Importance of Biologically Mediated Removal of Fine Sediments from the Fly River Plume, Papua New Guinea

Estuarine, Coastal and Shelf Science (1997) 44, 629–639

Importance of Biologically Mediated Removal of Fine Sediments from the Fly River Plume, Papua New Guinea T. Ayukai and E. Wolanski Australian Institute of Marine Science, PMB No. 3, Townsville M.C., Queensland 4810, Australia Received 26 October 1995 and accepted in revised form 16 April 1996 A freshwater plume from the Fly River, Papua New Guinea, stretched to the south-west over a distance of more than 150 km from the estuary towards Torres Strait. The highly turbid freshwater was mixed with the relatively clear water of the Gulf of Papua, producing a sharp boundary around 23 salinity. Sediment concentrations were typically more than 50 mg l "1 inside the turbid water mass, and were rapidly reduced to a few milligrams per litre in the area where salinity was still as low as 26. The area had relatively high levels of phytoplankton (>1 ìg chl l "1), resulting in rapid depletion of nitrate and phosphate. Biological as well as physico-chemical (flocculation) processes seemed responsible for the rapid removal of sediments from the plume. Settling particles collected near the estuary consisted largely of fine sediments bound loosely by flocculant organic matter, and contained very few plankton and their remains. Further away from the estuary, however, settling particles were dominated by colonies of phytoplankton, fecal pellets and macroscopic aggregates of apparently biological origin. Fine sediments were found firmly attached to or embedded in these settling particles. The area of high biological activities in the plume probably functions as a barrier, limiting a further offshore transport of sediments. ? 1997 Academic Press Limited Keywords: suspended sediments; removal rates; phytoplankton; river plumes; Fly River; Papua New Guinea

Introduction The Fly River has a catchment area of about 76 000 km2 and a suspended-sediment yield of 1500 tonnes km "2 year "1, about 10 times higher than the Amazon and Mississippi Rivers (Wolanski & Gibbs, 1995). This is largely due to rapid tectonic uplift and rainfall of up to 10 m year "1 in the south-western highlands of Papua New Guinea. Part of the sediment load also derives from mining activities in the highlands (Georg, 1989; Salomons & Eagle, 1990). A large proportion of sediments in the Fly River is retained in the estuary through flocculation dynamics, tidal pumping and the baroclinic circulation (Wolanski et al., 1995a; Wolanski & Gibbs, 1995). Sediments that escape the estuary consist largely of silt particles with a median size of 6–8 ìm (Wolanski & Gibbs, 1995). Such fine silt particles have a settling velocity of only a few metres per day. Silt-dominated flocs are structurally weak and are readily destroyed by turbulence. Fine silt particles, because of these attributes, may be transported offshore over a long distance (Wolanski & Gibbs, 1995). One aspect that has not been addressed in previous studies of the Fly River and the Gulf of Papua is biologically mediated removal of sediments from the 0272–7714/97/050629+11 $25.00/0/ec960172

plume (cf. Fowler & Knauer, 1986; Alldredge & Silver, 1988; Silver & Gowing, 1991). An area of relatively high phytoplankton biomass exists near the mouth of the estuary, where suspended-sediment concentrations are still as high as 70 mg l "1 (Robertson et al., 1993). Formation of large settling particles, such as macroscopic aggregates and fecal pellets, is potentially high in such an area (e.g. Alldredge et al., 1993; Kiorboe & Hansen, 1993; Kiorboe et al., 1994), yet no data are available for evaluating the significance of this process on removal of sediments from the plume. The water circulation in the Gulf of Papua has been described elsewhere by Wolanski et al. (1995b). Briefly, the Gulf has the shape of a half-moon and an average depth of <50 m. In calm weather, the circulation appears to be dominated by an anti-clockwise eddy generated by the Coral Sea Coastal Current. South-easterly trade winds prevail from autumn, winter through to spring months, generating a sealevel set-up of up to 30 cm in the semi-enclosed Gulf. This sea-level set-up, the Coriolis force, river runoff and oceanic forcing altogether tend to drive the bulk of freshwater from the Fly River to the east (see also Wolanski et al., 1984; Robertson et al., 1993). The plume observed in this study was, however, stretching ? 1997 Academic Press Limited

630 T. Ayukai & E. Wolanski

to the south-west over a distance of more than 150 km from the estuary towards Torres Strait. The distribution of sediments in this plume was investigated in relation to those of nutrients, organic carbon, phytoplankton and bacteria. Samples for microscopic observations of settling particles were also collected using the method described by Wolanski and Gibbs (1995).

