1. No category

JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 98, NO. C2, PAGES 2245-2257, FEBRUARY
15, 1993
Seasonal Distribution of Nutrients and Primary Productivity
on the Eastern
Continental
Shelf of Venezuela
as Influenced by the Orinoco River
JAIME BONILLA,1 WILLIAM SENIOR,1 JOHN BUGDEN,2
OLIVER ZAFIRIOU,3 AND RONALD JONES2
Nitrogenous nutrients, dissolved silicate, and salinity were measured in surface waters and shallow
hydrocastsalongsimilar cruisetracks duringthe spring(dry season)and fall (wet season)of 1988.Both
cruises transected the eastern Caribbean, transited the Gulf of Paria, ran parallel to the Orinoco Delta
and into the main channel of the Orinoco River. Trends in primary productivity were also measured
by daily carbon 14 incubations. In both seasons,samplescovered the range from highly oligotrophic
and transparentto highly productive and rich in biogenicand abiogenicparticulate matter. Most of the
I•rinnr, n n,,tflnw •ppo•re tn t,,m 1%/tn l•I•u•7•nrl r•molne in eho!!n• ,x;o•,•.o ngle ,1....... 1• •nA
surroundingTrinidad, permitting benthic regenerationof river-borne nutrients. However, the role of
the Orinoco and associated low-salinity coastal waters in fertilizing large areas of the eastern
Caribbean basin, as suggestedby satellite imagery, can be approximated crudely from the nutrient
compositionat Boca de Dragon, which is representativeof the nutrient statusof these waters as they
flow into deeper Caribbean waters. Additional nutrients may be supplied to the area primarily from
Amazon-derived water entering the Caribbean Basin further north, with some coastal upwelling along
the continental shelf in the dry season.
of 33.95x 103 m3 s-1 [Salazar,1989],second
onlyto thatof
INTRODUCTION
Coastal oceanic water masses,especially those located in
the tropics, are among the most fertile and productive. This
biological and organic richnessmay result from high loads of
nitrogen, phosphorusand silicon suppliedby hydrodynamic
processes resulting from continental drainage, or from
coastal upwelling processesduring seasonsof winds favoring this process. Within the context of coastal bioecosys-
the Amazon. The flow regime is characterized by precipitation throughout the year, with maximum flow in August and
minimum
flow in March.
The estuaries formed in the delta are known as Boca Ajies
and Boca Bagre in the Gulf of Paria, and Boca Grande or
Boca de Navfos, where the navigation channel is located
(Figure 1). Knowledge of the estuarine-deltaic system of the
Orinoco and the Gulf of Paria dates from 1954, when Van
rems, estuarine-deltaic zones are notable as transition enviAndel and Postma describedthe hydrographic, climatologironments between the ocean and the continent. The Orinoco
cal and sedimentary conditions of the area [Morelock, 1972;
River systemin Venezuela provides one of the largest-scale Eisrna et al., 1978]. The formation of muddy plains in the
examples of these interactions. It has a major influence on deltaic coast is due to depositionof sedimentstransported to
the hydrographic and biological conditions of the estuarine the northwest by the Guyana current. Butenko et al. [1981]
system of the Guff of Paria and the Caribbean Sea.
reported the formation of a clay wedge in the delta area up to
The Orinoco River begins near Brazil, at the junction of about 80 km into the adjoining Atlantic, with additional
Sierra Pfirima and Sierra Unturan, and flows over an area of
organic clays and inorganic input from the Amazon River.
2560km2, of which1670km2 are navigable.
It irrigatesa This was confirmed by Milliman et al. [1982], who found
floodplainof 981,000km2 in Venezuela,Colombia,and montmorillonite characteristic of the clay sediments of the
Brazil, is the only South American river with a true delta,
Amazon River in the Orinoco delta, also transported by the
and has an ocean front of 300 km. Its principal affluent is the
Guyana current. Herrera and Masciangioli [1984] deterCaroni River, with a brown-ochre color, which is mixed with
minedthat the predominantcirculationin the coastalarea of
the waters of the Orinoco, which are yellow. The Orinoco
the delta consistsof surface currents generatedby the trade
flow is turbulent and contains a large amount of suspended
winds moving to the west and northwest. The hydrographic
clay. These Orinoco-Caroni waters are rich in organic and
and chemical conditions of the sediments in the Gulf of Paria
inorganicmatter which is transportedto the Atlantic Ocean
were studied by Benitez and Okuda [1976], Bonilla [1977],
and the Caribbean Sea through the estuarine-deltaicsystem.
