MARINE ECOLOGY PROGRESS SERIES 1Published December 14 Mar Ecol Prog Ser

Distribution, biomass and community structure of megabenthos of the Gulf of Carpentaria,

Brian G. Long*, Ian R. Poiner, Ted J. Wassenberg

CSIRO Division of Fisheries, Marine Laboratories, PO Box 120, Cleveland, 4163, Australia

ABSTRACT: To describe the megabenthos communities in the Gulf of Carpentaria, Australia, beam trawls were taken from a systematic grid of 107 stations sampled during November and December 1990. Over 476500 individuals were collected, weighing over 1700 kg and representing more than 840 species from 150 families. The mean biomass was 15.9 kg trawl-', the average number of species was 59.8 trawl-' and the average number of individuals was 4405 trawl-'. The main taxa in terms of biomass were spatangoida (60.5%), porifera (13.7%). zoantharia (7.3%),bivalvia (2.8%), and decapoda (2.8%);they included 12, 102, 10 and 198 species respectively. The megabenthos species were mainly scavengers/carnivores (345),suspension-feeders (3311, deposit-feeders (121) and a few herbivores (37). Most species were classed as mobile (496) with lesser numbers classed as sessile species (334). Two deposit-feeding spatangoids accounted for 59% of the biomass but were sampled at only 9 stations. Classification and ordination using species presence or absence data indicated that there were 2 main communities in the Gulf: a community located in predominantly sandy sediments along the eastern and southeastern margins of the Gulf that comprised mainly sessile suspension-feeding sponges, zoanthar- ians, pennatulaceans, bivalve molluscs and ascidians; and a community located in the muddier sedi- ments in the central and western Gulf that comprised mainly deposit-feeding spatangoids and sand dollars. However, sessile suspension-feeders were also found in the central Gulf wherever suitable sub- strata were present.

KEYWORDS. Megabenthos . Gulf of Carpentaria. Australia . Community structure

INTRODUCTION future, the objective of this study was to describe the benthic communities before the area is trawled. The Gulf of Carpentaria, Australia (see Fig. l), sup- The megabenthos (benthos sampled by a beam ports a prawn-trawl fishery of over 130 vessels (Pow- trawl) of the Gulf of Carpentaria has not been describ- nall 1994) and a developing demersal fish-trawl fishery ed, with the exception of the penaeid prawns (Somers with 1 licensed vessel (Blaber et al. 1994). The prawn- 1994). The only other studies of the continental shelf trawlers operate in waters between 10 and 50 m deep, megabenthos of tropical Australia are of the decapod whereas the fish-trawl fishery is currently restricted to crustaceans of the North West Shelf off Western the deeper waters (>20 m) north of 13" 30' S. South of Australia (Ward & Rainer 1988) and the megabenthos 13" 30' S, the deeper waters (>50 m) of the central Gulf of the northern Great Barrier Reef lagoon (Blaber et al. of Carpentaria have been effectively closed to fishing 1993). This study is therefore the first description of for 14 yr; the seabottom is one of the few in shallow non-reefal megabenthos from tropical northern tropical that has rarely been trawled. Trawling Australia. can have a major impact on epi- and infaunal benthos (Messieh et al. 1991),and since demersal fish-trawling in the central Gulf is likely to expand in the near MATERIALS AND METHODS

Study area. The Gulf of Carpentaria is a large (approximately 3.7 X 105 km2), shallow (less than 70 m

0 Inter-Research 1995 Resale of full article not permitted Mar Ecol Prog Ser 129: 127-139, 1995

beam trawl or 'Church Dredge' rigged with a 30 mm mesh net bag. Each trawl lasted for 15 min at 6 km h-' from the stern of the ship. Sampling depths ranged between 20 and 63 m. The dredge sampled approxi- mately the top 150 mm of substratum as well as associ- S ated benthos and fish. One sample was taken at each station. The trawl samples were sorted on board into families and frozen for transport to the laboratory. The large sessile megabenthos, mainly sponges, were sorted immediately after collection, counted and their fresh weight recorded to the nearest 100 g A small piece of each was frozen and kept for identification. Samples were identified to species or putative taxon, counted and weighed to the nearest 1 g by taxo- S nomists at the Queensland Museum, Australia, where the samples were lodged. Colonies rather than individ- uals were counted and weighed for the colonial ani- mals. The species were also placed into feeding guilds - herbivores, deposit-feeders, scavengers/carnivores and suspension-feeders - by consulting Fauchauld & Jumars (1979) for polychaetes, Short & Potter (1987) for Fig. 1. Gulf of Carpentaria showing FRV 'Southern Surveyor' molluscs and Barnes (1974) for the remaining phyla. sampling stations Species were also placed into 3 size classes, large (>l kg), medium (l to 0.1 kg) and small (

