General characteristics of benthic faunas on the Amazon inner continental shelf with comparison to the shelf off the Changjiang River, East China Sea

(onlim'ltta1.1hellRese,rch.Vol 6. No. I/2. pp. 291 to 31(I. 1986.

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General characteristics of benthic faunas on the Amazon inner continental shelf with comparison to the shelf off the Changjiang River, East China Sea JOSEPtIINE Y. ALLER* a n d ROBERT C. ALLERt

(Received Jot publication 13 December 1985) Abstract--Major patterns of bacteria, meiofauna, and macrobenthos distributions were examined from muddy sediments on the inner continental shelf near the Amazon River. The spatial distributions and functional groupings of the fauna were compared with faunal distributions on the East China Sea shelf off the Changjiang (Yangtze) River and used to assess the impact of long-term, large-scale physical processes on benthic community structure in shelf environments. Both areas are influenced by large discharges of fresh water and suspended solids, and have inner-shelf mud deposits. On the Amazon shelf, macrofauna were generally small in size or absent from highly mobile, muddy sediments on the inner shelf. Significant numbers occurred only in relatively stable sediments offshore or to the south or northeast of the mud belt. Macrobenthos reached greatest densities (3915 m 2) in a general area of firm muddy sediments (Stas 79 and 88) interbedded with sand and burrowed by the ghost shrimp Callianassa sp. At these stations, diversity was greatest, particularly in the dominant polychaete taxon. Macrofauna in the East China Sea, particularly epifaunal species, were reduced in abundance and biomass in inner-shelf areas of most active accumulation near the Changjiang River. However, macrofauna were 50-100 times more abundant (ranging to 10,000 m 2) than at sedimentologically comparable locations on the Amazon shelf. For areas of reduced deposition on the East China Sea shelf, diversity was also greater, with 50 species per sample as opposed to a maximum number of 19 per sample of comparable size on the Amazon shelf. Bacterial and meiofaunal abundances were low nearshore on the Amazon shelf compared with farther offsbore or, in the case of meiofauna, other shelf environments. Average bacterial abundances for the top 10 cm of sediments ranged from 1.3 to 21 × 10<~g t on the Amazon shelf and from 7.1 to 22 x III<~g ~ on the East China Sea shelf. Meiofauna on the Amazon shelf varied from none in the nearshore mobile mud belt to 2045 (10 cm 2) offshore on firm silty clay sediments. These densities compare with 66 ( 10 cm 2) near the Changjiang River mouth and 4870 ( 10 cm -') farther offshore. Nematodes dominate the meiofauna on both shelves. The high degree of physical disturbance and unstable nature of the seabed coupled with reduced detrital food availability in bottom sediments appear to be the major factors limiting faunal abundances, controlling the taxonomic diversity of the meio- and macrofaunal species, and determining the functional groups of macrobenthos on the Amazon inner shelf, and, to a lesser extent on the East China Sea inner shelf.

INTRODUCTION TIlE SEDIMENTS a n d b e n t h i c f a u n a s o f t h e n o r t h e a s t e r n B r a z i l c o n t i n e n t a l s h e l f a r e s i g n i f i c a n t l y a f f e c t e d b y t h e v a s t A m a z o n i a n d r a i n a g e c o m p l e x (KEMPF, 1970; BARRETO, et al., 1975). T h e c o m b i n a t i o n o f t r e m e n d o u s a m o u n t s o f f r e s h w a t e r (6 x l 0 is

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* Department of the Biological Sciences, Wayne State University, Detroit, MI 48202, U.S.A. Present address: Marine Sciences Research Center, SUNY at Stony Brook, Stony Brook, NY 11794, U.S.A. t Department of the Geophysical Sciences, University of Chicago, IL 60637, U.S.A. 291

J. Y. ALLER and R. C. ALLER

292

GIBBS, 1967) and suspended solids (1.2 × 1015 g-l; MEADE et al., 1985) discharged from the Amazon River, together with a system of strong coastal currents (METCALV, 1968), and high amplitude tides near the river mouth (GraBS, 1976) control circulation of shelf waters and influence sediment transport and depositional processes (KuErtL et al., 1982; NIXrROUER et al., 1983). The interaction of benthic organisms with sedimentation processes on such physically active continental shelves is poorly understood. The main emphasis of this paper is to characterize the benthic biological communities on the Amazon shelf and to assess the impact of long-term, large-scale physical processes on these communities. The major distributional patterns of bacteria, meio- and macrobenthos from muddy sediments at 18 stations on the inner continental shelf near the Amazon River are examined and functional grouping of organisms and potential sedimentorganism interactions identified. To put into perspective the results of our Amazon shelf study, comparisons are then made between spatial distributions and trophic structure of the Amazon benthos with those on the East China Sea continental shelf near the Changjiang (Yangtze) River. This area is also influenced by large volumes of fresh water and suspended-solid discharge, and an inner-shelf mud deposit similar to that of the Amazon shelf extends along the c o a s t (CHIN, 1979; CHENGet al., 1983; STERNBERGet al., 1983). BACKGROUND