Purari River Fly River

10 20 30

Gulf of Papua

100 100 100

astal Curre a Co nt

Se Co r Torres Strait

Coral Sea

Parama Is

Stn R Stn 10

Bramble Cay

9.2°

Stn 35

r Re

efs

Stn 38

9.6°

War rio

A 2-week oceanographic cruise was conducted in September 1994. In total, 39 stations were occupied in the area between the estuary and Torres Strait (Figure 1). The vertical profiles of temperature, salinity and light attenuation were measured using a CTD-cum-transmissometer profiler (Sea-Bird Electronics, U.S.A.). At most of the 39 stations, water samples for chemical analyses and plankton counts were collected using an acid-washed 50-1 Niskin bottle from the sea surface (3 m). Additional water samples were collected from mid-depth (15–30 m) at eight offshore stations. Samples for microscopic observations of settling particles were obtained using a 50-l Niskin bottle coupled with a slide with well (Wolanski & Gibbs, 1995) at selected stations.

al

Methods

Nutrients, total organic carbon and dissolved organic carbon Duplicate 10-ml samples for dissolved inorganic nutrient (DIN) analysis were withdrawn from a Niskin bottle, filtered through 0·4-ìm glass-fibre filters (Acrodisc, Gelman Scientific) and kept in a freezer. Concentrations of silicate, nitrate and phosphate were determined in the laboratory using a multichannel segmented flow autoanalyser (Ryle et al., 1981). Duplicate 10-ml samples for total organic carbon (TOC) and dissolved organic carbon (DOC) analyses were also withdrawn, acidified immediately by adding 200 ìl of HCl and kept in a freezer. Total organic carbon and DOC concentrations were determined by the high temperature Pt-catalytic oxidation method with a Shimadzu TOC-5000 analyser (Sugimura & Suzuki, 1988). Phytoplankton and bacteria For each Niskin bottle sample, duplicate 100- or 250-ml samples were filtered onto Whatman GF/F filters and these filters were kept frozen for later chlorophyll analysis by fluorometry (Strickland & Parsons, 1972). About 200 ml of water was also taken at selected stations, and preserved with formalin for

10.0°S

km 0 10 20 10.4° 142.6°

143.0°E

143.4°

143.8°

F 1. Location of sampling stations in the southwestern Gulf of Papua and Torres Strait. The stations, where microscopic images in Figure 9 were taken, are numbered.

enumeration of phytoplankton under an inverted microscope. Duplicate 10-ml samples for bacterial cell counts were preserved with 0·2-ìm filtered, borate-buffered formalin at a final concentration of 2% and kept in a refrigerator. In the laboratory, bacterial samples were filtered onto 0·2-ìm black Nuclepore filters and stained by a fluorochrome DAPI solution (4,6diamidino-2-phenylindole, 0·05 ìg ml "1, Porter & Feig, 1980). These filters were mounted onto slides with a non-fluorescence immersion oil and examined under a Zeiss epifluorescence microscope with a filter

Biologically mediated removal of fine sediments 631

40 24

26

50 60

28

70 26

80

28 30

32

30 32

(b)

(a)

34

F 2. Horizontal distributions of (a) salinity and (b) light attenuation (%) at the sea surface.

set 487902. Some bacterial samples with a relatively large amount of sediments were processed with a biodetergent Tween 80 (Sigma) in advance to filtration (Yoon & Rosson, 1990). Settling particles The technique used for collecting settling particles has been described elsewhere by Wolanski and Gibbs (1995). Briefly, water samples were collected from 1 m above the sea-bed using a 50-l Niskin bottle. A specially manufactured hole slide was inserted from the slot on the side of this Niskin bottle and left still for about 1 h. The slide was then slowly pushed back under a cover glass. The seal between slide and cover glass was made water tight, so that settling particles intercepted into the hole were recovered without being disturbed physically. The sample was immediately examined under an Olympus inverted microscope with a Sony CCD video camera. The images were captured on an IBM-compatible PC with an interface video card. Results