and Bonilla and Lin [1979]. The geochemicalcharacteristics
The Orinoco River contributesa large amount of suspended
of
the sedimentsin parts of the river and delta were studied
sediments
(86.3x 106tonsyr-•), withanaverage
waterflow
by Bonilla et al. [1985]. Miiller-Karger and Varela [1988]
1Instituto
Oceanogrfifico
deVenezuela,
Universidad
de Oriente,
Cumanfi.
determined
that the nutrient
influence
of the Orinoco
River
on the Caribbean Sea reaches Puerto Rico, based on the
2Department
ofBiological
Sciences
andDrinking
WaterResearch concentrationof pigmentsduring the high flow seasonfor the
Center, Florida International University, Miami.
Orinoco (August-October). Salazar [1989] studied the hy-
3WoodsHole Oceanographic
Institution,WoodsHole, Massa- drogeochemicalconditionsof the estuarine-deltaicregion of
chusetts.
the Orinoco, from the island of Curiapo inside the fiver to
Copyright 1993 by the American Geophysical Union.
180 km into the Atlantic
Ocean.
In these tropical bioecosystemsthere are two seasons:a
dry seasonfrom December to April, and a rainy seasonfrom
Paper number 92JC02761.
0148-0227/93/92JC-02761 $05.00
2245
2246
BONILLA ET AL ' NUTRIENTS AND PRIMARY PRODUCTIVITYOF THE ORINOCO
64.0
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ß
I
':.•
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Fig. 1. Locationof the sampling
sitesfromColombus
Iselincruises
CI-8805andCI-8816,springandfall 1988(solid
line, E-W transect;dashedline, NE-SW transect).
May to November. Evaporation is highestin February (129
mm) and lowest in November (76 mm), while precipitationis
highestin August (127 mm) and lowest in April (19 mm).
Thus the timing of the two cruises reported here nearly
coincided with the two extremes. Westerly winds are prev-
the hydrochemicalmechanismsof the delta, conservative
and nonconservativeprocessesresultingfrom the runoff of
the Orinoco during high and low flow are considered.
METHODS
alent, with an easterncomponentduring most of the year. A
semi-diurnaltide regime dominatesin the estuarine-deltaic Source of Samples
region [Salazar, 1989].
Water sampleswere collected in the estuarine-deltaic
Here we summarize the saline stratification, nutrient distributions,and primary productivitymeasurements
from the systemof the OrinocoRiver, Gulf of Paria, and adjacent
spring (April 1988) and fall (September 1988) cruises. We areas(Figure1) duringresearchcruisesCI-8805in the spring
focus on longitudinaltransectsNE-SW and E-W from the (April 1988)and fall CI-8816 (September1988)on boardthe
mouth of the Orinoco. This paper focuseson the near-shore R/V ColumbusIselin. Seawater sampleswere taken using a
data (all data S of • 11øN), as studiesin that area were the rosettesampler(GeneralOceanics,Miami, Florida)consistmost intensive and the rivefine impact is strongest. For ing of twelve 5-L teflon-linedpolyvinyl chloride Niskin
effects in the Caribbean N of 1IøN, see the accompanying bottles. The rosette was fitted with a Neil Brown conductiv-
paper [Zika et al., this issue].In order to betterunderstand ity, temperature,anddepth(CTD) probe.Additionalsurface
BONILLAET AL.' NUTRIENTS
ANDPRIMARY
PRODUCTIVITY
OFTHEORINOCO
2247
(2 m) samples
werecollectedfromthethrough-the-hull
teflon limitedprimarilyto a shallowsurfacelayer and are hardly
detectable
evenat 50 m. Thisfactorcanbe seenclearlyby
comparing
salt,silica,andnitrateseasonaldifferences
along
Nutrient Analyses
bothtransects'NE-SW (Figure2, spring;Figure3, fall) and
Water sampleswere collectedin 60-mL Nalgenebottles E-W (Figure 4, spring;Figure 5, fall). The tendencyto
and refrigerateduntil analyzed.Sampleswere analyzed upwelling in the spring and its relaxation in the fall are
within24 hoursaftercollection.Turbidsamples
werefiltered clearlyindicatedin the NE-SW (onshore-offshore)
sections
using0.45 tampore-sizeAcrodiscdisposablefilter assem- by the slopingand horizontal(respectively)isolinesfor S =
blies(GelmanSciences,Ann Arbor, Michigan).