When binary data are used the Bray-Curtis measure is percentage of sand (r = -0.41. p < 0.001). The sandier equivalent to the Czechanowski measure. Dyer et al. sediments in the southeast Gulf graded gradually into (1983) used the Czechanowski measure to cluster ben- muds and sandy lnuds in the northwest Gulf (Fig. 2). thic trawl data in the North . The Gower metric was There was little evidence of large-scale patterns in used for calculating the dissimilarity between species turbidity or salinity at the time of sampling. A thermo- based on their presence or absence at stations (R-mode cline was present in the central Gulf at a depth of analysis). Station dissimilarities were sorted and the around 25 m, and surface-water temperatures were dendrogram was produced by the unweighted pair, 4 degrees higher than bottom temperatures (Somers & group-mean-average (UPGMA) fusion algorithm with Long 1994). a slightly space-dilating value of -0.1. A 2-way table (Belbin 1991) to cross-tabulate the species and station groups was used to assist in interpreting species and General station groupings. A visual summary of the 2-way table was created to Approximately 476 500 megabenthos weighing show graphically the relationship between the species 1700 kg and consisting of 846.species from 150 families groups and station groups. To do this the species were collected from the 107 stations in the Gulf of Car- occurrences in each station group were counted and pentaria. A checklist of the taxa collected in the Gulf is converted to a percentage of the station group total. given in Long & Poiner (1994a). The mean biomass was The percentages were displayed graphically in a 15.9 kg trawl-' (SE = 5.2); the average number of spe- species by station group matrix with 4 grey scales to cies was 59.8 trawl-' (SE = 5.8);and the average abun- represent: species absent, 1 to 10%, 10 to 30% and dance was 4405 ind. trawl-' (SE = 2154). The dominant > 30 % occurrence. taxa in terms of biomass were spatangoids, followed Only species found at more than 2 stations were used by sponges, zoantharians (mainly solitary, flabellid for the classification and ordination analysis so as to corals), bivalves and decapods (Table 1). Together reduce the number of chance CO-occurrences of rare these 5 taxa accounted for 87Y0 of the biomass. The species. Removing rare species has little effect on the major phyla were well represented: 223 (26%) were outcome of the analysis (Stephenson & Cook 1980). crustacea, 194 (23 'K) mollusca, 144 (17 %) echinoder- Station dissimilarities were ordinated by principal coordinate analysis (PCO) rather than multidimen- sional scaling (MDS) because PC0 separates the groups more clearly than MDS (Belbin 1991). Partial correlation analyses (Pearson's correlation coefficient) were performed for the main taxa, mobility and feeding classes between transformed (log,, + 1) megabenthos biomass (kg trawl-'), abundance (no. trawl-') and principal coordinate scores with the abi- otic variables, depth, mud, latitude and longitude. This was done to test whether there were Gulf-wide trends in the distribution and abundance of megabenthos which were statistically independent of underlying trends in depth and sediment grain size.

RESULTS *------m> 80% mud '- -S.---A Abiotic factors 1 50 to 80% mud 20 to 50% mud Water depth was positively correlated with the per- 3- c 20010mud centage of mud in the sediment (r = 0.33, p < 0.001) and , outsldestudv area negatively correlated with the percentage of sand (r = -0.36, p < 0.001). Depth was also negatively corre- lated with the bottom water temperature (r = -0.64, Fig. 2. Sediment grain size, expressed as percentage mud grouped into 4 categories, distribution in the Gulf of Carpen- p < 0.001). The bottom water turbidity was positively taria, Australia, during November-December 1990. Depth correlated with the percentage of mud in the sediment contours (m) overlaid Map adapted from Somers & Long (r = 0.38, p 0.001) and negatively correlated with the (1994) 130 Mar Ecol Prog Ser 129: 127-139, 1995