more than 150 km from the estuary. The longitudinal dispersion of the plume is usually limited by the presence of Warrior Reefs and the dense reef matrix in the east (Wolanski et al., 1984). The highly turbid water of the Fly River was mixed with the relatively clear water of the Gulf of Papua, producing a sharp boundary around a salinity of 23. Sediment concentrations inside the turbid water mass were typically more than 50 mg l "1 (also see Robertson et al., 1993). Surface light attenuation was less than 40% at Stn R, and increased to about 80% in the area where salinity was still as low as 26 [Figure 2(b)]. Data collected in repeated cruises to the area revealed a good linear relationship between light attenuation, Att (%), and sediment concentration, SC (mg l "1) (Wolanski et al., 1995c): SC= "0·266#Att+23·1 (n=50, r2 =0·749) Using this empirical equation, sediment concentrations were estimated to be about 13 mg 1 "1 at Stn R and 2 mg l "1 around the northern end of Warrior Reefs. This indicated that sediments were removed from the plume, much faster than would be predicted from its mixing with the surrounding water.

Salinity and light attenuation Surface salinity was about 23 near the estuary (Stn R) and increased gradually towards the south-west [Figure 2(a)]. The salinity-32 contour line was observed beyond the southern end of Warrior Reefs,

Nutrients and total organic carbon The horizontal distributions of silicate, nitrate, phosphate and TOC are shown in Figure 3. Silicate

632 T. Ayukai & E. Wolanski

0.4

20

0.3 0.2

10

0.1

10 0.05

10

5

(b)

(a)

0.2 3

7 5

0.1 0.05 1 0.05 1

1

(c)

(d)

F 3. Horizontal distributions of (a) silicate, (b) nitrate, (c) phosphate and (d) total organic carbon at the sea surface.

concentrations were above 23 ì near the estuary and gradually decreased towards Torres Strait. In contrast, nitrate and phosphate were rapidly depleted from the plume, with their concentrations being below their detection limits (about 0·05 and 0·02 ì respectively) at most offshore stations. Total organic carbon concentrations, except for one extremely high value at Stn R, changed little in the plume.

The mixing diagrams for nutrients and TOC are shown in Figure 4. A linear correlation between salinity and silicate concentration indicated that the distribution of silicate was largely regulated by mixing of the plume with the surrounding water. Nitrate and phosphate, on the other hand, showed a nonconservative behaviour. The distribution of TOC appeared to be explained again by conservative

Biologically mediated removal of fine sediments 633

(a)

20

10

0

(b)

Nitrate (µM)

Silicate (µM)

30

25

30 Salinity

0.4

0

35

25

30 Salinity

4

(c)

35

(d)

+ 7.34

TOC (mg l–1)

Phosphate (µM)

0.4

0.2

0

25

30 Salinity

35

2

0

25

30 Salinity

35

F 4. Relationships between salinity and concentrations of (a) silicate (ì), (b) nitrate (ì), (c) phosphate (ì) and (d) total organic carbon (mg l "1) in surface (+; 3 m) and subsurface (o; 15–30 m) waters of the south-western Gulf of Papua and Torres Strait.

mixing, if the high value observed at Stn R was excluded. Phytoplankton and bacteria An area of relatively high chlorophyll concentrations was formed in the south of Parama Island [Figure 5(a)] or around a salinity of 25 (Figure 6). Formation of such an area appeared to be primarily determined by light and nutrient availabilities. The horizontal distribution of bacteria [Figure 5(b)] did not always coincide with that of chlorophyll. There was a good linear relationship between chlorophyll and bacterial concentrations in a low chlorophyll range (<1 ìg l "1), but not in a high chlorophyll range (Figure 7). One unique phytoplankton species in the plume was the diatom Thalassiosira nana, of which the colony was embedded in a gelatinous sheath. This diatom commonly occurred in the low-salinity region (<28) of the plume (Figure 8). The average diameter of colonies was mostly in a range between 150 and 300 ìm.

