Deep-SeaResearch.Vol.29. No. 11A.pp. 1355to 1364,1982.
Printed in Great Britain.
0198--0149/82/I11355-10$03.00/0
O 1982 Pefgamot~Press Ltd.
The chemical mass balance of the Amazon Plume
and cadmium
II. Copper, nickel,
E. A. BOYLE,* S. S. HUESTED* and B. G R A N T *
(Received 22 June 1981; in revised form 4 February 1982; accepted 20 May 1982)
Abstract--Trace element samples spanning the salinity range were collected in the Amazon plume
in June 1974 and 1976. In 1976, laboratory mixing experiments using unfiltered fiver water and
seawater were undertaken. The studies show that copper and nickel are unreactive in the Amazon
plume on a time scale of a few days: in both field and laboratory studies the elements are related
linearly to salinity, so that neither adsorption and precipitation nor desorpfion reactions significantly
alter the net flux of these elements. The 1974 field data indicate there may be up to 25% removal of
copper, probably biologically, although a conservative interpretation could be entertained ff
systematic deviations from the resulting copper-salinity plot are ignored. Cadmium behavior was
not clearly defined. There is some indication of desorption, and the estuarine data can be used to set
an upper limit on the net flux. The net effective contribution of the elements to the ocean from the
Amazon is copper, 24 nmol kg-'; nickel, 4 to 5 nmol kg-~; and cadmium, <0.1 nmol kg-'.
INTRODUCTION
As RIVERSdischarge into the ocean, the dissolved and particulate materials they transport
encounter a pronounced physical-chemical boundary. The increasing salinity leads to the precipitation of some components such as iron colloids (SHOLKOVnZ, 1976; BOYLE,EDMONDand
SHOLKOVITZ, 1977b; MOORE, BURTON,WXLUAMSand YOUNO, 1979) and dcsorption of other
elements such as barium (HANOR and CHAN, 1977; EDMOND, BOYLE, DRUMMOND, GRANT
and MISLICK, 1978) and phosphorus (STALLARD, 1980; CHASEand SAYLI~ 1980). In addition, the estuarine circulation regime leads to enrichments for some elements by nutrient
trapping (ReX)FIELD, KETCHUMand RICHARDS, 1963), and diagenetic reactions in sediments
can produce elemental fluxes into and out of the deposited fluvial sediments, as observed for
manganese (EVANS, CUTSHALL,CROSSand WOLFE, 1977; Hess and MOORe, 1982). The construction of chemical mass balances for the ocean therefore requires a study of chemical
reactivity in estuaries. There are few reliable data on the trace element geochemistry of the
plumes of major rivers. This report describes data on copper, nickel, and cadmium in the
Amazon plume.
SAMPLING AND ANALYSIS
In June 1974, unfiltered surface samples were collected aboard the R.V. Chain by pumping
water through polyethylene tubing (from a towed bathythermograph shell streaming 5 m off
* Department of Earth and PlanetarySciences,MassachusettsInstitute
of Technology,Cambridge,M A 02139,
U.S.A.
1355
1356
E . A . BOYLE et al.
the side of the vessel) into an acid-cleaned polyethylene bag in a rigid container connected to a
vacuum pump. Through this procedure, the water encounters only clean well-flushed poly
ethylene and does not contact any moving parts. A similar scheme was used in June 1976
aboard the R.V. Alpha Helix, except that an acid-cleaned 4-1 polyethylene jug was used as the
receptacle. Also on the above cruise, samples were collected in wire-mounted 30-1 PVC
Niskin bottles with silicone rubber O-rings and an epoxy-coated stainless steel internal spring.
Most samples were filtered through Whatman G F / F filters (with a nominal pore size of
0.7 ~tm) except for a few samples at higher salinities where the particulate contribution was
negligible. In addition, samples were collected upstream for the river and its tributaries; the
samples were filtered through 0.4-1am Nuclepore filters.
The mixing experiments were with an unfiltered river-water sample collected within the
shallow section near the river mouth and seawater collected at the extreme eastern end of the
cruise track. The samples were stored in I0-1 polyethylene jugs in subdued light for 2 weeks
prior to the experiment. In the experiment the unfiltered samples were shaken (to distribute the
particles uniformly) and known aliquots of each added in varying proportions to fill clean
500-mi polyethylene containers. The mixtures were then shaken every 30 min over a 4-h
period; then they were vacuum-faltered through 0.4-1am Nuclepore f'dters into clean p o l y
ethylene containers, using a vacuum-filtration device similar to the method described above.