36 ppt, and silicate or nitrate 1 taM. The E-W transect
Nutrientdeterminations
(NO•-, NO•-, Si(OH)4)wereper- comparisonmost clearly showsthe much larger extent of
formedon an AlpkemRFA-300nutrientanalyzer(Alpkem freshwaterinfluence'the springtop-bottomcontrastis •31Corp., Clackamas, Oregon). All nutrient determinations 36.5 ppt, while in the fall the rangeis •13-36.6 ppt.
were measuredagainstinternalstandardsaccordingto the
The slopingisolinesindicatethe dry seasonupwellingof
RFA methodologyhandbook(Alpkem).
intermediate water from the southeastern Atlantic in the
bow pumping system.
•JIGLLM$1111
GL.I.)•/•,,,,CL
%JICllI•.I•5.
¾¾
•tL•l Ol bi:tlllllty ..•2.2 pp[ at
Primary Productivity Measurements
Surfacewater (•0-2 m) sampleswere collectedin BOD
(R1) ascendsto the surfacelayers (2-10 m), pushingthe
freshwater
massintothefiver (Figure2) fromthe surface(R3)
with changes
in salinity,delineating
thisareaof the estuary
from the salt wedgetowardsthe bottomat R5, where the
bottlesto which0.95mL of NaH•4CO3(2.5 taCimL-•
42.37nmolmL-• Amersham
Corp, Arlington
Heights, wedge'sapexis formed.The mixingzonewasbetweenR5 and
,
Illinois) was added. The sampleswere irradiated in full
R6 nearthe platformat BocaGrande.At a depthof 90 to 140
sunlightin a flowingseawaterbathfor 3 hours(darkcontrols
were covered with aluminum foil). After irradiation the
sampleswerefilteredthroughGF/F glassfiberfilters(WhatmanInternationalLtd., Maidstone,England),rinsedwith 50
m there was a cold, deep-watermass from the southeastern
Atlanticwhichwashigherin salinity(36.8-37.15ppt)between
R1 and R2, rich in nitrogenouscompounds(nitrite, 0.18-0.16
taM;nitrate,0.47-6.2taM)andlow in silicate(0.12-0.35taM).
mL of 1 N HC1 and storedfor furtheranalysis.Integrated Thus, duringthe dry season,when the Orinoco has its lowest
photosyntheticallyactive radiation (PAR) was measured
flow,itsinfluence
islimitedto a narrowfringestretching
along
with a LI-COR (model 1800)calibratedspectroradiometer.
Radioactivity was determined with a Beckman LS 3801
the estuarine-deltaic
area (Figures2 and 3).
liquid scintillationcounter(BeckmanInstruments,Fullerton, California).Primaryproductivityvalueswere calculated Nutrients and Primary Productivity
as tagCfixedper m3 h- •.
The verticalnutrientdistributions
are alsoshownin Figures2-5, while the surfacevaluesandprimaryproductivity
RESULTS
dataare givenin Figures6 and7. Table 1 presentsa highly
Spring and Fall Salinity Distributions
condensed summary of the data for both seasons in the
vicinity of the Orinoco River.
Giventhe restfictionsimposedin the springby navigation
The spring-fallsalinity distributions(Figures 2-5) are
dramaticallydifferent.In springtherewasa sharpfreshwa- requirementsand the strongtendencyof the fiver water to
sheet
ter-seawater interface within the navigation channel and spreadover the saltwater in a thin difficult-to-sample
in
both
seasons,
these
data
are
obviously
too
sparse
and
upstreamof Pt. Bafima,sothatthe NE-SW transect(Figure
2) neverencountered
purefiver water(Smin ----8 ppt at R5 at unrepresentativeof the fiver-delta-coastalsystemas a whole
ebb tide). The E-W transect(Figure4), as closeinshoreas to permit a detailedcharacterizationof the regionin either
comparison.