Table 1. Total catch, by weight and numbers, of the major highest biomass (64 X).Sponges made up the bulk of classes of ben1.hos, with the number of identified taxa from the suspension-feeder biomass, and spatangoids made each class up the bulk of the depos~t-feederbiomass. The majority of the Gulf megabenthos were classed as small (7911, Class No taxa Weight Numbers with medium (34) and large (9) species less prevalent. Most of the species were mobile (496). Sponges (101 Porifera 102 233.10 23 463 Hydroida 18 7.80 6 838 species) made up one-third of the sessile species (334). Pennatulacea 9 1.96 167 Gorgonacea 7 0.36 157 Alcyonacea 19 25.18 2 602 Distribution of megabenthos in the Gulf Zoantharia 10 123.61 229 957 Stomatopoda 21 2.81 1821 Decapoda 198 47.05 11767 There were significant correlations between mega- Prosobranchia 83 10.26 3 897 benthos abundance and biomass with percentage mud Cephalaspidea 2 0.13 82 and depth and latitude and longitude (Table 3). How- Nudibranchia 15 0.47 45 ever, there were also significant correlations between Bivalvia 82 48.10 6 604 Scaphopoda 2 0.12 29 depth and percentage mud and position in the Gulf Cephalopoda 10 5.78 395 (latitude and longitude). Hence partial correlation Bryozoa 28 28.88 10 274 analyses were necessary. These indicated that the Crinoidea 21 8.34 1296 biomass of scavengers/carnivores and zoantharians Asteroidea 25 24.45 888 decreased with increasing depth (p < 0.05). The bio- Spatangolda 12 1031.61 146321 Echinotda 8 3.87 505 mass of suspension-feeders, sponges, sessile, medium Diadematoida 1 3.65 44 and large-sized megabenthos decreased with increase Clypeasteroida 12 33.38 21 409 in percentage mud (Table 3). There were significant Holothuroidea 44 26.34 548 north-south trends in biomass: the biomass of zoan- Ophiuroidea 2 8 3.96 2 281 Asc~diacea 35 30.71 2 112 tharians and small megabenthos was higher in the Miscellaneous 63 2.49 3 028 southern Gulf than in the northern Gulf. There were Total 846 1704.41 476530 also significant east-west trends: abundance, number of species, and biomass of suspension-feeders, scav- enger/carnivores and zoantharians were higher in the eastern Gulf than in the western Gulf (Table 3). mata, 104 (12%) porifera, 63 (7 %) coelenterata and 48 Finally, there were significant north-west trends in the (6%) polychaeta. Two seagrasses - Halophila spinu- biomass of deposit-feeders, spatangoids and mobile losa and Halophila ovalis- and 10 algae were also megabenthos in the Gulf: all had a higher biomass in collected. the southeast than in the northwest Gulf. Most of the species occurred in low numbers:

41 l were represented by less than l0 Table 2. Species collected at more than 50% of the stations in the and 143 were Gulf of Carpentaria. A: Coelenterata, Alcyonacea; B: Molluscs, also very patchily distributed: 410 species were Bivalvia; D: ~rustacea.Decapoda; 0: ~chinodermata,Ophiuroidea. collected at l or 2 stations and of the 303 species n: number of stations sampled that were collected at 1 station, 224 were repre- sented by 5 or fewer individuals and 3 were rep- Species Taxonomic Abundance (trawl-') n resented by more than 100 individuals. The most order Mean Maximum abundant species, Flabellum sp. 3 (123 428 indi- Myra blconlca D 4.8 4 1 79 viduals), was sampled at only 1 station (401, in Bassina calophylla B 3.1 26 75 the southeast Gulf. Fourteen of the species were Metapenaeopsjs palmensls D 10.0 223 7 1 widely distributed in the Gulf (Table 2).All of the Parthenopus Iongimanus D 3.3 2 2 69 taxa in this gr0u.p have an Indo-West Pacific dis- Arcan~anovemsp~nosa D 2.6 2 7 65 tribution except Bassina calophylla, a blvalve Carc~noplaxpurpurea D 2.4 18 63 Alcyonacea sp. 4 A 9.3 173 63 found only in northern Australia and the East Phalang~pesaustral~ens~s D 6.6 11 6 1 Indies (Table 2). Oph~uro~deasp. 8 0 9.7 182 6 1 The Gulf megabenthos were mostly scav- Amus~umpleuronectes B 5.2 123 5 7 engers/carnivores (345) and suspension-feeders Charybd~struncata D 2.3 2 1 5 6 Portunus splnlpes D 2.7 3 0 56 (331),with fewer deposit-feeders (121)and herbi- Oratosqu~llalnorna ta D 1.5 14 54 vores (37). S.uspension-feeders were the most Ceratoplax sp 1 D 2.5 25 5 7 abundant (60%) but deposit-feeders had the Long et al.: Gulf of Carpentaria megabenthos 131

