1-s2.0-s1385110100000046-main

Zonation of benthic communities in a tropical tidal flat ofnorth-east Australia

S. Dittmann*Zentrum fur Marine Tropenokologie, Fahrenheitstr. 1, 28359 Bremen, Germany

Received 29 July 1999; accepted 25 November 1999

Abstract

Tropical tidal flats are relatively less well-known marine ecosystems. Therefore, the distribution and abundance of infaunalorganisms were surveyed in a tidal flat in the Haughton estuary, north-east Australia, testing several hypotheses on character-istics of intertidal faunal distributions. Using a stratified random sampling design, macrofauna, small macrofauna (mesofauna)and meiofauna were sampled at five sites along a transect from the high to the low intertidal in April and September 1991. Intotal, 77 macrobenthic species were recorded during this study, with polychaetes and crustaceans richest in species. While thisspecies record was low compared to other tropical tidal flats, the low individual densities confirmed previous findings of lowerabundances in tropical than temperate tidal flats. Along the transect, species densities were highest in the mid-intertidal muddysand and sandflats, with values ranging from 2.9 to 7.6 species 177 cm22 for macrofauna and from 2.2 to 3.8 species 10 cm22 formesofauna. At the Callianassa site in the mid-intertidal 35 species were recorded, while the lower sandflat site had the highestdiversity H 0 2:60: Macro- and mesofauna abundances were highest at the sandflat site (median values for macrofauna: 65and 69 ind. 177 cm22 in September and April, respectively, and 37 and 48 ind. 10 cm22 for mesofauna). There was littlevariation between the two sampling dates, although single taxa occurred with significantly higher abundances in one of the twomonths. Polychaeta and Amphipoda were abundant at the sandflat and Callianassa site, juvenile bivalves were most frequent inthe sandflat after a spatfall in September. There was no pronounced increase of suspension feeders in the lower intertidal, anddeposit feeders dominated the fauna. Meiofauna was abundant throughout the intertidal with median values up to 310 ind.5 cm22. Their densities were highest in the lower intertidal and lowest at the transect site with Avicennia mangroves. Nematodadominated the meiofauna at all transect sites, and only in April were copepodes more abundant at the Callianassa and sandflatsite. Altogether, the benthic fauna in this tropical tidal flat showed a zoned distribution between the tide marks. While nomeiofauna assemblages could be distinguished with multivariate analysis at the phylum level, defined groups were found for themacrofauna, corresponding to a zonation of distinct assemblages at the high intertidal mudflat, the mid-intertidal Callianassaand sandflat sites, and the lower intertidal sandflat. Based on this analysis and further information from qualitative mapping andliterature recordings, benthic communities in tropical tidal flats of north-east Australia are described. q 2000 Elsevier ScienceB.V. All rights reserved.

Keywords: benthic community; distribution pattern; macrofauna; meiofauna; tropics; tidal flat

1. Introduction

The distribution of benthic communities in tidal

flats is well known for temperate areas, especiallyfrom the northern hemisphere (e.g. the North Sea:Rees, 1940; Kay and Knights, 1975; Beukema,1976; Dorjes, 1978; the east coast of North America:Sanders et al., 1962; Whitlatch, 1977; Korea: Koh and

Journal of Sea Research 43 (2000) 3351

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E-mail address: [email protected] (S. Dittmann).

Shin, 1988; Shin et al., 1989), but also from southernChile (Jaramillo et al., 1985; Reise, 1991). Their tropi-cal counterparts are, however, less well explored, inspite of some early endeavours (e.g. Macnae andKalk, 1962; McIntyre, 1968; Vohra, 1971; Day,1974; Frith et al., 1976; Gibbs, 1978; Broom, 1982;Wells, 1983). Despite the high diversity reported inthese surveys, intertidal sand and mudflats havereceived less attention than coral reefs, mangrovesand tropical beaches (see reviews by Alongi,1989a,b; 1990). More recently, the ecology of tropicalintertidal soft-bottom communities has beenaddressed in several studies (e.g. Peterson andBlack, 1987; Vargas, 1987, 1988a; Alongi, 1988;Reise, 1991; Dittmann, 1993, 1996). Yet, a conciseunderstanding of tropical tidal flat communities, thebiology of their fauna, their role in the energy flowand as a stopover for migrating birds (Alongi, 1989b;Piersma et al., 1993; Dittmann, 1995) is still missing.

For a tropical tidal flat in north-east Australia, abasic description of the distribution and abundanceof benthic fauna is presented here. The distributionof benthic organisms is generally seen as a combinedeffect of several factors, including the physical envir-onment, animal-sediment relations as well as speciesinteractions (Dankers and Beukema, 1983; Reise,1985; Barry and Dayton, 1991; Snelgrove andButman, 1994). The following hypotheses, derivedmainly from studies in temperate tidal flats, weretested:

1. A zoned distribution between the tide marks is auniversal attribute of the benthic fauna regardlessof latitude. Throughout the intertidal, benthicfauna usually follows a zonation of varying envir-onmental conditions from the high towards the lowtidal level, corresponding with changes in sedimentproperties, wave exposure and the duration ofsubmergence and exposure (Sanders et al., 1962;Kay and Knights, 1975; Beukema, 1976;Whitlatch, 1977; Hertweck, 1994; Raffaelli andHawkins, 1996). The benthic animals are distribu-ted along this gradient of physiological stress inrelation to their tolerance of environmental condi-tions (Dankers and Beukema, 1983; Peterson,1991). In the tropics, consolidation of the soil,the level of the water table and shade are furtherfactors controlling species distributions (Day,

1974; Macnae, 1968). In the present survey, theinfauna was sampled along a transect from thehigh towards the low intertidal. Sediment grainsize and organic matter were determined alongthis gradient as well.

2. Highest species density is encountered at inter-mediate tidal level and sediment composition.From surveys in the Dutch Wadden Sea, Beukema(1976) concluded that the highest species densitiesoccur in the mid-intertidal at intermediate values ofsilt content, a finding that was confirmed by long-term studies (Beukema, 1988; Beukema andCadee, 1997). Armonies and Hellwig-Armonies(1987) also recorded the highest species richnessof macrofauna in muddy sand. Although not allspecies found in the studied tropical tidal flatcould be named yet, they were recorded separatelyto determine species numbers and densities.

3. The occurrence of suspension feeders is restrictedto the lower intertidal. Because suspension feederscan only feed at high tide, they cannot exist wheresubmergence is short and are therefore negativelycorrelated with the intertidal gradient from high tolow tidal levels (Beukema, 1976; Dankers andBeukema, 1983). To estimate the spatial distribu-tion of feeding types for this tropical tidal flat, themacro- and mesofauna were roughly classified intotrophic groups.

4. Interactions between species affect benthic assem-blages in tidal flats. Not only repressive, but alsopromotive ecological processes structure and regu-late tidal flat fauna (Reise, 1985). Biogenic struc-tures provided by macrobenthic species alsoaccommodate associated species in tropicaltidal flats (Dittmann, 1996; Dittmann, 1998).During this study, three size classes (macro-,meso- and meiobenthos) were surveyed simul-taneously to assess possible relations betweenvarious infauna.