The purpose
feasiblenavigationally,
alsoencountered
onlyrelativelysa- season,or a detailedinterseasonal
line waters (S > 30 ppt) becausethe saltwater-freshwater of gatheringthese data was rather to provide ancillary
gradientwas centeredcloseinshorein the nonnavigable informationfor the more detailedprocess-ofientedstudies
watersof the Orinoco'sextensivedeltaicsystemanddistrib- reportedin other papersin this Special Section.
utaries in spring.
In fall, both sections were within the core of the near-field
freshwater-saltwater
mixingzone (Figures3, 5). The E-W
transectand the Gulf of Paria was roughly50% saltwaterat
thattime,whiletheNE-SW transectencountered
S = 0 ppt
Nevertheless, a few issues of general interest to the
biogeochemistry
of this fascinatingbut relatively littlestudiedregioncanbe addressed
with thisdataset. They are
discussed below. Some of these same issues are also ad-
dressed
fromdifferentperspectives
in someof the accompanyingpapers(e.g., the Si budgetand Ra-Si relationships
[Moore and Todd, this issue];primaryproductivity,nutrigivenby Rodriguez[1975]for April, but in September
the ents,and specificpigmentdistributions[Bidigareet al., this
E-W transectandGulf of Pariawaterswere alreadyalmost issue]).
twiceassalineasthosereportedfor August,suggesting
that
DISCUSSION
the peakflow was alreadywell pastandthe gradualadvection and mixing of this large input with seawaterwere
underway.Wheretherewas sufficient
waterfor navigation Silicateand (Nitrate Plus Nitrite) in Springand Fall
by the R/V Iselin, the delta region of the Orinoco River
Coastalnitrogencyclingis intenseandcomplex[Carpenexhibiteda typical salt wedgecirculationwhich governs ter and Capone, 1983].The availabledata provide strong
hydrographicand chemicalconditionsin deeperwaters. qualitativeconfirmationof the expectedintensebiological
Thus the springversus fall effects of the rivefine inflow are cyclingin this region, as discussed
previously[D'Armas,
water from surface to bottom at R5. The surface distribution
of salt (not shown) on these two cruisesthus resembledthat
2248
BONILLA
ETAL.' NUTRIENTS
ANDPRIMARY
PRODUCTIVITY
OFTHEORINOCO
R1
$w
NE
$w
NE
n2
3O
4O
5O
6O
8O
•loo
Salinity
IO0
(%ø)
120
120
Nitrate
ß
(•LM)
140
140
160
160
180
180
2OO
2O0
3OO
$W
NE
$W
NE
o
lO
2O
20
3o
>0.05
4o
5o
0.1
Nitrite
120
140
160
180
(I•M)
Silicate
12o
(•LM)
14o
16o
18o
NE-SW Spring
Fig. 2. Depthprofileof salinity,
nitrite,nitrate,andsilicate
ona NE-SWtransect,
spring
1988.
BONILLAET AL ßNUTRIENTSANDPRIMARYPRODUCTIVITY
OF THE ORINOCO
NE
W
NE
ß
2O
2249
20
3O
E
1
2
Nitrate
o)
100
lOO
--6
,
120
• • --:36.8
-....
ß.._• •
120
ß
140
,.
140
160
ß
ß
160
180
o
ß
180
2O0
NE
SW
R1
o
,
4/
lO
20
•2
\
E
A
50
5o
-I
/d
•. so
C]
8o
Silicate
Nitrite
100
lOO
(IzM)
12o
120
140
140
160
ß
180
ß
200
ß
NE-SW
Fall
Fig. 3. Depthprofileof salinity,nitrite, nitrate,and silicateon a NE-SW transect,fall 1988.
1986; Salazar, 1989] for November conditions in the water
of Paria have been describedby Bonilla and co-workers, as
column of the delta region. Sedimentaryconditionsthere
have been discussedby Milliman et al. [1982], Eisma et al.
[1978], and Kennicut et al. [1987] and conditionsin the Gulf
referenced
in the introduction.