Settling particles The characteristics of settling particles collected at Stn R were visibly different from those collected at other stations. In detail, settling particles collected at Stn R consisted largely of fine sediments bound loosely by flocculant organic matter [Figure 9(a)]. These settling particles were relatively uniform in size and contained very few plankton and their remains. At the stations in the south of Parama Island or near Bramble Cay, settling particles were already dominated by colonies of phytoplankton, fecal pellets and macroscopic aggregates of apparently biological origin. Fine sediments, although not distinguished in captured images, were found firmly attached to or embedded in these settling particles [Figure 9(b–j)]. A large number of colonies of T. nana occurred among settling particles either singly or as part of macroscopic aggregates [Figure 9(c,d)]. These colonies were much larger than those found in water samples, sometimes reaching as large as 2 mm diameter. A variety of plankton or their remains

634 T. Ayukai & E. Wolanski

1.5

1.5

7 5

1.0 3

0.5

0.25

(a)

(b) 1

F 5. Horizontal distributions of (a) chlorophyll (ìg l "1) and (b) bacteria (108 cells l "1) at the sea surface.

10

2

1

0

(b)

Bacteria (108 cells l–1)

–1

Chlorophyll (µg l )

(a)

25

30 Salinity

35

0

25

30 Salinity

35

F 6. Relationships between salinity and concentrations of (a) chlorophyll and (b) bacteria in surface (+; 3 m) and subsurface (o; 15–30 m) waters of the south-western Gulf of Papua and Torres Strait.

occurred in association with settling particles [Figure 9(e–j)]. Colonies of large diatoms attaching fine sediments were also commonly observed. Discussion The bulk of freshwater from the Fly River usually disperses to the east, generating a complex three-dimensional circulation in the Gulf of Papua (Wolanski et al., 1984, 1995b; Robertson et al., 1993). The sea-level difference between east and west sides of Torres Strait generates occasional westward currents and may be responsible for the observed intrusion of Fly River water into the strait (Wolanski et al., 1995c).

Studies of these physical processes constitute a basis for elucidating the offshore transport of sediments from the Fly River. This study, however, shows that sediments do not behave simply as predicted from mixing of the plume with the surrounding water. The currents in the study area are topographically steered. The x-axis is alongshore, along the direction of the prevailing current, u. This current typically has a flow rate of about 0·1 m s "1 and is driven by large-scale oceanic forcing. The plume generates only weak currents of <0·02 m s "1 through barotropic plus baroclinic forcings (Wolanski et al., 1995b). In the study area, water depth is mostly <15 m and waters are vertically well-mixed in temperature and

Biologically mediated removal of fine sediments 635

(mg l "1), wf is the settling velocity, z is the vertical axis, and kz is the vertical diffusion coefficient. For estimating wf, Equation (1) is integrated over the depth (=the thickness of the plume), H (m), and several terms are estimated based on the field measurements of Wolanski et al. (1995b): i.e. u~0·1 m s "1, H=15 m, C¦50 mg l "1, kz~0·1 ut H, kx~10 m2 s "1, where ut is the root mean square tidal velocity. Inferring the horizontal scales also from the field measurements, it results in the m.k.s. units:

Bacteria (108 cells l–1)

8

4

0

2

1 Chlorophyll (µg l–1)

uH

100

200

0

25

30

Average colony diameter (µm)

–1

Colony concentration (no. l )

F 7. Relationship between chlorophyll and bacterial concentrations in surface (+; 3 m) and subsurface (o; 15– 30 m) waters of the south-western Gulf of Papua and Torres Strait.

0

Salinity

F 8. Occurrence (solid bars) and average diameter of (——) of the gelatinous sheath of the diatom Thalassiosira nana in relation to the surface salinity of the Fly River plume.

salinity. The plume thus extends to the bottom in the area and is largely passive. The vertical homogeneity is maintained by strong tidal currents of up to 2 m s "1 in the estuary itself (Wolanski et al., 1995a). The plume remains vertically well-mixed in coastal waters and it is only the Gulf of Papua that the plume lifts off the bottom in 15–20 m depth and the system stratifies. At steady state, the tidally-averaged equation for mass conservation of suspended sediments is: u

)C )x

+

) )z

(w f C) =

) )z

S D kz

)C )z

+

) )x

S D kx

)C )x

(1)

where the x-axis is the central longitudinal axis of the plume, C is the suspended-sediment concentration

)C )x

"3

z 3#10

k

±H

)C )z

) )x

S D kx

)C )x

"5

z 2#10

lbottom z 0·16

(2)