Within a few days of collection, samples were acidified to pH 1.8 with 2x vycor-distilled
6N HCI. Short-term adsorption onto container walls is not a severe problem: identical copper
concentrations were observed for samples filtered immediately and for the mixing-experiment
sample, which was filtered two weeks after collection. Nevertheless, acidification prevents
long-term adsorption and biological activity.
Salinity was determined using a refractometer on the 1974 cruise and with a standard
inductive salinometer on the 1976 cruise. The trace element determinations on the 1974
samples were made using the cobalt-APDC method of-BoYLe, and EDMOND(1977). The 1976
data were obtained using a modification developed in our laboratory. A Perkin-Elmer model
403 atomic absorption spectrometer and H G A 2100 heated graphite atomizer were used tbr
the 1974 data; a Perkin-Elmer model 5000 spectrometer and H G A 500 furnace were used
for the 1976 data. The reliability of the data was assessed by replicate analyses and by
independent analyses by two analysts (Boyle and Huested). We estimate the copper and
nickel precisions as +0.2 nmol kg -1 or +5%, whichever is larger. The cadmium data are less
precise because of a high blank and are probably good to 0.02 nmol kg -~ with a possibility of
systematic offsets of this magnitude due to inadequate blank correction.
The river data were also tested using a standard-additions direct injection graphite furnace
method. The method will determine total copper in the sample, while the APDC method will
only determine copper, which equilibrates with the ionic form at pH 2. From the data in Table
1 it is clear that the two methods agree and that there is no copper fraction that the APDC
method is missing. Relative to the earlier direct injection data on the copper concentration of
the Amazon and its tributaries (BOYLE, 1979), these data are about 3 to 4 nmol kg -~ lower.
We attribute this discrepancy to the recent improvements in sensitivity (4x) resulting from
three factors: the maximum power heating feature of the HGA 500 (resulting in higher peak
signals), the improved optics of the PE 5000 (resulting in an increased signal-to-noise ratio
and greater freedom from baseline shifts), and the increased sensitivity of Perkin-Elmer com
mercially pyrollized tubes. The Amazon tributary data of BOYLE (1979) probably should be
uniformly corrected by subtracting 3.5 nmol kg -I.
Some samples were obviously contaminated with iron, because the APDC precipitates
The chemical mass balance of the Amazon plume
1357
were black. We suspect that such samples were contaminated by particulate matter leaking
through the glass fiber filters, because our analyses for iron with Millipore" filters showed that
the iron concentration must be very low (SHOLKOVlrZ, BOVLr~and PpJc~ 1978). We rejected
these samples, of which there were only six. Otherwise, we report all o f the data; a few
analyses are flagged by question marks and are suspected of contamination and were not considered in the data analysis.
Table I. Comparison of cobalt-APDC data with direct injection analysis of several samples with data reported
by BOYLE(1979)
Bottle No.
Cu (APDC)
(nmol kg-I)
Cu (direct
injection)
(nmol kg-I)
23.7, 23.7
23.2, 23.9
-23.0, 24.0
--
BOYL~(1979)
(nmol kg-j)
ID
Location
At 19 N
At 22 N
$207
$207
Amazon, below Obidos
At 177 N
$203
Amazon, near mouth
25.3
25.3, 25.7
29.2
At 190 N
At 203 N
$209
$209
Amazon at Obidos
Amazon at Obidos
22.6
25.3
--
29.1
At 202 M
At 222 N
$216
$216
Negro,near Manaus
Negro,near Manaus
4.9, 4.6
4.4
At 219 N
At 226 M
At 232 N
$219
$219
$219
Solimoes,above Manaus
Sollmoes,above Manaus
Solimoes,above Manaus
7.0
3.9, 3.9
22.4
22.2
19.9, 20.7
~
29.3
N. samples filtered through 0.4-pro Nuclepore• filters; M, samples filteredthrough 0.45-ttm Milliporeefilters.
Table 2. Analytical data from the 1974 expedition
Date, Time
(local)
Depth
(m)
Salinity
( x 10s)
Copper
(nm kg-I)
Nickel
(nm kg-I)
16, 0905
16, 0835
16, 2230
12, 1500
16, 0310
12, 0830
17, 0330
17, 1750
18, 0810
19, 0930
19, 1330
0
0
4
0
0
0
5
0
0
0
0
5.6
8.6
10.6
12.5
16.3
18.7
19.3
22.0
28.8
30.5
32.5
19.1
20.9, 17.4, 19.0
15.6
14.5, 17.1
11.6
8.4, 11.0
18.7
7.2, 7.6
5.6
4.8
2.7
6.9
5.2, 5.9, 5.4
12, 1930
12, 1930
12, 1930
12, 2100
10
20
30
50
29.8
34.2
35.0
36.2
4.1
2.0
2.2
1.2
4.7, 4.5
4.4
3.6, 3.7
4.2, 4.3
2.9
2.7
2.8
1358
E.A. BOYLEet al.