There are few published studies of the benthos from the Brazilian continental shelf and these appear mainly to be qualitative reports of organisms collected from grabs or otter trawls (CoELHO, 1970; COELHOand KOENING, 1972; KEMPFand MATrHEWS, 1968; NONATO and LUNA, 1970a, b; TOMMASI, 1967, 1969; T1NOCO, 1971). KEMPF (1970) provides the most comprehensive discussion in English of the characteristics of bottom sedimentary environments and biological populations on the northeast Brazilian shelf. Hermatypic corals and calcareous algae form a coastal reef which parallels the northeast Brazilian shoreline about 2 km offshore from the Silo Francisco River northward. Limited regions of mud occur landward of the reef as a result of discharges from small coastal rivers. Seaward of the reef are regions of relict quartz sand grading into calcareous algal debris (KEMPF et al., 1968; MABESOONEand TINOCO, 1967). Farther north, there is a gradual disappearance of the typical northeast Brazilian shelf biological communities as a result of the Amazon River complex and large input of terrigenous debris (CAVALCANTIet al., 1965). The coral reefs and hermatypic corals are replaced by ahermatypic species. Off the Para and Amazon River mouths, calcareous algal bottoms, found at 20-m depth farther south, occur at depths below 40 m (KEMPV, 1970). Closer to shore, the large sediment discharge from the Amazon River causes high concentrations of suspended sediment in the water column (NrrrROUER et al., This Volume), fluid mud near the sea bed (FAAs, This Volume), a thick layer of reworked sediment and high accumulation rates in the sea bed (KUEHLet al., This Volume a). The result is that in the shallow water (<40 m) of the inner shelf of the Amazon region, macrobenthos are scarce. METHODS

Samples were collected during 27 May-15 June 1983 on the R.V. Iselin cruise 8306 from 18 stations on the Amazon continental shelf. The sampled area was bounded to the

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Fig. 1. (A) Map of northeastern South America showing position of study area on the Amazon inner continental shelf. (B) Station locations in relation to grain size distributions (after NITTROUER et al., 1983). (C) Distribution of bacterial standing stocks (× 10~ g 1) over the upper 10 cm of sediment. (D) Distribution of total and temporary 0 meiofaunal abundances (per 10 cruZ). Densities integrated over the upper 10 cm except at Sta. 47, where the depth is 25 cm. (E) Distribution of macrofauna (per m z) over upper 20 cm depth.

295

Benthic faunas of the Amazon continental shelf

southeast by the Para River (0 ° latitude) and on the northwest by the Brazil-French Guiana border (4°N) (Fig. 1A). The station locations are shown in Fig. lB. Water depths, and a description of selected, biologically significant sedimentary properties at each station are given in Table 1. Biological samples were taken using a 0.25 m 2 USNEL box corer (HESSLER and JUMARS, 1974) partitioned into four quadrants and equipped with the modifications originated by R. R. Hessler, P. A. Jumars, and J. Finger to reduce bow-wave effects. The base support frame of the corer tripod was sometimes outfitted with wooden 'snowshoes' to prevent overpenetration in very soft sediments. Bacteria and meiofauna were sampled with a 7.36 cm (i.d.) subcorer and processed as follows. At sea, the water overlying the subcore was drawn off, and the sediment extruded upward and sliced into the following layers: 0-1, 1-2, 2-3, 3-5, 5-10 cm. Sediment from stations in soft mud areas were divided into 5-cm depth intervals. Samples were fixed in buffered 0.5% gluteraldehyde (in 3% NaCI) and kept refrigerated. A 14.7-cm diameter subcorer was used to take macrofaunal samples to a depth of 20 cm from one of the four subcore quadrants. These sediment samples were washed on a 0.500-mm mesh sieve onboard using filtered seawater and the 0.500-ram fraction preserved in buffered 10% formalin containing rose bengal stain. Sediment water content was estimated by weight loss of wet sediment samples dried at 60°C. Organic carbon and nitrogen data for each station were obtained from kasten cores Table I. Station

t

Stations on the Amazon continental shelf with description of sedimentary characteristics