Table 3 Partial correlation analysis of megabenthos species richness (no. spec~estrawl-'), abundance (no. trawl-'), total biomass (kg trawl-'), scavengers/carnivores (kg trawl-'), suspension-feeders (kg trawl-'), deposit-feeders (kg trawl-') with abiotic factors, depth (m),percentage mud, latitude, longitude, and latitude X longitude. Biomass and abundances were loglo + 1 transformed, and percentage mud was arcsine square-root transformed. 'p < 0.05; "p c 0.001; ns: not significant

I B~ologicalvariables Environmental variables Depth (m)" % ~ud~LatitudeC LongitudeC Latitude X Longituder

Species richness ns ns ns 0.27" ns Abundance ns ns ns 0.21 ' ns Total biomass ns n s -0.23 ' ns ns Scavengers/carnivores -0.20' ns ns 0.25" ns Suspension-feeders ns -0.26" ns 0.25. ns Sessile megabenthos ns -0.27" ns 0.25. ns Mobile megabenthos ns ns ns ns 0.20' Small megabenthos ns ns -0.21 ns ns Medium megabenthos ns -0.20' ns ns ns Large megabenthos ns -0.22' ns ns ns Deposit-feeders ns ns ns ns 0.23' Spatangoida ns ns -0.19' ns 0.21' Porlfera ns -0.30" ns ns ns Zoantharians ns ns -0.23' 0.28 -0.48" Bivalvia ns ns 0.23' ns ns Decapoda ns ns ns ns ns

% Mud, latitude and longitude partialled out "~epth,latitude and longitude partlalled out '% Mud and depth partialled out of the correlation

Community structure and distribution patterns divided into 3 subgroups (station subgroups 1, 2 and 3) at a cut-off level of 0.85 (Fig. 3). The similarities between stations characterised by Subgroup 1 stations were in the western Gulf the presence or absence of megabenthos species, as (Fig. 4). There were 45 stations in this subgroup, the shown in Fig. 3, indicated that there were 2 main sta- largest in station group A. The average biomass, 4.1 kg tion groups at the cut-off level of 1.26. trawl-', was mostly made up of deposit-feeders (3.2 kg Station group A was composed of 28 stations sam- trawl-') (mainly spatangoid Maretia planulata and pled in the central and western Gulf. This group was spatangoida sp. 4, and the sand dollar Laganum sp. 1).

Fig. 3. Dendrogram showing sta- tion groups from the classification of the Gulf of Carpentaria beam Station Group 1 2 trawl presence or absence data Mar Ecol Prog Ser 129: 127-139, 1995

The suspension-feeders (0.49 kg trawl-') were mainly bivalve molluscs (0.17 kg trawl-l) and sponges (0.11 kg trawl-'). Most of the scavenger/carnivores were deca- pod crustaceans (0.27 kg trawl-') (Table 4). Subgroup 2 stations, made up of 16 stations in the central Gulf, had a megabenthos biomass trawl-' simi- lar to subgroup 1 stations, but the average abundance was lower. In contrast, the average number of species trawl-' was higher in the central (subgroup 2) than the western Gulf (subgroup 1).The biomass of subgroup 2 stations consisted mainly of suspension-feeders (2.8 kg trawl-'), which were predominantly sponges (1.66 kg trawl-') and sea pens (0.24 kg trawl-'). Deposit- feeders, which were less prevalent (0.76 kg trawl-'), were mainly spatangoids and holothurians (0.55 and 0.16 kg trawl-' respectively). The scavengers/carni- vores (0.63 kg trawl-') in subgroup 2 stations, as in subgroup 1 stations, were dominated by crustaceans (0.31 kg trawl-'). Together these 2 subgroups included 61 of the 65 stations in station group A. The 4 stations in subgroup 3 were scattered through- out the central and western Gulf (Fig. 4). The average biomass was similar to subgroup 2, with suspension- feeders dominating (Table 4).In this subgroup, sea Station group 1 Station group 2 Statlon group 11 pens dominated the suspension-feeders (0.37 kg Fig. 4. Station groups formed by the classification of mega- trawl-') whilst holothurians and deposit-feeding benthos presence or absence beam trawl data bi valves constituted most of the remaining biomass (0.2 and 0.19 kg trawl-' respectively). As with all the