Table 3. Analytical data from the field, 1976 expedition. Samples identified by hyphenated
numbers refer to station number and depth. The station locations are given in EDMONDet al.
(1981)
Bottle No.
Salinity
(x 103)
Cu
(nmol kg-')
Ni
(nmol kg-')
Cd
(nmol kg-')
I 1-0
11-20
11-25
67U
72 F
7I F
30.41
30.41
36.13
5.6, 5.9
5.7
6.9,5.9
2.1, 2.2
1.5
2.9,2.6
0.61, 0.063
0.052
0.278?,0.245?
14-0
82 F
13.02
14.6
2.8
0.070
25-0
25-12
137 F
139 F
12.52
35.03
20.4
3.9
4.2
1.7
0.123
0.040
29-0
146 U
28.46
7.4
2.7
0.115
30-60
149 U
36.11
1.6
1.8
0.037
41-0
160 U
23.68
11.5
2.6
0.133
$46
$60
$62
$66
$68
$69
$84
$85
$86
$87
$88
$90
$92
$94
$95
$96
$97
S106
85 F
88 F
90 F
86 F
95 F
97 F
109 F
104 F
111 F
115 F
113 F
119 F
124 F
128 F
130 F
132 F
t 34 F
158 F
19.67
9.55
6.45
3.64
2.15
0.85
2.30
2.35
3.10
3.43
4.53
9.16
3.90
5.01
6.62
7.63
9.13
15.23
11.9, 12.3
17.8, 17.7
19.0
20.8
22.9
25.8, 25.8
22.3
25.9
24.2
22.1
22.7
15.3, 18.4
24.1
20.5
20.0
18.8
17.8
13.6
2.6, 2.3
3.9, 3.3
3.1
3.3
3.5
3.1, 3.0
2.9
6.3
3.9
3.4
3.8
2.7, 2.9
4.1
3.2
3.1
3.1
3.3
1.7
0.044, 0.049
0.095, 0.103
0.068
0.082
0.085
0.195?, 0.047
0.058
0.048
0.085
0.400?
0.059
0.056, 0.053
0.079
0.060
0.075
0.075
0.074
0.041
Sample
U, unfiltered sample; F, glass-fiber-filteredsample.
RESULTS
The analytical data are given in Tables I to 4. The results are systematic, similar from year
to year, and show good agreement between the mixing experiments and field data. Conclusions from the data set can be expected to be generally representative o f the A m a z o n plume.
EDMOND, BOYLE, GRANT and ST^LLARD (1981) present a detailed discussion o f the sample
locations and the nutrient and salinity distributions in the A m a z o n plume in June 1976.
DISCUSSION
BOYLE et al. (1974) presented a model for deducing chemical reactivity in estuaries. The
flux of an element across an isohaline surface (Qc) is determined from the relation
The chemical mass balance of the Amazon
1359
plume
Table 4. Analytical data from the 1976 mixing experiment
Salinity
(x I0 "~)
0
0
0.25
0.50
0.75
1.00
1.50
2.0
2.5
3.0
4.0
5.0
7.5
I0.0
15
20
25
30
36
36
Cu
(nmol kg-')
Ni
(nmol kg -t)
24.9
27.8
24.2
28.3
24.3
23.7
23. I
21.3
23.6
21.3
22. I
19.5
19.5
19.4
14. I
11.6
4. I
4.8
4.3
4.6
4. I
4.4
4.5
4.5
4.8
4.3
4. I
4.0
3.8
3.5
3. I
2.8
0.032
0.029
0.016
8.4
5.2
1.6
0.8
2.6
!.9
!.5
1.7
0.569?
0.020
0.004
0.008
0.018
0.026
0.034
0.1397
0.045
0.529.?