Water depth

Sample depth

Water content

Bulk* density

(m)

(em)

(%)

(g cc-')

23

33 13

24

18

25

52

41

26

42

16

47

10

52

13

79 Inshore B 88 112 113 121

13 3 50 13 8 12

123 118

40 12

119

9

0-5 0-10 10-20 0-10 10-20 0-10 10-20 0-10 10-20 0-10 10-20 0-15 15-25 25-35 35-45 0-5 0-10 0-5 0-1 0-1 0-10 10-20 0-1 0-10 10-20 0-10 10-20

46.8 61.4 61.1 61.7 59.2 54.8 42.0 59.6 58.4 62.7 55.7 74.6 54.6 66.3 56,9 37.0 51.5 31.1 42.9 23.4 46.8 32.7 55,5 55,8 47,4 64.3 52.0

0.796 0.508 0.513 0.503 0.547 0.629 0.908 0.540 0.561 0.486 0.612 0.302 0.633 0.426 0.589 1.04 0.695 1.21 0.886 1.46 0.796 1.16 0.616 0.610 0.782 0.459 0.685

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N%

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4.6 6.5 6.5 6.6 10.2

0.061 0.0116 (I.094 0.050 0.1111 0.088

10.5 7.2 10.1 7.2 8.5 7.4

C/N

(mole ratio)

0.110 0.101

7.7 6.9 6.3 6.6 5.5 7.4 7.1 6.6 6.4

8.7 9.11

296

J.Y. ALLER and R. C. ALLER

taken at the same locations as the 0.25-m z box cores. Sampling procedures and analytical techniques for organic carbon are described in ALLER et al. (This Volume). Total nitrogen was measured using a modification of micro-Kjeldahl procedures described by ALLEN et al. (1974). In the laboratory, bacterial samples were counted directly, using the acridine orange/ epifluorescence method of HOBBLEet al. (1977) as modified by WATSON et al. (1977). Abundances are expressed per gram dry weight of sediment. Weights were determined from aliquots of known volume collected on preweighted filters which were then washed with distilled water to remove salts and dried at 50°C. The precision of bacterial values was estimated from 5 independent counts of one sediment sample. The 95% confidence interval for the mean number of cells was 17.64 _ 0.688 × 109 g-1. Meiofaunal samples were washed on 0.041-mm mesh sieves, stained with rose bengal and sorted under a dissecting microscope. Macrofauna were sorted under a dissecting scope and identified to the finest taxonomic category possible. The rarefaction method of HURLBERT(1971) was used to examine the number of macrofaunal species at different population levels at each station. Meiofauna and sediment bacterial samples collected from the East China Sea in November 1981 (using a 0.25 m 2 USNEL box corer) were processed in the same way as Amazon samples (ALLER et al., 1985).

RESULTS

Bacteria

Bacterial abundances in the top 10 cm of the sea bed ranged from an average of 1.3 × 109 g-1 at Sta. 25 to 21 × 109 g-1 at Sta. 47. Figure 1C summarizes the average abundances over the top 10 cm at all stations while Fig. 2 illustrates vertical profiles of total sediment bacteria at stations located in four cross-shelf regions (Fig. 1B). Lowest abundances were found at the inshore stations which lie within the physically reworked mud belt. Station 47 in the north-central region was an exception, with abundances twice as high as the station with the next highest standing stocks (Sta. 18). Bacterial numbers at Sta. 47 reached 25 × 109 g-1 at 5-10 cm depth correlating with a peak abundance of copepod nauplii larvae. The range of standing stocks is sufficiently large at the bestsampled water depth (-13 m) that no regular bathymetric pattern was evident to >40 m (Fig. 1C). At most stations standing stocks were highest in the upper-most depth interval in spite of the high water content of a fluid mud layer on some cores, often as much as 20 cm thick. In general, standing stocks decreased with depth in the cores. Increased bacterial numbers with depth at Stas 47 and 52 corresponded to the appearance of a relatively firm substratum underlying the 15-20 cm thick fluid mud layer at Sta. 47 and 25 cm thick fluid mud layer at Sta. 52. Bacteria decreased below a depth of 20-200 cm (sampling limit). This is probably related to depletion of reactive organic matter (Fig. 3). Meiofauna

