12 Dittmann Abundance

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1986, Micheli, 1993). Meiofauna, on the otherhand, occurs in low abundances in mangrovesediments (Alongi 1987a, b, c). Further infaunalcomponents of mangrove sediments were oftenrecorded in a qualitative way only or with theuse of a 1mm sieve size. Small-sized infaunahad not yet been studied in mangrove sediments.These organisms could play an important role inthe food web of mangrove ecosystems, as smallmacrofauna can constitute a major food sourcefor juvenile fish and prawns, which usemangroves as nursery sites (Robertson 1988,Daniel and Robertson 1990). Therefore,abundance and distribution of small-sizedinfauna were surveyed in mangrove andadjacent tidal flat sediments.

The study sites were arranged along atransect, as earlier studies on mangrove faunahave shown clear zonation patterns in relation totidal elevation, forest type and sedimentproperties (Macnae and Kalk 1962, Sasekumar1974, Frithet al. 1976, Wells 1983). Thesesurveys crossed through mangroves into ambientsand- and mudflats and generally showed lowerdensities and species numbers in mangroves thantidal flat sediments (see Alongi 1989 and Alongiand Sasekumar 1992 for further references).

Only Frithet al. (1976) found more species in amangrove than mudflat and Schrijverset al.(1995a) recorded higher macrobenthic densitiesin mangrove than sandflat sediments. This studypresents the first quantitative records of small-sized infauna from mangrove sediments.

MATERIALS AND METHODS

The study was carried out in the mangrovesof Missionary Bay at Hinchinbrook Island(1813S; 14611E), North Queensland,

Australia. Four transect sites were arranged alongthe boardwalk of the Australian Institute of Marine Science at Coral Creek (Fig. 1), coveringa distance of about 500 m. Site I was ca. 10 minside the forest from Coral Creek. This site wasfrequently inundated and the sediment black andsolid mud. The second and third sites lay furtherwithin the mangrove forest. Site II was on a more

elevated part of the mangrove peninsula, abouthalfway along the boardwalk. The mangrovecanopy was thin here so that more light reachedthe sediment, which was a dry and firm mud richin mangrove detritus. Site III was ca. 10 mbeyond the end of the boardwalk. Here, thesediment was rather soft mud. Site IV waslocated in a mudflat seaward of the mangrovefringe. At this site, the sediment was knee-deepmud which also contained shell fragments.Mudwhelks (mainlyCerithidea cingulata ) andTheodoxus oualaniensis were abundant on thesediment surface of the mudflat. Themacrofauna in the mangrove forest wasdominated by sesarmid crabs (Robertson 1986,Micheli 1993). The forest consisted of several

Rhizophora spp. and Brugiera gymnorrhiza ,Ceriops tagal co-occurred at the more elevatedsite and Avicennia marina grew on the mangrovefringe towards the mudflat. Detailed accounts onthis mangrove area and environmentalparameters of the studied sites are available fromAlongi (1987a, 1988), Boto and Wellington(1984) and Botoet al. (1989).

Fig. 1. Location of the study area in Missionary Bay,Hinchinbrook Island, North Queensland. The start (site I) andend point (site IV) of the transect are indicated in the map.


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Samples were taken on 17 November 1988and in 1989 on 3 May, 2 August and 13 October.The mudflat site IV could not be sampled inNovember 1988 due to the incoming tide. Atevery sampling occasion, six replicate sampleswere taken at each site. To extract sedimentsamples from the mud, a cut off syringe (50ml)with piston was used as a corer. This corer had asurface area of 6.61 cm-2 and sediment wasextracted to 5 cm depth. Usually several trialswere necessary to get an entire sediment core, ascrab burrows were often hit with the corer. Thesamples were sieved over 0.25 mm mesh size toassess the small sized infaunal organisms(mesofaunasensu Dittmann 1995), which wouldbe lost by the use of larger mesh sizestraditionally taken for macrobenthic samples(see also Bachelet 1990 and Schlacher andWoolridge 1996 for adequate choice of sievesize). The animals were sorted alive and countedunder a dissecting microscope. The sedimentcontained many small particles of mangrovedetritus, which made the sorting verycumbersome. Distinguishable species werealways recorded separately and determined tothe lowest possible taxonomic level. Forpolychaetes, genus or species could be identified,

while most gastropods were treated on familylevel with few species named. Oligochaeta andnemertines could not be determined further.

Species similarity between sites wascalculated by the Srensen Index QS (Srensen1948). To test for significant differences inabundances between the sites and samplingmonths, Kruskal-Wallis H-test and the U-test byWilcoxon, Mann and Whitney were applied(Sachs 1992). Multivariate analyses were carriedout using the PRIMER software package by thePlymouth Marine Laboratory. Cluster analysis(Bray-Curtis similarity, group-average linkage)

were made on double square root transformeddata in search of species assemblages.Differences in assemblages between sites weretested with ANOSIM and species discriminatingsample groupings were analysed with SIMPER(Clarke 1993).

RESULTS

Species: Based on the records from allsamples and sites, 39 taxa were distinguishedduring this survey, with polychaetes richestin species (19), followed by gastropods (9).Species numbers differed between transectsites and most species were recorded in themudflat (Table 1). Diversity was higher at themudflat site than in the mangroves, withlowest diversity at site II. On average, onlytwo species were found per sample andspecies densities were low at all sites (Table1). Species similarity was higher between themangrove sites I, II and III (QS values of 0.43 to 0.5) than between the mudflat andmangrove sites I and II (QS = 0.29 and 0.3resp.). However, the mudflat and the adjacentmangrove site III were similar in theirspecies composition (QS = 0.46).

Abundances and distribution:Abundances of small infauna rarely exceeded10 ind. 10 cm-2 along the studied transect(Fig. 2). Extrapolating individual densities tom-2 (beware of high variations and unknownspatial distributions), the overall meanamounts to 5 477 ind m-2, varying from 4 796

(site I) to 6 687 ind m-2

(site III).There was little variation in abundancesover time at the single transect sites (Fig. 2).Only at site II abundances were significantlyhigher (H-test, p


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TABLE 1

Abundance and diversity of small infauna in a mangrove forest, Missionary Bay.

site I site II site III site IVnumber of species

total 13 11 17 22Polychaeta 8 6 8 10Gastropoda 3 3 2 4

diversity (H) 1.78 1.18 1.81 2.38species density 1.83 (1.01) 1.5 (1.1) 2.33 (1.52) 2.28 (1.9)

Oligochaeta sp. 2 df 1. (46.1%) 1. (71.4%) 1. (54.7%) 4. (6.6%)Capitella sp. 4 df 2. (15.8%) 4. (3.6%) 4. (5.7%) -Capitellidae indet. df 3. (14.5%) 2. (8.3%) 4. (5.7%) -

Heteromastus sp. df 4. (5.3%) < < < Leitoscoloplos sp. df 5. (4.0%) - 5. (2.8%)

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12 Dittmann Abundance