. pejleri

F. opoliensis

F. passa

D. unguiculatum

C. sphaericus s.1.A. rectangula

B. rigidicaudis

I. sordidus

M. spinosa

M. micrura

D. excisum

C. eurynotusC. sphaericus s.1.A. rectangula

M. micrura

B. meridionalis

C. corn uta D. carinata s.1.

Simocephalus .spp.C. cornuta

C. quadrangula

B. triarticulata

C. ampulla

B. fiuvialisC. lucasi

M esocyclops spp.Microcyclops spp.Eucyclops spp.

1983) is represented by the cosmopolitan B. angularis and B. falcatus, with B.budapestinensis, B. calyciflorus, B. diversicornis, B. novaezealandia and B.urceolaris also common. All are summer forms, widely distributed in tropicaland sub-tropical alkaline waters (Koste 1978).Other Brachionus taxa are restric-ted to particular habitat types or single localities: B. plicatilis and B. bidentataoccur in saline waters, and the endemic B. dichotomus, B. kostei and B. lyratusoccur in isolated billabongs. Keratella, generally more widespread and eurytopicthan Brachionus, is represented by 11 species, seven of them widely distributed(K. australis, K. cochlearis, K. procurva, K. slacki and K. tropica, with K.quadrata and K. valga in high-altitude reservoirs or in winter river plankton).

The common synchaetids are cosmopolitan forms (Synchaeta oblonga inwinter; S. pectinata, S. stylata, Polyarthra vulgaris and P. dolichoptera as peren-nials). The asplanchnids also are cosmopolites (A. brightwelli, A. priodonta andA. sieboldi). Most of the Conochilidae, Hexarthridae and Filinidae are regularcomponents of the plankton, particularly C. dossuarius, H. intermedia and fourof the seven recorded species of Filinia (F. australiensis, F. pejleri grandis, F.longiseta and F. terminalis). Filinia opoliensis and F. pejleri are warm steno-therms typical of Darling waters. Facultatively pelagic species from severalpredominantly littoral families are those common elsewhere (e.g. Euchlanisdilatata, Lecane bulla, L. luna, L. lunaris, Trichocerca similis, T. stylata); theseusually accompany algal blooms.

Cladocera

The predominant cladoceran microcrustacea in Murray-Darling waters areherbivores or detritivores ranging in size from 0.3--4.5mm. Notable absenteesfrom Australia are the Holopedidae, Polyphemidae, Cercopagidae and Lep-todoridae (Smirnov & Timms 1983), which include predatory C1adocera. Some60% of the identified cladoceran species, most of them chydorids, are of restric-ted distribution and epiphytic or epibenthic in habit, occurring in billabongs orflushed into pe1agic regions during high flows. Table 1 lists the families withlimnetic or facultatively limnetic representatives.

Of the Sididae, only Diaphanosoma unguiculatum appears frequently in openwater. D. excisum and two species of Latonopsis occur rarely in the limneticregion of billa~ongs. .

Among 43 species of Chydoridae, few are facultatively limnetic. Alona rectan-gula, A. guitata and Biapertura rigidicaudis are common in the Murray plank-ton, together with a number of species grouped in Table 1 as Chydorus sphaeri-cus s.l. The status of these taxa is uncertain, but certainly more than one speciesis present (D. G. Frey, Indiana Univ., Bloomington, pers. commun.).

Although macrothricids generally are littoral in habit, Ilyocryptus sordidusand Macrothrix spinosa often occur in the plankton (cf. Shiel et al. 1982). Three

664

moinids are recorded, with Moina australiensis confined to billabongs, M.tenuicornis occasionally in reservoir plankton, and M. micrura, the most wide-spread species of the genus, a summer occurrence (November to April). Bosminameridionalis is common, and the sole representative of the Bosminidae.

Daphnidae are represented by the pan tropical Daphnia lumholtzi and severalspecies in the D. carinata complex (cf. Hebert 1977). Daphnia lumholtzi iscommon in lakes and reservoirs, and D. carinata s.1. in billabongs. The latterspecies-group is absent from shallow habitats in summer, when water tem-peratures may attain 40°C, but occurs at higher altitudes or in the deep watersof reservoirs. Two species of Ceriodaphnia (c. quadrangula and C. cornuta) areubiquitous in the basin, and at least four other species occur in billabongs (c.dubia and C. rotunda, with C. laticaudata and C. cf. pulchella recorded frombillabongs of the Goulburn River).

