The influence of freshwater and material export on sedimentary facies and benthic processes within the Fly Delta and adjacent Gulf of Papua (Papua New Guinea)

ContinentalShelfResearch,Vol. 12, No. 2/3,pp. 287-326, 1992.

0278-4343/92 $5.00+ 0.00 © 1992PergamonPresspie

Printedin GreatBritain.

The influence of freshwater and material export on sedimentary facies and benthic processes within the Fly Delta and adjacent Gulf of Papua (Papua New Guinea) D. M. ALONGI,*P. CHRISTOFFERSEN,*F. TIRENDI*and A. I. ROBERTSON* (Received 12 December 1990; accepted 6 February 1991) Abstract--Large volumes of freshwater and suspended material debouch from the Fly River in southwestern Papua New Guinea into the Gulf of Papua, greatly influencing the hydrography and sedimentary processes within the river delta and adjacent shelf region. Sedimentary facies within the subtidal regions of the Fly Delta are composed mainly of compacted and eroded very fine black sand, and highly laminated, muddy sand and sandy mud, progressing to prodelta mud with intermixed primary and biogenic structures in the inner Gulf of Papua. These prodelta muds grade further to mixed terrigenous-carbonate deposits southwards into the northern Great Barrier Reef and Torres Strait, and to well-bioturbated, fluid mud northwards into the Gulf of Papua. The transition from physically-dominated, estuarine conditions within the delta to more quiescent, marine conditions on the shelf leads to concomitant changes in sediment chemistry, microbial activity and infaunal and epifaunal communities. Particulate (C, N, P) and dissolved inorganic and organic nutrient concentrations were a function of sediment type (higher in finer deposits) rather than location (delta vs gulf). C: N: P ratios of solid-phase nutrients varied greatly, but were usually less than those predicted by the Redfield ratio. Mean interstitial concentrations of dissolved inorganic nutrients were low (~M range), but dissolved organic carbon, nitrogen and phosphorus levels were equivalent to those found in higher latitude systems. Fluxes of dissolved inorganic nutrients were generally low ~mol m 2 day- 1). Flux rates were mostly negative (into the sediment) in the delta suggesting that these deposits are a sink for nutrients. In the offshore deposits, dissolved inorganic fluxes were higher and mostly positive indicating that they are a source for dissolved nutrients. Standing crops of bacteria (range: below detection limits--2.5 × 10l° cells g-l dry wt), meiofauna (range: 5-750 individuals 10cm-2; 9-1006/~g dry wt 10 cm-2) and infauna (range: 86-5555 individuals m-Z; 0.10--5.85 g AFDW m -2) were generally lower in the delta than in the gulf. The infauna was dominated by nematodes, copepods, foraminifera and small, tube-building, deposit- and suspension-feeding polychaetes and amphipods. Rates of bacterial productivity were very erratic with sediment depth across stations, ranging from 0-2108mg C m -2 day 1 (DNA synthesis) and from 0-228mg C m-2 day-1 (protein synthesis), respectively. Rates of benthic respiration and DOC flux across the sediment-water interface were generally high, ranging from 63-780 mg C m -2 day 1 and from -797 to 514 mg C m -2 day -1, respectively. Epibenthos were more diverse (at the phyletic level) at the mid-shelf than inshore, composed mainly of sponges, crabs, crinoids, echinoids, bivalves, hydroids and asteroids. Demersal nekton abundance was low, dominated by the leatherjacket, Paramonacanthus filicauda, the pony fish, Leiognathus splendens and the grunter, Pomadasys argyreus, suggesting limited transfer of infaunal biomass to higher trophic levels. The response of the benthic regime to the export of freshwater and material from the Fly River generally conforms to the RHOADSet al. [(1985) Continental Shelf Research, 4, 189-213] model of * Australian Institute of Marine Science, PMB No. 3, Townsville MC, Queensland 4810, Australia. 287

288

D.M. ALONGIet al. benthic response to effluentderived from the ChangjiangRiver in the East China Sea and is similar to infaunal and sedimentary patterns off the Amazon. Nutrient release from the delta sediments contributes little to water-column production, but in the gulf, nutrient efflux from the benthos contributes, on average, 38 and 61% of the annual N and P requirements of phytoplankton production, reflectingcloser benthic-pelagiccouplingand enrichment of biologicalproductivityin the Gulf of Papua due to nutrient export from the Fly River. INTRODUCTION

THE major tropical and subtropical rivers contribute approximately 70% of the freshwater and 74% of sediments delivered into the world's oceans from the continents (MILLIMAN and MEADE,1983). Most of the mud and organic detritus in the marine tropical biosphere is derived from these rivers, and deposits mainly in the riverine deltas as intertidal flats and on the adjacent inner continental shelves as fluid mudbanks (WRIGHT,1989). Considering their importance to estuarine and nearshore processes, it is surprising that so little information is available about the effects of these high freshwater and sediment discharge rates on benthic processes in proximity to the major tropical rivers (see review of ALONGI,1990a). Most previous studies have been conducted in proximity to the Amazon (ALLER and ALLER, 1986; KUEHL et al., 1986), Ganges, Brahmaputra, Irrawaddy and Indus Rivers (NEYMAN,1969; SAVICH,1971, ANSARI et al., 1977), primarily to examine the distribution and abundance of macrofauna and, to a lesser extent, bacteria and meiofauna. Similar infaunal studies have been carried out off smaller river mouths in India (e.g. ANSARI et al., 1982) and Indonesia (DE WILDE et al., 1989; KASTOROet al., 1989), but only the Amazon studies offer a comprehensive picture of benthic structure and function as influenced by runoff from a major tropical river. The RHOADS et al. (1985) model for the effects of the Changjiang River on benthic standing stocks and sedimentary facies offers a conceptual framework to examine benthic processes in the vicinity of major tropical rivers. Based on their work in the East China Sea off the Changjiang, their model indicates that close to the river mouth episodic deposition and erosion events occur, leading to alternating recolonization and extinction of benthic communities and highly laminated sedimentary structures. With distance from the river delta, rates of sediment deposition decrease and rates of primary production increase as a result of the export of river-derived nutrients. In these inner and mid-shelf areas, conditions allow for the development of abundant benthic communities with burrows, tubes and feeding pockets obliterating physical laminations. Most of this infauna is surface-feeding on bioseston (deposited, dead phytoplankton). Benthic standing stocks are then predicted to decline to the outer shelf because of oligotrophy. The response of infaunal communities off the Amazon is very similar to the model suggested for the benthos off the Changjiang, but more comprehensive investigations are necessary to determine the universality of the RHOADS et al. (1985) model for other major rivers in the tropics and subtropics. Such research is particularly urgent as many of these river systems may be irreparably altered in the near future by extensive mining and/or agricultural activities (HATCHERet al., 1989). This paper reports on such an investigation of infaunal and epifaunal benthos, including demersal fishes, sedimentary structures and metabolic and biogeochemical processes from within the delta of the Fly River out to the adjacent, mid-shelf area in the Gulf of Papua off Papua New Guinea. Since 1984, mining activities on the upper reaches of the Fly River system have been adding as much as 70 million tonnes per annum to the total suspended

Benthic dynamics in the vicinityof the Fly River

289

sediment load (GEORG, 1989). This work was part of a project to determine the influence of freshwater, sediment and particulate and dissolved nutrient export from the river on pelagic and benthic processes within, and adjacent to, the river delta before mining activities p e a k in the mid-1990's.

THE STUDY AREA The Fly River, in Papua New Guinea, is approximately the tenth largest tropical river in the world (Fig. 1) with annual freshwater and sediment discharge rates exceeding 235 km 3 and 70 x 106 tonnes, respectively (ALONGI, 1990a). The river drains most of the southwestern region of Papua New Guinea where precipitation varies from - 2 0 0 0 m m y-1 on the coast up to - 13 m y - i at the headwaters in the Star Mountains of central New Guinea. The river is - 1 0 0 0 km long and the catchment area is - 7 6 , 0 0 0 km 2. The lower catchment area is a very flat, alluvial floodplain of swamps, backswamps and lakes giving way to the delta front consisting of several large and numerous small prograding islands. This lower region is subjected to tidal bores during spring tides. The intrusion of salt water is limited to - 1 0 0 km from the mouth of the delta. Mangrove forests cover 87,400 hectares of vegetation on delta islands, with Rhizophora-Bruguiera forests dominating in areas where river salinities are >10%o

"

':;:if:, o o,o .

-.

._].++ T2

.ii:

*'

+,+,-o. . . . . .

GP5

Fig. 1. Chart of the 14 benthic stations and six grids sampled for demersal trawl catch in the Fly Delta, Gulf of Papua and northern Great Barrier Reef (Bramble Cay). Isobaths are in metres. Dashed areas represent sand bars at the mouth of each entrance to the delta.

290

D.M. ALONGIet al.

(ROBERTSONet al., 1990). In regions where salinities are - 1 to <10%o, forests of the palm Nypa fruiticans are extensive, covering more than 38,000 hectares of island area. ROBERTSONet al. (1990) estimated total daily net primary production of mangroves within the delta on the order of 2216 tonnes C with approximately 678 tonnes C exported daily to delta waters. Mudbanks are well developed and extensive on the leeward sides of most of the delta islands with low to moderate densities of infauna, but with very high rates of bacterial productivity (ALONGI, in press). Microbial and biogeochemical evidence suggests that these intertidal mudbanks act as sinks for nutrients rather than as exporters of dissolved materials. Hydrographic and geological conditions in the adjacent Gulf of Papua are not well understood, but the small amount of information available indicates that much of the water and particulate material debouching from the Fly River is dispersed laterally, mainly in a northeasterly direction toward the mouths of the adjacent river deltas bordering the Gulf (MACFARLANE,1980; WOLANSKIet al., 1984). There is little flow into the Torres Strait (WOLANSKIand RUDDICK,1981; WOLANSKIand THOMSON,1984; WOLANSKIet al., 1988) as southwards dispersion appears to be somewhat limited by the complex currents in the Torres Strait and by large-scale water mass motion in the Coral Sea (ANDREWSand CLEG6, 1989). The circulation in the Gulf of Papua is characterized by strong tidal currents driven by the trade winds, freshwater runoff and, to a lesser degree, by the Coral Sea coastal current (ANDREWS and CLEG6, 1989). The available geological evidence supports this conclusion, as indicated by the sharp transition from terrigenous to reefal carbonate facies southwards away from the Fly Delta (HARRIS, 1988). Seismic data and drill holes have indicated the occurrence of subsurface, Miocene reefs in the Gulf of Papua which shares its geological evolution with the northern Great Barrier Reef (DAVIESet al., 1989). METHODS

Station locations, field sampling and sedimentological analyses Fourteen stations were visited within the Fly Delta and out into the Gulf of Papua and northern Great Barrier Reef near Bramble Cay (Fig. 1; see Table 1 for station locations, water depths and bottom water salinities). Five delta stations (D1, D2, D4, D5, D7) were sampled in July-August 1989 and the remaining stations (D9, D10, GP1, GP3-GPS) were visited in February 1990. The delta stations were located in the freshwater, upper delta (Stas D2, D4), the oligohaline-mesohaline, middle delta (Stas D1, D5) and at the delta entrances (Stas D7, D9, D10). The gulf stations are designated as either inner (Stas GP1, GP3, GP5, GP7) or outer (GP4, GP6, GP8) gulf locations. Trawling for epibenthos and demersal fish was undertaken in the Gulf of Papua during February 1990. Three replicate trawls were taken within each of three inshore (T1, T2, T3) and four offshore (T4, TS, T6, T7) grids (Fig. 1). Two boxcores were taken at each site using a modified 0.027 m 2 Bouma boxcorer (maximum depth of penetration -- 20 cm) for measurements of sediment granulometry, sediment temperature, pH, redox potential, chlorophyll a and phaeopigments and porosity (as per cent water content). The complete details of methods used for these measurements are described in ALONGI (1989). A further 12-16 boxcores were taken to complete the analyses described below.

291

Benthic dynamics in the vicinity of the Fly River p,,.

09

~ + ~ :>

©

:>

P~

c~

o

C~9

o

ao

~N

N %

'~.

~9

+~

E ;>

q~

"0

o ;>

~

292

D.M. ALONGIet al.