In addition to salt, other semiconservative tracers are
usefulin estuarinestudies,mostclassicallydissolvedsilicate
2250
BONILLA
ETAL.:NUTRIENTS
ANDPRIMARY
PRODUCTIVITY
OFTHEORINOCO
(us)qtdea
(us)qtdoG
BONILLA
ETAL.' NUTRIENTS
ANDPRIMARY
PRODUCTIVITY
OFTHEORINOCO
(w) qld•a
I
I
(w) qld•a
I
(LU)•dea
(m) qldea
2251
2252
BONILLAET AL.' NUTRIENTS
ANDPRIMARYPRODUCTIVITY
OFTHEORINOCO
i iß
lOB
ß te
lO'
SEA
Spring
Nitrite
(•u)
OCEAN
.
Spring
ß-'
Silicate
./'
Spring
/
t/,ø'
o----
(•U)
.••
Primary
Productivity
(•gc/re')
AT•ANTIC
•
/
OCEAN
/
,/
/
/
, /
;
/
(
,/
:
/
/
.
.
/
ß
ß
\
io
.. , t
'd
• •
:f
r••,
•1 •
•.",,"
5'0'..'
'
• 0ß
Fig. 6. Spatial
distribution
of nitrite,nitrate,silicate,
andprimary
productivity
inthesurface
waterduring
thespring
1988.
Primary
productivity
values
areexpressed
as/xgC
m-3 h-] .
behavioron spatialand
[Burtonet al., 1970;Liss, 1976],as exemplifiedrecentlyfor lead to seriouslynonconservative
the Mackenzie Shelf Estuary by MacDonald et al. [1989]. temporalscalesthat needto be definedempiricallyin each
However, silicatedistributionsmustbe interpretedwith care system[e.g., Edmond et al., 1981].
However, use of Si as a tracer has a history in the greater
becausebiologicaluptakeand subsequent
sinkingandredissolutionof biogenicopal in the water and on the bottomcan easternCaribbeanbasin with respectto understandingthe
BONILLA ET AL.' NUTRIENTSAND PRIMARYPRODUCTIVITYOF THE ORINOCO
ii-
i1,'
-
C&l•1111
Fall
Nitrate
All
, , ß
Fall
/•f
Nitrite
•
(izM)
2253
'
ii I
ii ß
OCEAII
/ -''
io
.
CAlllie
,/'
/
/
/, '"'
4111
I
ß
Fall
Silicate
•M)
Fall
Primary
Productivity
c/re')
IULLOF
PAllIA
,\
Fig. 7. Spatialdistribution
of nitrite,nitrate,silicate,andprimaryproductivity
in the surfacewaterduringthe fall
1988.Primaryproductivity
valuesareexpressed
as/xgCm-3 h-] .
influenceand seasonalvariabilityof the greatriver systems
to the southeast(Amazonand Orinoco)versusrain inputs.
Froehlichet al. [1978]first usedSi sectionsto suggestthat
rivers, not rainfall, were the principalcauseof the seasonal
ine inputsto this region.The radiochemistryapproachwas
alsoappliedon CI-8805 and 8816;the combineduse of Si and
surface fresheningas far away as Puerto Rico. More re-
radiochemical data to determine the "estuarine
cently, Moore et al. [1986] used Ra-Si relationships to
construct quantitative, sensitive tracers of the remote river-
end mem-
2254
BONILLA ET AL.: NUTRIENTS AND PRIMARY PRODUCTIVITY OF THE ORINOCO
TABLE
1.
Nutrient Content and Primary Productivity in Different Offshore Regions of Estuarine-Deltaic Systems of the Orinoco
Spring
Fall
Parameter
Region
Measured
Dragon's Mouth
NO/NO•Si(OH) 4
31.0
NO•NO•Si(OH) 4
0.16
0
4.90
PP
Serpent's Mouth
14.0
NO•NO•Si(OH) 4
Shelf
River (surface transect DM)
Ocean
Maximum
0.27
0.55
3.21
PP
Gulf of Paria
Minimum
0.12
0
0.93
PP
......