The results of the survey are also compared withinand between latitudes to test the hypothesis that abun-dances tend to be lower in tropical tidal flats whilespecies richness is higher than in temperate tidal flats(Macnae and Kalk, 1962; Morton and Morton, 1983;Reise, 1991).

A further objective of this study was to identify andcharacterise species assemblages in a tropical tidal

S. Dittmann / Journal of Sea Research 43 (2000) 335134

flat. Following a generalised zonation pattern ofbenthic communities based on observations andsurveys in several tidal flats along the North Queens-land coast (Dittmann, 1995), the quantitative study ofassociated fauna presented here allowed a detaileddescription of benthic communities in tropical tidalflats of north-east Australia.

2. Material and methods

Benthic communities were studied in the tidal flatsat the mouth of the Haughton River. This estuary islocated in tropical North Queensland, Australia, andopens into Bowling Green Bay (19825 0S, 14785 0E). Itcovers over 5 km2 of tidal flats, partly fringed bymangroves (Fig. 1). The tidal flat itself was withouthigher plants despite for some Avicennia marina andseedlings of Rhizophora spp. Tides are semidiurnalwith a mean tidal range of about 2.3 m.

To assess the infaunal distribution from the hightowards the low intertidal, five sites were arrangedalong a transect of about 1 km length (Fig. 1a andb). Using a stratified random sampling approach, thesites were chosen to cover the major gradient frommuddy sediments at the high tide line over muddysand in the mid-intertidal and sandflats towards thelower intertidal. Previous observations had shown asequence of mega- and macrobenthic organisms alongthis gradient (Dittmann, 1995). The transect sites werecharacterised as follows:

Mmudflat in upper intertidal, with crabs(Macrophthalmus latreillei and Uca spp.) andmudskippers (Periophthalmus spp.).

Cmid-intertidal Callianassa site, muddy sand-flat with Callianassa (Trypaea) australiensis indensities of 165 ind. m22 (Dittmann, 1996). Furthermacrobenthic species were an unknown echiuridand terebellid polychaetes (Loima medusa).Tubes of smaller sedentary polychaetes occurredhere, too. Occasionally, benthic anemones wereobserved in the muddy sandflat.

A a grove of Avicennia marina mangroveslocated in the mid-intertidal. Littorina scabra wasfound on branches of the trees and barnacles grewon the tree trunks. There was often a thin layer ofwater retained among the pneumatophores at lowtide. A few mudskippers occurred here andanenomes were seen in the sediment. Uca dussu-mieri and Dotilla sp. occurred on the fringe of theAvicennia grove.

S sandflat, faecal mounds of a benthic holothurid(cf. Paracaudina sp.) were seen here frequently.

L sandflat in the lower intertidal near the river,faecal mounds of enteropneusts occurred here;sand dollars (Arachnoides placenta) and small

S. Dittmann / Journal of Sea Research 43 (2000) 3351 35

Fig. 1. (a) Map of the study area in the Haughton River estuary inNorth Queensland, Australia, with the location of the transect acrossthe tidal flat. The transect sites were Mmudflat, C Callianassasite, A Avicennia grove, S sandflat, L lower intertidal. (b)Profile of the transect with major features of the sites (not drawnto scale).

hermit crabs were frequent on the sedimentsurface.

Mobile surface fauna was not restricted to certainsites. Soldier crabs (Mictyris longicarpus), predatoryand scavenging snails (Naticidae, Nassaridae) roamedthe sediment surface in the mid- and low intertidal.

The transect sites were sampled on 24 and 26 April1991, after a heavy wet season in the course of whichthe Haughton River had flooded. A second samplingwas carried out on 4 and 5 September 1991 towardsthe end of the dry season.

On both occasions, 15 samples were taken at eachof the five transect sites: 5 replicates for each of themacro-, meso and meiofauna groups. The sampleswere taken at random within an area of about100 m2 at each transect site. For macrofauna, a corerof 177 cm2 surface area was used, samples were takento 20 cm sediment depth and sieved through 0.5 mmmesh size in the field. In the lab, the animals weresorted alive from sorting trays.

To allow for the smaller individual size of infaunain tropical tidal flats (Dittmann, 1995), mesofaunasamples were taken with a corer of 10 cm2 surfacearea to a depth of 5 cm. Only at the mudflat site, asyringe with 6.6 cm2 surface area had to be used to getthe mud out of the corer. These samples were sievedover 0.25 mm mesh size and the animals were sortedalive under a dissecting microscope.

Both macro- and mesofauna were determined to theclosest possible taxonomic unit. Since the fauna of thetidal flats in North Queensland was not well known,specimens were preserved as a reference collectionand taxonomic descriptions are still under way. Speci-mens of clearly distinct species were always recordedseparately and although not many names could begiven, the benthic communities were thus treated atspecies level.

For meiofauna, a corer of 5 cm2 surface area wasused, except at the mudflat site, where smallersyringes (with 1.8 cm2 surface area in April and1.2 cm2 in September) were employed. Sampleswere taken to a depth of 5 cm. Meiofauna wasextracted alive by diluting the sediment with filteredseawater and repeated shaking and decanting througha set of sieves with 1258062 mm mesh size, follow-ing narcotisation with MgCl2. Major meiofauna taxawere quantified under a dissecting microscope. The

distribution of Platyhelminthes, recorded at specieslevel, has been reported elsewhere (Dittmann, 1998).

Further samples were taken at each site for grainsize analysis (dry sieving) and determination oforganic matter (loss of ignition after 5 h at 5508C, in% dry weight).

Differences in densities were analysed with thenon-parametric U-test, using two-way hypotheses(densities are smaller or larger between the sites ormonths compared). The infaunal assemblages at thetransect sites were analysed using the PRIMER soft-ware package from the Plymouth Marine Laboratory(PML). Multivariate analyses were carried out ontransformed (double square root) data using theBrayCurtis index and group average linkage forcluster analysis and non-metric multidimensionalscaling (MDS). The discrimination of infaunalcommunities along the transect was tested with one-way ANOSIM and species typifying assemblageswere identified using the SIMPER program (Clarke,1993).

3. Results

3.1. Sediment parameters

The sediment in the tidal flats of the Haughtonestuary consisted of very fine to fine sand with anincrease in grain size towards the lower intertidal(Table 1). Sediments were not well sorted. Theproportion of organic matter was higher at the mudflatsite and the Avicennia grove than at the muddy sandand sandflat sites. Only in September was the percen-tage of organic matter high in the lower intertidal aswell.

3.2. Species numbers

While a total of 77 infaunal species (excluding themeiofauna) were recorded from the Haughton estuaryduring this transect study, the species numbers atsingle transect sites varied from 23 to 35, with thelowest values in the lower sandflat and the highestat the Callianassa site (Table 2). However, calculatedfor all recordings from the two sampling dates, diver-sity (ShannonWeaver index) was highest at thelower sandflat and lowest at the sandflat site (Table2). The lower index resulted from the numerical


dominance of the opheliid polychaete Armandia cf.leptocirrus, tellinid bivalves and amphipods.