Copepoda

The Calanoida are the most abundant copepods; all 16 species recorded in the1981 survey are representatives of the Centropagidae. Boeckella triarticulata,the most widespread Australasian species, is ubiquitous and eurytopic in

Murray waters, and B. fiuvialis and B. minuta are common in s~all lentichabitats. Other Boeckella species are restricted to smalllentic habitats (B. major,B. pseudochaele), high altitudes (B. delicata) or are seasonally present inthe limnoplankton (B. symmetrica). Hemiboeckella searli is known only inGoulburn River billabongs.

Species of Calamoecia often are asociated with the larger Boeckella. Four ofsix recorded species are of restricted distribution, and only C. ampulla and C.lucasi are eurytopic.

Cyclopoids are poorly represented in lakes and rivers, but are essentiallylittoral and benthic in habit, and common in billabongs and other smalllentichabitats. Of the taxa listed in Table 1, the most common in the plankton areMesocyclops (M. thermocyclopoides and M. notius, following Kiefer's (1981)revision), Eucyclops agilis, an undescribed species of Eucyclops, and two speciesof Austrocyclops (Morton 1985). The latter are carnivorous (Shiel1981), and inthe absence of predatory Cladocera and Calanoida are the largest predatoryzooplankters in Murray-Darling waters. Harpacticoids are represented byundescribed species of Canthocamptus from billabongs and lakes (Dr R.Hamond, Univ. Melbourne, pers. commun.).

Ostracoda

Ostracoda are collected infrequently. Of 32 species in 22 genera, only Cypretta

665

~m..

! 3!~

fl\~!

~!! ~'}/P 0

~ i ~11 ~~! 0, --==gL ~I

~I~, ~ ...""..

...~~!h1. . , . . ~

~~i-I~J

Figure 1. Zooplankton communities of the Darling (upper panel) and Murray rivers (opposite)and their main tributaries and reservoirs. The histograms show dominant species and zooplankton

composition at 29 points over 20-24 May 1978. Scales are as in the lower example above. Rotifera

are shown as horizontally-hatched bars, Cladocera as solid bars and Copepoda as vertically-hatched

bars. The histograms show numbers of species (front bars, left axis) and percent composition (rear(open) bars, right axis). Habitat types are R = river, B = billabong and L = lake. Shannon-Weaver diversity (H') is shown below the habitat type.

666 F.F.7

occurs in the limnetic regions oflakes or, occasionally, in the potamoplankton.Species of Cypretta are adapted to a planktonic habit (Dr P. De Deckker, Aust.Nat. Univ., pers. commun.).

Zoogeography

Three major groups of zooplankters dccur in the Murray-Darling system,namely Australian endemics, circumtropical and cosmopolitan taxa. Endem-icity is lowest in the Rotifera (c. 15%), followed by Cladocera (c. 40%), cyclo-poids (c. 60%) and calanoids (c. 90%). Systematic work undoubtedly willincrease the known endemicity, particularly among the Cladocera.

Most endemics are restricted to billabongs and smalllentic waters; only a feware eurytopic and widespread (cf. Table 1). A remarkably large proportion ofMurray-Darling plankters is tropical in affinity. The Darling undoubtedly hasbeen a transport corridor for warm stenothermal taxa from northern Australia;however, many of these now are perennial in reservoirs on the Murray (e.g.Brachionus spp., K. tropica, F. pejleri, C. cornuta, D. lumholtzi), extending thesouthern latitudinal distribution of "pantropical" taxa.

Geographical differences in zooplankton composition across the basin areillustrated in Fig. 1, which shows species dominants, numbers of species in eachgroup, percent composition and community diversity of plankton communitiessampled 20--24 May 1978 (Shiel & Walker 1985). The contrasts betweenriverine, floodplain and reservoir communities are clear, as are differencesbetween the two river systems.

Zooplankton of the Darling River

Fig. 1 shows the major tributaries of the Darling. The western tributaries(Paroo, Warrego rivers) are episodic, as were the eastern rivers prior to con-struction of headwater dams in the late 1960s. Important features are the2500-km unregulated channel between the tributary dams and the Murrayconfluence, the long travel times (2-3 months), and high turbidity associatedwith suspended particulate clay.

