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Ecology of tropical soft-botlom benthos: a review with emphasis on emerging concepts * Daniel M. A10ngi Australian Institute of Marine Science, P.M.B. No. 3, Townsville M.C., Queensland 4810, Australia. (Ree. 9-1-1989. Acep. 9-111-1989) Abstraet: A rcview of the tropical soft-bottom literature reveals that several general concepts in benthic ecology. formulated mainly from temperate work, are either in need oí modification oc are no! readily applicable to tropica l benthic ecosystcms. Severa! coocepts emerge from the pIesent tropical líterature suggesting that in comparison with temperate cornmunities: (1) species diversity and faunal densities are no! necessarily grcater in the teopies, (2) environmental stress (excluding anthropogenic input) is genera1ly more sevcre, (3) ¡nfaunal communities are composed of proportionately more small opportunistic species (4) predation by demersal fishes and crustaceans is more intense, (5) microbes may be a carbon sink in sorne shallow-water habitats. notably mangroves. (6) production is general1y high, but breeding and reproduc tíon are frequently nor continuous and,(7) the distributlon and abundances al' tropiCal benthos, like most other communities, rcflcct temporal and spatial mosaies of major regulatory factors (competition, predation, food supp!y, environmcntal disturbances). Severa! tropical marine ecosystems such as rnangroves and coral reefs are unique, and other environments such as continental she!ves possess several common features which distinguish them to sorne degree from their tempcrate counterparts. To confirm, reject or modify these emerging coneepts, severa! aspects oC tropical benthic ecosystems require further study. including effects of wet season activity, physiological tolcrances, nutrient recycling, secondary production, bentluc-pelagíc coupling and pollution. Such infor mation and emerging conccptualizations are neeessary to permit proper and ¡nformed conservation and management of these uniquc ccosystems. MDst research on benthic cornmunities has been conducted in temperate, and to a lesser extent, subtropical and boreal latitudes (Gray bottom benthic assemblages in (Longhurst lhe tropics & Pauly 1987). Kecent reviews of tropical estuarine and 198 1). Lack of benthic research in tropical marine ecosystems (De La Cruz 1986; A10ngi regiDns is due to rnany reasons, including dis 1989a; Hatcher, Johannes tance of major oceanographic centers from the tropics, lack of funding and inadequate facili ties. Research into tropical marine benthic eco these systems for the future of mankind, both as significant contributors to the global carbon cycle and as nursery grounds for commercially systems has, over the past four decades, been viable sporadic and, until recently, conducted mainly Hatcher, Johannes during short-term developed visits countries. by Most workers & Robertson 1989) have c1ear1y underscored the importance of species. Indeed, as pointed out by & Robertson (1989), conser from vation and management of tropical soft-bottom early (pre-1970) systems is critical, but the information neces work was necessarily taxonomic or consisted of sary compilations of faunal lists. Hence, little com conservation strategies is insufficient. to pennit the development of proper prehensive work exists detai1.ing factors affect The purpose of lhis review is to briefly ing the distribution and abundance of soft- examine the existing tropical infaunalliterature * reefs, see review of A10ngi 1989a) and to make (but gene rally excJuding mangroves and coral Contribution No. 451 from thc Australian lnsti tute of Marine Sciencc. evident how unique these cornrnunities are, to 86 REVISTA DE BIOLOGIA TROPICAL formulate a preliminary synlhesis of eeological concepts presently emerging from the study of infaunal cornmunities in the trapies. Table I provides a comprehensive, but not faunal and macroinfaunal assemblages in tropi cal intertidal habitats. Abundances of hetero trophic microbial communities are not included because nearly all microbial slUdies to date in the tropics have utilized antiquated techniques such as plate counts. Only recent studies have used modern, more eeliable, techniques 5uch as diceet counts by epifluorescence microscopy and uptake of radionucHdes to estimate micro bial activity (Aiongi I 988a.b). Data on tropical microalgal abundances (as chlor9phyll a) does exist, bUl most of the informatian is incompa rable because of lack of standardized units (e.g., ,..g Ch! a per g_1 sediment dry weight, per g_1 sediment wet weight, per wet ce, per dry ce, etc). However, lhe comparable intertidal data, mostly from mangrove and coral reef sediments, 1 O ,..g ChIa g_l (see review conditions are more Early tropical studies of intertidaJ macroin exhaustive, compilation' of densities of roeio densities environmental quiescent. Abundances. community composition and intertida! zonation. suggests low « algae and plant detritus (Grelet el al. 1987), and where Tropical Intertidal Assemblages algal abundances are