The Deep Indian Ocean Floor

Chapter 7

THE DEEP INDIAN OCEAN FLOOR

Amanda W.J. DEMOPOULOS, Craig R. SMITH and Paul A. TYLER

INTRODUCTION

The Indian Ocean is the world’s third largest, stretching9600 km from Antarctica to the Bay of Bengal and7600 km from Africa to Australia. Compared to theAtlantic and Pacific, the Indian Ocean remainedpoorly explored scientifically until the InternationalIndian Ocean Expedition (IIOE) in 1962–1965, theresults of which were published in the Atlas of theIndian Ocean (Wyrtki, 1971). Prior to the IIOE, thevoyages of the ‘heroic age’ of deep-sea explorationhad sampled relatively little in the Indian Ocean.During the circumnavigation of the world’s oceans byHMS Challenger (1872–1876), study of the IndianOcean was confined to circumpolar waters south ofthe Polar Front. The Valdivia expedition (Chun, 1900)sampled the free-swimming larvae of the deep-waterbrachiopod Pelagodiscus in surface waters of the IndianOcean, benthic adults being collected from depthsas great as 2490m (Helmcke, 1940). In the 1930s,the John Murray Expedition to the Indian Oceanworked mainly in shallow water, but some deep-water sampling provided material for monographs (e.g.,Knudsen, 1967). The Swedish Deep Sea Expeditionof 1947–1948 on the Albatross sampled extensivelyin all the oceans (Pettersson, 1957), taking bottom-living specimens and cores from the deep sea (for a listof publications see Menzies et al., 1973). The last ofthe ‘heroic age’ cruises, on the Galathea (1950–1952),sampled deep-water fauna between Sri Lanka (formerlyCeylon) and the Kenyan coast, as well as along theMozambique Channel to South Africa (for a list ofpublications see Menzies et al., 1973). The taxonomicstudies of these major expeditions were supplementedby Soviet cruises to the Indian Ocean, particularly onthe Vityaz, again resulting in monographic treatments(e.g., Pasternak, 1964, 1976). The IIOE also gave the

opportunity for quantification of the bottom fauna ofthe Indian Ocean (Neyman et al., 1973). A historicalperspective of the major early cruises, including thoseto the Indian Ocean, has been given by Mills (1983).Since the IIOE, deep-sea research in the Indian

Ocean has focused on the Arabian Sea. Because fewsystematic surveys or conceptually integrated studiesexist, deep-sea ecosystems in the Indian Ocean remainpoorly known (Banse, 1994).The Indian Ocean is characterized by a number of

unusual oceanographic features. Patterns of circulationare unlike those in any other oceans, owing to large-scale monsoonal shifts in wind stress and currentdirections north of 5ºS. In addition, the Indian Ocean islandlocked to the north and lacks temperate and polarregions north of the equator, further modifying oceaniccirculation and hydrology. The basin also harbors oneof the largest and most intense intermediate-depth oxy-gen minimum zones in the world ocean (Kamykowskiand Zentara, 1990; Rogers, 2000). These unusualfeatures of the Indian Ocean substantially affect thespatial and temporal variability of primary production,the deep flux of particulate organic carbon (POC) andthe oxygen concentration, profoundly influencing thenature of the deep seafloor habitats.Deep-sea habitats studied with modern techniques in

the deep Indian Ocean include the Kenya slope, theoxygen minimum zone of the Oman Margin, and theabyssal Owen Basin in the Arabian Sea. In addition,limited deep-sea data exist for continental-rise andabyssal habitats in the central Indian Ocean and inthe Bay of Bengal. In this chapter, we first describephysical characteristics and key habitat variables of thedeep Indian Ocean; we then summarize the limiteddeep-sea benthic data available from this ocean, andhighlight topics of interest for future research.

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220 Amanda W.J. DEMOPOULOS et al.

PHYSICAL CHARACTERISTICS OF THE DEEPINDIAN OCEAN

The Indian Ocean, including adjacent seas (e.g., theArabian Sea, the Bay of Bengal, and the SouthernOcean), covers 73 426 000 km2, roughly one-fifth ofthe total world oceanic area. It has an average depthof 3890m, which is approximately equivalent to theaverage world-ocean depth.

Morphology

The morphological features of the Indian Ocean aresimilar to those of the Atlantic Ocean (see Chapter 6),and include mid-ocean ridges, abyssal plains, and fewdeep-sea trenches. The Indian Ocean has very fewseamounts and islands (see Rogers, 1994), but containsnumerous submarine plateaus and rises (Fig. 7.1).Two types of continental margins are present in thisbasin: divergent and convergent. Divergent marginsare the most common, and are characterized by widecontinental shelves, broad continental rises, and littleseismic activity. Such margins are found along EastAfrica, the Arabian Peninsula, much of the Indiansubcontinent, and Western Australia. The Indian Oceancontains only one convergent margin in its northeastcorner, the Java (Sunda) Trench, where oceanic crustis subducted beneath a continental plate (Fig. 7.1). The7500m deep Java Trench is part of the Indonesian Arc,which contains 14% of the world’s active volcanoes.Because of large riverine inputs of terrigenous

sediment, particularly from the Indus and GangesRivers, onto gradually sloping divergent margins, theIndian Ocean has vast continental rises and abyssalplains (Kennett, 1982). The rises are gradually slopingplains of terrigenous sediment several kilometers thick.Beyond the continental rises lie level abyssal plains; theabyssal plain south of the Bay of Bengal is the flattestlarge area of the earth’s surface (Tomczak and Godfrey,1994). Much of this plain has arisen from a turbidityflow down the northern slopes of the Bay of Bengal,extending 3000 km southwards into the deep sea of theBay.Like the Atlantic Ocean, the Indian Ocean is

subdivided into a number of major basins by longsections of mid-ocean ridge (Fig. 7.1). In the IndianOcean, some of these ridges (e.g., the Ninety-EastRidge, the Mascarene Ridge and the Chagos-LaccadiveRidge) are aseismic; they do not appear to be sitesof active seafloor spreading. Active ridges include the

Carlsberg Ridge and the Mid-, Southwest and SoutheastIndian Ridges, the last two of which extend beyond thelimits of the Indian Ocean, connecting with the worldMid-Ocean Ridge system. The abyssal Indian Oceanis divided into several smaller basins by meridionalridges. The West Australian Basin and the Mid-IndianBasin are separated by the Ninety-East Ridge whilstto the west of the Mid-Indian Ridge are a series ofbasins including the Somali, Mascarene, Madagascarand Natal Basins. The Carlsberg Ridge lies north ofthe Arabian Basin (Fig. 7.1).

Surface circulation

Three important factors make the circulation andhydrology of the Indian Ocean different from those ofany other ocean: the closure of the Indian Ocean in thenorthern subtropics; the seasonally-reversing MonsoonGyre; and the blocking effects of the equatorial currentsto the spread of water masses along the thermocline(Fig. 7.2). Owing to seasonal heating and cooling ofthe vast Asian landmass, winds vary seasonally northof the equator, resulting in the Indian Ocean monsoons.From November to March the Northeast Monsoon isaccompanied by the northeast trades, which are rein-forced by the rapid winter cooling of air over Asia. Asa result, the westward-flowing North Equatorial Currentfrom 8ºN to the equator is prominent in Januarythrough March, generating a small anticyclonic gyrenorth of the equator (Fig. 7.2). Very little upwellingoccurs during the Northeast Monsoon, and hydrologicaleffects are generally superficial (Wyrtki, 1973). FromApril to September, the Asian landmass warms fasterthan the ocean, drawing moist air ashore from over theocean, and creating the Southwest Monsoon. Duringthis period, eastward surface currents north of theequator combine with the Equatorial Countercurrent,establishing the Southwest Monsoon Current between15ºN and 7ºS (Fig. 7.2) and a strong westward-flowingSouth Equatorial Current around 5ºS. This reversalof surface currents gives rise to the greatest seasonalvariation in hydrography of any ocean basin (Burkillet al., 1993). Strong upwelling occurs off the Somaliand Oman coasts, resulting in substantial increases insurface production.The South Equatorial Current forms a marked

hydrographic boundary between the monsoon-drivencirculation in the north and the Southern HemisphericSub-tropical Anticyclonic Gyre to the south. Thecirculation pattern to the south of 5ºS is analogous to