Table 4. Biological and abiotic character~stics of station groups based on the presence or absence of species at the Gulf of Carpentaria stations. No. of stns: number of stations in each station group; SE: standard error; SD: standard devi.ation

Station group: 1 2 3 4 5 6 7 8 9 10 11

No. of stns 45 16 4 1 1 1 3 2 2 4 28 Biomass (kg trawl-') 4.1 4.2 1.4 0 01 60.0 168.3 210.1 0.15 13.2 59.2 11.9 (SE) (11.3) (3.6) (1.4) (-1 (-) (- (251.2) (0.06) (18 4) (55.6) (19.5) Abundance/trawl 15179 571.8 228.6 2.0 2592.8 30115.8 19876.2 46.5 275.7 61842.6 2035.1 (SE) (4726) (424) (241) (-) (-) (- (98950) (2.1) (346) (107042) (3796) Speciedtrawl (n) 48.2 69.0 34.5 1.0 47.0 11.0 18.6 20.5 27.0 41.0 94.3 (SE) (16.0) (19.8) (4.8) (-1 (-) (-1 (4.0) (0.7) (5.7) (11.0) (23.4) Depth (m) 47.6 53.0 50.3 36.8 29.0 24.2 52.9 48.9 34.9 23.0 44.2 (SDI (9.1) (5.7) (3.9) (-1 (-1 (-1 (4.0) (0.7) (1.4) (6.0) (11.2) Substrate Sand (%) 3 0 5 3 3 7 54 85 55 22 26 67 78 67

(SDI (19) (12) (15) (-1 (-1 (-1 (11) (31) (4) (8) (111 Mud (%) 6 6 4 0 5 7 31 5 4 1 7 7 71 26 13 25 (SDI (17) (11) (18) (-1 (-) (-1 (11) (35) (5) (10) (10) Bottom turb~dlty(NTU) 1.7 1.5 1.6 1.3 1.0 1.2 2.7 2.1 1.1 0.8 1.4 (SDI (0.73) (0.52) (0.52) (p) (-1 (-1 (0.26) (0.84) (0.57) (0.17 (0.72) Mobility Mobile (kg trawl-') ('XI) 94 32 4 9 0 88 100 100 63 7 56 23 Sessile (kg trawl-') (?G) 6 68 5 1 0 12 0 0 37 93 44 77 Feeding mode Suspension (kg trawl-') (X)12 67 67 0 61 0 0 52 93 4 5 8 1 Carnivore (kg trawl-') (%) 9 15 15 100 23 0 0 3 6 1 2 13 Depos~t(kg trawl-') (%) 78 18 18 0 16 100 100 12 6 53 4 Herbivore (kg trawl-') ('%) 1 0 0 0 0 0 0 0 0 0 2 Long et al.: Gulf of Carpentaria megabenthos 133

subgroups in station group A, crustaceans dominated Species groups the scavengers/carnivores (0.2 kg trawl-'). 123 45 6 78 91011 Station group B was the largest, with 79 stations. It was divided into 8 subgroups (station subgroups 4 to 11) at a cut-off level of 1.00. Subgroup 4, a single station (11) 50 km east of Groote Island, had the lowest biomass (

similarity measure of 1.27 (dendrogram not shown). sent: the sea urchins Echinoida spp. 2 and 3 and the Below this level there were many small and apparently gastropod Strombus vittatus. The major phyla were biologically meaningless groups, and above this level evenly represented (10 bryozoa, 10 coelenterata, 15 much information would be lost. The species and sta- crustacea, 15 echinodermata, 13 mollusca, 14 porifera). tion groups were arranged in a 2-way matrix to visu- Most of the species (70) were small (