greater in coral reer lagoons where sed,iments are rich in micro· and macro· DW) miero Alongi 1989a). fauna were concerned mainly with zonation of 'species associations', c1assifying low, mid and high intertidal zones on the basis of the domi nant speeies (Dahl 1953; Gauld & Buchanan 1956; Berry 1964; Mclntyre 1968; Vohra 1971). This reOects the application of Thorson's (1957) parallel-bottom community concept -dominant during this time period- to tropical intertidal habitats. Later studies (e.g. Dexter 1972,1974,1979; Shehon & Robertson 1981) focussed habitats on in comparing terms of different species intertidal composition, abundance, biomass and diversity. It is elear that tropical intertidal habitats differ greatly in macrofaunal community composition from temperate intertidaJ assem· blages. Tropical sandy beaches and sand f1ats are dominated by crustaceans. mainly crabs, and bivalve molluscs and tropical mudOats have a high proportion of polychaetes and micro crustaceans, whereas temperate intertidal habi· tats consist mainly of polychaetes and gastropod molluscs (see Swennen, Duiven & Spaans 1982; Vargas 1987). Why? Sevoral behavioural studies The data summarized in Table I must be have suggested that tropical crustaceans and considered with caution owing to the different sampling techniques and sieve sizes used, and rapid escape mechanisms than their temperate sediment depths. pre-1975 papees examined did nol gjve sufficient infor In fact, many mation to assess densities per unit area or vol· ume. Nevertheless, it is reasonably elear that the highest faunal densities occur in sheltered habitats, whereas the lowest densities are found in exposed, coarse sandy beaches. As reflected in lhe tables, most tropical intertidal studies have been conducted on sandy beaches. Densities of meiofauna and small « macroinfauna in tropical 5 mm) unvegetated tida! flats and beaches are generally greater than in mangroves where densities of both faunal groups average from < 500 individuals 10 cm-' and < 10 to several hundred animals m -', respectively (Grelet el al 1987; Alongi 1989a). bivalves are more motile and possess more counterparts to avoid high temperatures, salinity and dessication in the tropics (Ansell & Trevallion 1969; McLusky el al 1975; Jones 1979). Ocypodid crabs and teUinid bivalves, espeeially of the genus Donax appear to be particularly well adapted to life in lhe tropical intertidal and severaJ studies have documented their unique life histories (Wade 1967; Ansell & Trevallion 1969; Ansell & Trevallion 1969; Ansell el al 1972; McLusky el al 1972). However, differeoces do exist amoog tropical intertidal regioos, as showo in the comparative studies of Dexter (1972, Shelton into 1974, 1979) and & Robertson (1981) and must be taken account before generalizations can be made. The lower densities in mangroves are probably due to a variety of factors, including possible negative Seasonality and effeets of monsoons. effects of maUgI'ove-derived tannins and lower concentrations of interstitial oxygen Maoy of the above·cited studies were con and surface microalgae. In cuntrast, infaunal ducted during only one season, further limiting TABLE 1 Densities of meiofauna (No. lndilliduals 10 cm-1) and macroinfauna (No. Individuals m-2) in sorne tropical intertidal habitats x = mean: range Macroinfauna Reference x = 4245;0-40,064 Gauld & Buchanan 1956 ¡¡ = 2626; 420-3815 ¡¡ = 1405; 395-1010 x = 1305; 176-3360 ¡¡ = 2688; 2656-2720 McTntyre 1968 ¡¡ = 1307; 968-1960 x=208; 127-368 Meiofauna Locality Habitat Gold coast. Africa sandy beach Porto Novo,lndia estuarine sand bar marine sandy beach moderately exposed sandy beach Lebanon Tunisia Moroeeo sandy beaches x=27;7-68 x= 196; 101-409 x= 148; 29-570 Shimmary Beach, Panama, Caribbean Sea carbonate beach x Naos Is. Panama Pacific ccast quartz sandy beach x = 2439;1507-4067 Singapore sheltered sandy beach x=5614; 1678-9236 Andaman & Nicobar Istand. Indian Dccan sandflat 600-800 Rao 1975 Shertallai. India sandy beach X =467; 215-1337 Muruo, WeUs & Mclntyre 1978 Cebu. Phillipines sandy beach x= 739;417-1142 Natividad 1979 Texas. USA sandy beach Hulings 1971 � 193; 25-463 x= 2700; 335-6616 Dextcr 1972; 1974; 1979 Vohra 1974 Shelton & Robertson 1981 x = 219;118-381 Rama Sarma& Chandra Mohan 1981 Bay of BcngaL India coarse sandy beach Malaysia mudflats Sungei Bulok Kuala Selangor x = 129 x =479 Broom 1982 Surinam, S. America exposed mudflat x=245 Swennen, Duiven & Spaamo 1982 Mombasa. Kenya sandy beaches x = 1790; 1013-2·560 i =993; 592-2736 Ansari, Ingole & Parulekar 1984 Punta Morales, Gulf of Nicoya, Costa Rica mudflat ¡¡ =898; 653-1078 Gulf of Aquaba. Arabia Sea seagrass beds; fringing coral sands x=2369;922-4239 Gulf of Nicoya, Costa Rica mudflat Queensland. Australia sandflat x =841; 324-4000 de la Cruz & Vargas 1987 x = 1547; 815-4106 Grelet et 011987 x= 13,827; 3787-41086 Vargas 1987 Alongi 1988b comparisons between and arnong intertidal habitats. Extensive studies conducted in the Indo-Paeifie (Ganapati & Rao 1962; Trevallion & & Solís-Weiss extant (J ackson 