THE DEEP INDIAN OCEAN FLOOR 221

20°

10°

0°

20°

30°

40°

50°

120°100°80°60°40°20°

S

N

10°

E E

Fig. 7.1. Topography of the Indian Ocean including major ridges and basins. OM, Oman Margin. Modified from Tomczak and Godfrey(1994). The 1000, 3000, and 5000-m isobaths are shown, and regions less than 3000m deep are shaded.

the gyres of the South Atlantic and South Pacific. Inthe southern Indian Ocean, western boundary currentsinclude the East Madagascar, Mozambique, Agulhas,and Zanzibar Currents (Fig. 7.2). All of these currentsextend to great depths, disturbing sediments withtheir high velocity flow to depths as great as 2500m(Tomczak and Godfrey, 1994). Off South Africa, theAgulhas Current forms a cyclonic loop, and, althoughsome water is lost to the Atlantic, most flows eastwardforming the northern boundary of the CircumpolarCurrent.

Upwelling

A significant consequence of the seasonally changingcirculation pattern in the north Indian Ocean ispronounced upwelling along the western coastlinewhen the Southwest Monsoon produces strong Ekmantransport away from the coasts of Somalia and Arabia(Swallow, 1984). During the Southwest Monsoon,the strong winds from the northwest force intenseupwelling and deep mixing, which reduce coastalsea-surface temperatures by approximately 5ºC, and


E 30o 60o 90o 120o E90o

60o

30o

S0o

30o

30o

N

S0o

N

E 30o 60o 90o 120o E

North Equatorial Current

Equatorial Counter Current

South Equatorial Current

Subtropical Gyre

South Indian Ocean CurrentAgulhas C.M

oza

mb

ique

Curre

nt

SC

ZC

SJC

WGB

CircumpolarCurrent

STF

SAF

PF

Lee

uw

inC

.

Ea

st

India

nC.

EM

C

EAC

Som

aliC

.

Moza

mb

ique

C.

ZC

Southwest Monsoon

Current

South Equatorial CurrentSJC

A.

B.

Fig. 7.2. Major surface currents of the Indian Ocean (A) during the Northeast Monsoon season (March–April), and (B) during theSouthwest Monsoon season (September–October). The circulation south of 20ºS remains unchanged. Abbreviations: STF, Subtropical Front;SAF, Subantarctic Front; PF, Polar Front; SJC, South Java Current; ZC, Zanzibar Current; SC, Somali Current; EAC, East Arabian Current;EMC, East Madagascar Current; WGB, Weddell Gyre Boundary. Modified from Tomczak and Godfrey (1994).

cause high primary productivity and a substantial fluxof sinking particles in this region (Cushing, 1973;Banse and McClain, 1986; Nair et al., 1989). Primaryproduction is exceptionally high and widespread dur-ing this period, reaching levels of 547.5 gCm−2 y−1

(Krey, 1973; Nair et al., 1989). Because upwellingis embedded in the swiftly moving western boundary

current, nutrient upwelling has a reduced effect onlocal primary production. The strong current removesmuch of the additional planktonic biomass from theupwelling system before it can be utilized, with aconsequent enhancement of biomass and secondaryproduction in the open Arabian Sea. As a result,zooplankton concentrations in the upwelling system of


the Arabian Sea are not as high as those of coastalupwelling systems in the Pacific and the AtlanticOceans. Upwelling also occurs off the southeastmargin of Oman, associated with enhanced biologicalproductivity (Hermelin and Shimmield, 1990). Thesediments of the Oman Margin and northwest ArabianSea vary laterally between facies rich in organic carbon,in biogenic silica, or in carbonate, deposited under thishighly productive upwelling regime (Kennett, 1982;Tomczak and Godfrey, 1994).Upwelling along the eastern boundary of the Indian

Ocean is uncommon, because winds favorable forupwelling are weak during the Northeast Monsoonand absent during the Southwest Monsoon. This is incontrast to the eastern boundary of the Pacific, wherethere is significant upwelling in both the northern andsouthern hemispheres (see Chapter 6). A small amountof upwelling occurs along the coast of Java during theSouthwest Monsoon and weak upwelling also occursoff southwest India (Wyrtki, 1973; Burkill et al., 1993).Unlike typical eastern boundary currents, the LeeuwinCurrent (Fig. 7.2) off the west coast of Australia flowspoleward against the wind, and the undercurrent isequatorward, instead of moving toward the continent, asin the Peru–Chile Margin. For these reasons, upwellingdoes not occur on the Western Australian shelf, andthus overall biological productivity in this region isrelatively low.

Deep water masses

The hydrology of the deep Indian Ocean is muchless affected by the seasonal monsoon cycle thannear-surface currents. Monsoonal influence is restrictedto the surface mixed layer and western boundarycurrents. Three mediterranean-type seas influence thehydrographic properties of the Indian Ocean: thePersian Gulf, the Red Sea, and the AustralasianMediterranean Sea. In the Persian Gulf and Red Sea,evaporation exceeds precipitation and warm, densewater flows out of the Persian Gulf and Red Sea intothe northern Indian Ocean forming North Indian OceanIntermediate Water. This intermediate water does notreach the deep sea bed. In the southern Indian Ocean,south of 10ºS, between the depths of ~500 and 1000m,the northward flow of Antarctic Intermediate Water isblocked by the equatorial current system (Tomczak andGodfrey, 1994).Abyssal flow in the Indian Ocean is divided

into three components associated with three western

boundary currents. Western basins are penetrated bybottom waters derived from the Indian and AtlanticBasins (Warren, 1981). Like the Atlantic and PacificOceans, the water masses in the Indian Ocean below3500m consist mostly of cold Antarctic Bottom Wa-ter (AABW), with a potential temperature T of 0.3ºC.Antarctic Bottom Water leaves the circumpolar currentto enter and fill the Indian Ocean below a depthof 3800m at two locations: the Madagascar Basin,and gaps in the Southwest Indian Ridge. AntarcticBottom Water then flows across the Madagascarcontinental slope and forms a deep western boundarycurrent (Swallow and Pollard, 1988). Recirculation ofAntarctic Bottom Water in the Madagascar Basin isfast, bottom currents flowing northward at a speed ofapproximately 0.2m s−1. The bottom water continuesalong the western pathway in the Somali Basin, whereit eventually enters the Arabian Basin and disappearsthrough gradual mixing into overlying Deep Water. Inthe eastern Indian Ocean, Antarctic Bottom Water, alsocalled Circumpolar Water, enters the South AustralianBasin via the Australian-Antarctic Discordance. It fillsthe Great Australian Bight and moves west then north,forming a western boundary current along the Ninety-East Ridge. The oxygen concentration follows the flowpattern, with the concentration in the bottom waterdecreasing towards the north from 5ml °−1 at 60ºSto 0.2ml °−1 in the Arabian Sea and Bay of Bengal,as the water mass increases in age and isolation.This situation is in contrast to that in the Atlanticand Pacific, where the most notable regions of lowoxygen are on the east side around the equator. Froma depth of 1500 to 3800m, the Indian Ocean is filledwith Indian Deep Water formed from North AtlanticDeep Water (NADW) carried into the Indian Oceanwith the upper Circumpolar Current. It spreads northin the western boundary current to the Arabian Seaand the Bay of Bengal. The water properties consistof a moderate oxygen concentration (4.7ml °−1), lowtemperature (2ºC), and high salinity (35.85), similar tothe North Atlantic Deep Water. The northern ArabianSea is closed by land mass in the north, and is semi-enclosed altogether (Fig. 7.1). The Arabian Sea isdeeper than 3000m, and the basin is closed in the southby the Central Indian Ridge, the Carlsberg Ridge, andthe Chagos–Laccadive (or Maldive) Ridge (Fig. 7.1).Therefore, bottom water must enter in the west, throughthe Owen Fracture Zone, rather than from the south(Tomczak and Godfrey, 1994).