Group 10 was a suite of species that were mostly l collected at Stn 57 near the western entrance to Torres Stralt. This species group represented the shallow- water megabenthos of the seagrass and algae beds of western Torres Strait. The biomass consisted mainly of algae (Caulerpa mexicana and C. racemosa), seagrass (Halophila spjnulosa) and 3 large holothurians (Bohad- schia marmorata, Holothuria scabra and Stichopus horrens). The pearl shell Pinctada sugillata was also abundant. Group 11 was an assemblage of species found mainly at Stn 70 in the shallow inshore waters of the southeastern Gulf of Carpentaria. This species group included the commercially fished prawn Penaeus esculen tus. -0.4 -0.2 0.0 0.2 0.4 The 11 species groups and 2 main station groups defined a coastal zone along the eastern and south- eastern margins of the Gulf with sandy (%= 62 % sand) Fig. 6. Plots of axes 1 and 2 of the princ~palcoordinates analy- sediments and a megabenthos with a comparatively sis of 107 stations, based on species presence or absence, (+) (0) high abundance (9240 ind. trawl-') and biomass showlng main group A, composed of subgroup 1, sub- group 2, and (A)subgroup 3, and main group B, composed of (33.9 kg trawl-'). Suspension-feeders dominated in numbered subgroups 4, 5 and 6, (0) subgroup ?,(m)sub- terms of biomass (10.1 kg trawl-' after excluding the group 8, (*) subgroup 9, (0) subgroup 1, and (.) subgroup 11 large hauls of spatangoida sp. 4 from Stns 20 and 89 in station subgroup 7), with sponges, alcyonarians, zoantharians and ascidians comprising most of the centage mud and longitude correlated significantly suspension-feeder biomass. with the first principal coordinate (Table 5). Thus the The central-western Gulf zone, in contrast, had first principal coordinate represented an axis related to muddier sediments (%= 59% mud), low abundance changes in species composition with respect to depth, (1200 ind. trabvl-l) and low biomass (4.0 kg trabvl-l). percentage mud and east-west trends across the Gulf. Deposit-feeders rather than suspension-feeders domi- In contrast, PC0 2 measured changes in species com- nated in this . However, sessile suspension-feed- position that were related to depth, or factors associ- ers (sponges, pennatulaceans, hydroids, zoantharians ated with depth only. The third principal component and ascidians) were found in both the coastal and the (PC0 3) measured changes in species composition central-western zones wherever hard substrate was with depth and with changes in a north-south and a present. southeast-northwest direction. Thus there were signif- Species richness was higher in the coastal zone icant changes in species assemblages in the Gulf (70 species trawl-', SE = 5.9) than in the central- which were correlated with depth and sediment grain western Gulf zone (52 species trawl-', SE = 2.4),due to size. Moreover, when depth and sediment were statis- a significant east-west trend in species richness across tically controlled in the correlation analysis, there were the Gulf. The proportion of the catch that was scav- significant changes in species assemblages across engers/carnivores was similar in the coastal and the (east-west) and down (north-south) the Gulf. central-western zones. There were few herbivorous species, and their biomass and abundance were both low. DISCUSSION

There were 2 well-defined megabenthos communi- Community structure parameters in relation to ties in the Gulf of Carpentaria: an eastern-southeastern abiotic parameters coastal community (station group B) and a central- western Gulf community (station group A). The 2 main station groups - A and B - were well The existence in the Gulf of 2 distinct zones has also separated along the first 2 axes from the principal coor- been observed for phytoplankton (Rothlisberg et al. dinate analysis of the species presence or absence data 1994) and demersal fish (Blaber et al. 1994), but the (Fig. 6). Partial correlation analysis of the 3 principal zonation was not always clear-cut for some taxocenes: coordinates (PC0 1 to PC0 3) with depth, percentage the distribution and abundance of macrobenthic mud, latltude and longitude indicated that depth, per- infauna was mainly correlated with sediment grain 136 Mar Ecol Prog Ser 129: 127-139, 1995

Table 5. Partial correlation analysis of principal coordinate scores 1 to 3 (PC0 1 to PC0 3) from the ordination of station presence or absence data, with depth (m), percentdge mud ('%Mud), latitude, longitude and latitude X longitude. ";Mud was arcsine square-root transformed. 'p < 0.05; "p < 0.01: ns: not significant

Principal coordinates Depth (m)" mud Latitudec LongltudeC Latitude xLongitudeC

PC0 1 0.35" -0.51 " ns 0.43 m ns PC0 2 0.25" ns ns ns ns PC0 3 0.45" ns -0.26" ns -0.22'