(3)

This result is valid throughout the plume, since plume-driven currents are much smaller than the prevailing coastal current, u. Thus, the two dominant terms in Equation (1) are the vertical diffusion terms (keeping sediments in suspension) and the vertical settling term. From this balance, it results: wfz0·001"0·005 m s "1

(4)

under different, realistic assumptions in the estimate of various parameters. This would imply a median floc size d50 of about 100–500 ìm (Gibbs, 1985). Typical flocs at the mouth of the Fly River estuary have a median floc size of only 10–20 ìm (Wolanski & Gibbs, 1995). In other words, the observed distribution of suspended sediments in the study area is explained by increases in effective median floc size by a factor of more than 10. As discussed below, physico-chemical and biological processes seem responsible for such increases. Adsorption of dissolved organic matter on particles occurs during various stages of estuarine mixing of freshwater with seawater, promoting their flocculation and removal from the water column (e.g. Boyle et al., 1974; Sholkovitz, 1976; Church 1986). In this study, the TOC concentration near the estuary (Stn R) was 7·34 mg l "1, about 80% of which was in a dissolved phase (5·71 mg l "1). While the proportion of DOC to TOC remained relatively constant, TOC concentrations were reduced to less than 1·5 mg l "1 at the neighbouring station. Settling particles in the area consisted largely of fine sediments bound loosely by flocculant organic matter. As also noted by Wolanski and Gibbs (1995), the physico-chemical adjustment of organic and inorganic matter to changing ionic

636 T. Ayukai & E. Wolanski

Biologically mediated removal of fine sediments 637

strength (salinity), namely flocculation dynamics, plays an important role in determining the distribution of sediments near the estuary. The importance of biologically mediated removal of sediments from river plumes is probably conceivable, yet has received little attention (Cloern et al., 1983; Nelsen & Trefry, 1986; Monaco et al., 1990). In this study, the characteristics of settling particles dramatically changed over a relatively short distance from the estuary. At the stations in the south of Parama Island or near Bramble Cay, for example, settling particles were already dominated by colonies of phytoplankton, fecal pellets and macroscopic aggregates attaching a tremendous variety of plankton and their remains. Fine sediments were found firmly attached to or embedded in these settling particles. These observations support the hypothesis that an area of high biological activities in the plume functions as a barrier, limiting further offshore transport of sediments. Location of high biological activities in river plumes is affected by the availability of light to phytoplankton, which is in turn affected by freshwater and sediment discharges, and mixing characteristics of individual estuaries. This has been demonstrated for large rivers, such as the Amazon (Milliman & Boyle, 1975), Changjiang (Ning et al., 1988), Huanghe (Turner et al., 1990), Mississippi (Lohrenz et al., 1990) and Zaire Rivers (Cadee, 1978, 1984), and is probably true for the Fly River. Processes regulating the magnitude of biological activities in the Fly River plume are, however, still to be investigated. Robertson et al. (1993) have suggested that biological processes in the delta and plume are driven largely by the supply of organic matter from the river and fringing mangrove areas. This is a view based on the results that bacterial biomass and production varied almost independently of those of phytoplankton. In this study, however, bacterial biomass was positively related to phytoplankton biomass. Although the linearity in their relationship was skewed in a high chlorophyll range (>1 ìg l "1), the data do not suggest any significant effect of allochthonous organic matter inputs on bacterial biomass. The highest chlorophyll concentration reported by Robertson et al. (1993) is 5·07 ìg l "1, about three