Sediment radiography Sediment radiographs were made from cores taken by inserting, as gently as possible, two Plexiglas liners (30 cm long, 2.5 cm thick, 19 cm wide) into each of two, undisturbed boxcores taken at each station. Cores for radiographs were not taken at Stas D1, D2 and D10 because the sediments were very compacted and did not permit undisturbed penetration of the plexiglas liners. After coring, the liners were capped and sealed with rubber bands, cleaned and put into a refrigerator (4°C) until processing. The cores were X-rayed with a medical X-ray unit using either standard X-ray film or Agfa-Gevaert structurix industrial film in an Agfa Curix cassette. Cores were then dissected to examine sedimentary and biogenic structures using criteria outlined by COLLINSONand THOMPSON (1989).

Infaunal analyses and macroparticulate detritus Two boxcores per station were sampled for meiofauna by inserting three plastic core tubes (6.6cm 2 surface area) to a depth of 10cm. Each core was preserved in 5% formalin-seawater mixture containing Rose Bengal (0.5 g l-l). In the laboratory the samples were washed through 0.5 and 0.045 mm sieves. Animals retained on the smaller mesh were considered meiofauna, sorted and major taxa enumerated. Dry weights of all taxa were estimated by drying (90°C for 6 h) representative individuals and weighing (+ 1/~g) on a Mettler ME-22 microbalance. Three boxcores per station were used to estimate infaunal numbers and biomass, and standing amounts of large (->0.5 mm) particulate detritus. Each core was sieved (0.5 mm) and preserved in 10% buffered formalin. The animals were sorted to major taxa, enumerated, patted dry, preserved wet weight determined, then dried (80°C for 16-18 h) and ashed (450°C for 6h) to estimate ash-free dry weight (AFDW). Shell-bearing organisms were decalcified in dilute phosphoric acid prior to drying and weighing. Detritus was captured by continued decantation onto a 0.5 mm sieve until the sediment was visibly clean. The material was dried (80°C for 24 h) and weighed.

Demersal trawl catch Trawling was undertaken using a 3 fathom Yankee Doodle try net dragged at - 2 knots for 20 min behind the research vessel. The net was constructed of 21 ply twine with 5.0 cm stretch mesh size. Headline distance was 6 m and the sweep was 120 cm. At the conclusion of each trawl, the contents were sorted onboard, where fish and prawns were identified to species level, and counted and weighted (fresh) to the nearest gramme. Other epifauna were sorted into major taxa, counted and weighed. For dietary analyses, the stomach was dissected out of each fish (only the 20 numerically dominant species) and opened under water in a Petri dish. Volume of the total stomach of each fish was estimated by arranging the food to a constant depth over a plastic grid and scoring the area covered. The per cent contribution of each prey item to the total volume of food was then estimated by eye. Food items were identified to broad taxonomic categories owing to the broken-up state of prey (see legend, Table 2). Based on their diets, fish species were classified as macroinvertebrate feeders, invertebrate/fish feeders, fish feeders, or detritus/microinvertebrate feeders using the criteria of ROBERTSON(1984). Normal hierarchical classification of all fish

Benthicdynamicsin the vicinityof the Fly River

293

species was performed on square root-transformed data using the Bray-Curtis similarity measure and Ward's incremental sums of squares strategy (BELBIN, 1987) to fuse groups. Bacterial analyses

At each station, replicate (n = 3) 0.2 cc syringe samples were taken at the sediment surface (0-2 cm) and at 2 cm intervals to the maximum depth of boxcorer penetration (for bacterial numbers, all stations; for rates of DNA and protein synthesis, Stas D1, D2, D4, D5, GP3, GP4, GP7, GP8). Samples were taken from acid-washed, stainless steel cores sampled from boxcores. Samples for bacterial numbers were extruded into acid-washed scintillation vials containing 5 m110% buffered formalin and 0.001% Tween 80 to disperse clay aggregates. Sample processing was completed using the epifluorescence microscopy method of HoagIE et al. (1977) as detailed in ALON6I (1988). Bacterial production was measured by the rate of [3H-methyl]thymidine incorporation into DNA (POLLARDand MORIARTY,1984) and the rate of [3,4,5-3H]leucine incorporation into protein (KIRCr~MANet al., 1985, 1986). For both procedures, each 0.2 cc sample was dispensed into a separate, acid-washed test tube. Control samples (n = 3-5) were killed with 80% (v/v with water) ethanol saturated with either unlabelled thymidine (for DNA synthesis) or unlabelled leucine (for protein synthesis). Each tube was immediately placed in a glove bag under a N2 atmosphere (samples >2 cm depth), the isotope added (see below) and incubated in the shade at in situ temperature for 10 min. The rate of DNA synthesis was measured by adding 50.4/~Ci of [3H-methyl]thymidine (specific activity 10 Ci mmo1-1; total of 5/~mol Tdr per sample) with 100/~1 of sterile water from each site to each sample. Sample processing was completed following the extraction procedures of MORIARTY(1990), modified by filtering samples onto 0.2~m Nuclepore filters. An isotope dilution experiment was run concurrently on surface samples to check that the amount of thymidine added was sufficient to overcome dilution effects (POLLARD and MORIARTY, 1984). Production estimates were calculated using the most conservative conversion factors available: 1.0 × 10TM cells dividing per mole of thymidine incorporated (see MORIARTY, 1988) and a cell carbon factor of 1.7 × 10 -14 g C per cell (RUBLEE, 1982). Specific growth rate,/~, was calculated by dividing mean production estimates by the mean standing crop (ALONGI, 1988). The rate of protein synthesis was measured by adding 23.6~Ci of [3,4,5-3H]leucine (specific activity 78.6 Ci mmol-1; total of 0.3 nmol leucine per sample) with 100/d of sterile water to each 0.2 cc sample. Sample processing was completed following the methods of KIRCHMANet al. (1985, 1986) as modified for sediments by MORIARTY(1990). Briefly, each incubation was terminated with 80% ethanol and centrifuged and washed with EtOH twice. The sample was then transferred onto a 0.2#m Nuclepore filter and washed five times with ice cold 5% trichloroacetic acid (TCA). Each filter was transferred to a new test tube containing 2 M NaOH, heated for 2 h at 100°C, cooled and centrifuged. A portion of the supernatant was added to PCS II scintillation fluid and counted upon return to the laboratory on a Beckman scintillation counter. An isotope dilution experiment was run as for DNA synthesis to ensure that enough label was added. Rates of protein synthesis and carbon production were calculated using the factors reported by KIRCHMAN et al. (1986) and RUBLEE (1982), where cell production=leucine incorporation × 5.4 × 1017 cells produced h -1. Specific growth rates were calculated as described earlier.

~

Lt~

C~

© Z Lr~

Lf~

L~ o~

~J

~

Lr~

t~

0

II

II 3

II r~

~t Lt~

~z

~S s~

296

D.M. ALON~Iet al.

For both methods, recovery efficiency was assessed by the labelled cell procedure of FALLON et al. (1983). Recovery of labelled DNA ranged from 64 to 100% and labelled protein recovery ranged from 52 to 100%. Counts were corrected accordingly.

Solid-phase and porewater nutrients

Samples for solid-phase nutrients were taken at all 14 stations, but porewater nutrients were not sampled at some stations because of compaction and subsequent low water content (Stas D 1, D 10). For all of the delta stations (excluding Sta. D9), porewater volume was insufficient to permit both dissolved inorganic and organic analyses, so only inorganic nutrients were measured. Boxcores were subsampled for porewater and solid-phase nutrients by inserting two stainless steel cores (as described above) into each boxcore to maximum depth of penetration. Three cores (7 cm inner diameter) each were sectioned at 2 cm intervals, placed immediately into acid-washed Petri dishes and the porewater extracted as soon as possible using a Teflon porewater extractor (RoBBINS and GUSVINIS, 1976). Porewaters were squeezed through 0.4 zm Nuclepore filters under an applied Ar pressure of 100 kPa for 10-15 min to collect 10-15 ml of interstitial water. Samples were stored in acid-washed scintillation vials and frozen immediately. Samples for DOC were stored cooled (2-5°C) until analysis in acid-washed, Teflon-capped glass test tubes containing 100~1 of 25% (wt/v) HC1 to remove carbonates. DOC analyses were completed following the methods described by ALONG1et al. (1989). Dissolved inorganic nutrient analyses were performed by standard automated techniques described by RVLE et al. (1981) and RYLE and WELLINGTON(1982). DON and DOP analyses were carried out on a separate subsample of that collected for inorganic nutrients [NH~, NO2 + NO•, PO]-, Si(OH)~-]. Following 8 h digestion in a La Jolla UV photo-oxidation apparatus (STRICKLANDand PARSONS, 1972), samples were analysed for nitrate + nitrite. The original NH~ and NO~ + NOr concentrations were subtracted to derive the organic N concentrations. DOP concentrations were similarly determined from analysis of inorganic P before and after UV oxidation. Separate cores were taken and sectioned as described above for solid-phase nutrients. After sample processing (see ALONGI, 1989, 1990b), total carbon and nitrogen were measured on a Leco CHN 600 Analyzer. Total organic carbon was determined on a Beckman Total Organic Carbon Analyzer as described by SANDSTROMet al. (1986). Total extractable phosphorus was measured on a Spectrometrics V DC plasma emission spectrophotometer following a perchloric/nitric acid digestion (ALLENet al., 1974).

O x y g e n and dissolved nutrient flux measurements

Total community metabolism measurements were made for each site from opaque glass bell jars placed into intact, undisturbed boxcores incubated under in situ temperature conditions for 3 h in a continuously flowing water bath. Clear bell jars were used for nutrient flux [DOC, DON, DOP, NH~-, NO2- + NO~-, PO]-, Si(OH)~-] measurements. DON and DOP measurements were not made at Stas D1-D7. There were three replicate jars each for metabolic and nutrient measurements. For the DOC analyses, there were three replicate bell jars (unpoisoned) and two additional chambers in which HgCI2 was

Benthic dynamicsin the vicinityof the Fly River

297

added to exterminate the benthic fauna in order to determine DOC uptake by bacteria (see detailed explanation for the procedure in BOTOet al., 1989). The bell jars (1 litre volume; 0.007 m 2 surface area) were gently fitted into the boxcores and pushed 2-3 cm into the sediment. A full description of the bell jars and stirring instruments are provided in ALONrI (1989, 1990b) and BoTo et al. (1989). In the nutrient flux experiments, 10 ml syringe samples were taken from a sampling port immediately upon insertion of bell jars and at 30 min intervals over 3 h. The samples were filtered through 0.4/~m Nuclepore filters, frozen and later processed as described above for dissolved organic and inorganic nutrients. In the oxygen flux experiments, each bell jar was fitted with a temperature-compensated ICI a Model 411 Oxymeter (see ALONGI, 1989, 1990b). Oxygen readings were taken at 15 min intervals for 3 h. At Stas D 1, D5, GP1 and GP8, separate bell jars were poisoned with HgC12 to determine the extent of chemical oxidation. The results were not significant (regression analysis; P > 0.05) and thus chemical uptake was assumed to be negligible at all sites.

RESULTS AND DISCUSSION Sedimentary facies Based on X-radiographs of sediment cores and granulometric analyses (Table 1), five sedimentary facies were recognized within the study area: (1) compacted and eroded, very fine sands (Stas D1, D2, D10); (2) highly laminated, muddy sands and sandy muds (Stas D4, D5, D7); (3) transitional prodelta muds with both primary and biogenic structures (Stas GP1, GP3, GP5, GP7, D9); (4) mixed terrigenous-carbonate sand facies (Stas GP4, GP6); and (5) mottled, fluid mud (Sta. GP8). (1) Compacted, very fine sands. No X-radiographs were taken of cores at any of these stations (D1, D2 and D10) because of very high compaction and low water content (see grain size data in Table 1). These sands are black in appearance due to high particulate iron content (4-7% by dry weight, ROBERTSONet al., 1990) and are similar to subaqueous iron sands mined off Indonesia (POLUNIN, 1983). They were nearly devoid of benthic infauna (Tables 3 and 4) and are probably eroded due to the high shear stresses from the high rates of water flow out of the river. This facies appears to dominate the mid-channels of the three entrances to the river. (2) Deltaic interbedded mud and sand facies. The X-radiographs of cores from Stas D4 (not shown), D5 (Fig. 2) and D7 (not shown) indicate facies characterized by alternating bands of laminated mud and fine sand. The alternating bands contain laminae of coarse silt and of sandy silt (light areas) and the darker bands contain coarse silt and fine sand. This type of interbedding has been found in the vicinity of other major river deltas (e.g. the Amazon, KUEHL et al., 1986) and are thought to be the result of fluctuations in current velocity. Repetition of alternating periods of intense and weak current flow would form the alternating bed structure. Interbedding of mud-sand laminae is often accompanied by a history of erosional contacts. The truncated burrows in the X-radiograph from Sta. D5 suggest the possibility of scouring episodes leading to previous extinction of infaunal

298

D . M . ALONGI et al.