Polychaetes and crustaceans were richest inspecies, followed by molluscs (Table 2). Most poly-chaete species were recorded at the mid-intertidalsites (C, A and S, Table 2) and crustaceans repre-sented about one third of the species found at thehigh and mid-intertidal sites (M, C and A).

The highest species density for macrofauna wasfound at the Callianassa site and for mesofauna atthe sandflat site (Table 2). Macrobenthic speciesdensity was twice as high at the sandflat site as inthe lower intertidal. Species densities for the smal-ler-sized mesofauna varied from 2.2 to 3.8 species10 cm22, with no clear pattern over the transect sites(Table 2). Compared between April and September,species densities did not vary, except for a signifi-cantly higher species density of macrofauna at theCallianassa site in September 5:2 ^ 1:2 species177 cm22 in April, but 10 ^ 2:4 species 177 cm22 inSeptember, p , 0:05 and a significantly lowerspecies density of mesofauna at the sandflat site inSeptember 4:8 ^ 0:8 species 10 cm22 in April, but2:8 ^ 0:8 species 10 cm22 in September, p , 0:05:

3.3. Macro- and mesofauna abundancesIndividual densities of macro- and mesofauna were

rather low throughout the studied transect and showeda high variation at most sites (Fig. 2a and b). Alongthe transect, macrofauna abundances were lowest inthe lower intertidal, the mudflat and Avicennia grove(Table 3). Highest abundances were always recordedat the sandflat site (S), where variance was also


Table 2Species numbers of major macrobenthic taxa, diversity indices and infaunal species densities (mean with standard deviation, n 5 per site)along a transect in the tidal flats of the Haughton estuary, based on samples taken in April and September 1991. Other taxa include nemertines,sipunculids, oligochaetes, anthozoans as well as insect larvae, which were occasionally found in benthos samples. Mmudflat, CCallianassa site, A Avicennia grove, S sandflat, L lower intertidal. See Methods for further detail on the transect sites

M C A S L All sites

Species numbersTotal 26 35 32 25 23 77Polychaeta 8 12 11 11 8 23Crustacea 8 10 9 6 6 22Gastropoda 3 4 6 3 0 10Bivalvia 2 4 2 2 3 11Echinodermata 0 1 0 1 3 4Other taxa 5 4 4 2 3 7

Diversity (H 0) 2.31 2.43 2.54 1.72 2.60 2.83Species densityMacrofauna 4.6 ^ 1.3 7.6 ^ 3.0 4.8 ^ 1.7 6.9 ^ 2.0 2.9 ^ 1.4 5.4 ^ 2.6(species 177 cm22)Mesofauna 3.0a ^ 0.9 2.2 ^ 1.4 3.1 ^ 1.6 3.8 ^ 1.3 2.8 ^ 1.2 2.9 ^ 1.4(species 10 cm22)

a Species 6.6 cm22.

Table 1Sediment grain size, sorting coefficient and amount of organicmatter at the transect sites sampled in April and September 1991in the tidal flats of the Haughton estuary, North Queensland, Austra-lia. Mmudflat, C Callianassa site, A Avicennia grove, Ssandflat, L lower intertidal. See Methods for further detail on thesites

Parameter M C A S L

Median grain size (mm)April 0.10 0.19 0.09 0.11 0.18September 0.11 0.15 0.11 0.14 0.22

Sorting coefficientApril 1.67 1.53 1.32 1.11 1.62September 1.51 1.28 1.22 1.24 1.37

Organic matter (% afdw)April 5.0 1.4 3.5 1.0 0.4September 2.4 1.4 1.7 1.2 2.7

highest (Fig. 2a and b). Here, macro- and mesofaunaabundances were significantly higher than at theother sites p , 0:01; p , 0:05 only for mesofauna(M, A) in September), but no significant differencewas found between the macrofauna of sites S and Cin September. At site C, macrofauna was signifi-cantly more abundant than in adjacent areas p ,0:05 (A) and p , 0:01 (M)) and the lower intertidal

p , 0:01 in September. However, mesofaunacounts at site C were low (Fig. 2b and Table 4).

Looking at the densities of single macrobenthicgroups, many taxa were encountered with only fewindividuals (Tables 3 and 4). Polychaeta, Amphi-poda and Bivalvia (juveniles) were numerous andoccurred with highest abundances in macro- andmesofauna samples at sites C and S. Decapod crus-taceans were recorded in low numbers throughoutthe transect.

There was little difference between the surveys inApril and September. Only at the Callianassa andlower intertidal site, macrofauna abundances hadsignificantly p , 0:05 increased by September.Single taxa were, however, more abundant in one ofthe two months (Tables 3 and 4): Polychaeta at theCallianassa site in September (macrofauna, p , 0:01as well as at the mudflat and lower site (mesofauna,p , 0:05; but more abundant at the sandflat site inApril (macrofauna, p , 0:01; Amphipoda at thesandflat site in April p , 0:01 for macro- and meso-fauna) and bivalves at the Callianassa, sandflat andlower sites in September (macrofauna, p , 0:05 resp.p , 0:01; but higher in the mudflat in April (meso-fauna p , 0:05:

The relative contribution of the major taxonomicgroups to infaunal abundances changed along thetransect sites and varied between the two monthsstudied (Fig. 3). Polychaeta accounted for about 30to 40% of the macrofauna at most sites and a lowershare was only recorded in April at the Callianassasite, where amphipods dominated the assemblage innumerical terms, and at the sandflat site in September,where a spatfall of bivalves had occurred and resultedin a dominance of molluscs. Polychaetes accountedfor 10 to 28% of the mesofauna in April, while theirshare increased to over 30 to 60% at all sites inSeptember. Oligochaetes occurred almost exclu-sively at the mudflat site, where they were wellrepresented in the mesofauna samples. The shareof 25% of oligochaetes at the lower sandflat sitein April is based on three individuals only. Crusta-ceans accounted for 1/4 to 2/3 of the individualnumbers at the mid-intertidal sites and this rela-tively large contribution was mainly made up byamphipods (see Tables 3 and 4). Molluscs consistedmainly of bivalves and the mesofauna samplesrecorded almost exclusively juvenile bivalves. The


Fig. 2. Box plots of benthic abundances of (a) macro-, (b) meso- and(c) meiofauna along the transect in the tidal flat of the Haughtonestuary from April 1991 (left box plot at each site) and September1991 (right box plot at each site). Transect sites are Mmudflat,C Callianassa site, A Avicennia grove, S sandflat, L lowerintertidal. Abundances are based on five replicate samples per siteand month; only a single macrofauna sample with 510 juvenilebivalves taken at the sandflat site in September was omitted fromthe calculation.

large share of molluscs in the macrofauna at theAvicennia and sandflat sites in September weredue to the spatfall of bivalves mentioned above.Note the low contribution of molluscs to the macro-fauna at the Callianassa site.