Slow flows, nutrient enrichment from irrigation (particularly on the Namoiand Gwydir rivers) and moderate physico-chemical conditions favour develop-ment of a complex potamoplankton. Each tributary supplies a different assem-blage to the Darling; the northern tributaries contribute mostly pantropicalwarm stenothermal species, and the eastern tributaries contribute eurytopicendemic forms typical of cooler waters (and tropical species in summer).

Although microcrustacea may be seasonally abundant in upstream reservoirs,the dominants in the downstream rivers invariably are rotifers (ShielI985). The

668

disparity in species composition between the Gwydir and Barwon rivers wasillustrated by Shiel & Walker (1985), who in May 1978 recorded 27 rotiferspecies from the Gwydir and 24 from the Barwon, 100km distant, with onlyeight of a total 65 zooplankton taxa shared between the rivers (cf. Fig. 1).Dominants in the Gwydir were Keratella australis and K. slacki, with Brachionusurceolaris, Synchaeta oblonga, and Filinia pejleri subdominant, and the domi-nants in the Barwon were Keratella tropica, K. procurva and K. cochlearis, withBrachionus calyciflorus, Synchaeta tremula, Filinia australiensis and F. longisetasubdominant. Some 200km south, on the same day, the most abundant planktersin the Castlereagh River were Keratella procurva robusta and Lecane ohioensis,and more than half the recorded plankters were absent from the northern river.

The mixed Darling assemblage, typically of 20--40taxa (5 Filinia, 4-6 Brach-ionus, 4 Lecane and seasonally Ceriodaphnia cornuta and Bosmina meridionalis),persists to the Murray junction at Wentworth. Little is known of changes enroute. The assemblage at Wentworth is more diverse than in the tributaries, butdensities are less (less than 100 1-1) than in some of the headwater tributaries(e.g. Gwydir: > 4001-1).

Shiel & Walker (1985) speculated that the high turbidity of the Darling maylimit algal photosynthesis (cf. Walker & Hillman 1982), thereby limiting thedevelopment of herbivorous zooplankton and perhaps accounting for the absenceoflarge herbivorous species. Alternatively, obligate herbivores in the head waterstreams may remove the phytoplankton by grazing, so that only those'planktersable to utilise bacteria and detritus (i.e. rotifers) survive to pass downstream. Athird possibility is that the high suspensoid levels (montmorillonite-kaoliniteclays) limit feeding and locomotion in larger zooplankters.

The influence of Murray flows on the plankton of the Darling is unstudied (cf.Shiel et al. 1982), although "blocking" effects have been reported for other largerivers (cf. Rzoska 1978), with algal blooms occurring as a response to flow

. reduction. This was apparent in July 1982,when a dense algal bloom in theMurray below Lake Mulwala extended north for several kilometres into theDarling, where there was no obvious flow. The phytoplankton at Wentworthwas dominated by Melosira spp. (including M. granulata v. angustissima), withthe diatoms Synedra, Asterionella, Diatoma and Fragillaria, the chlorophytesClosterium, Chlamydomonas, Staurastrum, Spirogyra, ?Mougeotia, Pediastrumand Eudorina, the chrysophyte Dinobryon and the Cyanobacteria Anabaena andOscillatoria. In the Darling 20 km upstream of Went worth on the same day onlyEudorina was present, while all except Eudorina occurred in the Murray atCurlwaa, 10km upstream from the confluence.

A similar disparity occurred in zooplankton composition. At the upstreamDarling site 16 taxa were present (22 1-1, H' = 2.29), with Keratella australisand Brachionus angularis comprising 80% of the plankton. At Wentworth 25taxa occurred, with K. cochlearis, K. tropica and SynFhaeta spp. comprising70% of the assemblage (571-1, H' = 3.00). In the Murray at Curlwaa the

669

dominants were K. cochlearis, K. tropica, Synchaeta spp. (70%) with two coldwater taxa, K. quadrata and Polyarthra dolichoptera subdominant (16%).Thirty-one taxa were recorded (density 3241-1, H' = 3.00). The Wentworthplankton was a mixed Darling-Murray assemblage, and the Darling communityone of relatively low density.