Table 3. Densities(No, Individuals 10 cm 2) and biomass (,ug dry wt 10 cm -2) ofmeiofauna in the Fly Delta and Gulf of Papua. Values depict means _+1SD Taxa Stations

Nematoda

Copepoda

Foraminifera

Others*

Total biomass

Total densities

D1 D2 D4 D5 D7 D9 D10 GP1 GP3 GP4 GP5 GP6 GP7 GP8

12+8 1+1 8_+4 11 _+5 1-+2 25 +__18 67 _+ 13 142 _+ 67 135 _+ 66 153 _+ 45 131 _+ 40 131 _+ 32 25 _+ 22 240 _+ 105

13+_4 2+3 13_+3 9_+4 0 40 _+ 32 16 _+ 8 170 +_ 91 438 _+ 171 200 _+ 66 122 _+ 92 112 _+ 49 46 _+ 48 221 _+ 123

0 2+3 5_+ 1 6+_3 3-+2 35 _+ 19 3 _+ 2 12 _+ 10 62 _+ 23 100 _+ 22 55 _+ 13 109 _+ 39 0 255 _+ 161

7+3 2_+1 4_+2 8_+2 1_+1 7 -+ 6 11 _+ 9 23 _+ 11 19 _+ 16 31 _+ 18 17 _+ 6 38 _+ 16 10 _+ 3 34 _+ 19

25_+8 10+9 32_+9 38-+ 12 9_+2 139 _+ 86 60 _+ 26 251 _+ 236 582 _+ 238 511 _+ 148 310 _+ 129 465 _+ 171 61 -+ 50 1006 _+ 565

31_+10 7_+7 30_+6 34-+7 5___3 107 _+ 71 97 _+ 14 347 _+ 166 654 _+ 258 484 ___117 325 _+ 141 390 _+ 122 8l _+ 75 750 _+ 389

* Includes ostracoda, polychaeta, oligochaeta, archiannelida, hydrozoa, kinorhyncha and amphipoda.

communities.

The lack of biogenic structures

at these stations supports (3)

and low densities of infauna (Tables 3 and 4)

this observation.

Transitionalprodelta muds. T h e s e p r o x i m a l g u l f ( S t a s G P 1 , G P 3 , G P 5 , G P 7 ) a n d r i v e r

mouth

(Sta. D9)

stations

possess

sediment

fabrics of prodeltaic

muds

that have

been

Table 4. Densities of macroinfauna (No. Individuals m -2) and ash-free dry weight and wet weight biomass (g m -z) in the Fly Delta and Gulf of Papua Taxa Stations

Polychaeta

D1 D2 D4 D5 D7 D9 DI0 GP1 GP3 GP4 GP5 GP6 GP7 GP8

lll 62 12 86 173 311 419 456 333 1653 858 777 1073 407

+ 30 + 35 + 17 + 76 + 139 _+ 344 + 172 _+ 87 +__265 + 97 _+ 414 + 336 + 302 _+ 265

Total biomass

Crustacea

Oligochaeta

Others*

49 + 46 49 + 17 0 32l + 166 333 + 184 540 + 207 271 4-_ 155 839 + 201 281 _+ 86 728 + 46 918 _+ 940 678 _+ 337 866 +_ 184 4884 _+ 7005"

12 _+ 17 0 62 + 63 25 + 17 25 + 35 0 0 0 0 0 0 0 0 0

38 -+ 17 0 12 + 3 0 74 + 2 30 + 6 38 + 21 174 + 76 112 ___39 371 + 132 157 + 38 359 _+ 79 259 _+ 81 264 _+ 102

Total densities 210 111 86 432 605 811 728 1469 726 2852 1933 1814 2198 5555

+ 70 + 52 +- 76 + 167 + 257 + 395 + 257 +_ 223 _+ 446 _+ 184 + 1081 +- 759 -+ 276 -+ 749

AFDW 0.14 0.16 0.15 0.12 0.42 0.10 5.85 0.33 0.77 1.52 0.54 1.28 0.84 0.80

+ 0.05 + 0.10 -+ 0.11 + 0.02 + 0.19 -+ 0.05 + 5.29 -+ 0.08 -+ 1.17 + 0.34 _+ 0.27 -+ 0.60 -+ 0.60 -+ 0.11

* Includes bivalvia, gastropoda, echinodermata, sipuncula, bryozoa, hydrozoa and chordata. t > 9 0 % were the calanoid copepod, Undinula darwinii.

WW 1.05 0.66 1.2l 0.95 2.58 1.37 30.86 5.01 5.54 23.84 9.17 20.56 10.13 11.56

_+ 0.07 -+ 0.31 _+ 0.89 + 0.19 -+ 1.31 + 0.91 _+ 23.3 _+ 0.47 _+ 5.15 _+ 8.99 _+ 5.70 _+ 8.31 -+ 4.51 -+ 1.70

Benthic dynamics in the vicinity of the Fly River

Fig. 2. X-radiograph of core from Sta. D5. Length of core = 24 cm; m = mud laminae; s = sand laminae; b = burrow backfilled with coarse silt or sand. There is some distortion at distal ends due to drag of liner into boxcorer during sampling. Arrows depict possible erosional contacts.

299

300

D . M . ALONGI et al.

Fig. 3. X - r a d i o g r a p h of c o r e from Sta. GP1. L e n g t h of core = 21 cm; fp = f e e d i n g p o c k e t ; s = shell; t = t u b e . N o t e relict t u r b i t e l l i d shells in l o w e r l e f t - h a n d c o r n e r a n d in u p p e r right area.

Benthic dynamics in the vicinity of the Fly River

Fig. 4. X-radiograph of core from Sta. GP5. Length of core = 20 cm. Arrow depicts possible erosional contact and transition from a high to a low sedimentation regime. T o p b = spiral burrow; t = tube; bottom b = back-filled burrow coming out of the plane of the radiograph. Note n u m e r o u s tubes at sediment surface.

301

302

D . M . ALON6I et al.

Fig. 5. X-radiograph of core from Sta. D9. Length of core = 24 cm. Thick alternating bands of clay (dark) and silt-fine sand (light areas), b = burrows at extreme left- and right-hand sides appear backfilled and truncated by erosional contacts; t = relict tubes. Arrow depicts one of several erosional contacts.

Benthic dynamics in the vicinity of the Fly River

Fig. 6. X-radiograph of core from Sta. GP4. Length of core = 22 cm. Large particles (S) are fragments of bivalve shell and foraminifera remains. There is little evidence of biogenic structures, particularly below top 3 cm.

303

304

D . M . ALONCIet al.

Fig. 7. X-radiograph of core from Sta. GP8. Length of core = 24 cm. This fabric is thoroughly mottled with burrows and tubes. Contrast is poor in top 5 cm due to very high water content. Burrows tend to be larger deeper into the fabric. There is no evidence of primary physical structures.

Benthic dynamicsin the vicinityof the Fly River

305

subjected to very variable rates of sedimentation as indicated by the presence of both primary and biogenic structures. The X-radiographs from Stas GP1 (Fig. 3) and GP3 (not shown) consist of some laminal bands (mostly silt-clay) disrupted by bioturbation. The laminae at Sta. GP1 appears to be totally destroyed within the top 10-15 cm by feeding pockets and tubes. Some of this mixing may have been caused by tidal forces or currents as the fabric is well-churned and shells are oriented randomly. Laminae were better preserved, though disrupted by large burrows and some small tubes, at Sta. GP3. Sediment profiles of Stas GP5 (Fig. 4) and GP7 (not shown) are very similar with distinct subsurface laminae below the top 10 cm, but with a mottled, bioturbated fabric within the surface layers. At both sites, these units are separated by a distinct erosional contact. The lower, laminated fabrics are probably relict and suggest a history of high sedimentation, whereas the mixed surface fabrics indicate a lower sedimentation regime that is active today. This transition may preserve evidence of variations in river flow due to changes in climate, especially precipitation. The X-radiograph from Sta. D9 (Fig. 5) indicates a higher sedimentation rate at the mouth of the northernmost entrance as denoted by the thick alternating bands of clay (dark) and silt-fine sand (light areas). The biogenic structures truncated within the subsurface bands suggest that this site once had an established benthic community that was eroded (or possibly buried) in the past. (4) Mixed terrigenous-carbonate sands. The sediments of Stas GP4 and GP6 are mixed terrigenous-carbonate deposits (37-47% CaCO3) reflecting the transition from the terrigenous influence of the Fly deposits to the carbonate deposits originating from the northern Great Barrier Reef. X-radiographs from Stas GP4 (Fig. 6) and GP6 (not shown) are homogenous and devoid of physical laminae suggesting low rates of sedimentation. The fabric at both sites is comprised of quartz and carbonate sand, and fragments of bivalve shell, foraminifera and calcareous algae. These two stations are nearly identical to mid-shelf areas in the central Great Barrier Reef lagoon (ALONGI, 1989) that are mid-way between the nearshore muds and quartz sands, and the reef proper. (5) Mottledfiuid mud. The sediment fabric at Sta. GP8 (Fig. 7) is extensively bioturbated, indicating a regime where the rate of bioturbation exceeds the rate of sedimentation. The numerous permanent and temporary burrows and tubes reflect the very high (>5000 individuals m 2) densities of infauna (Table 4). Physical structures are absent, probably destroyed by the high rates of bioturbation. This fabric is very similar to mid-shelf areas of wetl-bioturbated, fluid mud found off the Amazon (KuEFIL et al., 1986) and the Changjiang (RI-IOADSet al., 1985).

Vertical profiles of particulate and dissolved nutrients Total organic carbon concentrations (Fig. 8, top) ranged from 0.14 to 1.39% (by DW), total nitrogen concentrations (Fig. 8, middle) ranged from 0.05 to 0.24% (by DW), and concentrations of total extractable phosphorus (Fig. 8, bottom) ranged from 0.062 to 0.113% (by DW). Two-level analyses of variance ( S o ~ L and ROHLF, 1981) of TOC and TN concentrations revealed exceedingly large interaction terms, indicating no general pattern in concentrations with sediment depth. Significant product-moment correlations of carbon and nitrogen vs grain size (for C, r = -0.87; for N, r = -0.72; P < 0.01) indicate that nutrient concentrations were a function of sediment type (higher in muds than in

306

D.M. ALONGIet

al.

sands) rather than location (delta vs gulf). Phosphorus values were not related to other edaphic characteristics as the range of concentrations was narrow (Fig. 8, bottom). C: N, N: P and C: P atomic ratios (Fig. 9) varied widely among stations and were not close to those predicted by the Redfield ratio (C: N: P = 106:16:1; REDHELD et al., 1963). Some stations (Stas D1, D2, D10, GP4, GP6) had very low (<6:1) C:N ratios. BARRETOet al. (1975) found similarly low C: N ratios in sediments off the Amazon and ascribed the phenomenon either to detritus in tropical areas containing less carbon relative to nitrogen or that these sediments are anomalously high in natural cyanide and humic nitrogen. There is no evidence to support either supposition. The low solid-phase N: P ratios (Fig. 9, middle) are probably due to preferential storage of particulate inorganic P by iron and manganese oxyhydroxides. Iron and manganese concentrations are high (4-10% Fe; 400-800 ppm Mn) in these sediments (ROBERTSONet

08

BW)

ORGANIC CARBON (% by

o,

o,,, , ;

04 •

I

~

I

12 ,

I

)

0~07

Oj4

eOf

OGPr AGP3

• 02

•

.

OGP7

I

12

I

08

i

04

I

q

i

08 J

~

r2 ,

I

AGP6 • GP8

GP5

TOTAL NITROGEN (% by DW)

°o

,

olo

,

o2,o

)

,

o:o

,

o2o

,

,

olo

,

o2,o

,

,

o,o

,

o2o

,

16

2O TOTAL PHOSPHORUS (% by

,o o?