Clay or no Deposit Calcareous OozeSiliceous Ooze

Shelf and Slope DepositsDeep-Sea MudsGlacial Debris m

0 1000 2000

MILES

0Km

3000

m

Fig. 7.3. Distribution of surface-sediment types in the deep Indian Ocean. Modified from Berger (1974).

Substratum type

Substratum types in the deep sea can influence thedistribution patterns of benthic organisms. Specifically,the organic-matter content of deep-sea sediments canbe correlated with the abundance, composition, anddiversity of benthos (Grassle and Grassle, 1994;Rice and Lambshead, 1994; Snelgrove and Butman,1994). In general, organic-rich sediments often arelow-diversity habitats, containing mostly tube-dwellingpolychaetes, whereas organic-poor sediments typicallycontain a diverse community of deposit feeders (Levinand Gage, 1998). Hard substrata in the deep sea provideniches for a broad variety of sessile organisms (seeChapter 2).Most of the Indian Ocean seafloor, particularly

that remote from land, is covered with calcareousooze (Fig. 7.3) (Berger, 1974). Calcium carbonateconcentrations are typically intermediate between thoseof the carbonate-rich Atlantic and the carbonate-poorPacific. The carbonate critical depth, CCRD, wherecalcium carbonate drops to <10% of the sedimentmass, is deepest in the equatorial region, and shoalsto 3900m between 50º and 60ºS. Carbonate sedimentsare relatively poor in organic carbon (<1%) and havecoarse grain size (Berger, 1974; Kennett, 1982). Nearlyall river discharge occurs in the northern part ofthe Indian Ocean adjacent to Asia, thick terrigenoussediment being deposited in the northern and westernparts of the Indian Ocean, especially the Arabian Seaand the Bay of Bengal. These sediments have highconcentrations (2–5% by weight) of organic carbon,


and are composed mostly of terrestrial plant material,phytodetritus, and mineral grains transported by rivers(Kennett, 1982; Levin and Gage, 1998; Levin et al.,2000). In the Bay of Bengal, terrigenous sedimentationfrom the Ganges is particularly extensive, reachingdepths of 5000m. In the Arabian Sea, there is anorganic carbon maximum (4.9%) at 400m, owing ap-parently to preferential preservation and accumulationof organic matter under low-oxygen conditions in thebottom water (Levin et al., 2000). The particle flux tothe deep Bay of Bengal is enhanced by the freshwaterinput from the major rivers entering at the north of theBay (Ittekkot et al., 1991).Sediments are at most very thin on the crests

of mid-ocean ridges, and essentially absent on theridge axes (Fig. 7.3). Thick siliceous oozes, composedprimarily of radiolarian and diatom tests, occur atdepths of ~5000m south of the polar front and alonga few mid-ocean ridges, where there is high biologicalproductivity (Berger, 1974; Kennett, 1982). However,because of the oligotrophic nature of the equatorialIndian Ocean, siliceous sediments are rare in lowlatitudes of the Indian Ocean compared to the PacificOcean (Kennett, 1982). Red clay is present mostly inthe eastern and southern Indian Ocean, near the equatorand high latitudes. It is composed of fine-grained,organic-poor sediments resulting from volcanic activityat ridges (Berger, 1974; Pilipchuk et al., 1977; Kennett,1982).Hard substrata in the Indian Ocean consist of basalt

rocks, rock faces, and the surfaces of ferroman-ganese concretions. The morphology of the centralIndian Ocean Basin is composed of abyssal hills andseamounts, as well as valleys and abyssal plains.The topographic highs, which are in the proximityof three major fracture zones, are composed of hard,massive basalts occurring at the crests, along theslopes and on the foothills as talus deposits (Sharmaet al., 1997). Owing to strong geostrophic currents andconsequent scouring of the sediments, the WhartonBasin, the southern Mascarene Basin, and parts of theSouthwest Indian and Australian–Antarctic Basins havelittle or no sediment (Kennett, 1982). Sediment in theseareas, when present, is mostly brown clay. Along theSouthwest Indian Ridge, German et al. (1998) haveidentified hydrothermal activity. The associated faunainhabiting these hydrothermal regions is dominated byshrimps and anemones (T. Shank, pers. comm.). Inthe southeast and southwest Indian Ocean, and in theMozambique Basin, there are extensive pavements of

manganese nodules at depths of about 4000m (Kollaet al., 1980). It has been suggested that their presenceat the sediment–water interface is a result of benthicbiological activity and strong bottom currents. Benthicorganisms may nudge the nodules upward, maintainingthem near the sediment–water interface, while strongcurrents may limit the deposition of sediment, allowingthe nodules to grow (Berger, 1974; Paul, 1976; Kennett,1982).

Near-bottom currents

Currents in the deep sea influence sediment depositionand the organisms that inhabit the seafloor (Nowell andJumars, 1984). In regions where there are strong cur-rents, sediment deposition is minimal. These currentscan smother sessile and suspension-feeding organismswith sediment grains. Where currents are weak, sed-iment chemistry and biology may be controlled bydiffusive processes. In these environments, suspensionfeeders may suffer owing to the inadequate supplyof advected particles (e.g., Jumars and Gallagher,1982). Thus, near-bottom currents not only may controlsediment deposition, but can also manipulate sedimentchemistry and the structure of the benthic communities(Nowell and Jumars, 1984).Both bottom and deep waters of the Indian Ocean

are derived from the Atlantic, and spread throughoutthe deep sea by active flow (Wyrtki, 1973). AntarcticBottom Water warms as it spreads north, and fills thedeep basins of the central Indian Ocean. Therefore, theabyssal Indian Ocean is an area of active deep-sea cir-culation. Antarctic Bottom Water supplies fine-grainedsediments from the Africa–Madagascar source to themarginal areas of the Mozambique Basin and generateswavy bedforms in this region. In addition, eddiesappear to penetrate to great depths in the Arabian Sea,and the circulation influences the bottom topography,causing depressions and rises (Das et al., 1980). Thereare strong bottom currents, with speeds approaching10–20 cm s−1, in the Wharton Basin and the southernMascarene Basin, and in parts of the Southwest IndianBasin and Australian–Antarctic Basins, resulting inminimal sediment deposition (Kennett, 1982; Gage andTyler, 1991). The active circulation ensures that theabyssal bottom waters of the Indian Ocean remainoxygenated.There is no evidence for benthic storms (see

Chapter 2) in the deep Indian Ocean although they are


predicted to occur in the extreme southwest region ofthe ocean (see Gage and Tyler, 1991).

Bottom-water oxygen

Oxygen concentration in the bottom water can bea controlling factor in the preservation of organiccarbon and benthic fauna assemblages. For example,at depths between ~100 and 1000m, oxygen minimumzones may develop below productive waters and coastalupwelling zones, where the average annual flux oforganic matter to the seabed is high. The resultinghypoxia can reduce abundance and biomass of manybenthic animals, altering species composition andrichness (Diaz and Rosenberg, 1995).In the Northern Indian Ocean, an oxygen minimum

zone occurs between depths of 100m and 1000m,where oxygen concentrations are <0.5ml °−1. This zoneresults from a combination of high surface productivitydriven by upwelling, inflow of oxygen-poor watersfrom the Persian Gulf, Red Sea, and Banda Sea, andslow deep-water circulation (Wyrtki, 1973). Such ahypoxic layer at intermediate depths has significantconsequences for the quantity and quality of organicmatter reaching the deep sea from surface production.