1973; Heek 1979; Young Young 1982; Ibañez-Aguirre 1986). el al 1970; Ansell el al 1972; Natividad 1979; Despite variations in sieve sizes and sampling & Chandra Mohan 1981; Broom gea!. it is clear that the eastem Indian coastline 1982; Rarkantra & I'arulekar 1985) have, however, examined seasonality and the effects (e.g. Bay of Bengal) harboes a numerieally rieh of stressed western eoast (Datta Rama Sanna monsoons season on intertidal infauna. Whidl (or seasons) organisms exhibít theír infauna compared to the more highly polluted, & Sarangi 1986). As discussed later, species diversity is gene rally highest abundances appeaes to be dependent on low refleeting whether the habitat in question is in the wet or species. Outside India, the data indica te poor to dry tropies. Meiofauna and maeroinfauna suffer moderately increased mortality during monsoons in the wet 1972a,b) with numerieal riehness apparently related to various stresses such as low salinity tropies due to lhe fluetuations in salinity and sediment erosiono In the dry tropics, organisms usually attain peak densities in autumn and and lhe dominanee of opportunistie rieh oxygen infaunal assemblages (Wade and low plankton produetion (Holm 1978). The winter; lowest abundances occur in spring and Indian studies reveal that tropical eover subtidal infauna respond negatively to freshwater Infaunal eommunities also respond differ inputs during the monsoons, but recolonize quiekly and are dominated by small, surfaee summer when the absenee of cloud results in sediments wanning to > 30° C. ently depending upon lhe intensity and fre quency of the monsoons. For example, in India where cyclones are very intense and prolonged, meiofaunal densities northeastern Australia decrease rapidly. In where monsoons are deposit- and suspension-feeding polyehaetes and bivalves. As with temperate benthos, eoastal lagoons in the tropies have also been poorly studied with most studies eomprised of faunal lists rather lhan information on generally not as prolonged or as intense as lhose density. biomass and species diversity (Moore in Soulheast Asia or on lhe Indian subeon el al 1968: Holm 1978). The study by Holm tinent, meiofaunal densities in tropical man� (1978) indicates a potentially rich epi- and grove estuaries were most abundant during the infauna in moderately flushed lagoons Iined summer monsoons occur in different seasons in these vegetation. Densitites of with submerged maerofauna ranged from 1059 to 4700 per m' in a seagrass flat (Moore el al 1968). In other different tropical lagoons not well flushed, high saline wet season (Alongi 1989a). These eontrasting results may refleet lhe faet lhat climatologieal zones. Interestingly, both meiofaunal and macroinfaunal cornmuni waters up to 300 ties appear to be able to repopulate quickly Priee following climatologieal disturbanees in lhe tropies. The magnitude of seasonal fluetuations is also dependent upon distanee from lhe equator. Habitats bordering lhe equator, for instance, in Malaysia (Broom 1982) exhibit less seasonal variability lhan tropical assemblages closer to lhe poles, refleeting lhe faet lhat elimate and its effeet on intertidal and shallow subtidal bentho, varie, greatly wilhin lhe tropies. o /00 are common. Jones, & Hughs (i 978) found a moderately abundant maerofauna (> 500 m-') in a saline lagoon nn lhe Saudi Arabian coast of lhe Arabian Gulf. Diveesity Cas number of species) was low ( < 15). These moderately high densi ties, despite the high salinity, were attributed to eomparatively high primary production. No! surprisingly, lhere is very Httle informa tion on funetional relationships among tropical subtidal assemblages. In one of the few ,ueh studies, Blaek & Peteeson (1988) examined possible adult-Iarval interaetions and competi Tropical coasta! assemblages (estuaries, lagoons, seagrass beds and neushore subtidal). tion between suspension.feeding bivaJves and smaller found Most studies of benthie eommunities tropical estuaries and shallow « in infauna in Western Australia, that the suspension-feeding They bivalves exhibited no evident competitive interactions 50 m) near with, or provided any positive enhancement of, shore regions have been conducted in India the establishment of the smaller infauna. These (Table 2). Comparatively few eomprehensive results, if found in other tropical subtidal habitats, would help to explain the coexistence benthie studies of tropical seagrass beds are 89 ALONGl: Tropical soft-bottom benthos. TABLE 2 Densities 01 meiolaulUl (No. Individuals 10 cm'1) and macrolaurw. [No. m'l) in some tropical shallow·water, coastal habitats (estuarine and marine) x�mean, range Meiofauna Macroinfauna Reference x = 240; 148-433 Wade 1972a Locality Habitat Depth 1m) Kingston Harbor, Jamaica marine muds 6-16 Shatt-al-atab, Arabian Gu1f carbonate muds 7·15 x =227; 24-599 Saad & Arlt, 1977 Karwar. central west coast of India estuarine muddy sands and mud 6·' i= 1098; 1022-1250 Ansari 1978 Madovi,Cumbarjua Zuari estuanes. Goa, India estuarine sand to mud 4·7 Nannada Estuary, India estuaríne