NO•NO•Si(OH)4
0.20
0
2.05
Average
Minimum
Maximum
0.45
5.34
4.76
m
•
m
m
•
•
12.9
12.9
12.9'
0.06
0.26
10.53
1.01
7.03
68.64
0.41
3.25
40.75
0.56
8.12
6.42
31.0
31.0'
0.60
1.00
7.43
0.34
0.41
6.33
14.0
14.0'
26.0
52.8
38.0t
0.42
0.98
6.01
•
•
•
•
•
m
m
•
•
0.97
9.30
17.92
9.55
3.75
6.83
0.15
0
4.61
35.0
35.0
35.0*
NO•NO•Si(OH)4
0.36
0.13
10.84
2.09
4.52
98.26
1.38
3.24
41.46
0.02
0
12.20
PP
50.0
50.0
50.0*
21.6
0.02
0
0
PP
1.0
•
•
•
0.92
3.37
11.86
PP
NO•NO•Si(OH) 4
Average
0.36
22.75
12.00
1.0
0.71
3.89
28.00
2.9
0.12
4.12
1.50
0
0
0
1.0'
•
0.46
1.45
9.41
2.9
2.9*
1.01
6.55
149.80
0.30
5.00
110.46
31.2
26.45
0.86
16.48
9.86
0.08
3.58
3.52
•
•
PP,primaryproductivity
(/zgCm-3 h-l), NO•-, NO•-, andSi(OH)4(/aM).
*One value only, no average, depth 5 m.
?Average of four values, depth 5 m.
SAverage of two values, depth 0 m.
gles), often with NO/-/NO/-
approachingunity. This com-
ber" by this approach is discussedin their paper [Moore and
Todd, this issue]. Hence it is of interest to examine its
behavior also in the estuarine region which may alter this
bination suggeststhat very active nitrification was convert-
water before
Saunders [1989] report •2.5
it reaches
the eastern
Caribbean.
ing ammoniaand possiblyDON to NO/- + NO/-. Lewis and
Figure 8 shows silicate versus nitrate plus nitrite plots for
both seasonsalong with the mixing lines expected for spring
and fall Orinoco fiver water plus oligotrophic seawater
(nutrient-salinity plots cannot be used in spring, when all the
low-salinity sampleswere acquired in rapid successionfrom
the pumping system and could not be accurately matched
with salinities). In spring (solid symbols) the data are from
the vicinity of R4-R5, whereas in fall the same [Si] range was
covered
over much of the E-W
transect
O Fall (transect)
ß Spring(R4-R5)
ß [NO2']>21aM
ß CH4>0.1gM
/
(•) Orinoco
River
Means
7
(8)
Spring
//
//
ß
/
outside of the Gulf of
Paria. As noted above, the highest Si values in both seasons
closely approximate those expected from the riverine data of
Lewis and Saunders [1989]. In contrast, in both seasons
NO/- + NO/- clearly cycles dynamically in waters of lower
salinity (higher Si). The dominant cycling mechanisms appear to differ seasonally, as described below.
Proceeding from high to low Si values in spring, we
/xM ammonia and >11
ß
/
//
/
•, 6
ßßß
////Slope-0.075
O
initially encounter waters with very low NO/- + NO/-/Si
compared to that expected from the river-seawater mixing
(Figure 8). These samples were from within the turbidity
maximum, where photosynthetic uptake cannot be the nitrate sink. Rather, the associated high methane values in
some samples (square symbols) strongly suggestthat interactions with anoxic bottom and pore water environments
were intense. Loss of N0•- + N0•- by denitrification(and
possibly Si injection by redissolution) are likely the domi-
nant processesredefiningthe NO/- + N0•-/Si ratios here.
At intermediate [Si] the data fall on the high N0•- +
N0/-/Si side of the mixing line and the highestvalues are
associatedwith unusually high nitrite concentrations(trian-
/•
IY
II//./
0
//
/
I/'
/
.
20
o
.o
40
.
60
ß
80
100
120
140
Silicate•uM)
Fig. 8.
Plot of nitrite and nitrate versus silicate in surface water for
spring and fall 1988.
BONILLA ET AL.' NUTRIENTS AND PRIMARY PRODUCTIVITY OF THE ORINOCO
Surface
E-W Fall Transect
- 140
2255
Boca Grande (R4-R5) may not typify the myriad other
unsampleddistributariesto the NE and enteringthe Gulf of
Paria.