Sinking flux of particulate organic carbon (POC)

Benthic organisms are fueled by sinking organic matterfrom surface waters. Therefore, it is important toevaluate what controls the flux of particulate organiccarbon to the deep sea. In general, regional flux ofparticulate carbon decreases with depth and distancefrom continents, and is directly controlled by overlyingprimary production, the depth of the water column(Suess, 1980; Smith and Hinga, 1983; Jahnke, 1996),and the freshwater supply (Ittekkot et al., 1991).Therefore, along continental slopes, where primaryproduction is high and the water column is shallow,the flux of particulate organic carbon to the seaflooris high over annual periods. Specifically, over theOman Margin of the Arabian Sea, particle flux isstrongly seasonal, with peaks during the Southwestand Northeast Monsoons (Nair et al., 1989; Honjoet al., 1999). High monsoonal primary production(912.5 gCm−2 y−1), resulting from wind-induced mix-ing and nutrient injection into the euphotic zone, isthe main factor controlling the observed pattern ofparticle flux (Burkill et al., 1993). For example, offthe Oman coast during the Southwest Monsoon, the

rate of sedimentation is approximately 365 gCm−2 y−1

at depths between 100 and 500m (Burkill et al., 1993;Pollehne et al., 1993). At 1500m, the total annual fluxof particulate organic carbon drops to 53 gCm−2 y−1,decreasing with depth to 23 gCm−2 y−1 at 3500m(Honjo et al., 1999). In general, particle fluxes duringthe Southwest Monsoon are greater than during theNortheast Monsoon (Honjo et al., 1999). Because ofthe lack of upwelling, the spring inter-monsoon periodis the most oligotrophic season. Low sedimentationrates are recorded during the inter-monsoon period,corresponding to 6% of the total annual flux (Nair et al.,1989). Therefore, there is high seasonal variability inthe total flux of particulate organic carbon, as a resultof monsoonal forcing.Rates of primary production in the central abyssal

Indian Ocean, however, are similar to those of thesouth Atlantic Ocean, which is characterized by lowproduction rates (Steeman Nielsen, 1975). For example,in the oligotrophic gyre of the Indian Ocean, netprimary production is less than 109 gCm−2 y−1. Thecorresponding organic carbon flux in this region isapproximately 36.5 gCm−2 y−1 (Burkill et al., 1993;Pollehne et al., 1993).

REPESENTATIVE DEEP INDIAN OCEAN HABITATS

Oxygenated slopes and basins on the Kenyamargin

An oxygenated slope and basin region occurs alongthe margin off the coast of Kenya. Preliminaryinvestigations of the benthic fauna have been conductedhere, making this the best-studied region of its type inthe Indian Ocean.

Habitat and community descriptionSeveral rivers discharge terrigenous material onto the

Kenya shelf, and during monsoon periods the outflow islarge. The continental-shelf region of Kenya is narrow,and the ocean is very deep near the coastline. Althoughknowledge of the benthic communities is limited, somedata exist for densities and biomass of macro- andmeiofauna from sediments collected by a boxcorer-respirometer (belljar). Along the Kenyan Slope, thedensities and biomass of macrofauna (animals retainedon a 500mm sieve) decrease with increasing depth to1000m. Macrofaunal densities decrease from 7590 in-dividuals m−2 at 500m to 2960 individuals m−2 at1000m, and the biomass decreases from 26.0 gCm−2


(500m) to 4.9 gCm−2 (1000m) (Duineveld et al.,1997). The meiofauna (animals retained on a 32mmsieve) follow the same spatial patterns as the macroben-thos, with densities ranging from 806 individuals m−2

(500m) to 223 individuals m−2 (1000m). Nematodesare the dominant taxon among the meiofauna, butforaminifera are also present, especially branchingforms (Duineveld et al., 1997).

Carbon sources and trophic typesThe Kenya shelf lacks the pulses in primary pro-

ductivity driven by upwelling (Feldman, 1989), andthus this area has a low rate of primary production,ranging from 109.5 to 182.5 gCm−2 y−1 (Kromkampet al., 1997). The concentration of organic carbonin the sediment is 0.4 to 1.6% and concentrationsof organic nitrogen range from 0.05 to 0.2% (Ev-eraarts and Nieuwenhuize, 1995). Most of the or-ganic matter present in the sediments originates frompelagic production (Duineveld et al., 1997; Kromkampet al., 1997). Sediment pigment concentrations decreasedownslope along the Kenyan margin (Duineveld et al.,1997), suggesting that benthic faunal distribution maybe influenced by the concentration of organic matter inthe sediment and/or the flux of organic matter to theseafloor.

Rates of key ecological processesIn order to understand the biological and chemical

processes of benthic communities, it is important toestimate rates of key ecological processes includingrespiration, production, bioturbation, and recoloniza-tion following disturbance. On the Kenyan slope, cross-shelf and downslope transport of particulate organiccarbon (POC) adds to the flux of sinking particles onthe slope (Duineveld et al., 1997). These processessupplying organic matter to the seafloor are tightlycoupled with benthic metabolism (Duineveld et al.,1997). On the Kenyan margin, oxygen consumptionby the sediment community (SCOC) ranges from1 to 14.2mmolm−2 d−1, decreasing with increasingwater depth down to 1000m. In addition, there appearsto be little temporal variation in the rate of oxygenconsumption from June to December.

Oxygen minimum zones

Oxygen minimum zones are found in the Arabian Seaand Bay of Bengal (Fig. 7.4). The most studied ofthese is on the Oman Margin of the Arabian Sea. Theinitial systematic investigation of the entire Arabian

Fig. 7.4. Locations of major oxygen minimum zones in theIndian Ocean. In the shaded areas, dissolved bottom-water oxygenconcentrations are less than 0.2ml °−1. Modified from Diaz andRosenberg (1995).

Sea was conducted within the scope of the IndianOcean Expedition (IIOE), from 1959 to 1965 (Wooster,1984; Banse, 1994).

Habitat and community descriptionUpwelling of nitrate-rich water along the south-

ern Arabian coastline during the Southwest Mon-soon gives rise to high rates of primary production(304 gCm−2 y−1) based on the newly-supplied nutrients(Burkill et al., 1993), making the basin one of themost productive oceanic regions in the world (Nairet al., 1989). High productivity during the upwellingseason affects more than one-third of the Arabian Sea(Ryther et al., 1966; Wyrtki, 1971, 1973; Banse, 1973).Sediments accumulating on the Oman Margin underthe oxygen minimum zone have a high content of or-ganic matter, owing to the high settling flux of organicmatter, supported by monsoon-driven upwelling andredistribution of the organic material by hydrodynamicinfluences after deposition (Pedersen et al., 1992).Because of the semi-enclosed nature of the northwestArabian Sea and the resulting sluggish intermediate-depth circulation of water from the Red Sea and PersianGulf, microbial decay of the high standing crop of


organic matter promotes an intense oxygen minimumzone between the water depths of 50 and ~1000m.Bottom-water oxygen concentrations within this zonerange from 0.5ml °−1 near the boundaries of the zoneto ~0.02ml °−1 within the core at depths of 400–700m (Fig. 7.5) (Smith et al., 2000). Concentrations

1000 2000 3000 400000

1

2

3

4

Water Depth (m)

Oxyg

en

(ml/l)

Fig. 7.5. Profile of oxygen versus water depth from the Oman Margin.Modified from Smith et al. (2000).