" %Mud, latitude and longitude partlalled out bDepth, latitude and longitude partialled out ' ?&Mudand depth partialled out of the correlation

size and depth (Long & Poiner 1994b), and the numer- oxygen and temperature in the water column become ically dominant squids and cephalopods in the Gulf did stratified in the central Gulf over spring and summer not show any clearly preferred depth range (Dunning and oxygen levels below 1.5 m1 1-' have been recorded et al. 1994). in the bottom waters of the central Gulf during summer The 2 main station groups were each divisible into at (Forbes 1984). Dyer et al. (1983) considered that sub- least 1 large subgroup that characterised the group stratum type, water masses and their associated plank- and several small subgroups with distinctive mega- ton, depth, temperature range and circulation patterns benthos and sediment types. The smaller subgroups were important factors which interacted in complex were mainly scattered along the nearshore margins of ways to affect the distribution patterns of the the Gulf or situated between the large subgroups and megabenthos within the . represented transition areas in the Gulf. Storm and cyclonic disturbances may also affect the The distribution of the numerically dominant Gulf distribution of the Gulf megabenthos; an average of megabenthos was correlated with sediment type, 2 cyclones pass through the Gulf every year (Lourensz depth and geographic position in the Gulf. Sediment 1981) and wave surge from a cyclone can disturb the grain size and depth were also significant factors sediments on the seabed down to 30 m (Krby-Smith & correlated with the distribution of commercial prawns Ustach 1986). Furthermore, benthos down to 40 m may in the Gulf (Somers 1994),and the megabenthos off the be affected by waves (Hall 1994). west coast of (Longhurst 1958), Israel (Gilat 1964), False Bay, South Africa (Field 1970) and the Northumberland coast, England (Buchanan 1963). In Comparisons with other studies this study, however, there were also significant spatial trends within the Gulf (east-west, north-south and The main factors that govern the abundance of ses- southeast-northwest) that were statistically indepen- sile megabenthos in the Gulf of Carpentaria appear to dent of depth and sediment grain size. be the amount of substratum suitable for sessile The spatial trends in megabenthos abundance, bio- megabenthos to attach to, and possibly the quality and mass and species richness in the Gulf may be quantity of food in the water column. Sponges, zoan- explained by the pattern and nature of the water circu- tharians, alcyonarians and ascidians were characteris- lation, which is linked to sediment distribution, cou- tic and often dominant features of coarse sediments in pled with the climate. The surface-water salinities in the eastern and southeastern coastal zones of the Gulf. the coastal (c30 m depth) may be reduced to The coastal zone has high phytoplankton production 20 psu during the wet monsoon by freshwater input equal to that of upwelling areas on continental sheIves from a 1200 000 km2 water catchment basln that emp- (Rothlisberg et al. 1994). In contrast, there were few ties via rivers into the southeast and southern Gulf sessile megabenthos in the muddy sediments of the (Torgersen et al. 1983). Moreover, the coastal waters central and western Gulf. Pennatulaceans were the (10 to 20 km offshore) are apparently largely indepen- most common sessiles in the soft sediments, and dent of the central Gulf waters due to a coastal water sponges were present when the substratum was suit- boundary layer that traps freshwater runoff, nutrients able; however, neither group was abundant. and suspended sediments (Wolanski & Ridd 1990), The link between substrate and sessile megabenthos which may partly explain why the waters of the central may be a general feature of continental shelves. Gulf are biologically independent of the coastal Sponges, alcyonarians and gorgonlans were the domi- boundary layer (Rothlisberg et al. 1994). Furthermore, nant sessile megabenthos in the coarse sediments of Long et al.. Gulf of C:arpentaria megabenthos 137