Zooplankton of the River Murray

The major tributaries and impoundme.nts of the Murray system are shown inFig. 1. Two shallow storages (Mulwala on the Murray and Nagambie on theGoulburn), both with short retention times, are distinguished from the head-water reservoirs as "middle-reach" reservoirs.

A feature of the limnoplankton of the Murray reservoirs is the disparity inspecies composition between reservoirs on any samplinK date (cf. Fig. 1).Although the dominants shown in Table 1 occurred in most reservoirs, theywere often not coincident, and peripheral taxa in adjacent impoundments wereof different species, and occasionally different genera. A total of 126 Rotifera,37 Cladocera, 27 Copepoda and 10 Ostracoda were recorded in Murray reser-voirs between 1976-82, although individual collections usually contained 10-16taxa.

Lake Hume

Plankton collections were taken from this reservoir between 1973-76 (Walker& Hillman 1977), and supplemented by collections in 1976-80 (Shiel 1981).There was little variation in species composition over the seven years: thezooplankton was a stable assemblage dominated by copepods (B. triarticulata,C. ampulla and Mesocyclops) with cladocerans subdominant (B. meridionalis, C.quadrangula, D. unguiculatum, and seasonally M. micrura (summer) andD. carinata s.l. (winter-spring». Rotifers were notably scarce; on only twooccasions (summer) did they comprise numerically more than 20% of thelimnoplankton. Fig. 2 shows temporal variation in the composition of the LakeHume limnoplankton.

On any sampling date a single species (C. ampulla) comprised 50-90%of the plankton, generally with only two or three species making up morethan 90% of the assemblage. The average momentary species composition(N = 26) was 1.8 rotifers, 3.3 cladocerans and 2.4 copepods. Species com-position generally was more similar between collections than for any otherreservoir, as was synchrony of appearance of major species each year. Thesewere true plankters; only in shallow arms of the reservoir were pseudoplanktersor littoral incursions recorded, and these did not extend into the mid-lake

670

. .'M . . . .0. o.0. 0.00M'. ' 0," 0Iv 0v 0" 0n, . .. ," 0'" 0 ' . '" ," . . ,'. ,'. , ,. . ., 0---

._hanosoma unguiculatum - -~ ~~- -- - - 4.

- - - -~~~~~a~~ ~~ -~..

Boeckella triarticulata - J

- Mtlsocyclopssp. -9"T- -~-

calanoid capepodites

Figure 2. Temporal variation in zooplankton composition, Lake Hume.

plankton, which provided the major source of plankton for the Murray down-stream of the dam.

Community diversity, relative to other reservoirs, was greater and more stable(mean Shannon-Weaver H' = 1.99), with a winter maximum. Generally lowpopulation densities were recorded (4-1231-1, mean 44.4 1-1), with autumnmaxima. Densities of the same order of magnitude were recorded by Walker &Hillman (1977). Nutrient loadings recorded in that study suggested that thereservoir is meso-eutrophic, but expected algal responses were inhibited byadverse environmental conditions (e.g. high turbidity). Inhibition appears tofollow through to the grazing zooplankton, resulting in lower population den-sities and greater community diversity than might otherwise be expected.

671

- -Brachionus urceolaris .-- -

- Asplonchna priodonta - - + ...- - -- Conochilus dossuarius --

Lake Eildon

This impoundment has a capacity comparable to that of Lake Hume, buta greater retention time, and it is oligotrophic (Powling 1980). Temporalvariation in the limnoplankton composition is similar to that in Lake Hume.Despite differences in nutrient status and phytoplankton assemblages, the samespecies dominated in the zooplankton, namely a group of perennial, multi-voltine microcrustacea (B. triarticulata, C. ampulla, Mesocyclops sp. and B.meridionalis), with other taxa perennial and univoltine (D. unguiculatum) ormarkedly seasonal in occurrence (M. rriicrura,D. carinata s.1.,D. lumholtzi andC. cornuta in summer). Generally, two or three species made up 60-80% of thelimnoplankton. Unlike Lake Hume, rotifers were important, with severalspecies perennial (K. cochlearis, P. dolichoptera, C. dossuarius and A. bright-welli), occasionally comprising more than 20% of the community. Momentaryspecies composition (N = 13) was 4.8 rotifers, 3.9 cladocerans and 3.9copepods. There was less similarity in temporal species composition due to aseasonal succession of cold water taxa (e.g. C. lucasi), or rare seasonal species(brachionids; B. fluvialis, B. minuta) and pseudoplank~onic species. In mostsamples the subdominant species were not those in Lake Hume at the same time.Of92 taxa recorded from the two storages only 20, mostly microcrustacea, wereshared. Diversity was lower (mean H' = 1.96), and densities were similar(44.51-1).