of organic carbon in sediments within the zone reach~4% (Levin and Edesa, 1997; Levin et al., 2000). Inthe upper part of the oxygen minimum zone, the highflux of particulate organic carbon coupled with the lowoxygen concentration and free hydrogen sulfide appearsto modify the distribution of benthic organisms (Gageet al., 2000). Within the core of the oxygen minimumzone (i.e., at oxygen concentrations <0.3ml °−1), themacrofaunal assemblages are characterized by highdensities and low diversities (Levin et al., 1997).Macrofaunal species diversity is limited within theOman oxygen minimum zone, apparently because onlya relatively small number of species can tolerate oxygenconcentrations below 0.2ml °−1 (Levin et al., 2000).Between depths of 400 and 700m within the

Oman oxygen minimum zone, sediments are frequentlyspeckled with remarkable worm tubes created bycirratulid polychaetes. These worms, in the genusTharyx, produce cigar-shaped mudballs, 4.5–25mmlong, which protrude several millimeters above thesediment–water interface. Mudball densities reach~16,000 individuals m−2 and they provide a habitat fora variety of benthic organisms, including cirratulids,epizoic polychaetes, and agglutinated and calcareous

foraminifera. Polychaetes, nemerteans, and nematodesare also found inside the tests. Mudballs appear toinhibit colonization by certain tube-building taxa (twopolychaetes and an amphipod), possibly because tube-building organisms compete for food and space. Inaddition, the mudballs may provide effective refugesfrom predation, both for the cirratulids inside, andfor nearby burrowing taxa (Levin and Edesa, 1997).Distribution of mudball-building cirratulids appears tobe highly restricted in terms of depth and location;they are abundant in at least two other margin settingswith low oxygen concentrations (the San Diego Troughand the Santa Catalina Basin). Other biogenic featureswithin the Oman oxygen minimum zone includesediment mounds and burrows. Burrow diameter andthe diversity of burrow types are positively correlatedwith oxygen concentration in bottom water within thedepth range of the oxygen minimum zone (Smith et al.,2000).

Carbon sourcesThe flux of small organic particles to the deep

Arabian Sea is the best documented source of carbonto the benthos. The results of short-term and long-term measurements with sediment traps indicate aseasonal pulse of particulate organic carbon to theArabian Sea bottom (Nair et al., 1989; Passow et al.,1993; Honjo et al., 1999). The seasonal fluctuationin particulate organic carbon is the result of intensebiological productivity in the surface waters driven bythe monsoon (Nair et al., 1989; Passow et al., 1993;Honjo et al., 1999). Particle flux is most intense duringthe Southwest Monsoon (Honjo et al., 1999). However,one study of sediment-community oxygen consumption(SCOC) revealed no significant differences between theSouthwest and Northeast Monsoon periods (Duineveldet al., 1997). There are seasonal fluctuations inphaeopigment concentrations on the slope; however,high concentrations occur during the non-upwellingseason (Duineveld et al., 1997). In general, the highestvertical fluxes of particulate organic carbon occurduring the upwelling season, so it is interesting that thesediment phaeopigment concentration does not show apeak thereafter (Duineveld et al., 1997). It is possiblethat there is a delay in chloropigment degradation, thusproviding a pool of labile organic matter available tothe benthos over long time periods (Duineveld et al.,1997).Aggregates of phytoplankton detritus (phytodetritus)

occur within the sediments of the oxygen minimum


zone. Phytodetritus accumulations are usually thickestin the fall, (up to 2 cm thick), but have also beenobserved as a thin layer on the sediment surface duringthe spring months (Prell and Murray, pers. comm.). Thedeep oxygen minimum layer allows great quantities ofdetrital material to sink to the deep sea, without beingrecycled by mid-water consumers; this results in anintense flux of labile organic material to the deep-seabenthos (Gage et al., 2000). Thus, metazoan densitiesand biomass observed in November may be a responseto the abundant phytodetritus available on the surfacesediments.

Faunal compositionMost of the quantitative data available for deep-

water benthic metazoans in the oxygen minimum zonewere collected during November, which corresponds tothe onset of the Northeast Monsoon upwelling periodand is five months after the onset of the SouthwestMonsoon (Levin et al., 1997, 2000; Cook et al., 2000;Gooday et al., 2000). Therefore, the abundance ofmetazoans found during this period may result fromthe seasonal pulse of organic matter. Sampling needsto be conducted during other time periods to evaluateany seasonal fluctuation in metazoan abundance andbiomass as a result of monsoonal forcing.

Megafauna: On the highly productive Oman mar-gin, in the most intense oxygen-deficient layer, themegafaunal assemblage has a high-biomass but lowdiversity. Between 300 and 700m, the community isdominated by the spider crab Encephaloides arm-strongi, a cocoon-dwelling mytilid (Amygdalum sp.),and an ascidian (Styela gagetyleri) (Creasey et al.,1997; Young and Vazquez, 1997). At depths between900 and 1000m, oxygen levels and megafauna diver-sity increase, the community consisting primarily ofophiacanthid ophiuroids, spider crabs, and galatheidcrabs, including an abundance of Munidopsis spp.(Gage, 1995; Smallwood et al., 1999; Creasey et al.,2000). The ophiacanthid Ophiolimna antarctica has adensity of 51 individuals m−2 (Smallwood et al., 1999).Encephaloides armstrongi is abundant throughout theoxygen minimum zone on the Oman Margin andappears to tolerate low oxygen concentrations (Creaseyet al., 1997; Smallwood et al., 1999). At 1000m, thedensity of crabs in general is approximately ten timesgreater than that at 800 and 1250m, with spider crabsaveraging 47 individuals m−2 (Smallwood et al., 1999).It is possible that this depth is a boundary between themore significant oxygen minimum zone above and the

increasingly oxic conditions below (Smallwood et al.,1999).The high densities of spider crabs and ophiuroids in

the oxygen minimum zone have implications for theburial of deposited organic material. Spider crabs andophiuroids may be highly mobile, and may resuspendfine organic material from surface sediments; inaddition, phytoplankton-derived sterols are altered bytheir digestive processes (Smallwood et al., 1999).These megabenthic activities may influence the qualityof organic matter in organic-rich sediments on thecontinental slope (Smallwood et al., 1999).

Macrofauna: The macrobenthos (for the ArabianSea, defined as animals retained on a 300mm sieve)are represented in the oxygen minimum zone by anabundant, low-diversity soft-bodied fauna. Polychaetesare the dominant group within this zone, defined bythe depth range 100–1000m and oxygen concentra-tions <0.5ml °−1 (Herring et al., 1998). Spionids andcirratulids are most common in the upper part of thezone (400–700m), where oxygen concentrations arelow (0.13ml °−1), and ampharetids and paraonids in thelower portion (850–1000m), where oxygen concentra-tion increases to 0.29ml °−1. Between 400 and 1000m,macrofaunal density ranges from 5818 to 19 183individuals m−2, the highest densities being found at700m (Fig. 7.6; Levin et al., 2000). Macrofaunal

3400

1250

1000

850

700

400

3400

1250

1000

850

700

400

0 5 10 15 20 25 0 20 40 60 80

Number of Macrofauna

(m x 10 )-2 3

Wate

rD

epth

(m)

Biomass

(g m )-2

A. B.

Fig. 7.6. (A) Mean density and (B) biomass for macrofauna sampledat six water depths within and beyond the oxygen minimum zoneon the Oman Margin. Error bars represent standard error. Modifiedfrom Levin et al. (2000).