A classification of the macro- and mesobenthicorganisms into trophic groups gave a numericaldominance of subsurface and surface depositfeeders at all transect sites (Fig. 4). More thanhalf of all infaunal organisms at the Callianassaand sandflat sites were surface deposit feeders,while this trophic mode only accounted for asmall share of the abundances at the mudflat andlower sandflat sites. Suspension feeders comprisedabout 21% of all individuals at these latter sites. Thespatfall of bivalves in the sandflat consisted mainlyof tellinid bivalves classified as surface-deposit-feeders. Carnivores had a higher numerical abun-dance only at the lower sandflat site.

3.4. Meiofauna abundances

Meiofauna was abundant at all transect sites in bothsampling months (Fig. 2c). No significant differencesof total meiofauna numbers were found between Apriland September at any of the sites. Yet, nematodeswere significantly more abundant at the Callianassaand sandflat sites in September p , 0:01; whilecopepods were significantly less abundant p ,0:01 in September at the transect sites C, A, S andL, and platyhelminthes occurred also with fewer indi-viduals at the Callianassa site p , 0:01 in the samemonth (Table 5).

The highest meiofaunal densities of the transectsites were recorded in the lower intertidal (in April:p , 0:05 compared to S, p , 0:01 compared to sites Aand C; in September: p , 0:05 compared to S and A;Table 5). The lowest meiofauna values were found inthe Avicennia grove (in September: p , 0:05


Table 3Abundance (individuals 177 cm22) of major macrobenthic taxa and total macrofauna at the transect sites in the Haughton estuary in April andSeptember 1991. Mmudflat, C Callianassa site, A Avicennia grove, S sandflat, L lower intertidal. The median value of fivereplicate samples per site is given together with the lowest and highest number recorded (in brackets). not recorded. Other Crustaceainclude Tanaidacea and Isopoda

M C A S L

AprilPolychaeta 3 (07) 2 (03) 2 (09) 19 (1544) 1 (02)Oligochaeta 2 (03) Amphipoda 9 (420) 0 (05) 49 (869) 0 (01)Decapoda 1 (02) 1 (01) 1 (01) 0 (01) Other Crustacea 0 (01) 1 (03) Gastropoda 0 (01) Bivalvia 5 (19) 0 (02) 1 (04) 0 (01) 1 (03)Nemertinea 1 (04) 0 (01) Sipunculida 0 (04) Total macrofauna 14 (616) 13 (1025) 7 (514) 69 (33113) 2 (16)SeptemberPolychaeta 5 (16) 12 (623) 4 (35) 7 (39) 2 (04)Oligochaeta Amphipoda 11 (234) 2 (03) 0 (02)Decapoda 1 (01) 1 (03) 1 (02) 0 (01) 0 (01)Other Crustacea 0 (01) 0 (01) 0 (01) 0 (01) 0 (01)Gastropoda 0 (01) 1 (02) 1 (05) 1 (01) Bivalvia 2 (14) 3 (07) 4 (013) 53 (0510) 3 (18)Nemertinea 0 (01) 2 (02) 1 (01) 1 (02) 0 (01)Sipunculida 0 (01) Anthozoa 0 (03) Echinodermata 0 (01) 0 (02) 0 (05)Total macrofauna 9 (412) 26 (2468) 13 (621) 65 (38523) 8 (611)

compared to M, C and L). The high variation at theCallianassa site in September and at sites M and L inApril (Fig. 2c) resulted from a patchy distribution ofnematodes (Table 5). Nematoda accounted for over 2/3of the meiofauna at most transect sites on bothsampling occasions, while Copepoda had a highshare to the meiofauna composition at sites C, Aand S in April (Fig. 5). Platyhelminthes were mostabundant at sites C, S and L. Ostracoda occurredthroughout the intertidal with higher numbers in thesandflat and the lower sandflat site.

3.5. Community analysis

The macrofauna was distributed in distinct faunalgroups along the transect (Fig. 6). The high (M) andlow (L) intertidal sites were clearly separated bymultivariate analyses. Both mid-intertidal sites C(Callianassa site) and S (sandflat) formed definedclusters each. These two clusters were also more simi-lar to each other than to other transect sites. The faunaat the Avicennia grove was less well defined and moresimilar to the other mid-intertidal sites in April, but

closer to the mudflat site in September. This distinc-tion of assemblages is further illustrated with theMDS plots, showing the clear similarity within aswell as between the Callianassa and sandflat sites(Fig. 7). Tests for differences in the macrofauna ofall transect sites confirmed the similarity of replicateswithin sites (ANOSIM results: April: R 0:730; p 0:000%; September: R 0:744; p 0:000%).Species typifying the clusters were identified withthe SIMPER analysis. Mainly bivalve, amphipodand polychaete species were important for the simi-larity within the respective transect sites (Table 6).The species listed in Table 6 were also responsiblefor dissimilarities between groups.

The cluster analysis of the mesofauna data fromthe transect gave defined groups as well, but theywere not related to the transect sites in April (Fig.8). In September, replicate samples from themudflat, sandflat and lower sandflat were groupedinto clusters each, while the mesofauna at the othermid-intertidal sites was not separated into cleargroups (Fig. 8). The ANOSIM test results for themesofauna indicated less similarity within sites


Table 4Abundance (individuals 10 cm22) of major benthic taxa in mesofauna samples (see Methods) and total mesofauna at the transect sites in theHaughton estuary in April and September 1991. Mmudflat, C Callianassa site, A Avicennia grove, S sandflat, L lower intertidal.The median value of five replicate samples per site is given together with the lowest and highest number recorded (in brackets). notrecorded. Other Crustacea include Tanaidacea and Isopoda

M C A S L

AprilPolychaeta 1 (01) 0 (04) 0 (03) 6 (114) 1 (01)Oligochaeta 1 (04) 0 (02) 0 (02)Amphipoda 2 (02) 4 (08) 14 (1224) 0 (01)Decapoda 0 (01)Gastropoda 0 (01) 0 (01) Bivalvia 6 (012) 0 (02) 4 (36) 16 (1441) 0 (01)Nemertinea 0 (01) 0 (01) 0 (01)Echinodermata 0 (01)Total mesofauna 7 (316) 3 (09) 10 (417) 48 (3259) 2 (14)SeptemberPolychaeta 3 (16) 1 (02) 4 (16) 20 (144) 3 (17)Oligochaeta 7 (49) Amphipoda 0 (01) 0 (011) Decapoda 0 (02) Other Crustacea 0 (03) 0 (01)Gastropoda 1 (02) 0 (05) 0 (01) Bivalvia 0 (03) 1 (02) 2 (08) 12 (1035) 1 (01)Nemertinea 0 (01) 1 (05)Total mesofauna 11 (616) 3 (16) 9 (221) 37 (1157) 8 (210)

(ANOSIM results: April: R 0:452; p 0:000%;September: R 0:569; p 0:000%).

The meiofauna, treated here only at phylum level,was not separated into faunal groups along the trans-ect by multivariate analyses.