sand to mud 3-19 x=53;0-3164 X =592; 0-4049 Krishna,Godavari, Mahanadi, Hooghly rivers,E. coast India marine muds 15-50 x =813; 502·2149 x=1529;90-4785 Aman et al 1982 Carrie Bow Cay, Belize Thalassio meadows 1·2 i=12,167 Young & Young 1982 Carrie Bow Cay, Belize bore sand 1·2 ;=16.750 Young & Young 1982 Gautami-Godavari estuaries,India estuaríne muddy sands <10 x = 1386; 245-2194 South Gujarat estuaries, India estuaríne sands to mud < 10 x = 138; 0-5334 x = 122; 0-880 Vellar Estuary. Porto Novo,India estuarine sand 2·5 x=90; 33-213 x= 5 1 (premonsoon) x =87 (monsoon) x = 46 (postmonsoon) x=94; 78- 1 1 2 ¡¡ =80 (premonsoon) x=99 (momoon) x =75 (postmonsoon) Fernanoo. Khan & Kasínathan 1983 Gulf of Nicoya, Costa Rica marine mUddy sands +mud 6-65 x"" 1269; 0-8744 Maurer & Vargas 1984 Goa,Coast, India estuarine muddy sands 10 i = 1703; 475-310 (premonsoon) Harkantra & Parulekar 1985 \Vest Bonga!, India estuarme .and, i =620; 100-2420 Dalla & Sarangi 1986 < x = 1653; 492-4382 (total) x =528; 93-2120 (monsoonal) of �lIrface suspension-and deposit-feeders in tropical seafloors. Varshney. Govinúan & Desai 1981 Kondalarao 1983 r=810; 71·1495 (postmonsoon) 10 Parulekar, Dhargelkar & Singba1 1980 Govindan, Varshney & Desai. 1983 limiting inter-shelf comparisons. Macroinfaunal densities naturally vary widely depending upon sediment type and water depth. There is very little information on meiofaunal densities and biomass on tropical shelves (Table 3). Tropical continental shelves Benthic nental inforrnation shelves on tropicaJ conti have likewise be en obtained mainly by Soviet and Indian scientists work Despite the lack of data, tbere appear to be several characteristics common to most if not all tropical shelves which distinguish tbem, to a limited degree, from temperate continental ing in the lndian ocean (Table 3). Consequently, shelves: sampling procedures (different sediment depths; 1. Faunal composition appeaes to be domi the same problems regarding sieve sizes and dry weight vs. wet weight biom.ss) prevail. nated by large epibentbos, lancelets (e.g. TABLE 3 Densiries (No. JOcm'l) and biomass (mg DW 10 cm-') ofmeio/auna and macroinfauna (No. m-l and g DW or g WW m'l ) on sorne tropical continental she/lles. x =mean: range Locality Depth (m) Guinea & Senegal shelves, West Afriea Mozambique Channel, S.E. ACríea N.W.lndian shelf Density Macroinfauna Hiornass Density Meiofauna Biomass 2().350 < 31 Reference 15-\386 1.5-74.2 Longhurst 1958,1959 J2.3ll 5.8-25 Makarov & Averín 1968 x =20; Neyman 1969 20-350 I().IOO (WW) S.W. Indian shelf 2().350 West Pakistan shelf 20-200 Persian Gulf ;;; =5; ().IO (WW) 196-224 2().50 Bay of Bengal -2().170 -x= 163; 13-963 Cap Blanc, Spanish Sahara Coa Coast, India Savich 1970 x = 25 30-35 (WW) Neyman & Kondritskiy 1974 Ansari et 01 1977 ;<=2.5; x= 517: x=92; 0.2-11 58-1644 O,Of-36.5 x=8124; x= 26.3; 2.4-94.4 (WW) 3().1830 2().840 10-15 1635-35200 :i = 1217 Nichols & Rowe .1977 Ansari, Parulekar & ,.250-2925 West Indian shelf 10-70 Andaman Sea. Indian Ocean 1().200 Malacca Strait. Indian Ocean 8().320 Arabian Sea Andaman Sea Bay of Bengal 10-250 10-250 Hong Kong Shelf x=708; 543-985 ¡ :::::248 ; 68-438 ¡=149; 7-200 2().l50 "-- - ¡= 14: 3.6-32.8 (WW) ;: = 9; 1-27 (WW) x = 258; 118-528 = 1.4 (DW) =0.9 (DW) Jagtap 1980 x= 10; 1-37 (WW) Parulekar.l Ansari x = 15; 0.2-71 (WW) Ansari &. Paro lekar x =0.9 (DW) Rodrigues, Harkantra. Parulekar 1982 ;¡ =0.8 (DW) =0.2 (DW) 13-23 >;:= 1 1 . 5; 5.5-24.5 (WW) ;¡ = 1.8 (DW) x= 101: 55-183 1981 1981 x= 35; 12-155 (WW) Shin tl Thomspon 1982 Ea!t African Shelf(l2-170N) lO-lOO x =3.5; 0.5-17.6 (DW) Dornain 1982 Pero shelf 35-350 x=45.3; 1-247 (WW) Rosenberg el al Java. Indian Ocean 5-30 1983 Warwick &. Ruswahyuni 1987 x=463; llO-739 BranchioSloma) and 1aege Foearninifeea such al watee-column tumovee (Bukatin as Marginopora and Alveoline/la (Buchanan 1982; 1958: Longhuest 1958: Gosselck 1975); 2. Across shelf variations are due to severa! Longhuest 1983; Alongi el al 1989b); 3. Infaunal communities aee composed mainly of small ( < 5_0 mm) oppoetunists that aee ate production and sedi.mentation, infringe. sueface deposit-and suspension-feedees_ This appears to be an adaptive strategy lo re ment of water movement and lack of season- spond quickly to erratic inputs of nutrients, factoes, including gradients in depth, caebon 91 ALONGI: Tropical 8Oft-bottom bcnthos. low water content (i.e. hlgh compaction) of mostly quartz and carbonate sand and gener ' ally warm ( > 20 C) sea temperatures (Longhurst 1959: Harkantra el al 1980; Alongi 1989b); 4. One common feature of subtropical and tropical shelves is an abundant demersal fisheries, shrimps including (Ghana, penaeid Buchanan Leone, Longhurst prawns 1958; and Sierra 1983; India, Harkantra however, densities of bacteria, meiofauna and macroinfauna are comparatively high due to a more stable seafloor phytoplankton and sedimentation of blooms caused by nutrient runoff from lhe Amazon (Table 4). AlIer & AlIer (1986) conc1uded lhat lhe hlgh degree of physical disturbance coupled wilh low food inputs appear to