0.046
0.036
0.037
0.035
0.029
0,030
dC(S) q
I-
=
Cd
(nmol kg -t )
[c(s)- s
/
dS' J '
where Qw is the river-water discharge, C(S) is the concentration of the chemical eonstitutent
of interest at a given salinity, and S is the salinity. The variation of the flux with salinity (the
reactivity) is therefore
dQ,
d 2C(S)
dS
dS 2
Where there is no reactivity (the element is conservative)
dQ c
d2C(S)
- - - -
- - - - 0
dS
dS
and the concentration vs salinity plot is linear. Therefore, linear eoneeatration--salinity
diagrams are useful for determining the chemical reactivity in estuaries and are used in the
following discussion.
Copper
Both the field and mixing-experiment data from 1976 (Fig. la) fall along a line defined by
the river-water concentration (24 nmol kg -~) and the coastal ocean-water concentration
(2 nmol kg-t). This shows that copper was essentially unreactive in the Amazon plume on
that occasion, with evidence neither for desorption from particles (as observed for barium,
EDMOr~D et al., 1978), nor precipitation along the iron--organic colloids (as is ob~rved for
iron, SHOt.Kovrrz et aL, 1978), nor biological removal (as observed for silicate,, MnJ.n4AN and
BOYLe, 1975; EDMONDet aL, 1981). In the field data a few low-salinity points fall above the
line, but the high values probably were caused by a leakage of f'me-grained particulate matter
1360
E.A. BOYLEet aL
through glass fiber filters. The excellent agreement between field and mixing-experiment data
shows that copper was unreactive on this occasion.
In the 1974 data (Fig. 2), copper appears to be defined by two trends. One line, at low
salinities, trends towards a river concentration of 24 nmol kg-l; the other, at high salinities,
trends towards a river concentration of 18 nmol kg -1. Taking the data literally, the flux model
of BOYLE et al. (1974) implies that about 25% of the river-borne dissolved copper was
removed from solution in the Amazon plume on that occasion. The two-line model also
results in good agreement between the 1974 and 1976 river concentrations. It would be
possible to argue that the data could also be fitted by a single line passing through a river concentration of 22 nmol kg -n, although this interpretation would ignore systematic deviations
from the line as a function of salinity. The 1974 samples were unfiltered, so it is also possible
that the higher concentrations at low salinities were due to particulate copper. But it is
important to bear in mind that the Amazon plume in 1974 was substantially different
physically than in 1976. In 1976 (and presumably, more commonly, GraBS, 1970) the plume
was confined to a narrow ribbon along the continental border, being swept northwest by the
Brazil coastal current. But in 1974, the trade winds collapsed, allowing brackish waters to
301( 0 )
1--
i
field
20[- ~ ) ' ~
e~'l.. A~-o
|
Copper,
n tool/kcj
e'"z~: ° ~4)-o
I0
0
)
i
J
mixing
30
~o
2O
• "o. o
Copper,
nmol/kg
,
0
I
I0
,
I
20
30
40
Solinity
Fig. I. Trace element data from the 1976 field and mixing experiment. Dark circles arc surface
samples collected with the towed fish. Triangles arc surface samples collm:ted with wire-mounted
Niskin bottles; the numl~rs nearby arc the station numbers and the depth. Hexagons arc deep
samples collected with the wire-mounted Niskins.
(b)
!
I
field
~
&25-0
A29-0
Nickel,
nmol/kg
OIl-Z'
O 0 ~-6~
,
t
L
I
,
I
i
mixing
Nickel,
nmol/kg
%
I
I
I
20
30
i
I0
40
Salinity
(c)
L
field
I00
Cadmium,
pmol/kg
•
•
$
50
027-30
,
o
I
I
mixing
i
I
I
I00
Cadmium,
pmol/kg
50
oo
•
P
oc
J
I
IO
i
I
20
Salinity
i
I
30
I
$
4O
1362
E.A. BOYLEel
al.
Copper
I',lk'ket
2°t
•
loL
I
1
I
O~
I
I
I
80
Fig. 2. Copper and nickel data from the 1974 expedition. The vertical axis is the element con
centration, in nmol kg-I, and the horizontal axis is salinity.
30
extend 200 km offthe coast over deep waters (M1LLIMANand BOYLE,1975; Fig. 3). Theretbre
considerable qualitative differences were observed in the plume during the two expeditions,
and it seems possible that the sluggish circulation during 1974 m a y have allowed more time
for biological uptake of copper. It is well established that copper is biologically removed from
the surface waters of the open ocean (BOYLE,SCLATER and EDMOND, 1977a; BRULAND,1980).
Although we cannot make a conclusive argument for such biological removal, the evidence is
sufficient to consider the process to be a plausible mechanism accounting for the observations.