Meiofaunal abundances varied from 2045 (10 cm 2) at Sta. 123, located directly east from the river mouth (40 m of water; firm silty clay) to 0 at Sta. 52 (13 m of water; fluid mud) in the north-central region (Figs 1D and 4, Table 2). Nematodes dominated the

Benthic faunas of the Amazon continental shelf

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meiofauna at the majority of stations, constituting over 78% of total individuals at 8 of the 15 stations sampled. At Sta. 123 they made up 97% of all individuals. With the exception of Sta. 47, meiofaunal abundances at the inshore stations (depth of <13 m), located within fluid muds, averaged 10 (10 cm-2) (range 0-59) and consisted of nematodes and an occasional juvenile tubicolous polychaete. At Sta. 47 total meiofaunal numbers were 169 (10 cm-2), with copepod nauplii the dominant taxon, comprising 86% of all individuals. These larvae were most abundant at depths between 5 and 10 cm suggesting instability of the surface muds due to recent deposition. Copepods, the second most abundant group overall, never made up more than 28% of all individuals at any other station. Juvenile polychaetes, the third most abundant group, generally averaged <16% except at Stas 42 and 23 where 1 or 2 tubicolous individuals were the only organisms found. In most fluid muds, animals tended not t o occur in the top 2 cm sampled. Depth distributions in some cases are not meaningful because of the transient and fluid nature of the surface layers. At other stations with more consolidated sediments (for example Stas 25, 113, and 119) meiofauna were found from the surface to below 5 cm, although only polychaetes and nematodes were found below 3 cm. Peak abundances of copepods, polychaetes, and bivalves always occurred in the uppermost depth interval. On the other hand, nematode densities were highest between 1 and 3 cm. Juvenile polychaetes found between 2 and 5 cm are predominantly tubiculous, and many were maldanids. It is likely that these tubes extended to the sediment surface.

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Macrobenthos

At stations within the inshore, fluid muds (Stas 119, 121, 123, 118, 41, and 25) no macrofauna were found (Fig. 1E). In contrast, at Stas 79 and 88 in an area of firm muddy sediments interbedded with sand and well burrowed by the ghost shrimp Callianassa sp., densities reached 3915 m -2 (Sta. 88) with diverse taxonomic groups (Table 3) and considerable diversity within the dominant polychaete taxa (Table 4). Figure 5 shows rarefaction diversity at stations on the Amazon shelf. The curves separate into two groups corresponding to the relative stability of the bottom. In spite of large numbers of individuals at Sta. l l G , diversity is low with individuals living epifaunally attached to shell debris or in abandoned shells. No infauna were present. Life habits and feeding strategies of the macrobenthic polychaetes- (based on FAUCHALDand JUMARS, 1979; and our personal observations) at these and other stations on the shelf are included in Table 4. Although the only macrofauna at Sta. 47, which lies north and downcurrent from the interbedded mud and sand area of relatively stable deposits, were maldanid polychaetes (59 m-Z), numerous juvenile maldanid polychaetes [14.8 (10 cm) -2] were also found. At Sta. 123, (40 m, southern region) no macrobenthos were recovered although numerous large shells of the protobranch Yoldia sp. were found and appear to represent a recent death assemblage. Biogenic sedimentary structures were also abundant at Sta. 123 providing additional evidence for infaunal populations in the recent past. Juvenile Table 3.

Macrofauna (individuals per m z calculated from sample size o f 170 cm 2) at stations on the Amazon continental shelf. Number o f species in each taxon is given in parentheses

Station 11G Water depth (m) Taxon Polychaeta Bivalvia Gastropoda Scaphopoda Holothuroidea Sipunculida Crustacea Amphipoda Ampeliscidae Caprellidae Gammaridae Tanaidaea Ostracoda Copepoda Anomura Callianassa sp. Pagurus sp. Cirripedia Lepas sp.