be the major factors regulat ing benthic infauna OR lhe Amazon shelf. In contrast, densities of lhe major groups el al 1980; Georgia Bight, Hanson el al 1981; are higher on lhe central Great Barrier Reef Gulf ofMéxico, shelf .probably due 10 lhe more stable seabed and gre.ter food input, particularly from export Yingst and Rhoads 1985; central Great Barrier Reef, Alongi 1989b), Demersal populations are probably abundant on these shelves due to the predominantly shallow sandy bottoms and abundant epibenthic and surface infaunal prey; 5. Tropical continental shelves are generally of detritus from individual reefs of the GBR on the outer shelf. Alongi ( I989b) conc1uded that intermediate levels of food input and physical disturbances from trawling and storms regulate benthic standing crops and processes on lhe shallow, driven by ¡ntermittent ¡ntrusions of central shelf. upwelled, nutrient-rich water and by estua rine outwelling of detritus (probably of low Although sedimentary organic carbon and nitrogen concentrations on the Amazon shelf nitrogen content) and input of reef detritus are on average much higher than on the central of quality. GBR tropical compared to levels on temperate shelves and generally Bentltic hlgher communities nutritional thrive in shelf (Table 4), !he arnounts are low upwelling regions off Panama (Lee 1978), other tropical shelves influenced by upwelling Peru (Tarazona e l al. 1988) and northeast Africa (Nichols & Rowe 1977), and in areas (Longhurst & Pauly, 1987). Iimitation and moderate to Detrital food hlgh levels of of massive estuarine outwelling such as off physical India (Neyrnan 1969; Ansari el al. 1977). features of the benthic regimes of both of these tropical continental shelves. Interestingly, lhese shelves are cornmonly disturbance appear to be common subjected to anoxia (with deleterious effects on lhe benthos) if estuarine inputs and upwelling events occur on a massive scale, too frequently, or occur simultaneousiy wilh periods of stagnant water mass, for instance, off lhe Amazon (Aller & Aller 1986) and off Peru (Rosenberg el al 1983). Emerging concepts It is hoped lhat the reader has developed, at thls stage, sorne appreciation of tropical sea floor habitats, the assembIages lhat tive in them, and how general concepts in benlhic ecology (diversily, stress, disturbances, see Gray 198 1) may have evolved wilh fewer Comparison of two tropical benlhic regimes: lhe Amazon and central Great Barríer misconceptions and to a more advanced state, if tropical information was obtained and/or Reef continental shelves. considered earHer. In trus section, I discuss The comprehensive studies of Aller and AlIer how sorne major preconceptions in ternpe· rate benthlc ecology, when applied lo Ihe ( I 986) on the Amazon shelf and by Alongi tropics, are either in need of modification or (1989b) on are not valid. the central Great Barrier Reef shelf provide a rare opportunily to compare and contrast tropical benthlc biocoenoses from l . Species diversity and faunal densities are different biogeographical provinces. The inner IlOI greater in the tropic� The tropics is not a Amazon shelf is dominated by massive runoff homogenous part of !he biosphere. Thus it is not surposing that lhe greatest range of faunal and sedimentation from the Amazon river and neae the over mouth, resulting in an unstable seabed causing either very low or totalIy absent densities and species diversity occurs here. Older ecological studies conducted primarily in infaunal communities. Away from the mouth, rainforests and on coral reefs indicated hlghest REVISTA DE BIOLOGIA TROPICAL 92 TABLE 4 Comparison ofmajor ¡n[aunal groups and sedimentary choracteristics on rhe Amazon (Aller & Aller and central Great Barrier Ree! (Alongi J 989b) continental shelves Depth range Amazon GBR O-52 15-46 (m) 1986) x= 7.5 ; 1 .3-2 1 x 10' Bacteria (g'1 DW) Meiofauna (No. 1 0 cm'l) x =282; 1l-2045 x= 1520; 810-3255 Macroinfauna (No. m-l) x= 1038; 59-3953 x = 2530; 2061l-5406 organic e (% DW) x =0.6; 0.07-0.84 ¡¡=0.25; 0.05-0.71 organic N (% DW) x = 0.10; 0.01-0.14 ;¡ =0.06; 0.02-0 . 1 3 6.0 4 .2 ----- e/N ratio Major SOUIces of detritus ¡ntermittant phytoplankton blooms; [iver runaff of pLant detritus river runaff of mangrove + terrestrial debris; low phytoplankton input; reef detritus + mucus Seafloor unstable. fluid muds to mid shelf; a high physical disturbances from IaIger river discharges: inner shelf muds to outer shelf quartz sand stable. compacted sand with low Hl O content; across-shelf gradient of terrigenous to carbonate facies; physical disturbances from trawling and cyclones; river input limited to inner shelf species richness per habitat in the tropics, espe disturbances than temperate habitats, aceount cially in the lndo-Pacific region (see MacArthur ing for the wider variation in species diversity 1 972). Sandees (1968, 1969) hypothesized Ihat of tropical benthos. For instanee} in rny experi physically stable, berugo envirooments such as ence, intertidal regions in the tropies are fairly the tropics favor development of