biomass at these depths ranges from 14.2 to 59.7 gm−2,and again the biomass is highest at a depth of 700m.Each taxon appears to have a threshold above whichoxygen concentration and organic matter supply arehigh enough for the animals to survive (Levin andGage, 1998; Levin et al., 2000).The families represented vary with depth as bottom-

water oxygen concentration varies within the oxygen


minimum zone. At 400m, where the oxygen min-imum is most intense, the dominant taxon is thetube-building spionid polychaete Prionospio (Minus-pio) sp. A (63%), followed by the cirratulid polychaeteAphelochaeta sp. A (27%) (Levin et al., 1997).Macrofauna are most abundant in the upper 5 cmof sediment, this layer accounting for 84% of theindividuals found and 77% of the biomass (Levin et al.,1997, 2000). Overall, 10 species were found at thisdepth. At 700m, two species of spionid polychaetes(Minuspio sp. A and Paraprionospio sp. A) dominate,whereas at 850m the most abundant species is aparaonid polychaete, Aricidea sp. A (21.3% of thetotal individuals in the macrofauna). The most abun-dant species at 1000m is an ampharetid polychaete,Eclyssipe sp. B. Crustaceans between 400 and 1000mprimarily consist of amphipods (Ampelisca sp., andan unidentifed gammarid), together with a few tanaids(Levin et al., 2000). In total, 28 species are foundat 700m, indicating an increase in species richnesswith increase in oxygen concentration within this zone.The oxygen minimum zone in the Arabian Sea ischaracterized by significantly lower species richnessthan in other oxygen minimum zones such as WalvisBay (Sanders, 1969) and that in the eastern Pacific onthe summit of Volcano 7 (Levin et al., 1991) whereoxygen concentrations range from 0.08 to 1.3ml °−1.Most (63%) of the taxa within the oxygen minimum

zone live in tubes, including Prionospio sp. A,ampharetid polychaetes, and mudball cirratulids. It ispossible that these organisms use the structures toprovide channels for pumping oxygen from above theseafloor (Levin et al., 1997). Certain organisms haveadapted to their low-oxygen environment by alteringtheir morphology. For example, spionid and cossuridpolychaetes from the Oman Margin have enlargedrespiratory surface area, and larger and more branchedbranchiae (Lamont and Gage, 2000). Within the oxygenminimum zone at about 400m, the most commonspecies have large numbers of long branchiae ortentacles, which most likely assist in oxygen utilization(Levin et al., 1997). The mussel Amygdalum, whichoccurs in low abundance, has a thin shell. Echinodermsand coelenterates are absent from the community. Thegeneral lack of taxa other than polychaetes within theoxygen minimum zone suggests that most molluscs,crustaceans, and echinoderms are intolerant of low-oxygen conditions; this results in the low diversityin these regions (Levin et al., 2000). The generalcommunity composition of the oxygen minimum zone

in the Arabian Sea conforms with the dysaerobicfacies described for bottom-water concentrations of 0.1to 0.5ml °−1 by Rhoads et al. (1991) and with theoxygen minimum zone macrofaunal structure presentat Volcano 7 (Levin et al., 1991, 1997), whereoxygen concentrations ranged from 0.1 to 0.2ml °−1.However, in the near-anaerobic conditions on the Perumargin (O2 ~0.02ml °−1), burrowing oligochaetes arethe dominant taxa, and no tube builders are present(Levin et al., unpubl.). In addition, at Volcano 7there is a mixture of burrowing, epibenthic and tube-building taxa (Levin et al., 1991). Therefore, it appearsimpossible to make generalizations regarding dwellingbehavior of low-oxygen macrofauna (Levin et al.,2000).Below the oxygen minimum zone, between 1250

and 3400m, macrofaunal densities range from 2485to 3190 individuals m−2 and biomass ranges from ~2to 10 gm−2 (Fig. 7.6; Levin et al., 2000). At 1250m,amphipods, tanaids, and cumaceans are present (2.7%of macrofaunal individuals). At 3400m, amphipods,tanaids, and isopods form 31% of the total macrofauna.At 1250m, the most abundant species was a syllidpolychaete, and at 3400m, a tanaid. Molluscs appearto be present only at depths �1000m (1.9%), theirproportion in the fauna increasing as oxygen concentra-tion increases with depth – 23% at 1250m and 18% at3400m (Levin et al., 2000). In general, low pH and lowoxygen concentration create an unsuitable environmentfor calcified taxa (Levin et al., 2000). The taxa presenton the Oman slope are broadly distributed throughoutthe deep sea (Smith and Demopoulos, Chapter 6, thisvolume). In the Arabian Sea, outside of the oxygenminimum zone, the macrofauna are not as abundant aswithin this zone, suggesting that there is a thresholdfor opportunistic organisms that can tolerate the lowoxygen conditions and utilize the abundant food supplyin this highly productive region (Levin and Gage, 1998;Levin et al., 2000).

Metazoan meiofauna: Meiobenthos are abundant inthe oxygen minimum zone, and have been well studiedin the Arabian Sea. However, few of these studies usedcomparable sampling and laboratory techniques. Ne-matodes and foraminiferans are the major meiobenthictaxa present, followed by harpacticoid copepods, poly-chaetes, and turbellarians (Qasim, 1982). Data fromcore samples (10 cm deep × 3.4 cm2 area) taken fromgrab samples indicate that meiobenthic biomass in theoxygen minimum zone between 200 and 1000m ranges


from 2.01 to 42.30 gm−2, and at depths greater than1000m the biomass ranges from 16.55 to 119 gm−2

(Qasim, 1982). Since it has been documented that theoxygen concentration in the bottom water increasesfrom 0.13ml °−1 at a depth of 400m to 0.27ml °−1 at1000m (Smith et al., 2000), it appears that meiobenthicbiomass and abundance follow the same pattern as forthe mega- and macrofauna, increasing with increasingoxygen concentration.Nematode abundance, estimated from sediment sam-

ples collected with a multiple corer using 25 cm2 tubes,is positively correlated with macrofauna abundance.Between 400 and 700m, nematode abundance rangesfrom 1700 to 2495 individuals m−2, and the oxygenconcentration from 0.13 to 0.16ml °−1 (Cook et al.,2000; Smith et al., 2000). At the lower boundary of theoxygen minimum zone (1250m) and beyond (3400m),nematode abundance decreases, ranging from 860 to494 individuals m−2, respectively (Cook et al., 2000).Bottom-water oxygen concentration does not appear tobe the controlling factor for the nematode population –rather, food quality, as measured by the hydrogenindex1 (Patience and Gage, unpublished), appears to bethe major predictor of overall nematode abundance inthe Oman slope region (Cook et al., 2000).

Protozoa: In the oxygen minimum zone between 200and 600m, the dominant foraminifera present includeBolivina pygmaea, Bulimina sp., and Lenticulina iota(Hermelin and Shimmield, 1990). At 400m, corre-sponding to the core of the oxygen minimum zone, theforaminiferan taxa also include allogromiids, bathysi-phonids (Bathysiphon spp.), hormosinaceans (mostlyLeptohalysis spp.), saccamminids (Lagenammina spp.),spiroplectamminaceans, textulariaceans and trocham-minaceans (Gooday et al., 2000). From 600 to 1000m,Ehrenbergina trigona, Hyalinea balthica, Tritaxia sp.,and Uvigerina peregrina dominate the foraminiferanassemblage. These taxa appear to be closely related;they could be limited by low oxygen concentration, andpossibly by the organic-carbon concentration in the sed-iment (Hermelin and Shimmield, 1990). Foraminiferantaxa found in the oxygen minimum zone appear to besmaller in size (92.9% were <500mm) and have moreelongate tests (160mm) than foraminifera collected out-side the oxygen minimum zone at 3400m, which hadan average test length of 120mm (Gooday et al., 2000).

Below the oxygen minimum zone (3350m), very large,tubular, agglutinated species can be found, specificallythe genera Bathysiphon, Hyperammina, Rhabdamminaand Saccorhiza (Gooday et al., 2000). Foraminiferandensities in the oxygen minimum zone of the ArabianSea are among the highest reported from an oxygen-poor environment (Gooday et al., 2000). Foraminiferafrom the Santa Barbara Basin (590m, O2 ~0.1ml °−1)and from the Peru margin (300–1200m, O2 = 0.02to 1.6ml °−1), follow the same trend; in these areas,however, soft-shelled monothalamous taxa are rareand large agglutinated taxa are absent. Foraminiferaand metazoans show similar population responses tooxygen stress: species dominance increases, diversitydecreases, and the relative abundance of major taxachanges (Gooday et al., 2000).The benthic flagellates are significantly more abun-

dant in the sediments during the non-upwelling season.Although grazing rates are low, bacterivory at thatperiod has a significantly greater impact on bacterialstanding stock in the bottom water than duringupwelling (Bak and Nieuwland, 1997). Microbes arefueled by particle flux from the surface waters, andrespond to seasonal sedimentation of organic matter(e.g., Pfannkuche, 1993).