Fig. 3. Salinity distribution in the Amazon estuary, June 1974,
The chemical mass balance of the Amazon plume
1363
Because the normal mode of the Amazon plume is closer to the 1976 situation, copper in
the Amazon must generally be conservative. SHOt.KOVITZ(1978), using 'product mode' mixing
experiments on filtered Scottish river-water samples, reported a 40% removal of copper. The
difference between his data and those for the Amazon plume is probably due to the
differences in humic acid and particulate matter concentrations between the two rivers. The
Scottish stream water was high in humic and low in particulate matter. Although the Amazon
has abundant sources of humic material from its lowland tributaries (ST^LLXRD, 1980), the
material is partially diluted by upstream waters, and a considerable amount is probably adsorbed by the abundant fine-grained particles suspended in the river. As most of the world's
major rivers are also low in organic and high in particulate matter, it is likely that the Amazon
is more representative of the net effect of estuaries on the flux of dissolved copper into the
ocean than the Scottish stream water. The Sholkovitz reaction may be important for smaller
streams with high organic carbon contents.
Nickel
The 1976 nickel data (Fig. lb) generally suggest conservative behavior. The scatter in the
nickel field data is worse than that of copper. Also, the mixing experiment shows lower riverwater concentrations than the field data. From the mixing experiment data, however, it is
clear that there is no rapid desorption-adsorption or precipitation of nickel. The field data are
consistent with this interpretation, albeit the higher scatter allows for the possibility of some
reactivity. The 1974 data are also consistent with a conservative mixing modal, although there
is no convincing evidence from the river end member on this occasion. The observed concentrations are higher, however, indicating that the effective end member would be about
6 nmol kg -~. Based on the data from the two studies, the average flux of nickel from the
Amazon is probably about 5 nmol kg -1 of river water.
Cadmium
There were no reliable cadmium data from the 1974 expedition, and cadmium concentrations observed in the 1976 expedition were considerably lower than anticipated (Fig. le). The
data interpretation must be tempered by our poor precision due to high reagent blanks leading
to an uncertainty of about 0.02 nmol kg -~ and possible systematic offsets between the field
data and the mixing-experiment data, which were analyzed on different days and therefore
were calculated using slightly different reagent blanks. The mixing-experiment data tend to be
lower than the field data. The difference could be due to the storage of the sample for two
weeks prior to mixing. There is slight evidence for desorption of cadmium in the mixing
experiment, although the evidence is not especially convincing. However, Ggx~rr, MiNo-Hux,
BoYL~ and EDMOND(1982) report similar cadmium enrichments in the Orinoco and Yangtze
river plumes, so desorption of cadmium from fiver-borne particles may be a general
phenomenon. As phosphate is highly reactive and not simply related to salinity (EDraOr~D et
aL, 1981), and as in the open ocean cadmium and phosphate are well correlated, the cadmium
scatter in the field data might be due to biological recycling. However, a plot of cadmium vs
phosphate showed no apparent correlation.
The most useful function of the data is to place an upper limit on the effective flux of
cadmium from the Amazon, which must be <0.1 nmol kg -1 of river water. This is considerably lower than the estimate of 0.6 nmol kg -I based on a single glass-fiber-filtered sample
collected along the bank of the Amazon at Macapa (BOYLE, SCt~TEx and EDMOr~D, 1976).
The revised estimate increases the apparent residence time of cadmium in the ocean to more
than 250,000 years.
1364
E.A. BOYLEet aL
CONCLUSIONS
C o p p e r and nickel are usually u n r e a c t i v e d u r i n g the mixing o f river w a t e r and s e a w a t e r in
the A m a z o n plume, with e n d - m e m b e r c o n c e n t r a t i o n s o f 24 n m o l kg -I for c o p p e r and
5 n m o l kg -I for nickel. T h e reactivity o f c a d m i u m is not yet certain but an u p p e r limit o n the
effective flux of c a d m i u m f r o m the A m a z o n can be set at 0.1 n m o l kg -I o f river water.
Acknowledgements--We thank the officers and crew of the R.V. Alpha Helix for their help in the 1976 observations. The 1974 observations were with the assistance of the officers and crew of the R.V. Chain; our participation
in that cruise was encouraged by JOHN MILLIMAN.We thank LISA JAI3LONSKIfor assistance in the 1974 sampling
program. Sample collection in 1976 was supported by NSF Grant No. OCE75-21208 to J.M. EDMOND. The
sample analysis was supported by NSF Grant No. OCE8018665.