more complex, inhospitable plaees, where organisms are with eommonly subjeeted to very high ( > 300 C ) stressed and/or younger environments where temperatures, desiccation, low dissolved oxygen levels, low food supp1y, ehemic.al plant defenses species-rich communities compared physieal faetoes dominat. in regulating com munity evolution. Even eaclier, Thoeson (1955, and monsoons. These conditions are gene rally 195 7) suggested that species richness increases not borne markedly toward the equator for epifauna of Alongi (1987) found 10w to moderate species hard·bottoms, allhough infaunal diveesity and low number of species per habitat assemblages. .r or These hypolheses have had Iheir detraetoes ments in northem Australia, supporting Hilary & Waltees 1979), but modifieation Moore's (1972) earlier conlention Ihat tropical and refinement of Ihe stability-time idea by intertidal organismsare, on average, subjected to (e.g. Abele not foc by temperate organisms. lndeed, free-living nematodes in mangrove sedi Huston (1979) has attained a general level of greater physieal stress than their temperate acceptanee. Huston's hypothesis is not eentered eounterparts. Olher benthie studies have found around comparisons 10w or moderate species diversity in the tropies of temperate-tropical speeies diversity, but in predieting variations in again depending on degree of physieal stress within-habitat diveesity as eaused by varying (Johnson 1 970; Wade 1972a,b; Spight 1977; levels of disturbance. The model is latitudinally Maurer independent and has provided sorne relief from 1987). & Vargas 19 84; Warwiek & Ruswahyuni Ihe old ideas of tropical vs. temperate diveesity gradients. [ propose that benthie habltats in the tropies are subjected to a wider range of environmental 2. Envíronmental stress is generally greater in the tropies. [n addition to the evidence eited aboye} other examples can be provided to .A, T,ONGI: Tropical soft·bottom benthos. support this supposition. Shallow inshore eom munities in regions of ¡ntense wet season activ· ity and upwelling are frequently smothered by massive riverine sedimentation (AUcr & Aller 1986) or by erosion of estuarine mudbanks (Seshapp. 1953). They are frequently stressed by low sa!inity (Goodbody 1961) and by anoxia eaused by Ihe impingement of coasta! water masses during he.vy continental runoff and subsequent diatom blooms (Harper et al 1981; Nichols 1976a,b: Rosenberg et al 1983). It is of interest to note that most physiologieal studies have indicated tbat tropical organisms are generally not more tolerant to environmen· ta! stress than temperate organisms (Mayer 1914; Seholander et al 1953). 3. Tropical infaunal assemblages are composed of proportionately more opportunistic, small sized species. Nearly all of the benthic studies cited earHer suggest that conditions on tropical seafloors eommonly lead to the development of pioneering seres and retardation of the estab· Jishment of large, equilibrium assemblages. Low food supply (oligotrophy), freshwater input, heavy sedimentation or erosion, anoxia, poUution, compaction of carbonate sediments and ctimatic disturbanees (cyelones and monsoons) are generally more typiea! of sub tropical and tropical shelves and coastal areas than in temperate latitudes. Pioneering assem· blages are well adapted to respond quickly to patehy food inputs as well as to repopulate after disturbanee, Ihat is, they exhibit a high degree of resilieney. The faet Ihat many such organisms are surface or water·column feeders ,uggests advantages in eompeting for Ihe lirnited rood supply depositing to Ihe seafioor. The relative abundance of surfaee infauna! feeders can lead to greater rates of predation by epi fauna and demersal organisms. Indeed, as men· tioned eartier, demersa! fishing industries are reasonably well developed on most shallow, warm water shelve, (Longhurst & Pauly 1987). 4. Predation is more intense in the trapies, This early eeologieal paradigm (MaeArthur 1972) appears to hold true for tropiea! benthos. Nearly all of Ihe evidenee i, eireum,tantia! and based 00 studies of predation pressure on intertida! gastropods (Bertness et al. 1981 and referenees within). Yermeij (1978) argued Ihat narrow or oeeluded apertures, thiekened shells, 93 low spires and more pointed seulpture on molluse shells increases from temperate to tro pica! seas. Experiments conducted in Panarna and New England e1early indicate that shell erushing predation of intertida! gastropds is greater in Ihe tropies. Interestingly, greater predation appears to modify behaviour of tropiea! intertida! gastropod" sueh as eurtailing Iheir foraging time (Bertness, Garrity & Levings 1981). OnIy one similar experiment has been con dueted with tropiea! mudflat infauna (Vargas 1988), Ihe results of which suggest that Ihe role of maeropredators (birds, erabs, fish) in con trolling eommunity strueture is relatively minoro However, a large body of correlative evidence indicates a positive relationship between in fauna! biomass and total fish eateh on many subtropieal and tropical shelves (Longhurst 1959; Savieh 1971; Harkantra et al 1980; Rosenberg et al. 1983; Tarazona et al. 1988). Additiona! eireumstantia! evidenee is provided by Ihe greater proportion of predatory nematodes in coral reefs and mangroves in Austra!ia (Alongi 1987, 1989b) in eomparison with temperate nematode cornmunities. Other trophic relationships sueh as eompeu tion, cornmensalism and amensalism remain too poorly understood for tropiea! infauna to elueidate pattems and to compare wilh trophie interactions among temperate infauna. 