Nanobiota: Seasonal deposition of organic matterin the Arabian Sea results in seasonality of benthicmicrobial production. After the upwelling season,for instance, bacterial abundance and production arehigh (Ducklow, 1993). Bacterial density, biomass, andcell volume are larger during the August upwellingperiod than in the non-upwelling period (February).There is no obvious relationship between the biomassand abundance of microbes and the existence ofthe intense oxygen minimum zone, which is equallypresent in both seasons (Duineveld et al., 1997; Bakand Nieuwland, 1997). There is a decrease in thebiomass and abundance of benthic bacteria and benthicnanoflagellates with increase in depth in sediment, andalso with increasing ocean depth. Bacterial densitiesin the Arabian Sea decrease from 1.5×109 cm−3 insurface sediments to 0.8×109 cm−3 at a depth of10 cm within the sediment (Bak and Nieuwland, 1997).Within the core of the oxygen minimum zone (400m),average bacterial densities range from 25×107 cm−3 forthe upper 0.5 cm of sediment, to 10× 107 cm−3 at a

1 The hydrogen index has been suggested as a proxy for sediment food quality, and is a measure of the hydrogen content (and hence theredox state) of the organic matter. Its units are (mg hydrocarbon)/(g total organic carbon).


depth of 4.5 cm in the sediment (Levin et al., 1997).Colonies of Thioploca reach densities of 22 117m−2

(Levin et al., 1997). With increasing seafloor depth,bacterial densities range from 4×109 cm−3 at 200m to0.6× 109 cm−3 at 5000m (Bak and Nieuwland, 1997).

Trophic typesThe most prevalent feeding mode among the macro-

fauna in the oxygen minimum zone is deposit feeding –that is, the ingestion of sediment and associatedorganic matter. For the depth range from 400 to1000m within the oxygen minimum zone, most ofthe macrofauna (94%) are tentaculate, surface-depositfeeders. The nemerteans are likely to be scavengers orcarnivores, and the mussel Amygdalum politum is afilter feeder. Cossurid polychaetes (constituting 1.1%of the fauna) may be the only subsurface depositfeeders present in this region. Below 850m, subsurfacedeposit feeders constitute an increasing proportion ofthe total fauna, the largest figure being recorded at3400m (Levin et al., 2000). Subsurface deposit feedersare usually present in deep-sea or organically enrichedenvironments (Levin et al., 1997). However, in organic-rich oxygen minimum zones, opportunistic species thatcan survive oxygen stress are generally surface-depositfeeders. Organic-rich sediments resulting from highsurface production probably contribute to the highdominance of surface-deposit feeders in a relativelydense faunal assemblage, which has been observed(Levin et al., 1997, 2000; Levin and Gage, 1998).Nematodes generally feed on detrital particles, sed-

iment, and/or bacteria, although some nematodes arecarnivorous (Gage and Tyler, 1991). Food availabilityappears to govern foraminiferal abundance and biomass(Altenbach, 1988; Altenbach and Sarnthein, 1989;Herguera and Berger, 1991; Gooday et al., 2000).Generally, foraminifera consume phytodetritus, thebodies of small dead animals, bacteria associated withsediment, particulate organic carbon, and potentiallydissolved organic carbon (Gooday et al., 1992). Wherefood is plentiful, foraminifera succeed, but they alsomust tolerate the reduced oxygen availability that isconcomitant with abundance of organic matter (Goodayet al., 2000). The predominance of these organisms inthe oxygen minimum zone of the Arabian Sea suggeststhat the meiofauna, of which they constitute the majorpart, occupy low trophic levels.

Rates of ecological processesVery few data exist estimating the rates of key

ecological processes in the oxygen minimum zone ofthe Arabian Sea. Useful data exist for the oxygen con-sumption of the sediment community (SCOC) in theoxygen minimum zone, specifically from the sedimentbelow the Yemen–Somali upwelling region (~500–800m). During both the Southwest and NortheastMonsoons, the oxygen consumption of the sedimentcommunity ranged from 0.7 to 4.3mmolm−2 d−1 whenthe zone between 70 and 1700m was covered withwater with a low oxygen content (10–50mM) (Duin-eveld et al., 1997). These values are 3–7 times higherthan reported for oxygenated slopes in the Pacific(Hammond et al., 1996; Smith and Demopoulos,Chapter 6, this volume).It may be expected that the bioturbation activities

of benthos are linked with bottom-water oxygenconcentration (Pearson and Rosenberg, 1978; Rhoadset al., 1978; Diaz and Rosenberg, 1995; Smith et al.,2000). In the oxygen minimum zone on the Omanslope, rates and patterns of bioturbation have beenevaluated using profiles of 210Pb and X-radiography(Smith et al., 2000). The mixing depths for 210Pb withinthe oxygen minimum zone, with oxygen concentrationsof 0.13–0.27ml °−1, were half of those on oxygenatedslopes in other oceans (mean depths 4.6 cm and 11 cm,respectively). The reduction in the 210Pb mixing depthlikely results from the prevalence of surface-depositfeeders and tube builders within this oxygen minimumzone (Levin et al., 2000; Smith et al., 2000). Unlikeoxygen minimum zones in other oceans, there does notappear to be enhanced bioturbation at the boundary ofthe Oman oxygen minimum zone, possibly because ofthe gradual change in oxygen concentration from 0.13to 0.27ml °−1 over the breadth of the zone (Smith et al.,2000).

The Western and Central Abyssal Indian Ocean

Habitat and community descriptionThe deep abyssal zone of the Indian Ocean is an

area of active deep-sea circulation (Parulekar et al.,1982). It is a habitat with rich benthic biomass.Investigations on deep-sea benthos in the western andcentral Indian Ocean, in the depth range of 1500 to6000m, have revealed abundant biota but low speciesdiversity (Parulekar et al., 1992).Sediment samples have been collected by grab,

and macrofauna (retained on a 500mm sieve) andmeiofauna (retained on a 44mm sieve) from the deepArabian Basin and the Central Indian Basins have


been quantified (Parulekar et al., 1982, 1992). Thefauna from these abyssal sediments (3600–5300m) arecomposed of 12 macrofaunal and 3 meiofaunal inver-tebrate taxa (Parulekar et al., 1982). Specifically, theabyssal macrofauna consists primarily of polychaetes(41.6%), followed by peracarid crustaceans (31.7%),ophiuroids (12.2%), Echiura and Bryozoa (9.7%),molluscs (4.8%), and agglutinating rhizopod proto-zoans (which were not included in these percentagefigures). Macrofaunal densities range from 92 to462 individuals m−2, and biomass ranges from 0.47 to13.32 gm−2 (Parulekar et al., 1982, 1992). Megafaunalscavengers present include ophidiid fish (Lochte andPfannkuche, 2000).The abyssal plains of the Indian Ocean harbor rich

meiobenthic assemblages, meiofauna density rangingbetween 50 177 and 232 912 individuals m−2. Thesedensities are many times greater than densities reportedfor meiofauna of the central North Pacific and theeast and west Atlantic (Wolff, 1977), but are onlyone-tenth of that observed in the bathyal depths ofthe northwest Indian Ocean (Thiel, 1966). Meiofaunalbiomass ranges from 0.02 gm−2 to 0.41 gm−2. Nema-todes are the most abundant group, accounting for53.3% of the individuals, followed by foraminifer-ans at 17.6% and harpacticoid copepods (16.8%).Kinorhynchs, ostracods, and turbellarians were alsopresent in small quantities (Parulekar et al., 1992). Thedensity of meiofauna decreases with increasing waterdepth (Parulekar et al., 1982).In general, manganese nodules from the 3000–