- 120
ß . []
- 100
80 •
- 6O
- 40
- 20
I
I
I
ß
I
I
0
,•0
Salinity(%o)
Gulf of Paria and Inputs to the Caribbean
Although some Orinoco and coastally trapped Amazon
water enters the Caribbean by way of Galleons PassageN of
Trinidad, etllux out of the Boca de Dragon is thought to
represent a major component of the estuarine input to the
Caribbean, as shown strikingly by satellite photographs
[Miiller-Karger et al., 1990].
The gyre circulation in this Gulf [Bonilla, 1977; D'Armas,
1986]tendsto homogenizeits waters, includingthe inputs of
the northernmost distributaries discharging into the Gulf
itself. Hence it is especially interesting to examine the
behavior of Si on this scale. Taking our fivefine end-member
values and R1 as a seaward
end-member
for Atlantic
water
Fig.9. Plot
Ofnitrite
plus
nitrate
versus
salinity
andsilicate
versusentering the Caribbean in the vicinity of the Grenada passalinity from the surface water of E-W transect, fall 1988.
sage, conservative mixing of surface waters in spring predicts = 18 tam silicateat P1; <7 tam was present. Thus at low
discharge, the residence time of water in the Gulf is apparently long enoughthat substantialbiological removal occurs.
DON in the riverincend-member.
Displacement
of NHf/
In contrast, in fall Si appearsto be conservative along the
DON bound to clays by seawater cations in this zone of
intense mixing would provide a mechanism to free new coast and in the Gulf of Paria (Figure 10 inset), despite
substrate for subsequent oxidation to nitrite by bacteria evidence for localized Si removal (Figure 9). Presumably,
[Carpenter and Capone, 1983], while the high turbidity either this Si was regenerated and re-entrained in the flow
would minimize light inhibition [Olson, 1981; Vanzella et al.,
(perhaps in the course of turbulent mixing in the Boca de
1989].Finally, at <20 tam Si this highNO•- + NO•-/Si water Serpente), or the fraction of the total flow thus affected was
appears to mix conservatively with oligotrophic seawater, minor. This mixing line thus defines the Gulf of Paria source
perhaps with some photosyntheticuptake of N at the very water to the Caribbean (at this time) as having an endlowest [Si].
member value of =120 tam Si representing fiver water
Thus these data show that in spring, nonconservative (Orinoco plus coastally trapped Amazon flows) little modified by dilution by rainfall or biological removal. This differs
behaviors indicating biogeochemicalcycling and input of N
to coastal waters were intense in this very active zone, from Froehlich et al. [1978] estimate of average freshwater
generatingboth relatively N-poor and N-rich waters (N/Si input to the eastern Caribbean (for the entire eastern Caribratios =0.1-5 of the riverinc ratio). However, our sampling bean, a significantamount presumably being Amazon water
was far too sparseand unrepresentativeto permit estimating entering with the North Equatorial and Antilles currents
the relative amounts of these two types of water input to the through more northern passages) had an apparent endcoastal zone over any significant area, and the absence of member compositionof =60 tam Si, correspondingmostly to
ammonia and DON data prevents a comprehensive mass three-componentmixing of Amazon water (with nearly the
same Si content as Orinoco water, = 130 taM), rainfall, and
balance approach.
In fall (Figure 8, open symbols)the much larger river flow oligotrophicsurface seawater.
pushed high-Si water out well beyond the core of the
Figure 10 is an Si-salt plot with our end-membermixing
turbidity maximum; waters with stronglyenriched NO•- + line (dashed), that of Froehlich et al. [1978] (heavy solid),
NO•-/Si ratio were absent. The most Si-rich waters were
also higherin NO•- + NO•- than expectedfrom the data of
Lewis and Saunders [1989] but in the expected ratio. The fall
data are plotted against salinity in Figure 9, revealing estuarine Si consumption and strong NO•- + NO•- uptake,
especially near =7 ppt salinity. This presumably reflects
photosyntheticuptake by silicoflagellates(see Edmond et al.
[1981] concerningthis same situation in the Amazon plume).