5. Microbes may be a sink for carbon in some Reeent earbon flux models of mangrove and coral reef benthos suggest an enormous rate of reeyeling between microbes and sedimentary carbon pools, implying that mierobes are a sink or long term ,hunt for earbon in Ihese systems (Stanley et al 1987; Alongi 1988a,b,e; 1989b; Boto, Alongi & Nott 1989). This sink hypolhesis (Alongi 1988e) evolved from a large body of circumstantial evidence obtained mainly from mangroves in tropica! Australia: (1) bacteria! standing erop is very large and produetivity is high, often exeeeding Ihe rate of tata! primary produetion, (2) mieroa!ga! biomass is 10w 10 moderate, (3) Ihe biomas, of protozoan,. meiotauna and small ( < 5 mm) inlauna are low eompared to olher benthic systems, (4) the above- and below-ground partieulate earbon pools are large, but the below-ground DOC pool is low to moderate in size, (5) correlative evidence reveals relationships of tropical shallow-water benthic systems. REVISTA DE BIOLOGIA TROPICAL 94 bacterial numbers and growth rates with wide variation of PIB ratios for meiofauna and organic carbon, but not with supposed grazers, the faet that this information was derived from and, (6) amino acid and DOC fluxes across temperate organisms. Bacterial production rates the are high in the tropies, ranging from 0.01 10 5 g e m-' d-' (Alongi I989b,e). Produetion sediment-water interface are negligible unless the benthos is poisooed. After poisooing, fluxes of amino acid. and DOC from tropical estimates mangrove sediments are large, ranging from 0.08 to 0,2 and from 0.3 to 2.4 g C. m -' have been considered in another ceview (Alongi 1989b) and will not be detailed further. Need d-t, respectively. This latter evidence indicates less to say lower ¡nfaunal biomass and general1y no net export but large intemal recycling of warmer temperatures are probably at least two nutrient pools by microbenthos. for bacteria in tropical sediments I reasons for the higher baeterial standing erop and productivity in the tropics. In addition, �-glutamic acid, a non-protejo amina acid found in sorne anaerobic bacteria but not derived from mangrove root hairs, leachates, detritus or bacteria damaged during porewater extraction, is a major component of the interstitial dissolved free amino aeid pool, sugge.ting marker that this amino aeid may be lar natural baeterial mortality a and reeyeling of bacterial intraeellular pools. In faet, the composition of major amiDa acids found free in pare waters in mangroves and coral reeh and those found in intraeellular pools of sorne sulfate-reducing bacteria are very similar (Alongi 1988c). bacteria, replenishment of the DOC pool would have to depend on eontribu tiaD from bacterial consumers. However, based on the abov� ¡nfacmation, this mlght suggest that the eotice microbial-meiofaunal foad chain is, in tum, a sink for carbon particularly in subsurface sediments where consumer densities decline rapidly in (1976) for the lBP results, AnseU el al 1978; Parulekar, Dhargalkar & Singbal 1980). The study by Ansell et al (I978) indieates macro faunal turnover rates nearly ten times greater in SW India than in Seotland with annual PIB ratios ranging from 6 to 136. They suggest that these higher turnover rates reflect a lack of temperature eompensation (as suggested by early physiologieal studies) and higher rates of predation on tropical organisms. A larger volume of data indieates that in comparison with tempera te counterparts, tropicar species exhibit greater metabolism and If protozoa and meiofauna consume large quantities of Several studies have estimated production of tropical maeroinfauna (see review of Steele the absenee of biogenie structures Ctubes, burrows, sorne oxidized root hairs). This raises two questions: to what extent are protists and meiofauna consumed by higher organisms? Aow productive are they and their potential consumers? growth rates on a per weight basis, and shorter lifespans and greater marta lity (Nayer 1954; & & Ditte1 1982; Parulekar Edwards et al 1970; Lewis 1971; MeLusky Stirling 1975; Epifanio 1984; Riehardson 198 7). Breeding and repro ductive activity are not conttnuous for most tropical benthie speeies with the life eyde of most organisms activity oc interrupted excessive heat by wet season (Le., mortality I hibernation or eneystment) (Penzias 1969; Ansell et al. 1972a,b; Govidan et al. 1972; Vohra 1971) and lunar eycles (DeVries et al. 1983a). Tropical erustaeeans, Callinectes spp., frequently for example, undergo one or more reproductive peaks superimposed on a year-Iong background of eontinuous, low -Ievel 6. Bacterial and' infaunal production in the tropics is high, but breeding and reproduction are frequently not continuous. Valid estimates of benthie seeondary produetivity exist only for the two most size-divergent groups, bacteria and macroinfauna. Many workers have indirect Iy estimated meiofaunal produetivity using breeding (DeVries et al. 1983b). 