24

79

54

42

47

88

112

33

18

13

50

16

10

50

13

1941 (1) 0 0 0 0 0

0 0 0 0 0 0

529(5) 0 0 59(1) 59(1) 0

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0 0 0 0 0 0 0

0

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0 0

235(1) 0

0 0

118(1)

0

0

59(1)

0

0

2942

0 0 5911) 0 0 0

176(2) 0 0 0 0 0

59 0 0 0 0 0

2478( 11 ) 59( 1) 0 59(l) 0 413( 1)

0 0 0 0 0 0

0 0 0 0 0 0

0 0 0 0 0 0 0

0 53 ! ( 1) 59( 1) I 18( l ) 59( l ) 177(l) 0

0 0 0 0 0 0 176(1)

0 0

0 0

0 0

0 0

0

0

0

0

0

0

0

0

0

0

0

59

0

0

0

0

0

0

59

882

59

176

59

3953

176

l(calanoid)

Anthozoa

Madreporaria Astrangia sp. Teleost Anguilliformes 'mud eel' Total

301

Benthic faunas of the Amazon continental shelf

Table 4. Distribution of polychaetes in Amazon shelf sediments. Numbers per m 2 were calculated from a sampled area of 170 cm 2 Station 88

79

42

11G

47

Family Paranoidae Spionidae Serpulidae

112

Functional *

group 176

59 118 1941

DDF T,SDF T,S

Vermiliopsis sp. Cossuridae Amphinomidae

59 118

DDF P

Chloeia viridis

Arenicolidae Eunicidae

59

DDF T,P

118

P

59

Eunice sp. Goniadidae

Opioglycera sp. Hesionidae

235

P,Sc

Hesione picta Lumbrineridae

59

P,DDF

Lumbrineris sp.

Maldanidae Nephetidae

353 294

235

T,DDF P

Nephtys squamosa Orbinidae

471

DDF

Scoloplos agrestis Owenidae

59

T,S

59

T,DDF P

Owenia fusiformis Pectinaridae Syllidae

59 588

Typosyllis sp. Terebellidae

59

T,SDF

* D D F = deep deposit feeder, P = predator----carnivore, S = suspension feeder, Sc = scavenger, SDF = surface deposit feeder, T = tube dwelling.

bivalve densities were 5 (10 cm) -2 and juvenile polychaetes densities were 46 (10 cm) -~. At Sta. 11G (33 m, northern region, an erosional shell-lag mixed with mud) large numbers (1941 m -2) of the serpulid polychaetes Vermiliopsis sp. were found attached to shell debris. Gooseneck barnacles (Lepas sp.) (118 m -z) and the solitary ahermatypic coral Astrangia sp. (59 m -2) also were found attached to the shell debris. Epifaunal hermit crabs (824 m -2) were abundant. DISCUSSION

Amazon shelf benthic faunal deposits The nearshore deposits of the Amazon subaqueous delta complex (topset beds, NIJ'rROUERet al., 1986) generally have a surface layer of fluid mud (FAns, This Volume) and are generally devoid of macrobenthos. Significant numbers of macrofauna were present only in the region of sandy mud off the river mouth, on stable relict deposits, or north of the high accumulation rate areas of fine-grained sediment. Meiofaunal and bacterial abundances were also lower in the nearshore fluid muds than farther offshore

302

J . Y . ALLER and R. C. ALLER 18

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Rarefaction diversity for macrofauna at stations on the Amazon shelf. The termination of each curve is the actual number of individuals and species found in the sample.

or, in the case of meiofauna, many other shelf envi~-onments (WIGLEYand MCINTYRE, 1964; MCINTYRE and MURISON, 1973; TENORE et al., 1978; YINGST and RHOADS, 1978; TIEFJEN 1984). Although bacteria had low densities in the most fluid (-80% water) surface mud deposits, their average density taken over the uper 20 cm was not unusually low compared to those reported for various shelf environments (Table 4, YINGST and RHOADS, 1985). There was no evidence of deeper burrowing macrofauna below the sampled top 20 cm nor were biogenic structures apparent near the surface or at depth (KuEHL et al., This Volume b) except at Stas 79 and 88 in the Callianassa biotype (sandymud area) and Sta. 123. This suggests that the shallow faunal distribution patterns documented here are real and not an artifact of insufficient depth of sampling. Aside from low abundances, other important diagnostic characteristics of the macrofauna are small-size (polychaetes = <2 cm in length), low diversity, and high mobility. Suspension-feeding bivalves were absent at all sample sites. Inshore stations (<30 m) were dominated by juvenile and small adult polychaetes, most of which were mobile deposit feeders or carnivores (Table 4). Although 17 polychaete families were represented overall, only one species from each of these families was found. Other areas of the Brazilian shelf have often been characterized by multiple species in each polychaete family represented (NONATO and LUNA, 1970a, b). The largest abundances, but not necessarily greatest diversity of infauna, were present on relatively stable mud or sandy bottoms rather than in the fluid mud regions; for example, at Sta. 11G with a relict shell lag, large numbers of the serpulid polychaete Vermiliopsis sp. (1941 m-2) occurred, but that was the only polychaete present. Hermit crabs and other epifauna dominated at this station but diversity was low (Fig. 5). The greatest density as well as diversity of