~6

20

Fig. 8.

o ,o,

o ,'5

005,

0 I0

0 ~15

o 05

DW)

o,o

I,

op

005

0 fO

I!

Mean vertical depth profiles of organic carbon (top), total nitrogen (middle) and total phosphorus (bottom). Values are expressed on a percentage dry weight basis.

015

307

Benthic dynamics in the vicinity of the Fly River C N (atomic)

5

5

fO

10

~5

5

,o

,~

5

~0

,

I

• GP6

eD~ • GO5

~2,

6

• ";P8

o GP7

toO4

20 N P (atomic)

oo

~

e

,

6

r2

s

e

,

e

e

z

,

6

8

e

,.

6,_

e

.

1

20

C P (atomic) 0

t6

20

40

I

20

40

I

I

20 ,

,

40

iI

2O

Fig. 9.

Vertical depth profiles of mean atomic C: N (top), N : P (middle) and C: P (bottom) ratios.

al., 1990). P retention is also offered to explain the generally low C:P ratios (Fig. 9, bottom). There is no evidence of burial of C, N or P at most of the stations as the C: N: P ratios do not vary greatly with sediment depth, implying that these nutrients are mineralized at similar rates. The high rates of isotope uptake by bacteria with sediment depth (see microbial section) at most of these stations supports this idea. In comparison to other major river systems (e.g. the Amazon, Changjiang, Mississippi) these particulate nutrient concentrations are within the same ranges from sediments of equivalent grain size (BARRETO et al., 1975; ALLER and ALLER, 1986), but greater than concentrations in the central Great Barrier Reef shelf which does not receive nearly as much riverine input (ALONGI, 1989). Mean concentrations of dissolved inorganic nutrients in sediment porewaters were surprisingly low (within the #M range) considering the large quantities of freshwater and

308

D.M. ALONG]et al.

terrigenous material exported from the river, and compared with porewater data from other major river systems (e.g. ALLER et al., 1985). Ammonium concentrations ranged from <1 to >500ktM across stations (Fig. 10, top), whereas nitrite + nitrate concentrations (Fig. 10, bottom) ranged from <1 to 44/~M. Phosphate (Fig. 11, top) and silicate (Fig. 11, bottom) concentrations were very low, ranging from < 1 to 6.9 #M and from 22.5 to 180/~M, respectively. Vertical profiles of all inorganic species varied greatly among replicate cores and among sites, as indicated by very highly significant interaction terms. Most profiles showed no consistent increasing or decreasing trends with sediment depth. However, total mean concentrations (averaged over the entire profile) of NH~- and Si(OH)~- correlated positively (r = +0.78 and +0.69), and NO~ + N O 3 , negatively (r = - 0 . 7 0 ) , with redox potential indicating that their concentrations reflect the appropriate end products of either aerobic or anaerobic decomposition (REEBURG, 1983). In contrast, D O N and DOP concentrations in the porewater were moderate to high compared to other benthic habitats (YAMADAet al., 1987; TEAGUEet al., 1988; ZEHR et al., 1988). D O N concentrations were usually within 80-200/~M (Fig. 12). DOP values were lower, ranging from < 1 - 3 / ~ M (Fig. 12, bottom). Most profiles indicated no clear and consistent trends with depth. Concentrations of porewater DOC (Fig. 13) were low and variable among stations (1.8-16.4 mg C 1-1), and very variable with sediment depth at Stas D2, D4 and D9. Concentrations of dissolved organic C, N and P were significantly (P < 0.05) lower in the overlying water-column. The DOC values were within the range of values from other

N H 4(,uM) 0 ~ 3 0 15

t27 20

40

~60

80

03.06

05fo I00

200

300

400

500

"100

200

300

400

tOO

500

II\

• GP~

~

• GP3

• O2

• GP5

200

300

400

500

:;:

GP8

1 T6 NO 2 + NO 3 (uM) D7 6

l

0 05.0 2 I g .

0

l

0 t~.O ~7 ~ ~

t

2

3

4 |

t2 Fig. 10. Mean vertical depth profiles of porewater Nt-lg (top) and NO2- + NO3- (bottom). Arrows depict mean overlying watervalues.

309

Benthic dynamics in the vicinity of the Fly River PO' (#M) 0~ 2

4

6

8

0

2

4,

f ol

2.

;

f • GP4

104 005

i:tl 8

1 GP8

I07 lO 9

t37

i2i',00

Si(OH)4 (~M) 42.135 t50

50~

~00~

~50L

~°i88

50 •

loo L

~50

t 2O ~

Fig. 11. Mean vertical depth profiles of porewater PO43- (top) and Si(OH)~- (bottom)• Arrows depict mean overlying water values.

marine sediments (HERNDL et al., 1987; ALONGIet al., 1989; BOTOet al., 1989) and did not correlate significantly with other sedimentary characteristics. Nutrient and oxygen fluxes

Rates of dissolved inorganic and organic flux across the sediment-water interface were low (Table 5), within the ktM range, but equivalent to other tropical benthic habitats (see review of ALONGI, 1990a). Fluxes were dominated by silicate diagenesis with measurable rates ranging from 2572 to 9205#mol m -2 day -1. Ammonium fluxes were second in importance followed by low rates of pO34- and N O ( + NO3 release (Table 5). DON fluxes were usually not measurable. When detectable, flux rates were moderately high (-1509-3502 #mol m -z day-a). Detectable DOP fluxes were low (Table 5). Inorganic fluxes correlated positively with water-column salinity, with r values ranging from 0.55 to 0.69 (P < 0.05). NH~- fluxes also correlated negatively with sand content (r = -0.55; P < 0.05) and positively with silt-clay (r = 0.55; P < 0.05). These flux rates and correlations reflect a trend similarly found by TEAGVE et al. (1988) in a riverdominated estuary in Louisiana. In the upper, river-influenced areas of the estuary, fluxes were mostly negative (into the sediment), whereas in the lower, marine-influenced regions, nutrients were usually released (TEAGVE et al., 1988). In the Fly Delta, particularly in the middle and upper reaches, inorganic N and P fluxes were also usually negative. Near the river mouth and in the Gulf of Papua, fluxes were usually positive (Table 5). TEAGUE et al. (1988) hypothesized that this pattern is caused by more oxygenated

310

D . M . ALONGI et al.

freshwater in the upper bay with higher dissolved nutrient levels which would support high sediment uptake as well as nitrification-denitrification processes. This hypothesis is plausible for the Fly Delta as overlying water-column PO34-, NH~-, NO2 + NO3 are greater than or equal to porewater concentrations in surface sediments at some of the delta stations (Figs 10 and 11). Moderate to high oxygen levels (5-7 ppm) in the overlying water (RoBERTSON et al., 1990) may explain the generally low P fluxes, due to adsorption by sedimentary oxyhydroxides. Oxygenation may have facilitated the precipitation of PO34- , ensuring that PO43- would not diffuse across the sediment-water interface. It appears that sediments of the upper and middle reaches of the Fly Delta are a nutrient sink, whereas the more saline areas are a source of dissolved nutrients, probably for the same reasons as suggested by TEA6UE et al. (1988) for Fourleague Bay in Louisiana. Rates of dissolved organic carbon flux were consistently measurable at the river mouth and gulf sites, but less so in the delta (Table 6). Fluxes in unpoisoned chambers were generally high when measured, ranging from -797 to 514 mg C m -2 day -1. Most fluxes were into the sediment, suggesting supplementation of the high rates of bacterial DON (pM)

50

o~ /~ 4.

100 /

150

200 i

50 L

100 !

150 i

200 i

oGPI • GP3

8"

12

16,

20

DOP (I~M) 2

3

2O

Fig. 12.

Mean vertical depth profiles of porewater DON (top) and DOP (bottom). Arrows incorporate range of mean overlying water values.

DOC

311

Benthic dynamics in the vicinity of the Fly River

(mgC.I ')

I

4

I

8 i

I

12 I

I

16 I

o~I,

.

,

8,

I

'?

,

16

4~

8'

12

o

D~Sm,

16

I LU C~ • GO4 • GP I

~

• GP6

• GP3 • GP5 OGP7

12

16

2

Fig. 13. Mean vertical depth profiles of porewater DOC. Arrows depict mean overlying water values.

productivity measured at many of these sites (see next section). When poisoned, fluxes were usually in the opposite direction (net release) or not measurable, reflecting the importance of DOC utilization by sedimentary bacteria. These results are equivalent to findings by BoTo et al. (1989) and ALONGI et al. (1989) in mangrove intertidal and in adjacent nearshore sediments in tropical Australia, where DOC fluxes account for 40-50% of sedimentary bacterial production. Of course, this assumes that the difference between poisoned and unpoisoned fluxes can be equated to bacterial utilization in surface sediments. Rates of total community metabolism ranged from 63-780 mg C m - 2 day -1 (= 5.265.1 mmol 02 m-Z day- 1) and were highest at the muddiest stations (Table 7). Respiration rates correlated with per cent silt-clay ( r = +0.84; P < 0 . 0 0 1 ) , salinity ( r = +0.71; P < 0.01), fluxes of NH~-, NOy + NO3, and pO34- (r range = +0.53-0.84), infaunal densities (r = +0.74; P < 0.01) and bacterial numbers (r = +0.78; P < 0.01). The rates of oxygen uptake were high, within the upper range of values measured on temperate shelves ( e . g . HANSON et al., 1981; TENOREet al., 1984), but equivalent to rates in some temperate estuaries (e.g. BOYNTONand KEMP, 1985; TEAGUEet al., 1988).

312 Table 5.

Station D1 D2 D4 D5 D7 D9 DI0 GP1 GP3 GP4 GP5 GP6 GP7 GP8

D . M . ALONGt et al.

Rates o f dissolved inorganic and organic nutrient flux (~mol m -2 day -1) calculated from belljar measurements over a 3 h period. Values depict means ± 9 5 % C.I. NH~-172 -350 -73 140

± 38 ± 103 ± 53 ± 454 0 2160 ± 609 0 516 ± 43 1114±91 1029 ± 313 872 ± 114 0 1594 ± 94 276 ± 39

N O 2 + NO3

DON

84 ± 339 (1 0 0 0 345 ± 14 0 205 ± 18 279±10 457 ± 15 0 417 ± 23 354 ± 21 490 ± 22

* * * * * 3502 ± 632 1032 ± 194 0 - 1 5 0 9 ± 459 0 0 0 0 0

PO 3 0 ± 35 ± 27 ± 4 ± 13 0 0 70 ± 4 66±7 169 ± 2 843 ± 120 96 ± 15 267 ± 87 70 ± 4

-29 -37 -19 -60

DOP * * * * * 0 -59 ± 0 -57 ± 0 -59 ± 0 84 ± 0

Si(OH)~-

11 30 27 27

0 4608 _+ 1263 0 2572 ± 1278 0 3235 ± 239 9205 ± 2715 5785 ± 58 4 1 1 4 ± 188 6647 ± 1306 2897 ± 546 4875 ± 504 4361 ± 154 6542 ± 128

* Not measured.

Standing amounts of plant matter Standing amounts of chlorophyll a and phaeopigments in surface sediments were low (Table 7), ranging from 0 to 1.02pg g-1 sediment DW for chlorophyll a and from 0-8.2 pg g- 1 DW for phaeopigments. Values were generally greater in the gulf than in the delta, correlating positively with phytoplankton production ( r = +0.77 for Chl a; r = +0.72 for phaeopigments; P < 0.01). Large pieces of particulate plant matter, mainly Table 6.