4. Discussion

4.1. Species richness and abundances

Tropical soft-bottom communities are rich inspecies, but individual numbers are often lower thanin their temperate counterparts (Macnae and Kalk,1962; Morton and Morton, 1983; Reise, 1991). Thehighest yet recorded species numbers (over 350species) for macrobenthic invertebrates in tropical

intertidal soft-bottom habitats come from the coastof Mozambique (Macnae and Kalk, 1962; Day,1974). Somewhat lower species numbers wererecorded from sand- and mudflats in southeast Asiaand Australia (Vohra, 1971: 140 species; Gibbs, 1978:150 species; Wells, 1983: 112 species; Reise, 1991:118 species). The number of 77 macrobenthic speciesrecorded during the present study is thus a low valuefor a tropical tidal flat. However, in my three years ofstudy, I found a total of 96 species in the Haughtonestuary (Dittmann, 1995). With ongoing taxonomicdescription of the reference collection from tidalflats of North Queensland, some records may be sepa-rated further and the species number given so far willhave been conservative rather than too high. Consid-ering the low species frequency and exponentialspecies-area curves for tropical tidal flats (Reise,


Fig. 3. Relative contribution of major macrobenthic taxa to the individual densities at the transect sites in the Haughton estuary (Mmudflat,C Callianassa site, A Avicennia grove, S sandflat; L lower intertidal). Values are given for macrofauna and mesofauna sampled inApril and September 1991 and presented as percentages of the respective total individual number (n) of five replicate samples per site, a fullcircle being 100%. In September, one macrofauna sample at the sandflat site contained 510 individuals of juvenile bivalves and was omittedfrom the calculation. Note the otherwise sometimes low individual numbers. Other taxa include sipunculids, anthozoans and insect larvae.

1991; Dittmann, 1995), species numbers for theseareas will increase with further faunal surveys.

The diversity (H 0) in the tidal flat of the Haughtonestuary falls within the range of diversity recorded byVargas (1987) from a mudflat in Costa Rica.

A comparison of the taxonomic composition ofmacrofauna in tropical tidal flats shows that Crusta-cea, Polychaeta and Mollusca are richest in species.Crustacea are especially diverse in brachyuran crabs(Day, 1974), but amphipods, tanaids and isopodsoccur with several species as well (Schrijvers et al.,1995; Swennen et al., 1982; and the present study).Polychaeta accounted for over 1/3 of the species in thepresent study, as in several other tropical tidal flats(Vohra, 1971; Gibbs, 1978; Piersma, 1982; Vargas,1987; Reise, 1991; Michaelis and Wolff, in press).The dominance of Polychaeta in terms of speciesnumbers will become more apparent with the use ofsmaller mesh sizes for extracting infauna from tropi-cal soft-sediments (Reise, 1991; Dittmann, 1995).

Extrapolating the median abundances of the presentstudy to areas of 1 m2 gives 1133898 individualsm22 for macrofauna and 2000 to 48000 individuals


Fig. 4. Contribution of six major trophic groups in percentage of thetotal individual numbers at the transect sites in the tidal flat of theHaughton estuary, based on macro- and mesofauna counts of bothsampling dates. Transect sites are Mmudflat, C Callianassasite, A Avicennia grove, S sandflat, L lower intertidal.

Table 5Abundance (individuals 5 cm22) of major meiobenthic taxa and total permanent meiofauna at the transect sites in the Haughton estuary in Apriland September 1991. Mmudflat, C Callianassa site, A Avicennia grove, S sandflat, L lower intertidal. The median value of fivereplicate samples per site is given together with the lowest and highest number recorded (in brackets). 2 not recorded

M C A S L

AprilNematoda 147 (84466) 50 (2770) 55 (35140) 50 (3970) 220 (195630)Copepoda 53 (17121) 80 (69102) 50 (1792) 145 (80175) 33 (1642)Platyhelminthes 0 (08) 49 (3078) 2 (07) 13 (735) 8 (318)Ostracoda 3 (08) 2 (13) 4 (16) 10 (316) 12 (251)Kinorhyncha 0 (02) 0 (03)Gastrotricha 0 (01) 1 (07)Halacaroidea 2 (03)Tardigrada 0 (01) 0 (02)Total meiofauna 284 (118525) 177 (129243) 158 (81196) 227 (146268) 310 (245618)SeptemberNematoda 183 (162256) 220 (180445) 120 (50225) 105 (80170) 270 (205390)Copepoda 24 (057) 4 (38) 2 (013) 38 (867) 4 (213)Platyhelminthes 4 (08) 9 (228) 1 (05) 18 (728) 13 (520)Ostracoda 4 (412) 1 (02) 1 (02) 7 (518) 3 (18)Kinorhyncha 0 (01) Gastrotricha 0 (01) 0 (01) 0 (01)Halacaroidea 1 (02)Tardigrada 0 (01)Rotifera 2 (010)Total meiofauna 227 (191325) 250 (193451) 124 (56233) 175 (127267) 307 (220415)

m22 for mesofauna (lowesthighest values from allsites and both sampling dates). Such extrapolationsshould be considered with care because of the highvariation of infaunal abundances. For macrofauna, therange lies within reported values from other tropicalsand- and mudflats (see reviews by Alongi, 1989b;1990; Reise, 1991), although they are not as high asthe highest yet recorded macrofauna abundance froma mudflat in Costa Rica (Vargas, 1987). Low abun-dances of macrofauna were also reported from tropi-cal tidal flats in Indonesia (Warwick and Ruswahyuni,1987) and West Africa (Michaelis and Wolff, inpress). However, the mesofauna values per m2would even exceed the abundances recorded byVargas (1987) and come close to the higher individualnumbers known from temperate tidal flats (Reise,1991).

The hypothesis that abundances tend to be lower intropical tidal flats, while species richness is higherthan in temperate tidal flats is thus substantiated bythe present study.

4.2. Species densities along the intertidal gradient

In temperate tidal flats, a higher species density ofmacrofauna was recorded at mid-intertidal levels andintermediate silt content (Beukema, 1976, 1988;Armonies and Hellwig-Armonies, 1987). Species

numbers and densities along the studied transectwere highest at the mid-intertidal Callianassa,Avicennia and sandflat sites. Therefore, the presentstudy corroborates the hypothesis that highest speciesdensities are encountered at intermediate tidal levelsand sediment composition.

Diversity, however, was highest at the lower sand-flat site in the Haughton estuary, where speciesnumbers were lower. Diversity was also highest atthe lowest level of a sandflat in Singapore studiedby Vohra (1971), who suggested that the reducedexposure to the sun or a more continuous food supplycaused this distribution. Day (1974) found highestspecies numbers where habitat diversity for infaunawas highest (estuarine regions with sandbanks, opensandflats, seagrass beds, a few mangroves). Thus,favourable sediment properties, habitat variety andrelatively low environmental stress could promotehigher species numbers at mid- and low intertidallevels of tropical shores.