REFERENCES
BOYLE E. A. (1979) Copper in natural waters. In: Copper in the environment, J. R. NRIAGU, editor, WileyIntersciance, New York, pp. 77-88.
BOYLE E.A., R. COLLIER, A.T. D~GLEg, J.M. EDMOND, A.C. NG and R.F. STALLARD(1974) On the
chemical mass-balance in estuaries. Geochemica et Cosmochimica Acta, 38, 1719-1728.
BOYLE E.A., F.R. SCLATER and J.M. EDMOND (1976) On the marine geochemistry of cadmium. Nature,
London, 26;$, 42--44.
BOYLEE. A., F. R. SCLATr~ and J. M. EDMOND(1977a) The distribution of dissolved copper in the Pacific. Earth
and Planetary Science Letters, 37, 38-54.
BOYLE E.A., J.M. EDMOND and E.R. SHOLKOVlTZ(1977b) The mechanism of iron removal in estuaries.
Geochimica et Cosmochimica Acta, 41, 1313-1324.
BOYLE E. A. and J. M. EDMOND(1977) Determination of copper, nickel, and cadmium in sea water by APDC
chelate coprecipitation and flameh:~ atomic absorption spectrometry. A nalytica Chimica Acta, 91,189-197.
BRULANDK. W. (1980) Oceanographic distn'butions of cadmium, zinc, nickel, and copper in the North Pacific.
Earth and Planetary Science Letters, 47, 176--198.
CHASE E.M. and F.L. SAYLSS(1980) Phosphorus in suspended sediments of the Amazon River. Estuartne
Coastal Marine Science, ! 1, 383-393.
EDMOND J. M., E. A. BOYLE, D. DRt~IMOND, B. GRANT and T. MISLiCK (1978) Desorption of barium in the
plume of the Zalr¢ (Congo) River. Netherlands Journal of Sea Research, 12, 324--328.
E D M O N D J. M., E.A. BOYLE, B. G R A N T and R.F. STALLARD (1981) Chemical mass balance in the Amazon
plume h The nutrients.Deep-Sea Research, 28, 1339-1374.
EVANS D. W., N. H. CUTSHALL, F. A. CROSS and D. A. W O L F E (1977) Manganese cycling in the Newport River
estuary, North Carolina. Estuarlne and Coastal Marine Science, 5, 71-80.
GIBBS R.J. (1970) Circulationin the Amazon riverestuary and the adjacent Atlanticocean. Journal of Marlne
Research, 28, 113-123.
GRANT B., H U MINO-HUI, E. BOYLE and J. M E D M O N D 0982) Comparison of the trace metal chemistry in the
Amazon, Orinoco, and Yangtze plumes. Transactionsof the American Geophysical Union, 63, 48.
HANOR J. S. and L. H. C H A N (1977) Non-conservative behavior of barium during mixing of MississippiRiver and
Gulf of Mexico waters, Earth and Planetary Science Letters, 37, 242-250.
HESS J. and W. S. MOORE(1982) Manganese and iron geochemistries in Winyah Bay Estuary, South Carolina
(unpublished manuscript).
MILLIMANJ. D. and E. BOYLE(1975) Biological uptake of dissolved silica in the Amazon River estuary. Science,
New York, 189, 995-997.
MOORE R. M., J. D. BURTON,P. J. LE B. WILLIAMSand M. L. YOUNG(1979) The behavior of dissolved organic
material, iron, and manganese in estuarine mixing. Geochimica et Cosmochimica Acta, 43, 919-926.
REDFIELD A. C.. B. H. KETCHUMand F. A. RICHARDS(1963) The influence of organisms on the composition of
seawater. The sea. Vol. 2, M. N. HILL, editor, John Wiley, New York, pp. 26-77.
SHOLKOVITZE. R. (1976) FIocculation of dissolved organic and inorganic matter during the mixing of river water
and seawater. Geachlmica et Cosmochtmica Acta, 40, 831-845.
SHOLKOVlTZ E. R. (1978)The flocculation if dissolved Fe, Mn, AI, Cu, Ni, Co, and Cd during estuarine mixing.
Earth and Planetary Science Letters, 41, 77-86.
SHOLKOVrrz E. R.. E.A. BOYLEand N. B. PRICE (1978) The removal of dissolved humic acids and iron during
estuarlne mixing. Earth and Planetary Science Letters, 40, 130-136.
STALLARD R.F. (1980) The geochemistry of the Amazon River. Ph.D. Thesis, Massachusetts Institute of
Technology. Woods Hole Oceanographic Institution. 366 pp.