Rates o f dissolved organic carbon (mg

C m 2 d a y - l ) flux across the sediment-water

interface in unpoisoned and poisoned bell]ars. Values depict means ± 9 5 % C.I. Flux rates

Station

Unpoisoned

Poisoned

D1 D2 D4 D5 D7 D9 D10 GP1 GP3 GP4 GP5 GP6 GP7 GP8

0 0 - 7 0 7 _+ 399 - 2 1 2 ± 71 0 - 2 2 4 _+ 115 - 2 1 1 _+ 397 514 ± 134 - 1 2 3 ± 70 - 7 9 7 ± 389 171 ± 111 220 ± 218 - 3 4 1 ± 72 - 1 9 2 ± 100

0 0 - 7 3 8 _+ 378 0 0 0 0 0 444 +_ 186 695 ± 130 202 ± 116 0 153 ± 102 0

313

Benthic dynamics in the vicinity of the Fly River

macerated palm and mangrove litter, were most abundant within the delta (Table 7) with little material found at the most distal sites. ALONCI (in press) found large standing amounts of litter on intertidal mudbanks within the delta, and thus it appears that a large proportion of waterlogged plant litter is deposited there. The amount of material exported to the gulf may still be large, but probably diluted over a large area. A similar scenario has been suggested for mangrove litter export within the central Great Barrier Reef lagoon (RoBERrSON et al., 1988).

Bacterial numbers, productivity and specific growth rates Bacterial densities varied widely among stations and with sediment depth producing a very highly significant (P < 0.001) interaction term when a two-level analysis of variance test was performed. In surface (0-2 cm) sediments, densities ranged from 7.6 x 107 to 6.5 × 101° cells g-1 DW among stations (Table 8). Bacterial numbers were highest at the muddiest sites, correlating positively with both water depth (r = 0.65; P < 0.05) and percent silt-clay (r = 0.67, P < 0.05). Vertical profiles of bacterial numbers were erratic at most of the upper and middle delta stations (upper left graph, Fig. 14) and were below detection limits below the top 2 cm at Sta. D1 (Table 9). At the river mouth (Sta. D9) and in the Gulf of Papua (both bottom graphs, Fig. 14), bacterial numbers declined significantly (P < 0.01) with increasing sediment depth (one-way ANOVA's at each site). Averaged over the entire profile, the Gulf of Papua stations had densities ranging from 2.2-25.0 x 109 cells g-i DW (Table 9), higher than densities observed off the Changjiang and Amazon Rivers (ALLEl~ and ALLER, 1986) and on temperate shelves (HovrdNSON, 1987), but equivalent to values in the central Great Barrier Reef lagoon (ALoNGI, 1989). Rates of DNA and protein synthesis were very erratic, ranging in surface (0-2 cm) sediments from 0-387 mg C m -z day -1 (DNA synthesis) and from 0-854 mg C m -2 day -1 (protein synthesis), respectively (Table 8). Net uptake was negligible in surface sediments at most stations for DNA synthesis and in subsurface sediments at some of the sandy, delta Table 7. Rates (mg C m -2 day-l) of total community respiration (TCR) and concentrations of sedimentary chlorophyll a and phaeopigments ~ g g-1 sediment DW) in surface (0-1 cm) sediments. Concentrations of macrodetritus (g D W m -z) are over the entire 0-20 cm depth horizon. Values are means ± 1SD. Station D1 D2 D4 D5 D7 D9 D10 GP1 GP3 GP4 GP5 GP6 GP7 GP8

TCR 63 _+ 13 114 _+ 40 140 + 45 85 _+ 26 146 +_ 72 552 _+ 85 80 _+ 172 216 ± 67 282 ± 30 280 ± 76 570 _+ 204 252 ± 38 468 ± 641 780 ± 205

Chl a

0.05

0.69 1.02 0.21 0.91 0.18 0.52

0 0 +_ 0.11 0 0 0 0 +_ 0.48 + 1.43 ± 0.23 ± 0.63 _+ 0.24 0 ± 1.1

Phaeopigments

1.8 2.1 2.4

8.2 2.5 1.8 7.4 4.7 1.5 1.8

0 0 _+ 1.0 + 0.7 _+ 0.4 0 0 _+ 3.1 ± 1.8 ± 1.4 ± 2.4 ± 1.1 ± 0.7 ± 2.8

Macrodetritus 14.0 587.6 819.3 158.6 1537.7 89.8 62.2 74.5 7.2

± 6.9 +_ 650.5 _+ 100.8 _+ 49.9 ± 1(158.4 + 10.7 ± 22.8 ± 15.7 _+ 1.5 0 51.4 ± 20.6 0 59.5 + 6.2 0.7 ± 1.5

314

D . M . ALONGI etal.

stations (Tables 8, 9). When uptake was detected, rates of DNA synthesis (extrapolated to C) were high and erratic with sediment depth (Fig. 15, top), ranging at these stations from 325 to 2108 mg C m -2 day -1, averaged over the entire profile (Table 9). Rates of protein synthesis were equally erratic with sediment depth when detected (Fig. 15, bottom) but were lower than rates of D N A synthesis, ranging over the entire profile from 72 to 228 mg C m -2 day- t (Table 9). Specific growth rates were therefore highly variable, ranging from 0 to 3.2 day -1 based on thymidine uptake rates, and from 0 to 0.41 day -1 based on leucine incorporation rates (Table 9). Comparisons with other nearshore systems are difficult, owing to the few studies available. Nevertheless, these rates of bacterial productivity are generally high compared with data from tropical Australian habitats (ALONGI, 1989; MORIARTYet al., 1990; see review of A L O N G I , 1990a). The absolute values are, of course, dependent upon the applicability of the conversion factors used, but they are among the most conservative in the literature (see B E L L , 1990 and refs within). In addition, recent evidence has shown that anaerobic bacteria, particularly sulphate-reducers, do not incorporate exogenous thymidine ( G I L M O U R et al., 1990) suggesting that the DNA synthesis values reported herein are underestimates of total bacterial production. The values used for protein synthesis are particularly uncertain because the leucine methodology employed was designed for pelagic work, thus the appropriateness of the conversion factors used is unknown. Rates of protein synthesis should be higher than rates of DNA synthesis ( C H I N - L E O and KIRCHMAN, 1990) in theory, but such was not the case at some of these stations, particularly in subsurface sediments, suggesting some methodological limitations. Based on the thymidine uptake rates, DOC fluxes (poisoned minus unpoisoned rates) supplied, on average, 60% of the bacterial C requirements in surface sediments. The

Table 8.

Mean bacterial densities and productivity averaged over the top 2 cm at each station Production ( m g C m - 2 d a y lcc 1)

Station D1 D2 D4 D5 D7 D9 D10 GP1 GP3 GP4 GP5 GP6 GP7 GP8

N u m b e r s (cells g-1 D W ) 3.7 (+5.2) 7.6 (+6.9) 3.5 (_+1.3) 6.3 (-+3.5) 1.3 (-+1.1) 1.3 (-+0.4) 9.0 (_+5.2) 2.7 (_+1.3) 1.4 (_+0.4) 4.9 (_+0.4) 1.9 (_+0.2) 2.3 (_+0.4) 8.9 (_+1.9) 6.5 (-+2.3)

* No net uptake.

x x × × x x x x x x x x x x

108 107 109 108 108 10 t° 108 101° 10 m 101° 1010 101° 109 1010

Tdr * 323 + 10 * * 387 -+ 92 * 160 _+ 20 230 _+ 30 * 280 _+ 20 * * * *

Leu

371 408 854 296 60 72 40 113 364 193 48 34

* + 9 + 54 _+ 342 _+ 166 _+ 11 _+ 18 +_ 10 * _+ 3 _+ 165 _+ 71 _+ ] _+ 1

315

Benthic dynamics in the vicinity of the Fly River

leucine-derived rates suggest that as much as 74% of the bacterial C requirements were accounted for by DOC. Both isotope methods have problems, but both agree in suggesting a close coupling between DOC and bacterial growth in these sediments.

Meiofauna and macroinfauna Densities and biomass of meiofauna were generally lower in the delta than in the gulf (Table 3). Total densities and biomass of meiofauna in the delta ranged from 5 to 107 individuals per 10 cm 2 and from 9 to 139/~g dry wt per 10 cm 2, respectively. In contrast, densities and biomass in the gulf ranged from 81 to 750 individuals per 10 cm2 and from 61 to 1006ktg DW per 10 cm2, respectively. On average, nematodes were the most abundant taxon in the delta, whereas harpacticoid copepods and Foraminifera increased in greater proportion than nematodes in the gulf. Infaunal patterns were similar, with numbers and biomass in the delta being generally lower than in the gulf (Table 4). Small, surface-dwelling polychaetes and crustaceans

BACTERIA

2

4

o

6

8

i

u

cellsg

DWXIO

10

0

j

2

4

t

6

8

v

10

~

t2 i

.i • D7 • O2

~ ' ~

• D9

20

o. u4 £3

0 0

5 i

cellsg

DWXIO

10 i

15 o

cells g 20 ,

25 u

1 i

DWX10

2

3

I

I

~

5 i

f/~o

GO5 0 GP7 20

Fig. 14.

Mean vertical depth profiles of bacterial densities at the delta (top) and gulf (bottom) stations. Note change in scale for abundance values in lower graphs.

6 I

jjAI

316

D . M . ALONGI etal.

(mainly amphipods) were co-dominant at most stations. The small individual body size of most infauna was reflected in the low range of ash-free dry weight (0.10-5.85 g m -z) and wet weight (0.66-30.86 g m -z) biomass. Oligochaetes were found only in the delta. One unusual phenomenon was the occurrence of very high densities of the calanoid copepod Undinula darwinii found at Sta. GP8, the most bioturbated station. Mean total macroinfaunal numbers correlated with pelagic primary production ( r = +0.78; P < 0 . 0 1 ) , total meiofaunal densities ( r = +0.77; P < 0 . 0 1 ) , bacteria (r = +0.76; P < 0 . 0 1 ) , respiration ( r = +0.74; P < 0 . 0 1 ) , macrodetritus (r = - 0 . 6 4 ; P < 0.05), salinity (r = +0.89; P < 0.01) and clay content (r = +0.71; P < 0.01). Total meiofaunal numbers correlated with water depth ( r = +0.69; P < 0 . 0 5 ) , bacteria (r = +0.93; P < 0.001), respiration (r = +0.070; P < 0.01), chlorophyll a (r = +0.71; P < 0.01), pelagic primary production (r = +0.84; P < 0.001) macrodetritus (r = -0.81; P < 0.001), salinity (r = +0.81; P < 0.001) and per cent clay (r = +0.63; P < 0.05). These correlations indicate that the benthic infauna is sparse in the delta because of the unstable nature of the seabed (alternating episodes of erosion and deposition) and low food quantity and quality. Tidal stress is known to deplete benthic abundances (WARWICK and UNCLES, 1980) and such is probably the case in the Fly Delta. The distributional patterns of the infauna are similar to those of the Amazon and Changjiang Rivers (ALLER and ALLER, 1986; RHOADS et al., 1985), where infauna are absent (or nearly so) near the river mouth. Away from the delta, the water-column is deep and clear enough to permit significant water-column phytoplankton production, a stable seabed and mature benthic communities (including bacteria) to develop (e.g. at Sta. GP8).

Table 9. Grand mean bacterial densities, productivity and specific growth rates ( d a y - l ) averaged over the entire 0-20 cm sediment profile

Productivity ( m g C m 2day l c c - l ) Station D1 D2 D4 D5 D7 D9 D10 GP1 GP3 GP4 GP5 GP6 GP7 GP8

Numbers (cellsg 1 D W )

1.9 3.0 3.1 4.6 5.7 4.6 1.4 8.9 2.5 8.8 6.8 2.2 2.5

* × 109 × 109 x 109 x 108 x 109 x 10 s X 1010 x 109 x 101° × 109 x 109 x 109 x 10 m

k t ( d a y J)

Tdr

Leu

Tdr

Leu

+ 1795 t -~ $ :~ $ :~ 2108 724 ~ ~ 1185 325

+ 228 t ? $ ~ $ :~ 197 122 ~ :~ 72 198

0.0 3.2 0.0 0.0 ~: ~ $ ~, 1.1 0.19 :~ :~ 2.3 0.05

0.0 0.41 0.0 0.0 ~: ~: :~ 0.10 0.03 :~ :~ 0.14 0.03

* See Table 8 for densities in top 2 cm. Densities below 2 cm were below detection limits. ? N o net uptake. :~No samples taken below 0-2 cm.

Benthic dynamicsin the vicinityof the Fly River

317

DNA SYNTHESIS (IJgC.cm3.d ')

,oo

0

zoo

i

DI ~ 0

i

~

| ~C~

~

3o0

4o,0

• [.)2

0 GP4

• GP3

t3 GP8

500

PROTEIN SYNTHESIS (pgC.cm3.d ') ua

10

20 I

30 i

40

50 I

8

12

16'

2O

Fig. 15. Mean vertical depth profiles of rates of DNA synthesis (top) and protein synthesis (bottom) convertedto#g C cm-3 day 1. See Table 9 for grand means and for stationswhere no net uptake was observed.