the North West Shelf off (Sainsbury of species of decapods collected by dredges or trawls in 1987). In contrast, there was little sessile megabenthos the Gulf of Carpentaria (198) was close to the number in the pelagic carbonate muds on the Scott Reef-Row- in the South Atlantic Bight (184 species; Wenner & ley Shoals platform off northwest Australia (McLough- Read 1982), less than on the North West Shelf of west- lin et al. 1988). Sponges, ascidians, alcyonarians and ern Australia (308 species; Ward & Rainer 1988) and hydroids were abundant in the coarse sediments of the higher than off tropical West Africa (143 species; deeper (50 to 70 m) waters of the northern Great Bar- Longhurst 1958), in the northern Great Barrier Reef rier Reef lagoon, and the shallow banks (15 to 20 m) lagoon (108 species; Blaber et al. 1993), the were dominated by the bivalve Pinctada spp. and (24 species; Neyman 1960), the North Sea (18 species; macroalgae, mainly Caulerpa spp. (Blaber et al. 1993). Holme 1966), Scottish lochs (5 species; McIntyre 1956) In contrast, there were few sessile megabenthos in the and Antarctica (2 species; Dell 1972). muddy inshore areas. Over 840 species of megabenthos were recorded in If the availability of suitable substrate is a main the Gulf of Carpentaria trawl samples. In a similar factor, one would expect that, on a global scale, there study of the megabenthos of the northern Great Barrier would be a latitudinal gradient in the standing biomass Reef lagoon, more than 600 species were recorded of sessile megabenthos that correlates with the from 100 beam trawls (Blaber et al. 1993). In contrast, increase in rock and gravel on the continental shelves only 200 species were identified from 60 beam trawls with increase in latitude (Hayes 1967). This simple taken in the Bering Sea in a study area approximately relationship would be locally modified by the presence 550000 km2 (Neyman 1960), although the Bering Sea of niches, rugosity, patchiness and heterogeneity of the is an area of high primary and secondary production substrate, quality and quantity of food, and regionally with a large benthic standing biomass (McRoy et al. by the presence or absence of upwelling. 1972, Alton 1974). The relationship between standing biomass, turn- The results of the above species-richness compar- over rate and water temperature must also be consid- isons must necessarily be interpreted cautiously. How- ered. There is evidence that the standing biomass of ever, they indicate that the relationship between sessile megabenthos is high at high latitudes: in some megabenthos abundance and species richness, and areas of the , Antarctica, with upwelling, high geographical location and environmental factors is productivity and large numbers of ice-rafted rocks and complex; there may not be a simple link between spe- boulders, the megabenthos forms a dense and continu- cies richness and latitudinal gradients in the subtidal ous bed of sessile sponges, ascidians and bryozoans marine environment (see Longhurst & Pauly 1987, (Bullivant 1967). At Point Barrow, Alaska, the mega- Alongi 1990). fauna taken by dredge in the deeper water in areas of The megabenthos species that were widely distrib- rubble was dominated by ascidians and sponges uted in the Gulf of Carpentaria were also widely dis- (MacGinitie 1955). On the plateau of the grand banks tributed throughout the Indo-West Pacific. Blaber et al. off Newfoundland-Labrador, Canada, which has high (1994) found that the numerically dominant and wide- primary productivity and abundant ice-rafted boulders spread fish species in the Gulf were also characteristic and stones, a dense sessile megabenthos of porifera of shallow (<80 m), soft-bottom trawl grounds through- and mussels was trawled (Nesis 1970). This may partly out the Indo-West Pacific region. Similarly, Dunning et explain Tseythin's (1987) findings that the biomass of al. (1994) found that many of the demersal cephalo- megafauna in cold-water regions was greater than that pods of the Gulf of Carpentaria had an Indo-West in warm-water regions by a factor of 20. Pacific distribution. The results for the megabenthos The species richness of the sessile megabenthos was conform with the consensus that the dominant marine moderately high in the warm waters of the Gulf of fauna in the Gulf of Carpentaria is part of a wider Indo- Carpentaria (102 porifera, 35 ascidiacea and 28 bryo- West Pacific fauna. zoa) and the northern Great Barrier Reef lagoon (64 sponges, 48 ascidians and 24 bryozoans; Blaber et al. 1993). Species richness, however, was higher in the Acknowledgements. We are grateful for the assistance of all Antarctic where 294 porifera, 92 ascidiacea and participants in Cruise SS03/90 of FRV 'Southern Surveyor' The work was partly funded by FIRDC grants 88/77 and 321 bryozoa were collected mostly by trawl or dredge 90/29. Special thanks to S. Blaber, D. Brewer and 2 external (Dell 1972). 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This article was presented by G. F. Humphrey (Senior Manuscnpt first recejved: November 24, 1994 Edjtonal Advisor), Sydney, Australia Revised vei-sjon accepted. June 22, 1995