Benthic faunas of the Amazon continental shelf

303

macrobenthos were found at Sta. 88 located in poorly sorted muddy sands near the Callianassa biotype. At Sta. 88, 19 species were collected although not a single specimen of a callinassid shrimp was found. The meiofaunal component was dominated by nematodes at most stations on the Amazon shelf, comprising up to 85% of all meiofaunal individuals. This relatively high percentage compared to many shallow water environments is largely due to the absence of foraminifera (Table 2). Benthic foraminifera were found only at Sta. 123. Total abundances of nematodes tended to be lower than documented for other shelf environments (CouLL et al., 1977; WIGLEYand MCINTYRE, 1964; TENOREet al., 1978) except at Stas 24 and 123, where densities reached 900 and 1976 (per 10 cm 2) respectively. Nematode diversity was also limited. Individuals belonging to the species Theristus sp. (family Monhysteridae) were most abundant. Large numbers of harpacticoid copepod and decapod larvae were present at Sta. 47 (downcurrent from the macrofauna-rich muddy sand corridor) as were high standing stocks of bacteria (21 x 109 g-l, averaged over top 10 cm). There are at least three types of non-exclusive explanations for the observed depauperate faunal composition and its distribution on the Amazon inner continental shelf: (1) predation, (2) food limitation, and (3) physical disturbance. It is unlikely that predation could produce the observed patterns as no obvious major predators were recovered or observed. Fishing activity in the area was also not observed, suggesting the absence of large, bottom-feeding fish. Indeed, MABESOONEand TINOCO(1967) note that bottom fish were not abundant on the northeast Brazil shelf. Primary production in the Amazon shelf waters overlying stable bottom sediments is at least comparable to other coastal waters. A value of 0.86 g m -2 day-1 was reported for a site near the muddy sand region (Stas 112, 113, 79, and 88) (TEIXERIAand TUNDISI, 1967). Primary productivity is inhibited in the turbid nearshore waters (DEMASTERet al., 1983, This Volume). Any planktic detritus falling onto the sea floor is also diluted by the large volumes of terrigenous sediment supplied from the Amazon. Nevertheless, bacterial standing stocks (except in the most fluid mud stations) are comparable to those in other nearshore deposits (Y1NGST and RHOADS, 1985), and nitrogen regeneration rates (NH~production - 3 mmole m -2 day-~ in the upper 20 cm) are also comparable to many other nearshore areas which support greater abundances of infauna (ALLER et al., This Volume). This implies that the quantity of reactive or metabolizable organic matter is probably not the major limiting factor for bottom organisms. Near-bottom and pore water salinities were equal or close to open ocean values during the sampling period. This was during a time of high runoff of the Amazon so that bottom salinities are unlikely ever to be significantly lower. Bottom waters are also well oxygenated (TEIXERIA and TUNDISI, 1967; EDMONDet al., 1981). The large input of terrigenous debris is extensively reworked and remobilized by a combination of wave and current activity (KuEHL et al., This Volume a, b; N~ROUER et al., This Volume). It seems most likely that the major factor limiting faunal abundances, controlling the taxonomic diversity of meio- and macrofaunal species, and determining the life history and functional groupings of macrobenthos, is the physically disturbed nature of the innershelf environment. This is in agreement with previous interpretations of KEMPF (1970), who noted the bathymetric displacement of benthic communities found in shallow water south of the Amazon into offshore and deeper water north and east of the Amazon.