Demersal trawl communities Epifauna exhibited differences in community composition between the inshore and offshore stations in the Gulf of Papua (Table 10). Bivalves, crabs, asteroids, hydroids, crinoids, sponges and echinoids were significantly (one-way A N O V A s P < 0.05) more abundant at the offshore sites, being either absent or rare inshore. The total number and biomass of epifauna catch were generally higher at the offshore sites, although inshoreoffshore differences were not statistically significant. Large pieces of wood and detritus were found in trawls at most stations. Nekton communities were not significantly different in terms of numbers, total biomass and fish biomass per trawl among grids (Table 10). The leatherjacket, Paramonacanthus filicauda, was the dominant fish species caught (see list of all nekton species caught, Table 10), followed by the pony fish, Leiognathus splendens and the grunter, Pomadasys argyreus. Prawns and cephalopods were caught within all grids, but were not abundant, usually <1 per trawl. In 21 trawls, 1561 nekton individuals were caught comprising 70

318

D.M.

ALONGI etal.

species of fish, six species of penaeid prawns and several unidentified species of cephalopods (Table 11). Classification analysis of all fish species caught revealed a nearly complete segregation of fish communities between the inner and outer gulf (Table 11). This was due mainly to the distributional patterns of the top four species: P. filicauda was nearly absent in the inner gulf and L. splendens, P. argyreus and Upenaeus sulphureus were found at one grid in the inner gulf, but were absent at the offshore sites. Several species of less abundant fish were present only at the offshore stations (e.g. Samaris cristatus; see Table 11). Most fish species were categorized as macroinvertebrate feeders, feeding mainly on a variety of epibenthic crustacea (Table 2). The two most abundant species in our catches (P. filcauda, L. splendens) were detritus/microinvertebrate feeders with gut contents comprised mostly of plant detritus and copepods. The remaining fish were invertebrate/ fish feeders with stomach contents composed of fish and squid. The stomach contents of the prawns were not examined, but their diets can be inferred from the literature (e.g. WASSENBERG and HILL, 1987). These prawn species feed on bivalves, gastropods, ophiuroids, crustaceans and polychaetes; meiofauna are generally not a preferred dietary item. Table 10.

Summary of trawl data from Gulf of Papua stations. Figures are means (± 1 S E ) for three trawls per station I n s h o r e sites T1

T2

T o t a l d e t r i t u s (g) T o t a l a l g a e (g)

2 + 1 +

920 ± 187 -

Total epifauna: No. taxa per trawl Biomass per trawl

18 +_ 5 2571 ± 1354

Nekton only: ENo. per trawl EB. trawl EFishB. trawl No. spp. pertrawl

156 _+ 95 2422±1402 2380 _+ 1379 13±5

Major epifauna (minus nekton): Z B . Sol. A s c i d i a n s 116 + 34 EB. Col. Ascidians 9 + 9 ZB. Bivalves 1± 1 EB. Brachyura + ZB. Asteroids + EB. Hydroids EB. Crinoids ZB. Sponges ZB. Echinoids EB. Gastropods 9 _+ 7 + = mean <1 per trawl. - = a b s e n t f r o m all t r a w l s .

O f f s h o r e sites T3

T4

T5

26 ± 17 -

171 ± 71 -

214 ± 84 143 ± 81

9 +_ l 631 ± 134

13 ± 1 1086 ± 560

16+1 1045±161

32±5 3492±698

31±2 2643+659

25_+3 2119±96

6 _+ 2 151±96 73 ± 43 4±1

2 7 . 7 +_ 1 864±416 864 +_ 416 10±1

17 _+ 5 423+23 376 ± 61 8±1

216 ± 57 2554±697 2375 ± 638 21±4

54 ± 5 1415±237 1190 ± 146 18±1

42.3 ± 10 857+_173 700 ± 111 14+3

187 + 94 172 +_ 155 4 +_ 2 1± 1 -

9 + 5 1± 1 184 ± 158 -

10 + 8 19 _+ 12 35 +_ 7 53 ± 29 158 ± 52 46 ± 10 217 ± 167 2 ± 2

72 _+ 38 41 ± 19 35 ± 14 122 ± 117 144 ± 80 34 _+ 30 507 _+ 361 139 ± 121 39 ± 22

45 ± 16 22 _+ 15 22 +_ 9 294 ± 150 343 ± 129 85 ± 46 283 ± 234 147 ± 89 -

27 1 18 127 35 26 259 30 56 2

± ± ±. ± ± ± ± ± ± ±

T6

14 1 7 36 29 12 86 10 42 2

381 + 225 -

T7 998 ± 380 -

319

Benthic dynamics in the vicinity of the Fly River

Table

11. (F =

Total number and mean ( + 1SE ) number per trawl at each trawl grid in the Gulf of Papua for all nekton fish; P = prawn; C = cephalopod) + = mean is less than 1.O per trawl; - = absent from trawl Stations Inshore

Offshore

Total Taxa

number

Paramonacanthusfilicauda Leiognathus splendens ( F ) Pomadasys argyreus ( F ) Upenaeus sulphureus ( F ) Pseudorhombus arsius ( F ) Sorsogona tuberculata ( F ) Apogon septemstriatus ( F ) Saurida undosquamis ( F ) Suggrundus sp. ( F ) Penae~ semisulcatus (P) Apogoncarinata ( F ) Cephalopoda

(F)

(C)

Saurida tumbiI ( F ) Theraponjarbua ( F ) Saurida gracilis ( F ) Arnoglossus weighti ( F ) Upenaeus sp. ( F ) Apogon harzfeldi ( F ) Dactylopena sp. 1 ( F ) Epinephalus amblycephalus ( F ) Trixiphichthys weberi ( F ) Callyionomus japonicus ( F ) Callyionomus sp. ( F ) Grammatobothus polyophthalmus Lagocephalus spadecius ( F ) Penaeus merguiensis (P) Synodus hoshinonis ( F ) Trachinocephalus myops ( F ) Anodontostoma chacunda ( F ) Dactylopena sp. 2 ( F ) Lepidotrigula sp. ( F ) Metapenaeus ensis ( P ) Nemipterus nematophus ( F ) Scorpaenodes scaber ( F ) Ammotretis weighti ( F ) Dexilliehthys muelleri ( F ) Leiognathus equulus ( F ) Samaris eristatus ( F ) Psettodes erumei ( F ) Pseudorhombus diplosphilus ( F ) Pterois russelli ( F ) Uranoscopus cognatus ( F ) Alectes ciliaris ( F ) Johnius vogleri ( F ) Nemipterus isacanthus ( F )

(F)

T1

T2

599

+

-

200

66 _+ 45

.

T3

T4

-

T5

8 + 2

.

.

T6

168 ± 4 8 .

.

7 +_ 1

T7 16 -+ 9

.

121

40 ± 34

.

.

.

.

.

.

76

25 +_ 13

.

.

.

.

.

.

70

-

-

5 -+ 3

-

7 -+ 4

11 _+ 2

44

1 + 1

-

+

+

6 + 1

3 ± 2

3 _+ 2

43

2 ± 2

1 -+ 1

7 _+ 1

+

3 -+ 1

1 -+ 1

35

-

-

3 ± 2

-

2 _+ 1

5 _+ 3

1 + 1

35

-

-

-

+

2 ± 1

1 _+ 1

1 _+ 1

31

+

-

-

2 ___ 1

2 _+ 1

1 _+ 1

4 ± 2

30

1 + 1

-

1 + 1

1 + 1

6 ± 3

1 -+ 1

1 _+ 1

23

-

2 _+ 1

-

1 _+ 1

1 _+ I

2 +_ 1

1 _+ 0

16

1 ± 1

-

2 + 1

+

1 -+ I

1 _+ 1

16

5 ± 3

.

.

14

.

11

-

.

.

-

.

.

.

.

-

1 _+ 1

-

-

-

-

10

-

9

.

9

-

-

-

1 _+ 1

9

-

-

-

1 _+ 0

1 + 0

1 +_ 1

-

1 _+ 1

.

.

.

.

2 -+ 1

1 + 1

1 +_ 1

3 _+ 1

1 _+ 0

1 _+ 1

3 _+ 2 -

2 _+ 1 -

1 ± 1

8

+

7

.

.

.

.

7

.

.

.

.

2 + 1

-

-

7

.

.

.

.

2 ± 1

-

-

7

2 ± 2

.

7

1 ± 0

-

-

-

7

.

7

-

6

2 ± 2

6

.

6

-

-

-

+

6

-

-

-

1 _+ 1

6

1 ± 1

-

+

-

6

.

-

-

.

2 _+ 2

1 _+ 1

+

.

.

. -

. . .

.

.

.

-

-

1 + 1

-

-

-

1 +- 1

5

2 ± 1

5

.

4

+

4

.

4

1 + 1

4 3

.

.

.

. .

. .

.

1 ± 1 .

+

.

.

-

.

-

+

-

. 1 _+ l

. .

-

+

. .

+

1+ 1 1 _+ 1

2 _+ 1 -

.

1 ± 1

.

1 ± 1

.

-

-

.

. .

.

1 _+ 1

-

-

-

1 +_ 1

1 _+ 1 .

1 ± 1

.

1 +_ 1

.

+

-

.

3

. .

-

1 ± 1

3

. .

1 ± 1

+

1 _+ 1

-

+ -

.

.

5

.

.

+

-

5

.

2 ± 1

1 _+ 0

.

1 ± 1

.

2 + 1

.

.

.

. +

-

-

1 _+ 0

+ +

. .

. 1 +_ 0

.

-

-

+

. .

1 ± 1

. -

-

(ConKnued)

320

D.M.

Table

et al.

ALON6I

Continued

11.

Stations Inshore

Offshore

Total Taxa

number

Penaeus escualentus ( P ) Pomadasys hasta ( F ) Carangoides dinema ( F ) Carangoides s p . ( F ) Gerres oyena ( F ) Lutjanus malabaricus ( F ) Paraplagusia bilineata ( F ) Penaeus longistylus ( P ) Pomadasys maculatus ( F ) Priacanthus s p . 1 ( F ) Upenaeus vittatus ( F ) Apogon ellioti ( F ) Bothidae

sp.

T2

3

-

-

3

-

-

2

1 _+ 1

.

2

-

-

2

.

2

-

-

+

-

2

+

-

+

.

2 2

sp.

sp.

+

.

.

. .

.

.

.

.

1

.

1

.

1

.

.

.

.

.

1

.

.

.

.

.

1

-

1

.

.

1

.

.

1

-

1

.

1

.

1

+

(F)

.

+

. +

1

.

1

-

1

. +

.

-

.

1

.

1

.

1

.

.

.

1

.

.

.

.

. -

-

-

+

-

-

+

-

+

.

.

. .

.

.

.

.

.

.

.

.

.

.

.

+ +

.

.

+

.

.

.

. -

-

+

-

-

.

+

-

-

.

+

-

-

.

. .

.

.

.

-

+

.

.

-

.

.

.

.

.

-

.

+

-

+

.

.

1

.

.

.

.

+ .

.

.

.

-

.

. .

.

-

.

+

-

.

-

+

-

+

.

.

.

-

1 _+ 1

.

.

.

.

.

+

.

. .

.

.

.

T7

. .

.

.

.

.

.

-

.

.

-

. .

+

.

+

.

1

. .

T6

.

+ .

-

1

.

1 _+ 1

1 _+ 1

.

+ .

.

-

(F)

T5

.

1 _+ 1

-

(F)

Siganus canaliculatus ( F ) Stegostoma varium ( F ) Total No. nekton

.

1 _+ 1

.

1 _+ 1

T4

2

Dendrochirus brachypterus ( F ) Fistularia petimba ( F ) Gymnura australis ( F ) Halientaea indica ( F ) Leignathus decorus ( F ) Leiognathus leuciscus ( F ) Leiognathus stereorarius ( F ) Lutjanus sebae ( F ) Metapenaeus endeavouri ( P ) Parachaetodeon occelatus ( F ) Priacanthus s p . 2 ( F ) Rhynchostracion nasus ( F ) Scorpaenidae

.