4.3. Trophic groups

Although little is known on the biology of most ofthe tropical benthic fauna, a preliminary classificationinto major trophic groups could be made. The domi-nance of deposit-feeders in the tidal flats in north-eastAustralia (Dittmann, 1995, and the present study)


Fig. 5. Relative contribution of some major meiobenthic taxa to the total numbers of meiofaunal animals at the transect sites in the Haughtonestuary (Mmudflat, C Callianassa site, A Avicennia grove, S sandflat, L lower intertidal). Values are given for meiofauna sampledin April and September 1991 and presented as percentages of the respective total meiofauna number (n) of five replicate samples per site, a fullcircle being 100%. Other taxa include Kinorhyncha, Gastrotricha, Tardigrada, Halacaroidea and Rotifera.

complies with findings of Vargas (1987) and Wolff etal. (1993). Deposit feeders also dominate the fauna intidal flats of temperate latitudes (Sanders et al., 1962;Whitlatch, 1977). The abundance of deposit-feedersand especially surface deposit feeders implies a richfood supply on the sediment surface. The origin of thisfood source and the resulting food web in tropicaltidal flats are certainly promising subjects for futureresearch.

An increase of suspension feeders towards thelower intertidal as known from temperate tidal flats

(Beukema, 1976) was not pronounced in the tidal flatstudied and this trophic group was even present in thehigh intertidal mudflat. Thus, the hypothesis thatsuspension feeders are restricted to the lower interti-dal cannot be substantiated by the present study.

While bivalves, which are the main suspensionfeeders, are scarce in many tropical tidal flats (Macnaeand Kalk, 1962; Gibbs, 1978; Frith et al., 1976), theyare abundant (Day, 1974; Wells, 1983; Piersma et al.,1993) and even characteristic species of benthiccommunities in other regions (Warwick and Ruswa-hyuni, 1987; Reise, 1991). In tropical tidal flats ofThailand, a filter-feeding snail predominates in thelower reaches (Huttel, 1986; Reise, 1991). Thus, theabove hypothesis cannot be generally denied for alltropical tidal flats.

4.4. Zoned distribution

The sediment distribution along the studied transectwith increasing grain size towards the lower intertidalfollows a general pattern in tidal flats worldwide,which is related to the hydrodynamic regime andsettling behaviour of particles (Dankers andBeukema, 1983; Dronkers, 1984; Hertweck, 1994;Flemming and Ziegler, 1995). A similar fine-grainedsediment composition as in the Haughton estuary wasdocumented by Alongi (1988) from a sandflat nearbyin Bowling Green Bay. Day (1974) and Schrijvers etal. (1995) found a decrease in mud content andorganic matter from the mangroves to sandflat andbeach sites in East African estuaries. In my study,the organic content was also higher in the mudflatthan in the other transect sites, which agrees withthe well-known correlation of organic matter and silt(e.g. Dankers and Beukema, 1983). The reason for thehigh content of organic matter at the lower sandflatsite in September is unclear, but a nutrient input fromthe nearby river could play a role here.

Salinity is a further factor known to affect the distri-bution of fauna in estuaries (Hedgpeth, 1983). Unfor-tunately, salinity was not recorded in the presentstudy, but in a tidal flat nearby Alongi (1988)measured salinity ranges from 25 to 41, showingmainly fully marine conditions. The estuary of theHaughton River is tide-dominated and the studiedtidal flat, located at the very mouth of the estuary,


Fig. 6. Dendrograms of Bray-Curtis similarity for the macrofauna inthe tidal flat of the Haughton estuary as surveyed in April andSeptember 1991 at five transect sites: Mmudflat, CCallianassa site, A Avicennia grove, S sandflat, L lowerintertidal.

was flushed regularly. Thus, the transect did notfollow a salinity gradient in the estuary.

Distinct zonations of soft-sediment fauna alongtropical shores have been recorded in earlier surveysin the Indo-West Pacific (e.g. Macnae and Kalk, 1962;McIntyre, 1968; Vohra, 1971; Day, 1974; Sasekumar,1974; Frith et al., 1976; Gibbs, 1978; Morton andMorton, 1983; Warwick and Ruswahyuni, 1987;Schrijvers et al., 1995). These studies have shownthat the fauna of tidal flats was clearly differentfrom the one within mangroves or adjacent subtidalareas, and sand- and mudflats had a separate fauna aswell. Ndaro and O lafsson (1995) recorded higherpolychaete numbers in coarse than in fine sand inthe intertidal of Zanzibar and amphipods were absentfrom fine sand, just as they were lacking in the mudflatin the Haughton estuary. Vohra (1971) found that theinfauna in muddy sand was denser and more variedthan in clean sand. In the present study, the sandflatheld highest macro- and mesofaunal densities.

However, abundances of macro- and mesofaunawere generally low at most transect sites and showeda high variation, indicating patchy distributions.Therefore, the five replicate samples per site were a

minimum number to assess infaunal densities. Sincemost species in tropical tidal flats are represented insamples by few individuals only (Gibbs, 1978;Vargas, 1987; Dittmann, 1995), further surveys willyield more species as well as more detailed informa-tion on small-scale distribution patterns of tropicalbenthic fauna (Reise, 1991).

Records of meiofauna from tropical tidal flats showa composition of major meiofauna groups similar tothose of temperate zones, with a dominance of nema-todes (McIntyre, 1968; Kondalarao and Murty, 1988;Vargas, 1988b; Harkantra and Parulekar, 1989;Alongi, 1990; Ansari and Parulekar, 1998). Nematodeand copepod abundances recorded in the present studyfell within the range of densities reported by Alongi(1987a; 1988) from North Queensland, while Vargas(1988b) recorded nematode numbers at least twice ashigh from a mudflat in Costa Rica and Ndaro andO lafsson (1995) also found very high nematodenumbers in intertidal fine sand at Zanzibar.

The zonation of meiofauna in relation to tidal levelsdiffers between mangroves and tidal flats. Alongi(1987a,b) and O lafsson (1995) found increasing meio-fauna densities towards the lower intertidal of


Fig. 7. MDS plots for the macrofauna in the tidal flat of the Haughton estuary as surveyed in April and September 1991 at five transect sites:Mmudflat, C Callianassa site, A Avicennia grove, S sandflat, L lower intertidal. The numbers behind the site symbols refer toreplicate samples. Two outlying samples (A2 and L1, far right in Fig. 6) were omitted for the multidimensional scaling of the transect data fromApril. Stress values were 0.093 (MDS plot for April) and 0.123 (MDS plot for September).

mangrove areas. In the Haughton estuary, nema-tode and total meiofauna numbers were highestboth in the higher intertidal mudflat and thelower intertidal sandflat. The low meiofaunanumbers at the Avicennia site could perhaps beexplained by an inhibitory effect of tannins leach-ing from mangrove detritus (Alongi, 1987c),although A. marina does not contain as muchphenolic acid as Rhizophora. Further environmen-tal factors regulating meiofauna zonation arerelated to temperature and sediment properties(Alongi, 1987a; Ndaro et al., 1995). Comparedto macro- and mesofauna, a zoned distribution ofmeiofauna was not pronounced. This is similar tothe little variation of meiofaunal abundances

recorded by Armonies and Hellwig-Armonies(1987) from an intertidal gradient in a temperatetidal flat.