Trends were similar in other headwater reservoirs. Storages of longer reten-tion time (e.g. Burrendong, Burrinjuck) had a stable microcrustacean limno-plankton, with some local differences in species composition. Short retention-time storages (e.g. Wyangala, Keepit) have an unstable rotiferjcopepoditeplankton. Densities are lowest in higher altitude oligotrophic reservoirs (e.g.mean 13.01-1), and highest in seasonally eutrophic storages (e.g. Burrinjuck,mean 567 1-1).

Lake Dartmouth

Completed in 1977, Dartmouth Dam impounds 4 million Ml and is the largestreservoir in the system. There is no information on the pre-impoundmentplankton, although Powling (1980) recorded Ceriodaphniasp., Conochilussp. andDaphnia carinata s.1. during the filling phase. Fig. 3 provides more completeinformation. Of 61 zooplankton species recorded, half were present in the com-plex zooplankton assemblages of billabongs downstream of the damsite. Fromthe compositional changes which occurred in the reservoir as it filled, suchassemblages clearly contributed to the zooplankton community as the billa-bongs were inundated. The rapidity of these changes distinguished Darmouthfrom other storages.

672

1977 1978 1979 1980

,xi, xii, i , ii , iii , iv, v ,vi, vii ,viii, ix, x , xi , xii, i ,ii , iii , iv , v ,vi, vii ,viii, ix ,x ,xi, xii, i , ii , ill, iv,

Rotifera .Kerate/laprocurva -. - K tropica

Lophocharis salpina

.

. _Synchaeta ~es ~ <1IIIIIIIIS oblonga ~ ....--~ - -S.pectinato - - -- - Polyarthradolichoptera - ..

~ Lacinularia .~-. ~ ismaeloviensis~ Conochilusunicornis .-

H"Mhm:"". ~...~-Filinia longiseta

Cladocera Chydorus sphaericus 5.1. -

Copepoda

+-

~ - -Bosmina meridionalis

~ . . 4. Daphniacarinata5.1.

~ Ceriodaphniacornu/a -. - . ~ C quaqrangu~ -

~ckella triarticulata.~ --.- ~ "'Mesocyclops - spp.- - -

-...Ostracoda

Figure 3. Temporal variation in zooplankton composition, Lake Dartmouth.

673

The microcrustacean community which developed after closure of the dam(D. carinata s.1., B. triarticulata) was replaced early in 1978 by a rotifer assem-blage, with seasonal pulses of copepods. True limnoplankters were Lacinulariaismaeloviensis, Conochilus unicornis, Polyarthra dolichoptera and Hexarthramira (perennial), and Synchaeta pectinata (autumn), S. oblonga (winter) and S.longipes (spring). Rotifers predominated as the reservoir became eutrophic andanoxic following the decay of drowned vegetation. As the reservoir stabilised,a diverse rotifer assemblage persisted through 1981, and by July 1982 thecommunity dominants were as in other Murray reservoirs (B. triarticulata,C. ampulla, with subdominants DiaphaflOsoma unguiculatum and Synchaetapectinata).

Mean diversity (H' = 1.8) was lower than for other large storages. Popu-lation densities (24-173 1-1,mean 73 1-1) initially were higher and indicative ofeutrophy, but decreased steadily as the storage filled. There was variation inspecies composition between sampling dates due to rapid replacement ofspecies. The predominance of rotifers is reflected in the average momentaryspecies composition (N = 32; 5.3 rotifers, 2.9 cladocerans and 1.8 copepods).