4000m depth range in the abyssal zones are relativelybarren with respect to benthic biomass, possibly be-cause of the oligotrophic feeding conditions (Neymanet al., 1973; Parulekar et al., 1982). Benthic tunicatesrepresented 20% of the invertebrate species collectedwith a trawl in an area with polymetallic nodules(Monniot and Monniot, 1985). Nodules occur at lowabundance (1–2 kgm−2) in areas of thick sediments,compared to areas with thin sediments (3.5–5 kgm−2)(Sharma et al., 1997). Meiofauna and macrofauna havebeen quantified from sediment cores collected fromnodule areas. Meiofaunal density ranges from 0.3 to4.5 individuals cm−2, dominated by nematoda. Themacrofaunal density ranged from 8–64 individuals m−2,generally dominated by polychaetes (Sharma et al.,1997). In the abyssal region of the central IndianOcean in the 3500 to 4500m depth range, in brownoozy sediments with polymetallic nodules, meiofaunaldensities range from 0.4 to 1.5 individuals cm−2

(Parulekar et al., 1982). These sediments had almostthree times the macrofaunal biomass (5.16 gm−2) ofthe yellow calcareous oozy sediments without nod-ules (1.78 gm−2).The mean benthic population density (meiofauna

and macrofauna) from abyssal sediments varies from233 322 individuals m−2 in the 1500–1999m depthzone to 50 269 individuals m−2 in the 5500–5999mdepth zone (Parulekar et al., 1992). The abundanceof both macrofauna and meiofauna decreases withincreasing water depth. However, the proportions ofmeiofauna and macrofauna are reversed with increasingdepth; the proportion of meiofauna increasing withdepth. Benthic biomass in the abyssal Indian Oceanis relatively poor, ranging from 0.11 to 12.75 gm−2,compared to other deep sea habitats in the Indian Ocean(Parulekar et al., 1982, 1992).It has been suggested that the supply of organic

material to the abyssal plains of the deep Indian Oceanresults from deep-water circulation transporting organicmatter from the shelf and slope to abyssal depths(Parulekar et al., 1982). It is possible to find terrestrialplant debris at depths of 4500m in the abyssal centralIndian Ocean (Parulekar et al., 1982).The diverse benthic fauna and the high values

of standing crop in the western and central IndianOcean are dependent on high organic production in theoverlying water column. The correlation in the abyssalIndian Ocean between the total oxidizable organiccontent of the water column and the benthic standingcrop is statistically significant (Parulekar et al., 1992).In addition, microbial activity and biomass showsignificant linear correlations with the vertical flux ofparticulate organic carbon (Lochte and Pfannkuche,2000). The high biomass on the central Indian Oceanabyssal plain is probably a result of high biologicalproductivity in surface waters (Humphrey, 1972).A close relationship between primary productivity andbenthic standing crop was observed (Parulekar et al.,1982), which is not consistent with the suggestion thatthe deep-water supply of organic matter from the shelfand slope to abyssal depths promotes high standingcrops of deep-sea benthos (c.f., Gage, 1978).

The oligotrophic abyss

The seafloor of the Bay of Bengal is characterized bya deep-sea fan cut by many distribution channels ofturbidity current fanning out from the north (Curray


and Moore, 1971; Ohta, 1984). During present sea-level conditions, a majority of sediments from largerivers (the Ganges and Brahmaputra) are reported to betrapped in the subsiding deltas and on the inner shelf,and thus little sediment and nutrients are transported bythe turbidity channels (Curray and Moore, 1971). TheBay of Bengal experiences wind-generated upwellingalong the coast, promoting primary productivity. Aver-age primary productivity in the Bay is 109.5 gCm−2 y−1

(Pant, 1992). The carbonate compensation depth inthese waters is ~4500m; at shallower depths, thecalcium-carbonate sediments are covered with thingreenish brown flocculent material.Limited data from this region come from sedi-

ments collected by grab samples; the total benthicbiomass (meiofauna and macrofauna) in the Bay ofBengal ranges from 0.11 to 0.38 gm−2 (Sokolova andPasternak, 1962, 1964; Neyman et al., 1973). Theseabundances appear to be low compared to the rest ofthe Indian Ocean. Despite the relatively low biomassof deep-sea benthic organisms in the Bay of Bengal,distinct biogenic features can be observed on thesurface of the deep-sea floor. Specifically, star-shapedfeeding traces produced by echiuran worms can beobserved (Fig. 7.7). They live between the depths of

Fig. 7.7. Star-shaped echiuran feeding trace from ~4000m in the Bayof Bengal. Modified from Ohta (1984).

2635m and 5025m (Ohta, 1984). As the organismfeeds on surface detritus, its proboscis skims thesediment surface of the deep-sea floor radially, leavinga distinctive star-shaped feature. These features are alsofound in the deep Pacific and Atlantic Oceans (Gageand Tyler, 1991; Gage, Chapter 11, this volume).The surface productivity by phytoplankton is poor,

and therefore the zooplankton biomass is poor (Pant,1992). As a result, total transport of organic matter to

the sea floor may be expected to be low. Thus, benthicbiomass and abundance in the Bay of Bengal appear toreflect the low surface productivity.

CONCLUSIONS AND OUTSTANDING PROBLEMS

The deep Indian Ocean is composed of a varietyof habitat types, including abyssal plains, oxygenatedslopes and basins, oxygen minimum zones, seamounts,and trenches. This chapter summarizes the availabledata from a few of these habitats. The generalconclusions are that the deep Indian Ocean still remainspoorly known, and is waiting to be discovered andunderstood. We have identified below specific areas thatneed to be explored within the Indian Ocean, includinghabitats and ecological rates.

(1) Complete benthic habitat descriptions forseamounts, the Java Trench, and other oxygenminimum zones (e.g., the Bay of Bengal) are notavailable. In order to understand the productivity ofthe Indian Ocean and compare it with other oceans,extensive benthic surveys need to be conducted. Inaddition, the acquisition of reliable data for biomassand abundance from seamounts is important in fish-eries. Current knowledge of the deep-sea organismsconstituting the Indian Ocean benthos is very limited.

(2) Total energy budgets and biomass estimatesfor all size classes are not available for any deep-seahabitat in the Indian Ocean. Complete estimates ofbiomass production for benthic populations are scarce.

(3) Composition, variability, and flux rate ofparticulate organic carbon to the seafloor is poorlyquantified throughout the Indian Ocean. The natureand flux of other food sources to the deep sea, (e.g.,phytodetritus, nekton falls), is also unknown for theIndian Ocean. Not only are these important foodsources for the deep-sea benthos – quantifying them isnecessary for calculating the global carbon budget.

(4) Data on ecological rates, including thebenthic response to the intense seasonal (monsoonal)production cycle, are scarce. Bioturbation rateshave been evaluated in oxygen minimum zone ofthe Oman slope, but data for other habitats in thedeep sea are very limited. Because the mining ofmanganese nodules is becoming more important, moreintensive research involving the responses of benthiccommunities to disturbance, both anthropogenic andnatural, is imperative.

(5) Chemosynthetic environments. There is recent


evidence of hydrothermal venting on the SouthwestIndian Ridge (German et al., 1998), and it would beof great biogeographic interest to see if the dominantfauna is more related to the Atlantic or to the Pacificvents. There is also evidence of reducing conditions insediments near the base of the oxygen minimum zone,where the fauna may gain energy from chemosyntheticprimary production.The Indian Ocean remains an exciting area for

pioneering research. A complete understanding ofits habitats and processes will not be possible untilthe outstanding problems mentioned above have beenresolved.

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