.

T3

2

(F)

Choerodon s p . ( F ) Cirrhitichthys aprinus Cynoglossus s p . ( F ) Dasyatididae

TI

+

.

.

.

. .

.

.

.

.

.

+

1561

It is difficult to compare our estimates of trawl catch with other studies owing to differences in the trawl nets used, but it appears that standing stocks of epifauna and demersal fish are low off the Fly Delta, perhaps due to the generally low infaunal densities and unstable seabed. Trawls were not conducted within the delta, but considering the nearly complete absence of an established infaunal benthos and a physically-dominated seabed, it is possible that standing stocks of trawl catch in the delta are even lower. There is a reasonable similarity between the species composition of our catch and their diets with those communities found in other tropical nearshore areas (LE LOEUFF and INTES, 1973; LIU, 1976; LIV etal., 1978; RAINER, 1984; WATSON, 1984; CANNONetal., 1987; DREDGE, 1988). For instance, in the Gulf of Carpentaria, RAINER(1984) observed the same numerical dominance of leatherjackets, pony fish and grunters, noting that the gulf

321

Benthic dynamics in the vicinity of the Fly River

community shared many species with the demersal trawl communities of the Gulf of Thailand and the Gulf of Papua (WAXSON,1984). Nearly all of these studies (e.g. WAXSON, 1984; CANNONet al., 1987; DRED6E, 1988) have observed inshore mud versus offshore sand patterns similar to ours as well as the importance of detritus and epibenthic crustacea in the diets of most of the dominant demersal fishes (e.g. LE LOEUFFand INXES, 1973). The low abundance of demersal fish suggest that only a small proportion of benthic biomass in the western Gulf of Papua is transferred to higher trophic levels, including the indigenous population inhabiting islands within the Fly Delta and northern Torres Strait. Recent studies, however, indicate large potential fish and penaeid prawn yields in the northern and eastern regions of the Gulf of Papua (GWYTHER, 1982; DALZELLand PAULY, 1989). CONCLUSIONS

Changes in the benthic regime from the upper freshwater reaches of the Fly Delta out to the mid shelf of the Gulf of Papua (see summary Table 12) correspond well to the benthic response model of RHOAOSet al. (1985) considering the differences in the quantity of river Table 12.

Comparison of major attributes of the benthic regimes within the Fly Delta and in the Gulf of Papua Fly Delta

Attribute: Facies

Physical structures dominate; compacted very fine sands to laminated mud and sand.

Nutrients

Lowest C and N and stoichiometric ratios in compacted sands; high levels in laminated deposits; higher levels of porewater NO 2 + NO 3 than in gulf

Bacteria

Low densities at most stations; very variable rates of production but high when measurable; weak DOC-bacteria interactions. Most litter deposited in laminated muddy sands; low levels of chlorophyll and phaeopigments. Low densities and biomass; small pioneering seres. Low biomass, composed mostly of ascidians.

Detritus and plant pigments Infauna Epifauna

Demersal fish

Benthic-pelagic coupling

Not sampled in delta, but low diversity proximal to delta; composed of pony fish and grunter; cephalopods and prawns rare. Low rates of respiration and nutrient flux, mostly into sediment; inorganic fluxes account for 14% of N and 0% of P plankton requirements.

Gulf of Papua

Transitional prodelta muds grade to mixed terrigenous-carbonate sands near GBR and to mottled fluid muds northwards. Highest C and N and stoichiometric ratios in mottled muds; little vertical structure; concentrations function of grain size as in delta. NH~- levels increase with sediment depth. High densities; very variable rates of production, high when measurable; tight DOC-bacteria coupling. Little macroparticulate litter; higher levels of algal pigments than in delta. High densities and biomass, but still mostly pioneering seres. High biomass, more diverse and composed mostly of hydroids, crinoids, sponges, echinoids, asteroids and crabs. High diversity, composed of a variety of feeding types, mainly leatherjackets; penaeids and cephalopods rare. High rates of respiration and nutrient exchange, mostly release into water column; inorganic fluxes account for 38% of N and 61% of P plankton requirements.

322

D . M . ALONGIet al.

effluent and in the prodelta and shelf bedforms between the Changjiang and Fly River regions. Within the Fly Delta, it is apparent that episodes of erosion and deposition caused by variations in river runoff prevent the establishment of mature infaunal communities, except in quiescent areas (e.g. on leeward side of islands, intertidal mudbanks, ALONGI, 1991), leading to sedimentary facies dominated by primary physical structures. Deltaic sediments exhibited low to modest levels of bacterial remineralization and appear to be a sink for nutrients with low rates of dissolved nutrient uptake. Most of the suspended sediment and organic material deposits on the intertidal flats, on the leeward side of the delta islands, and within the inshore gulf, probably to be dispersed laterally along the shore. As this material is diluted seawards, light penetrates deeper into the water column fostering high rates of primary production stimulated by the high levels of dissolved river-borne nutrients. With increasing distance from the river delta, the water column deepens, leading to a progression of facies from prodelta muds with both physically and biologically-produced structures and moderate infaunal densities, to midshelf fluid muds that are thoroughly bioturbated by a very abundant infauna. Along this gradient, the contribution of nutrients regenerated from the seabed to pelagic primary production increases. This benthic-pelagic coupling is fostered by deposition of ungrazed "high quality" bioseston which stimulates infaunal population growth, higher rates of sediment mixing and microbial mineralization, and the release of nutrients back into the water column. The diversity of epifaunal taxa and demersal fish communities also increase along this gradient with more benign shelf conditions. The abundance of epifauna and demersal fish are low indicating limited transfer of benthic prey up the food chain to man, at least on the inner shelf. Potential fish yield to man may be considerably greater in the northern and eastern regions of the Gulf of Papua near the other major Papuan rivers (DALZELL and PAULY, 1989). This pattern is complicated by the presence of the northern Great Barrier Reef to the south, which results in a change from terrestrial to mixed terrestrial-carbonate to pure carbonate facies in this direction. This progression is nearly identical to the across-shelf gradient of terrigenous to carbonate facies within the Great Barrier Reef Lagoon (ALONGI, 1989). It thus appears that most of the particulate matter debouched from the Fly Delta is dispersed in an along-shore, northeasterly direction and diluted and mixed with sediment-laden water transported from nearby rivers (Bamu, Kikori, Purari) as an inshore band along the Papuan coast within the Gulf of Papua. This scenario may be altered in the near future. The mining activities on the Ok Tedi, a tributary of the Fly, have increased total sediment load by 55% since 1984 (GEORG, 1989; OK TEDI MINING LTD, personal communication). It is likely that the increased sediment burden, expected to peak in the mid-1990s, will increase accretion rates within the delta and on the adjacent shelf. In addition, the mine-derived sediment is contaminated with trace metals (mainly copper and cadmium). At present, dissolved copper in the sediment porewaters at our stations is at or below detection limits (-<0.01/~g m1-1) and mean particulate concentrations are still low (7-45 ppm) (ALONGI et al., 1991) and below concentrations thought to negatively impact on infaunal abundances and diversity (RYGG, 1985; WARWICKet al., 1988). The acceleration of mining activities in the near future may lead to dramatic changes in the benthic regime of this region. Acknowledgements--We are grateful to the masters and crew of the R.V. The Harry Messel for their able seamanship; J. Hardman for help in organizing both cruises; P. Daniel and P. Dixon for help during trawling

Benthic dynamics in the vicinity of the Fly River

323

operations; K. Hamilton for making the trawl nets; E. Seymour for help with analysis of fish stomach contents; J. Wellington and C. Payn for laboratory assistance; Gary Kershaw of the Townsville General Hospital for X-raying the cores; L. Richards and K. Handley for photography; D. Eddleston for typing the manuscript and two anonymous reviewers for their helpful comments. Mr Murray Eagle and his colleagues at OK Tedi Mining Ltd provided invaluable help with information and logistics. This project was supported by the Australian Institute of Marine Science and by a grant from Ok Tedi Mining Ltd. This is contribution No. 531 from the Australian Institute of Marine Science.

REFERENCES ALLEN 8. E., H. M. GRIMSHAW, J. A. PARKINSON and C. QUARMBY(1974) Chemical analysis of ecological materials, John Wiley, New York, 565 pp. ALLER J. Y. and R. C. ALLER (1986) General characteristics of benthic faunas on the Amazon inner continental shelf with comparison to the shelf off the Changjiang River, East China Sea. Continental Shelf Research, 6, 291-310. ALLER R. C., J. E. MACKIN,W. J. ULLMAN, W. CHEN-Hou, T. 8H1NG-MIN,J. JIAN-CA1, S. YONG-NIANand H. JIAZHEN (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. ALONGI D. M. (1988) Bacterial productivity and microbial biomass in tropical mangrove sediments. Microbial Ecology, 15, 59-79. ALONG1 D. M. (1989) Benthic processes across mixed terrigenous-carbonate sedimentary facies on the central Great Barrier Reef continental shelf. Continental Shelf Research, 9, 629-663. ALONGI D. M. (1990a) The ecology of tropical soft-bottom benthic ecosystems. Oceanography and Marine

Biology Annual Reviews, 28,381-496. ALONGI D. M. (1990b) Effect of mangrove detrital outwelling on nutrient regeneration and oxygen fluxes.in coastal sediments of the central Great Barrier Reef Lagoon. Estuarine, Coastal and Shelf Science, 31,581598. ALONGID. M. (in press) The role of intertidal mudbanks in the diagenesis and export of dissolved and particulate materials from the Fly Delta, Papua New Guinea. Journal of Experimental Marine Biology and Ecology, in press. ALONGI D. M., K. G. BOTO and F, TIRENDI (1989) Effect of exported mangrove litter on bacterial productivity and dissolved organic carbon fluxes in adjacent tropical nearshore sediments. Marine Ecology Progress Series, 56, 129-140. ALONGID. M., F. TIRENDIand A. I. ROBERTSON(1991) Vertical profiles of copper in sediments from the Fly Delta and Gulf of Papua (Papua New Guinea). Marine Pollution Bulletin, 22,253-255. ANDREWS J. C. and S. CLEGG (1989) Coral Sea circulation and transport deduced from modal information models. Deep-Sea Research, 36,957-974. ANSARI Z. A., S. N. HARKANTRA, S. A. NAIR and A. H. PARULEKAR(1977) Benthos of the Bay of Bengal: a preliminary account. Mahasgar-Bulletin of the National Institute of Oceanography, 10, 55-60. ANSARI Z. A., C. L. RODRIQUES, A. CHATTERJIand A. H. PARULEKAR(1982) Distribution of meiobenthos and macrobenthos at the mouth of some rivers of the east coast of India. Indian Journal of Marine Science, 11, 342-353. BARRETOL. A., J. D. MILLIMAN,C. A. B. AMARAZand O. FRANCISCONI (1975) Northern Brazil. Contributions to Sedimentology, 4, 11-43. B ELBINL. (1987) PA TN Pattern analysis package reference manual. CSIRO Division of Wildlife and Rangelands Research, Canberra, 337 pp. BELL R. Z. (1990) An explanation for the variability in the conversion factor deriving bacterial cell production from incorporation of [3H]thymidine. Limnology and Oceanography, 35,910-915. BOTOK. G., D. M. ALONGIand A. L. J. N o ~ (1989) Dissolved organic carbon-bacteria interactions at sedimentwater interface in a tropical mangrove system. Marine Ecology Progress Series, 51,243-251. BOYNTONW. R. and W. M. KEMP (1985) Nutrient regeneration and oxygen consumption by sediments along an estuarine salinity gradient. Marine Ecology Progress Series, 23, 45-55. CANNON,L. R. G,, G. B. GOLDENand P. CAMPBELL(1987) Community patterns revealed by trawling in the interreef regions of the Great Barrier Reef. Memoirs of the Queensland Museum, 25, 45-70. CHIN-LEo G. and D. L. KIRCHMAN(1990) Unbalanced growth in natural assemblages of marine bacterioplankton. Marine Ecology Progress Series, 63, 1-8.

324

D.M. ALONGIet al.