Middle-reach reservoirs

Goulburn Weir (Lake Nagambie, Goulburn River) and Yarrawonga Weir(Lake Mulwala, River Murray) (Fig. 1) impound shallow storages with shortretention times (2-3 weeks). A stable limnoplankton is precluded, and specieswith a rapid generation time (i.e. rotifers) predominate. Only in winter andspring is there sufficient time for microcrustacea to develop, and then only oneor two taxa occur (B. meridionalis, D. lumholtzi). In Mulwala, 98 taxa are known(67 rotifers, 21 cladocerans and 8 copepods), more than from any otherimpoundment. Of these, 60% are littoral in habit. Spatial and temporalheterogeneity are more pronounced, with higher diversities and lower popu-lation densities (5-25 I-I) than other reservoirs. Similar densities were recordedin 1974-75 by Walker & Hillman (1977), who related the low biomass to thelake's low retention time.

The zooplankton assemblages from two disparate sources are mixed inLake Mulwala: that of the Ovens River, with a largely littoral (pseudo plank-tonic) community derived from fringing reeds and other plants, and a sparsezooplankton from the Murray consisting of Hume limnoplankters and inoculafrom billabongs. The billabong species tend to predominate in Mulwala,which more nearly resembles a billabong than a reservoir, and has similarspecies-rich community diversity to nearby billabongs (cf. Fig. 1). Algal bloomsin the storage pass little changed, with a diverse zooplankton, into the down-stream river. The zooplankton of the lower Murray is described by Shiel et al.(1982).

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River plankton below dams

Except for hypolimnetic-release dams, most reservoirs supply plankton to thedownstream rivers. As elsewhere (cf. Hynes 1970) rotifers predominate, andmicrocrustaceans, with the exception of Bosmina meridionalis, evidently are ableto avoid outflows. As a result, downstream plankton densities are lower by oneor two orders of magnitude relative to those in the impoundment. The contri-bution of the plankton from floodplain habitats increases in proportion todistance downstream (i.e. H' increases, cf. Fig. 1), so that middle-reach reser-voirs have complex communities with elements of reservoir limnoplankton,billabong heleoplankton and a true potamoplankton.

Billabongs

Billabongs (oxbows) generally have been ignored in river regulation pro-grammes. The only information on the plankton of Murray billabongs is thatofShiel (1976), Walker & Hillman (1977) and Shiel & Koste (1983). The speciesoflimnetic plankters in billabongs generally are not those of reservoirs (cf. Table1).There is a high proportion of epiphytic or epibenthic taxa in open water, and

, high community diversity, with 15-40 taxa present at any time, as consequencesof shallowness and the presence of littoral plants. Momentary species com-position for one Murray billabong, for example, is 6.4 rotifers, 4.6 cladoceransand 3.1 copepods (N = 8), and for a Goulburn billabong 18.3 rotifers, 7.2cladocerans and 5.2 copepods (N = 8).

Conclusion

In the Murray-Darling system the zooplankton clearly is an integral part of anecological system. Despite arguments that food web and metabolic products arenot shared with other sections of a river (cf. Rzoska 1978), this is manifestly notso here. In the Murray, nutrients may be depleted in headwater storages by algalblooms which pass downstream, sometimes for several hundred kilometres, tobe grazed by zooplankton that also has been transported or has developed in theriver or floodplain habitats. In the Darling River particularly, the planktoncommunity subsists on allochthonous organic material, as high turbidity appar-ently limits photosynthetic production (cf. Shiel 1985).

The differences in plankton community composition between the two riversappear to be a direct response to river regulation, and are similar to thosereported from regulated rivers elsewhere. In the uncertain Darling flow regimethe rotifer plankton is rapid breeding, opportunistic, and adapted to extremehabitat variability. The ~tabilised Murray regime provides more suitable

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conditions for phytoplankton growth and favours the development of her-bivorous micro crustacea with relatively long generation times.

Acknowledgements

Most of this review is from a doctoral program supervised by Dr K. F. Walkerand Professor W. D. Williams, Department of Zoology, University of Adelaide.Dr Walker also assisted in chapter preparation. Financial support from theCommonwealth Government, University of Adelaide and Albury-WodongaDevelopment Corporation is gratefully acknowledged. Professors H. B. N.Hynes and C. H. Fernando gave support during a fellowship at the Universityof Waterloo and provided comments on a draft. Dr H. Duthie (Waterloo)provided phytoplankton identifications. The later stages of chapter preparationwere completed at the Department of Botany, University of Adelaide.

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