304

J . Y . ALLER and R. C. ALLER

Comparison of Amazon shelf with East China Sea The Amazon shelf faunal characteristics can be placed in perspective by comparing them with another shelf region influenced by a large river, the East China Sea (Figs 6A and B). In inner-shelf areas of most rapid sediment accumulation off the Changjiang River, macrobenthos abundances, biomass, and species diversity were low and small deposit feeders dominated with few epifaunal species present relative to areas of slow

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Fig. 6. (A) Map of eastern Asia showing China, the Changjiang River, and position of study area in the East China Sea. (B) Station locations are shown in relation to grain size distributions (after CHIN, 1979). (C) Distribution of bacterial standing stocks (x 109 g ~) over the upper 10 cm of sediment. (D) Distribution of total meiofaunal abundances (per 10 cm 2) over the upper 5 cm. (E) Distribution of macrofauna (per m 2) (from BOESCH et al., 1983).

accumulation, farther offshore (Ltu and Hsu, 1963; BOESCHet al., 1983; RHOADSet al., 1985). Although the relative pattern in the East China Sea is similar to that on the Amazon shelf, the macrofauna were 50-100 times more abundant in similar locations (>10,000 m-2 nearshore at Stas G8101, 8102 and 8103) and were distributed to depths of 40 cm in these deposits (BoESCH et al., 1983) (Fig. 6E). In areas of muddy sand with reduced sedimentation, species richness was greatest, with 50 species per sample. In contrast, on the Amazon shelf (i.e. bottomset region, which is most similar to offshore muds of the Huanghe (Yellow) River dispersal system), maximum diversity was 19 species per comparable size sample at Sta. 88. Polychaetes at inshore stations near the Changjiang were small capitellids and cirratulids; taxa characteristic of disturbed environments. These were replaced offshore by larger individuals and additional taxa, such as ophiuroids and infaunal crabs (Xenophthalmus pinnothoroides). Suspension feeding

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307

Benthic faunas of the Amazon continental shelf

bivalves, onuphid, nepthyid, and glycerid polychaetes became abundant in the offshore muddy sands with slow accumulation rates (BoESCHet al., 1985; J1AN6et al., 1983). In the East China Sea, meiofauna were dominated by nematodes but unlike the Amazon, foraminifera were also abundant (Table 5, Fig. 6D). Total meiofauna generally were more abundant in East China Sea sediments ranging (in the upper 5 cm) from 66 (10 cm -2) (83% nematodes) closest to the river mouth (Sta. G8128) to 4870 (10 cm -2) (50% nematodes) at Sta. M4, which lies in 49 m of water seaward of the region of rapid accumulation. Station M4 also had the greatest meiofaunal diversity of all stations (Table 5). FLEEGERet al. (1983) reported lower abundances, less variability, and lower diversity among harpacticoid copepods and kinorhynchs in nearshore vs outer-shelf deposits. Physical stability is thought to play a major role in allowing greater meiofaunal diversity in deep-water relative to shallow-water environments (T]ETJEN, 1984) and in structuring benthic communities in general (RHOADSand BOYER, 1982; THAYER, 1983; YINGSTand RHOADS, 1985). The meiofaunal distribution patterns relative to depositional and stability gradients are similar between the Changjiang and Amazon shelves but abundances and diversities are higher off the Changjiang. Comparison of ranked correlation coefficients using Spearman's method (ZAR, 1974) shows no correlation between meiofaunal abundances and bacterial standing stocks in either environment (Fig. 7). On both shelves, large changes in meiofaunal abundances occur over a relatively small range in associated bacterial numbers (Fig. 7). Although this suggests that the availability of bacteria as food is not a major factor limiting the distribution or abundance of meiofauna, standing stocks do not necessarily reflect production rates (Y]NGSTand RHOADS, 1978). In contrast to the macrofaunal and meiofaunal standing stocks, total bacterial abundances were similar in deposits of the East China Sea and the Amazon shelf (Figs 1C and 6C) and as compared to other shelf regions (Table 4 in YINGSTand RHOADS, 1985). At stations in the East China Sea, bacterial abundances (averaged over the top 10 cm) were AMAZON SHELF

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308

J . Y . ALLER and R. C. ALLER

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from 7 to 22 × 10 9 g-l, no higher than at some Amazon shelf stations. This is consistent with the similar concentrations of organic matter (organic C: 0.5% for the East China Sea; 0.6-0.8% for the Amazon shelf) and comparable remineralization rates as measured in surface sediments of the two environments (ALLER et al., This Volume). Exceptions to these generalizations are the extremely low bacterial numbers in the highly transient fluid mud surface layers (10-25 cm) at stations shallower than 13 m on the Amazon shelf. There appears to be a tendency for higher abundances in deeper water on both shelves in more stable sandy muds, although this correlation is not statistically significant as indicated by ranked correlation coefficients using Spearman's method (Fig. 8). The above comparisons suggest that the basic factors controlling the distribution patterns, abundances, and benthic community structure off these two large river systems are similar. The inner-shelf muds off the Amazon, however, are apparently more physically disturbed and unstable than those off the Changjiang, resulting in the virtual absence of macrobenthos and in reduced numbers of meiofauna over large regions. Acknowledgements--We thank those who assisted in the collection and processing of box cores, especially S. Rensing, J. Mackin and the crew of the R.V. Iselin. Laboratory assistance was provided by C. Ciokajlo, B. Negele, A. Ray, and L. Jasper. K. Sherman kindly confirmed identification of nematodes. We also appreciate the helpful comments of P. Jumars and D. Rhoads who reviewed this manuscript. This research was mainly supported by NSF grants OCE-8117709, OCE-8415413 and partly by NSF grant OCE-8309551 (to R. C. Aller) and E P A contract 807338-01 (to J. Y. Yingst).