There was little variation in abundances along thetransect sites between April and September, except forsingle taxa. The sampling in April followed a heavywet season, but a comparison of overall abundancesfrom the two dates shows that this had little effect onthe infauna. Only the low macrofauna abundances atthe lower sandflat site in April might have resultedfrom floods carrying freshwater down the river.Several authors report negative effects of monsoonalrains on intertidal benthic fauna (see reviews byAlongi, 1989b, 1990), followed by a good recoveryof meiofauna (Alongi, 1987a). Nematode numbers


Table 6Species typifying the transect sites (SIMPER-analysis), ranked in decreasing order of their importance to the similarity within the respectivesites. Species contributing about 90% to the similarity are listed. The transect sites surveyed in April and September 1991 in the tidal flat to theHaughton estuary were Mmudflat, C Callianassa site, A Avicennia grove, S sandflat, L lower intertidal. Genus names marked withp are tentative.

Site April September

M Laternula p sp. (Bivalvia, Laternulidae) Barantolla sp. (Polychaeta, Capitellidae)Oligochaeta indet. Laternula p sp. (Bivalvia, Laternulidae)Barantolla sp. (Polychaeta, Capitellidae) Siliqua cf. tenerior (Bivalvia, Solenidae)Nereidae sp. A (Polychaeta)

C Gammaridae indet. (Amphipoda) Armandia secundariopapillata (Polychaeta, Opheliidae)Haustoriidae indet (Amphipoda) Haustoriidae indet. (Amphipoda)Nemertinea indet. Nemertinea indet.Barantolla sp. (Polychaeta, Capitellidae) Tellinidae indet. (juveniles) (Bivalvia)Cumacea indet. Capitella sp. (Polychaeta, Capitellidae)

Nereidae sp. B (Polychaeta)Scolelepis sp. (Polychaeta, Spionidae)Ampeliscidae indet. (Amphipoda)

A Scolelepis sp. (Polychaeta, Spionidae) Bivalvia indet. (juveniles)Psammotaea p sp. (Bivalvia, Psammobiidae) Nemertinea indet.Armandia secundariopapillata (Polychaeta, Opheliidae) Naticidae indet. (Gastropoda)Decapoda indet. Mictyris longicarpus (Decapoda, Mictyridae)Gammaridae indet. (Amphipoda) Barantolla sp. (Polychaeta, Capitellidae)

Scolelepis sp. (Polychaeta, Spionidae)S Gammaridae indet. (Amphipoda) Tellinidae indet. (juveniles) (Bivalvia)

Armandia cf. leptocirrus (Polychaeta, Opheliidae) Armandia cf. leptocirrus (Polychaeta, Opheliidae)Nereidae sp. C (Polychaeta) Nereidae sp. C (Polychaeta)Haustoriidae indet. (Amphipoda) Haustoriidae indet. (Amphipoda)Glycera macintoshi (Polychaeta, Glyceridae) Nemertinea indet.

Gammaridae indet. (Amphipoda)L Anapellap sp. (Bivalvia, Mesodesmatidae) Anapellap sp. (Bivalvia, Mesodesmatidae)

Glycera oxycephala (Polychaeta, Glyceridae) Glycera oxycephala (Polychaeta, Glyceridae)Haustoriidae indet. (Amphipoda) Arachnoides placenta (Echinoidea, Arachnoididae)

were higher at the mid-intertidal sites in September.Alongi (1987b) recorded highest nematode densitiesat various tidal levels in different seasons. The highercopepod numbers in April could have resulted from aquick recovery in the early post-monsoon, as reportedby Ansari and Parulekar (1993). Vargas (1988b)found no seasonal pattern in the abundance of meio-fauna, whereas macrofauna responses to wet and dryseasons were species-specific and partly related toreproduction (Broom, 1982; Vargas, 1989, 1996).

Without long-term studies and knowledge of recruit-ment patterns, temporal variations of benthic fauna intropical tidal flats cannot be properly interpreted.

An indication that recruitment is relevant to spatialdistributions can be derived from the bivalve recordsalong the transect. Spatfalls of bivalves were recordedin the mesofauna samples from the mid-intertidal sitesand juvenile bivalves reached considerable numbersespecially in the sandflat. Adult bivalves, however,were found mostly in the mudflat and at the lowersandflat site. Heavy predation by predatory snails orpassive trapping in the burrows of Callianassa(Trypaea) australiensis (Peterson, 1977; Dittmann,1996) could have reduced the recruitment successof bivalves in the mid-intertidal, restricting theiroccurrence to the upper and lower reaches of thetidal flat.

Single species varied in their occurrence along theintertidal gradient and the distribution of majormacrofauna taxa, genera and even species was gener-ally in good agreement with records from other tropi-cal tidal flats. Macrophthalmus spp. and mudskipperswere recorded from mudflats in the Haughton estuaryand other tidal flats in North Queensland (Macnae,1967; Dittmann, 1995). They were also reportedfrom the upper and mid-intertidal level of tidal flatsat Inhaca Island (Macnae and Kalk, 1962; Guerreiro etal., 1996), foreshore mudflats in Malaysia (Saseku-mar, 1974) and Thailand (Frith et al., 1976), and aregenerally conspicuous members of the fauna ofmuddy areas on accrescent shores of the Indo-WestPacific (Macnae, 1968). Other ocypodid crabs(Ocypode spp., Uca spp., Dotilla spp.) occur withseveral species in the high and mid-intertidal (Macnaeand Kalk, 1962; Vohra, 1971; the present study). Tubeworms (Terebellidae (Loima medusa), Maldanidae,Oweniidae, Spionidae) and echiurids are reportedfrom the mid-intertidal of several tropical tidal flats(Macnae and Kalk, 1962; Gibbs, 1978; this study andunpubl. data). The dominance of callianassid shrimpsin mid-intertidal muddy sandflats of northern Austra-lia has not been described from other tropical tidalflats, but thalassinidean shrimps occur in a variety ofmarine environments (see Griffis and Suchanek, 1991;Reise, 1991; Tamaki and Ingole, 1993). Echinoderms(benthic holothurids, sand dollars, brittle stars) andhermit crabs occurred only at the mid- and lowerintertidal level in the Haughton estuary, which is in


Fig. 8. Dendrograms of BrayCurtis similarity for the mesofauna inthe tidal flat of the Haughton estuary as surveyed in April andSeptember 1991 at five transect sites: Mmudflat, CCallianassa site, A Avicennia grove, S sandflat, L lowerintertidal. One outlying sample (C5) was omitted from the dendro-gram in April.

agreement with their distribution in other tropical tidalflats (Macnae and Kalk, 1962; Day, 1974; Morton andMorton 1983).

The findings of the present study together with theliterature recordings corroborate the hypothesis that azoned distribution between the tide marks is a univer-sal attribute of the benthic fauna also for tropicallatitudes.