7. The distributian and abundances of infaunal benthos re/lect temporal and spatial mosaics af the major regulatory factors. lt is interesting to observe arguments in the literature concern ing whether competition, predation or distur annual tumover rates and mean biomass values. bance is the major factor regulating cornmunities. These unrealistic to any ecologist who works in the estimates must be considered highly questionable if not invalid beeause of the Such views natural are groundless environment. and Recent intuitively papers (c.g. 95 ALONGI : Tropical soft-bottom benthos. Dayton 1 984, Menge & Sutherland 1987) have interplay regulating adopted varying more reaHstic between portrayals envuonmental force s of the under eonditions. Not Another danger is that summarizing tropical literature mainly by the considering abundance data ignores Ihe fact Ihat species may function disproportionately as predicted from field experiments condueted in tropical from their abundances or biomass. In any event, one cannot expect tropical and temperate and temperate deciduous forests are similar to tro· surprisingly, many of these ideas have developed roeky intertidal areas where the relative ease success of simple manipulations yielded concrete results (Menge & Lubchen· cho 1 98 1 ; Garrity, Levings & Caffey Menge el al. 1986: An,ell have 1986; & Morton 1987). One of the more recent attempts to realisti· cally model community model of Menge regulation is the & SutherJand ( l 9 87). AppHca· ble to all habitats, the model predicts that, { l ) benthic ecosystems to be similar anymore than pical rainforests. Usted beIow are tapies in urgent need of more study: 1 . effects of freshwater discharge, storros and other climatic disturbances; 2. benthic·pelagic coupling predation and competition are absent o r weak in stressful environments, in moderate (2) habitats, predation pressure is stiU weak, but on continental shelves, including rate of detritus sedimen· tation; nutrient recycling (C,N'p) and geochemistry; 4. microbial biomass and productivity; 3. sessile animals are less affected by stress leading 5. physiological tolerances, especially to temo competition, ameliorates 6. effects of pollution on cornmunity structure 'predator bottlenecks') in benign environments. 7. secondary production of meiobenthos aod to population densities great enough to initiate exeessive and (3) competition predation for space (excluding Testing in soft·bottom environments is needed to aecept, reject or modify the model, but it is an important first step in realistically grappl· ing with food web interactions as affected by changes in the environment. Many of these changes can be as ephemeral as a tidal cycle, perature and salinity; and function; macroinfauna; 8. trophic studies particularly demersal feeding and groups; and interactions among fish benthlc 9. multidisciplinary, system·level studies further accentuating the concept of communi combining analyses of water mass movement with sediment chernistry, geology and ecoIo· many factors constantly changing in time and rare. ties regulated and structured by a patchwork of space. gy. Studies of Ihe entire benthic regime are Wilh increasing population growth and anthropogenic input to coastlines in tropical CONCLUS10NS AND RECOMMENDATIONS This review was written to provide counter generalizations and examples to concepts countries, information - eoncerning the structure and on. benthic and proper management and conservation of ingrained in the general soft-bottom literature season replication) aod mostly from temperate infonnation these unique ecosystems. ACKNOWLEDGEMENTS concepts formulated from often unreliable data (poor methodology, lack of site', sample and accurate communities is urgently needed for inforrned The author is supported by Marine Sciences dynamics of infaunal assemblages. It is probable, and Technologies Grant No. 86/0708 and the Australian Institute of Marine Science. 1 coocepts will be modified or proveo wrong to patiently that these counter-generalizations aod emergiog sorne degree in the future oc, in the opposite thank our Iibrarians, particularly B. Betts for finding many references, Truscott for typing Ihe manuscript. and l K. also veio, be uncritica11y accepted as fact. They are oversimplificatioos, but the poteotial benefits thank l.A. Vargas and two anonymous review in benthic ecology by considering Ihe tropical component, outweigh Ihe risks. and l. Tietjen over the years were instrumental of attempting to improve upon general concepts ers for their suggestioIls on the manuscript. Discussions with K.G. 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