At higher salinities, Si is more nearly conservative and N +
N is exhausted. These trends suggestremineralization of Si
and continued uptake of N + N by nonsiliceousprimary
producersat higher salinity. However, in both seasonsthese
interpretationsmust be tempered by the realization that the
system was probably not at steady state, that significant
vertical gradients which may have originated by nonconservativeprocessesare ignored, and that the "riverinc"
(near Barrancas, at the head of the deltaic region) endmember data of Lewis and Saunders [1989] or our samplesat
and the surface Si data for the entire east. Caribbean
cruise
track (Figure 1) in fall 1988. Much of the data lie near the line
of Froehlich et al. [ 1978], especially if adjusted to assumean
end-memberwith 0.5 tam Si at 36.2 ppt rather than the value
of 1 at 36.4 they used (light line). However, to the north (up
to about 13øN)many points lie in between these two or along
our mixing lines. If Si is nearly conserved in the E Caribbean, our Si data suggest a modified view of the threecomponentmixing concept to account for the fact that the
Gulf of Paria componentis apparentlylittle diluted by rain,
whereas the "offshore Amazonian" inputs supply raindiluted
fivefine
Si.
Thus our Si data more or less directly demonstrate the
existence in fall of two freshwater/Si inputs and show that
the sourceof high Si water is at least partly associatedwith
the Orinoco input through the Gulf of Paria, up to = 13øN.
Thesefindings
thusreinforce
theconclusion
from228Ra
data
by Moore et al. [ 1986]that two discreteroutes of delivery for
2256
BONILLA ET AL.: NUTRIENTS AND PRIMARY PRODUCTIVITY OF THE ORINOCO
14-
••
12 -
\\\xxx,'•'/
ß GulfLine
ofParia
ß
Froehlich,-/•'• oeec',, (inset)
10-
et al. (1978);
Venezuela
Basin
•
-
•
(67ow)
•
•,
'x
ß
•x•
'
6-
•
4
'•'•
GulfofPa•a
-••
return
toMiami
Track)
"Re-adjusted"/• •
• I
0•
No.•ibbean eo• •
øo•a/ini•
_..2o(•).... 4o
o
27
28
(O =no•hern
pa•,
•1oo•
I
29
30
(S>33.25)
'
31
32
33
34
O•
-- •
35
•
36
Salinity (%ø)
Fig. 10. Plot of silicate versus salinity for the surfacewaters of the East Carribean cruise track and the Gulf of Paria,
fall 1988.
the rivedfine waters exist [see also Moore and Todd, this
issue].
Spring and Fall Distributions of Primary Productivity
On a larger scale, Ryther et al. [1967] suggestedthat
Amazon water actually depressedproductivity off Brazil by
diluting N nutrients, whereas Mfiller-Karger et al. [1990]
suggesta massive fertilization of the eastern Caribbean by
Orinoco nutrients. As mentioned previously, the absenceof
ammonia and DON data do not permit us to evaluate the
nitrogenousinputs to the eastern Caribbean, though clearly
in fall there is massive nitrate depletion (Figure 9) before
waters exit the Gulf of Padfa. The absenceof small primary
productivity values close inshore in either season shows a
normally enhancedcoastal productivity, despitethe fact that
light, turbidity, and salinity stresses must be strong. The
data by Bidigare et al. [this issue] offer confirming evidence
from pigment analyses. Thus the primary productivity data
give an approximate indication that dfvedfnefertilization
takes effect close inshore (Figures 6 and 7). If sufficient N
nutrients survive passage through the Gulf of Padfa to
fertilize the Caribbean Basin (as assumedby Mfiller-Karger
et al. [ 1990]), this effect must be due to efficient biogeochemical recycling processesen route, not to an initially unproductive
water
mass.
Acknowledgments. This work was supported by National Science Foundation grants OCE 8700576 and OCE 8620249, ONR
contracts N0001-14-87-K-0007
and N00014-89-J-1258. We thank the
Captain and crew of the R/V Columbus lselin for assistanceat sea.
This work is partially the result of research sponsoredby NOAA
National Sea Grant College Program Office, Department of Commerce, under grant NA86-AA-D-SG090, Woods Hole Oceanographic Institution Sea Grant Project R/O-15-PD. The U.S. Government is authorized to produce and distribute reprints for
governmentalpurposesnotwithstandingany copyfight notation that
may appear hereon.
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