4.5. Synoptic patterns

The relevance of physical heterogeneity andspecies interactions to the organisation of benthiccommunities can vary with scale (Barry and Dayton,1991). Sampling three different infaunal size classesimplied a change in the scale of observation. Thus thedistribution patterns of macro-, meso- and meiofaunadescribed here can result from different physical andbiological processes as well as the supply of resources(space and food) for the respective faunal size.

Along an intertidal gradient in a temperate tidal flat,Armonies and Hellwig-Armonies (1987) foundsynoptic patterns in the distribution of macro- andmeiofauna. In the present study, the spatial distribu-tions of the three infaunal size classes were not alwayssimilar. Meiofauna was most abundant at the mudflatand lower sandflat sites where macro- and mesofaunaabundances were low. At the Callianassa site, macro-and meiofauna were abundant, but mesofaunanumbers low. Both macro- and mesofauna had highestabundances at the sandflat site. Whether these distri-bution patterns are related to sediment properties orthe result of biotic interactions, as they are known tostructure soft-bottom communities in other parts ofthe world (Commito, 1982; Ambrose, 1984; Reise,1985; Flach, 1992) cannot yet be answered.

In the Haughton estuary, the characterising speciesof benthic communities were mainly large bioturbat-ing infauna (Macrophthalmus, Callianassa, holothur-ids and enteropneusts). They will have comparableecological roles to their temperate counterparts (e.g.Arenicola marina on the NE Atlantic coast (Reise,1985)), as e.g. burrows of callianassid shrimps accom-modate meiofauna and smaller infauna (Dittmann,1996). Promotive and repressive interactions havealso been recorded from other tropical tidal flats(Morton and Morton, 1983; Black and Peterson,1988). The high macro- and mesofaunal numbers in

the sandflat could be related to biogenic structuresprovided here by benthic holothurids, but furtherstudies are needed on the biology of the species intropical tidal flats and their interactions with otherspecies. Thus, with the indications available, thehypothesis that interactions between species affectbenthic assemblages in tidal flats can be corroborated.

4.6. Community descriptions

The transect sites in the tidal flat of the Haughtonestuary had been chosen to cover a range of sedimenttypes and benthic habitats, based on prior qualitativemapping. Previous surveys had identified some of themajor first- and second-order characterising species(sensu Thorson, 1957) in the tidal flats of NorthQueensland (Dittmann, 1995). Further associatedspecies of these communities were assessed with thequantitative survey presented here. Along the transectsites, distinct benthic communities were identified inthe high-, mid- and low intertidal for macrofauna and,to a lesser degree, for mesofauna. Though no assem-blages were found for meiofauna at phylum level, aprevious analysis of Platyhelminthes on species levelhad shown distinct associations in the mid- and lowintertidal sites of this transect in relation to environ-mental conditions and biotic interactions (Dittmann,1998). Together with observations on species distri-butions from other Indo-West Pacific tidal flats (seeabove), the major benthic communities from thehigh to the low intertidal can be described asfollows.

Mudflat community: Macrophthalmus latreillei,fiddler crabs and mudskippers are the first- andsecond-order characterising species of this commu-nity. These larger macrobenthic species occurregularly and in great abundance in the muddysediment of the upper intertidal. Patches ofmudwhelks are often found in this community aswell. Oligochaetes and capitellid polychaetes wereabundant and among the characterising macro-fauna species for the infaunal assemblage of thissite, together with bivalves of the families Laternu-lidae and Solenidae. Meiofauna numbers were highwith a dominance of nematodes.

Callianassa community: Callianassa (Trypaea)australiensis is the structuring organism for theinfaunal community in mid-intertidal muddy


sandflats of north-east Australia. The burrows ofthis shrimp extend over 1 m deep into the sediment(Kenway, 1981). Experiments and investigationson small-scale distributions have shown thatmany infaunal species are associated with theshrimp, and amphipods live as commensals in theburrows (Kenway, 1981; Dittmann, 1996). In thepresent survey, several amphipod and polychaetespecies characterised the macrofauna assemblageat this site (Table 6). Meiofauna was also numeroushere and the abundance and species distribution ofPlatyhelminthes are linked to the biogenic struc-tures (Dittmann, 1996; 1998). Further large macro-benthic species in this community are terebellidpolychaetes (Loima medusa) and echiurids.Brachiopods (Lingula spp.), which occur in themuddy sand of other tidal flats in North Queenslandand southeast Asia (Morton and Morton, 1983;Baron et al., 1993; Dittmann, 1995), were absentfrom the Haughton estuary.

Avicennia mangrove: this site is characterised bythe mangroves and no infaunal organisms could beidentified as characterising species here. The sedi-ment in the Avicennia grove consisted of muddysand. The mangrove roots and pneumatophores areendo- and epibenthic structures, but the flora andfauna directly associated with them has not yetbeen studied. The infauna at this site was relativelyrich in species and resembled the mudflat andCallianassa communities. Wells (1983) recordedlow species numbers and densities from an Avicen-nia site of a transect in north-west Australia. Meio-fauna numbers were low, which might have beendue to tannins leaching out of the mangroves(Alongi, 1987c, see above).

Sandflat community: the characterising specieshere is a benthic holothurid (cf. Paracaudina sp.),which has a burrow system similar to that of Areni-cola marina in temperate tidal flats of the NEAtlantic. The associated fauna was relatively richin species and occurred with high individualnumbers. Several species of amphipods and poly-chaetes characterised the infaunal assemblage(Table 6). Glycera spp. and Armandia spp. werealso recorded in a sandflat community by Macnaeand Kalk (1962). A spatfall of tellinid bivalves inSeptember had the highest settlement here, butadult bivalves were rare. Nematodes, copepods

and platyhelminths were well represented in themeiofauna at this site.

Lower sandflat community: characterizing speciesof the lower sandflat are enteropneusts (which formthe major biogenic structure in the sediment here),sand dollars and hermit crabs. The species numberof the associated infauna was not high and abun-dances were low. Among the characterizingspecies of the infauna were a bivalve (cf. Anapellasp.) and a glycerid polychaete (G. oxycephala) notrecorded on the higher intertidal sites. Meiofaunawas very numerous and contained more phyla (e.g.Tardigrada, Rotifera) than at the other sites.This study demonstrates a high degree of similarity

in the intertidal zonation of benthic fauna in tropicaland temperate tidal flats. The concept of findingparallel communities within and between latitudes(Thorson, 1957) invites further investigation andcomparison of ecological functions in tidal flats withdifferent degrees of biodiversity around the world.

Acknowledgements

This study was carried out during a postdoctoralfellowship funded by the German Science Foundation(DFG grant III 02-Di 396/1-2). I very much appre-ciated the hospitality and excellent research facilitiesprovided by the Australian Institute of MarineScience. Ulrike Siebeck and Urte Bottcher were ofgreat help in the field and U. Siebeck also carriedout the grain size analysis. Jochen Zeil identifiedsome of the crabs for me and Markus Boggemannworked on the taxonomy of the glycerid polychaetes.Karsten Reise, Jose Vargas and an anonymous reviewergave helpful comments on the manuscript.

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