COLLINSON J. D. and D. B. THOMPSON(1989) Sedimentary structures, Unwin Hyman, London, 207 pp. DALZELLP. and D. PAULY(1989) Assessment of the fish resources of Southeast Asia, with emphasis on the Banda and Arafura Seas. Netherlands Journal of Sea Research, 24,641~550. DAVIESP. J., P. A. SYMONDS,D. A. FLAReand C. J. PIGRAM(1989) The evolution of the carbonate platforms of Northeast Australia. In: Controls on carbonate platform and basin development, P. D. CREVELLO,J. L. WILSON, J. F. SARCand J. F. READ,editors, Society of Economic Paleontologists and Mineralogists, Tulsa, pp. 233-258. DE WILDEP. A. W. J., W. W. KASTORO,E. M. BERGHUIS,1. ASWANDY,1. AL HAKIMand A. KoK (1989) Structure and energy demand of the benthic soft-bottom communities in the Java Sea and around the islands of Madura and Bali, Indonesia. Netherlands Journal of Sea Research, 23, 449-461. DREDGE M. C. L. (1988) Bycatch from the Central Queensland prawn fisheries. Part I. The prawn fisheries and their bycatch composition in terms of species and community assemblages. Unpublished Report to the Great Barrier Reef Marine Park Authority, Department of Primary Industries, 45 pp. FALLON R. D., S. Y. NEWELL and C. S. HOPKINSON (1983) Bacterial production in marine sediments: will cell-specific measures agree with whole-system metabolism? Marine Ecology Progress Series, 11, 119-127. GEORG D. (1989) Government accepts pollution levels from Ok Tedi. Engineers Australia, 61, 28-29. GILMOURC. C., M. E. LEAVITrand M. P. SHIARIS(1990) Evidence against incorporation of exogenous thymidine by sulfate-reducing bacteria. Limnology and Oceanography, 35, 1401-1409. GWrrHER D. (1982) Yield estimates for the banana prawn (Penaeus merguiensis de Man) in the Gulf of Papua prawn fishery. Journal du Conseil, 40, 245-258. HANSONR. B., K. R. TENORE,S. BISHOP,C. CHAMBERLAIN,M. M. PAMATAMATand J. H. TIETJEN (1981) Benthic enrichment in the Georgia Bight related to Gulf Stream intrusions and estuarine outwelling. Journal of Marine Research, 39,417-441. HARRIS P. T. (1988) Sediments, bedforms and bedload transport pathways on the continental shelf adjacent to Torres Strait, Australia--Papua New Guinea. Continental Shelf Research, 8,979-1003. HATCHER,B. G., R. E. JOHANNESand A. I. ROBERTSON(1989) Review of research relevant to the conservation of shallow tropical marine ecosystems. Oceanography and Marine Biology Annual Reviews, 27, 337-414. HERNDL G. J., J. FAGANELI,N. FANUKO, P. PEDUZZIand V. TURK (1987) Role of bacteria in the carbon and nitrogen flow between water-column and sediment in a shallow marine bay (Bay of Piran, northern Adriatic Sea). Publicanzion della Stazione Zoologica di Napoli 1: Marine Ecology, 8,221-236. HOBBLEJ. E., R. J. DALLYand S. JASPER(1977) A method for counting bacteria on Nuclepore filters. Applied and Environmental Microbiology, 33, 1225-1228. HOPKINSONC. S., Jr (1987) Nutrient regeneration in shallow-water sediments of the estuarine plume region of the nearshore Georgia Bight, USA. Marine Biology, 94, 127-142. KASTOROW., l ASWANDY,I. AL HAKIM,P. A. W. J. DE WILDEand J. M. EVERAARTS(1989) Soft-bottom benthic community in the estuarine waters of East Java. Netherlands Journal of Sea Research, 23,463-472. KIRCHMAND., E. K'NEESand R. HODSON(1985) Leucine incorporation and its potential as a measure of protein synthesis by bacteria in natural aquatic systems. Applied and Environmental Microbiology, 49,599--607. KIRCHMAND., S. Y. NEWELLand R. HODSON(1986) Incorporation versus biosynthesis of leucine: implications for measuring rates of protein synthesis and biomass production by bacteria in marine systems. Marine Ecology' Progress Series, 32, 47-59. KUEnL S. A., C. A. NIrrROUERand D. J. DEMASTER(1986) Distribution of sedimentary structures in the Amazon subaqueous delta. Continental Shelf Research, 6, 311-336. LE LOEUFFP. and A. INTES (1973) Note sur le regime alimentarire de quelques poissons demersaux de Cote d'Ivoire. Documents scientifiques du recherches oceanographiques, Abidjan, 4, 17-44. Liu H. C. (1976) The demersal fish stocks of the waters of North and Northwest Australia. Acta Oceanographic Taiwanica, 6,128-134. Liu H. C., H.-L. LAt and S.-Y. YEn (1978) General review of demersal fish resources in the Sunda shelf and the Australian waters. Acta Oceanographic Taiwanica, 8,109-140. MACFARLANEJ. W. (1980) Surface and bottom currents in the Gulf of Papua and western (;oral Sea. Department of Primary Industry, Papua New Guinea, Research Bulletin No. 27, 128 pp. MILLIMAN J. D. and R. H. MEADE (1983) World-wide delivery of river sediment to the oceans. Journal of Geology, 91, 1-21. MORIARTYD. J. W. (1988) Accurate conversion factors for calculating bacterial growth rates from thymidine incorporation into DNA: elusive or illusive? Archivfi~r Hydrobiologie Beihefie Ergebrisse Limnologie, 31, 211-217.

Benthic dynamics in the vicinity of the Fly River

325

MORIARTYD. J. W. (1990) Techniques for estimating bacterial growth rates and production of biomass in aquatic environments. In: Methods in microbiology, Vol. 22, J. R. NORRIS and R. GRIGOROVA,editors, Academic Press, London, pp. 211-234. MORIARTY D. J. W., D. G. ROBERTS and P. C. POLLARD (1990) Primary and bacterial productivity of tropical seagrass communities in the Gulf of Carpentaria, Australia. Marine Ecology Progress Series, 61,145-157. NEYMAN A. A. (1969) Some data on the benthos of the shelves in the northern part of the Indian Ocean. Oceanology, 9, 861-866. POLLARD P. C. and D. J. W. MORIARTY(1984) Validity of the tritiated thymidine method for estimating bacterial growth rates: measurement of isotope dilution during D N A synthesis. Applied and Environmental Microbiology, 78, 1076-1083. POLUNIN N. C. (1983) The marine resources of Indonesia. Oceanography and Marine Biology Annual Reviews, 21,455-531. RAINER S. F. (1984) Temporal changes in a demersal fish and cephalopod community of an unexploited coastal area in Northern Australia. Australian Journal of Marine and Freshwater Research, 35,747-768. REDF1ELD A. C., B. H. KETCHUMand F. A. RICHARDS(1963) The influence of organisms on the composition of sea-water. In: The sea, M. HILL, editor, Wiley-Interscience, New York, pp. 26-77. REEBURG W. S. (1983) Rates of biogeochemical processes in anoxic sediments. Annual Review of Earth and Planetary Sciences, 11,269-298. RHOADS D. C., D. F. BOESCH,T. ZHICAN, X. FENGSHAN,H. LIQIANGand K. J. NILSEN (1985) Macrobenthos and sedimentary facies on the Changjiang delta platform and adjacent continental shelf, East China Sea. Continental Shelf-'Research, 4,189-213. ROBBINS J. A. and J. GUSTINIS (1976) A squeezer for efficient extraction of porewater from small volumes of anoxic sediment. Limnology and Oceanography, 21,905-909. ROBERTSON A. 1. (1984) Trophic interactions between the fish fauna and macrobenthos of an eelgrass community in Western Port, Victoria. Aquatic Botany, 28, 125-153. ROBERTSON A. I., D. M. ALONGI, P. A. DANIEL and K. G. BoTo (1988) How much mangrove detritus enters the Great Barrier Reef Lagoon.'? Proceedings of the Sixth International Coral Reef Symposium, 2, 601-606. ROBERTSON A. [., D. M. ALONGI, P. CHRISTOFFERSEN, P. A. DANIEL, P. DIXON and F. TIRENDI (1990) The

influence of freshwater and detrital export from the Fly River system on adjacent pelagic and benthic systems. Australian Institute of Marine Science Technical Report No. 4, Townsville, 199 pp. RUBLEE P. (1982) Bacteria and microbial distribution in estuarine sediments. In: Estuarine comparisons, V. S. KENNEDY,editor, Academic Press, New York, pp. 159-182. RYGG B. (1985) Effect of sediment copper on benthic fauna. Marine Ecology Progress Series, 25, 83-89. RYLE V. D., H. R. MUELLER and P. GENTIEN (1981) Automated analysis of nutrients in tropical seawater. Australian Institute of Marine Science Oceanography Series, OS-81-4, 24 pp. RYLE V. D. and J. WELLINGTON (1982) Reduction column for automated determination of nitrates. Analytical Laboratory Note No. 19, Australian Institute of Marine Science, Townsville, 3 pp. SANDSTROM M. W., F. TIRENDI and A. N o ~ (1986) Direct determination of organic carbon in modern reef sediments and calcareous organisms after dissolution of carbonate. Marine Geology, 70, 321-329. SAVlCH M. S. (1971) Quantitative distribution and food value of benthos from the West Pakistan Shelf. Oceanology, 10, 113-119. SOKAL R. R. and F. J. ROHLF (1981) Biometry, W. H. Freeman, San Francisco, 859 pp. STRICKLAND J. D. H. and T. R. PARSONS (1972) A practical handbook of seawater analysis. Bulletin of the Fisheries Research Board of Canada, 167, 1-311. TEAGUE K. G., C. J. MADDEN and J. W. DAY, Jr (1988) Sediment-water oxygen and nutrient fluxes in a river-dominated estuary. Estuaries, 11, 1-9. TENORE K. R., R. M. CAL, R. B. HANSON, E. LOPEZ-JAMAR,G. SANTIACOand J. H. TIETJEN (1984) Coastal upwelling off the Rias Bajas, Galicia, Northwest Spain. lI. Benthic studies. Rapports etproces verbaux des reunions, conseil international pour l'exploration de la mer, 183, 91-100. WARWICKR. M. and R. J. UNCLES(1980) Distribution of benthic macrofauna associations in the Bristol Channel in relation to tidal stress. Marine Ecology Progress Series, 3, 97-103. WARWICKR. M., M. R. CARR, K. B. CLARKE,J. M. GEE and R. H. GREEN (1988) A mesocosm experiment on the effects of hydrocarbons and copper pollution on a sublittoral soft-sediment meiobenthic community. Marine Ecology Progress Series, 46, 181-191. WASSENBERC,T. J. and B. J. HILL (1987) Natural diet of the tiger prawns Penaeus esculentus and P. semisulcatus. Australian Journal of Marine and Freshwater Research, 38, 169-182.

326

D . M . ALONGIet al.

WATSON R. A. (1984) Trawl fish composition and harvest estimates for the Gulf of Papua. Report No. 8141. Department of Primary Industry, Port Moresby, 25 pp. WOLANSKI E. and B. RUDDICK (1981) Water circulation and shelf waves in the Northern Great Barrier Reef Lagoon. Australian Journal of Marine and Freshwater Research, 32,721-740, WOLANSKIE., G. L. PICKARDand D. L. B. JuPP (1984) River plumes, coral reefs and mixing in the Gulf of Papua and the Northern Great Barrier Reef. Estuarine, Coastal and Shelf Science, 18,291-314. WOLANSKI E. and R. E. T,oMsoN (1984) Wind-driven circulation on the Northern Great Barrier Reef continental shelf in summer. Estuarine, Coastal and Shelf Science, 18, 271-289. WOLANSKIE., P. R1DD and M. INOUE(1988) Currents through Tortes Strait. Journal of Physical Oceanography, 18, 1535-1545. WRI~rIT L. D. (1989) Dispersal and deposition of river sediments in coastal seas: models from Asia and the tropics. Netherlands Journal of Sea Research, 23, 493-500. YAMADAH., M. KAYAMAand K. FUJISAWA(1987) Seasonal changes of concentrations of inorganic and organic nitrogen in coastal marine sediments. Estuarine, Coastal and Shelf Science, 24, 585-598. ZEHR J. P., S. G. PAULSEN, R. P. AXLER and C. R. GOLDMAN(1988) Dynamics of dissolved organic nitrogen in subalpine Castle Lake, California. Hydrobiologia, 157, 33-45.