times higher than the highest value measured in this study. This appears due to the difference in availability of nutrients between two studies. Silicate is, however, an exception. Silicate concentrations measured in this study were similar to the values reported by Robertson et al. (1993) (about 1–25 ì). Uptake of silicate across the salinity gradient was insignificant in both studies. Independence of silicate concentrations on freshwater discharge or other parameters has also been reported for other large rivers (Gibbs, 1972; Van Bennekom et al., 1978; Edmond et al., 1981). Robertson et al. (1993) have noted that the distributions of nitrate and phosphate in the Fly River plume appear to be explained by conservative mixing, with predicted values at a salinity of 25 being 1·5 and 0·5 ì, respectively. In this study, however, the mixing diagrams for nitrate and phosphate indicated nonconservative mixing, and such high concentrations of nitrate and phosphate were not observed in any part of the plume (Figure 4). Rather, these nutrients appeared to be taken up rapidly by phytoplankton at 24–25 salinity and essentially became depleted in a high salinity (>28) region of the plume. Robertson et al. (1993) have suggested that the amount of freshwater discharge has a significant influence on nitrate and phosphate concentrations in the plume; however, limited data available to date do not preclude the importance of the other processes. Nitrate concentrations in the Fly River plume are much lower than those reported for plumes of the world’s large river (Table 1). Human activities along catchments have a strong influence on river end-member concentrations of nitrate (e.g. the Changjiang, Huanghe and Mississippi Rivers vs. others). Nitrate concentrations in the high-salinity region (25–30) are also affected by processes, such as localized regeneration in bottom layers (e.g. the Amazon and Mississippi Rivers), coastal upwelling and/or entrainment of nitrate-rich subsurface waters (e.g. the Amazon and Zaire Rivers). The importance of these processes in the Fly River plume is yet to be evaluated, but may depend on its flow direction. The study area, in particular, is isolated from the influence of nitrate-rich subsurface waters of the Gulf of Papua by the dense reef matrix in the east.

F 9. Microscopic images of settling particles in the Fly River plume: (a) silt particles bound loosely by flocculant organic matter; (b) different forms of settling particles of apparently biological origin; (c)(d) a colony of Thalassiosira nana (arrows) occurred singly or as part of a larger settling particle; (e)(f ) association of plankton or their remains with settling particles. dt, diatoms; df, dinoflagellates; fp, fecal pellets; tc, tintinnid ciliates. Horizontal bars represent 100 ìm. a, b, 20 September 1994, sites R and 10, respectively; c, d, 25 September 1994, Site 35; e, f, 25 September 1994, Site 38C.

638 T. Ayukai & E. Wolanski T 1. Approximate range of nitrate concentration (ì) in the low- and high-salinity regions of the plumes of world’s large rivers Salinity

Fly River Amazon River Changjiang River Huanghe River Mississippi River Orinoco River Zaire River a

0–5

25–30

References

4–12 — 7–14 20 50–65 — 65 60 6–8 5–8

Trace–2 Trace Trace–5 Trace 12–20 6a Trace–15 0–50 Trace Trace–6

Robertson et al. (1993) This study Edmond et al. (1981) Edmond et al. (1985) Edmond et al. (1985) Ning et al. (1988) Turner et al. (1990) Lohrenz et al. (1990) Edmond et al. (1985) Van Bennekom et al. (1978)

Average of five measurements.

Biological filter

Physical filter 10 Sal

0 Sal

. . . . .. . .

.. . flocs .Small

Large flocs .Unflocculated particles . ..

. .

.

20 Sal

. . . .. . .. ....... ...... .. .......

Clay

Clay trapping

30 Sal

Silt

.. .................... ...

Setting Return flow

Estuary

Ocean Mouth

F 10. Conceptual model for retention of Fly River sediments in the estuary and the adjacent coastal water.

A conceptual model for the fate of Fly River sediments is shown in Figure 10. In the freshwater region of the estuary, sediments containing 20% clay and 80% silt particles are not flocculated (Wolanski & Gibbs, 1995). On reaching the higher salinity region, sediments encounter a physical filter; i.e. flocculation and a number of physical processes, including the baroclinic circulation and tidal pumping, result in formation of a turbidity maximum zone and the preferential trapping of clay particles in the estuary (Wolanski & Gibbs, 1995). In the coastal water, fine silt particles that have escaped trapping in the estuary encounter a biological filter and are precipitated through formation of large settling particles, such as macroscopic aggregates and fecal pellets (this study). The high turbidity in the Fly River estuary prevents much plankton growth, thus clearly separating the physical filter in the estuary from the biological filter in the coastal water. In less turbid estuaries, these two filters may co-exist.

Acknowledgements This study was supported by the Ok Tedi Mining Limited and the Australian Institute of Marine Science. The authors thank R. McAllister, D. Galloway, J. Soles and the captain and crew of the RV Western Venturer for their able assistance in the field. S. Clarke and S. Spagnol assisted in drawing and processing figures and images. G. Brunskill and D. Miller provided comments on the manuscript. AIMS Contribution No. 739.

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