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Benthic faunas of the Amazon continental shelf

309

ALLER R. C., J. E. MACKIN, W. J. ULLMAN, C. H. WANG, S. M. TSAI, J. C. JIN, Y. N. Su1 and J. Z. HONG (1985) Early chemical diagenesis, sediment-water solute exchange, and storage of reactive organic matter near the mouth of the Changjiang, East China Sea. Continental Shelf Research, 4, 227-251. ALLER R. C., J. E. MACKIN and R. T. Cox, JR (This Volume) Diagenesis of Fe and S in Amazon inner shelf muds: apparent dominance of Fe reduction and implications for the genesis of ironstones. Continental Shelf Research, 6,263-289. BARRETO L. A., J. D. MILLIMAN, C. A. AMARAL and O. FRANCISCONI (1975) Upper continental margin sedimentation off Brazil: northern Brazil. Contributions to Sedimentology, 4, 11-43. BOESCH D. F., A. TANG, F. XU and K. J. NILSEN(1983) Macrobenthos and biogenic structures in sediments of the East China Sea In: Sedimentation on the continental shelf with special reference to the East China Sea, Vol. 2, China Ocean Press, Beijing; Springer-Verlag, New York, pp. 937-946. CAVAL('ANTI L. B., P. A. COELHO, M. KEMPF, J. M. MABESOONE and O. C. DA SILVA (1965) Shelf off Alagoas and Sergipe (NE Brazil). 1. Introduction. Trabalhos Oceanograficos Universidade Federal de Pernambuco, 7/8, 137-150. CHENG G., J. Q1AN, C. A. NITFROUER, D. J. DEMASTER and B. A. McKEE (1983) Modern sediment structure of East China Sea. In: Sedimentation on the continental shelf with special reference to the East China Sea, Vol. 2, China Ocean Press, Beijing; Springer-Verlag, New York, pp. 511-521. CHIN Y. S. (1979) A study on sediments and mineral composition of the seafloor of the East China Sea. Oceanic Selections, 2, 130-142. COEIJtO P. A. (1970) A distribuicao dos crustaceos decapodos reptantes no norte do Brasil. Trabalhos Oceanograficos Universidade Federal de Pernambuco, 9/11,223-238. COEI,HO P. A. and M. C. KOENING (1972) A distribuicao dos crustaceos pertencentes as ordens stomatopoda, tanaidacca, e isopoda no norte e nordeste do Brasil. Trabalhos Oceanograficos Universidade Federal de Pernambuco, 13,245-260, COULL B. C., R. L. ELLISON, J. W. FLEEGER, R. P. HIGGINS, W. D. HOPE, W. D. HUMMON, R. M. RIEGER, W, E. STERRER, H. THIEL and J. H. TIETJEN (1977) Quantitative estimates of the meiofauna from the deep sea off North Carolina, U.S.A. Marine Biology, 39,233-240. DEMASTER D. J., G. B. KNAPPand C. A. NITTROUER (1983) Biological uptake and accumulation of silica on the Amazon continental shelf. Geochimica et Cosmochimica Acta, 47, 1713-1723. DEMASTt-R D. J., S. A. KUEHL and C. A. NITFROUER (This Volume) Effects of suspended sediments on geochemical processes near the mouth of the Amazon River: examination of biological silica uptake and the fate of particle-reactive elements on the Amazon continental shelf. Continental Shelf Research, 6, 1(17-125. EDMOND J. M., E. A. BOYLE, B. GRANT and R. F. STALLARD (1981) The chemical mass balance in the Amazon plume--l. The nutrients. Deep-Sea Research, 28, 1339-1374. FAAS R. W. 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