SPECIAL ISSUE
Organic carbon, phosphorus and nitrogen in surface sedimentsof the marine-coastal region north and south of the PariaPeninsula, Venezuela
Marly Carolina Martinez-Soto • Gregorio Martınez
Received: 24 June 2010 / Accepted: 12 August 2011 / Published online: 27 August 2011
� Springer-Verlag 2011
Abstract The organic carbon, phosphorus and nitrogen
content of silt and clay fractions of surface sediments from
the marine-coastal region north and south of the Paria
Peninsula (PP) were quantified. Organic carbon concen-
trations (Corg) were determined by dry combustion after
decarbonation with 10% hydrochloric acid, and total
phosphorus (TP) and total nitrogen (TN). This information
was then used to produce maps of the iso-concentrations of
the distribution of these elements in the sub-marine conti-
nental shelf north of the PP and in the Gulf of Paria (GP).
In the silt fraction, the Corg concentration, TP and TN
showed average values of 1.53, 0.04 and 0.03%, respec-
tively. The highest Corg, PT and NT values were recorded
from silts from the PP with a gradual increase towards the
west and the lowest figures were found in the GP. In the
clay fraction, Corg, TN and TP had mean values of 1.64,
0.13 and 0.04%, respectively, and showed a spatial distri-
bution very similar to the silt fraction, indicating the
influence of ocean currents and coastal upwelling patterns.
The C/N ratio had an average of 23.67 and showed that the
Corg present in the PP sediments is of marine origin,
resulting from primary productivity, especially towards the
west. This zone has been identified as the most productive
in this region due to coastal upwelling and the influence of
the Orinoco and Amazon rivers (Gomez 1996; Monente
1997). In contrast, a greater variability in the parameters
measured was found in the GP sediments, probably due to
the mixing of marine and continental Corg, confirming the
influence of the Orinoco and Amazon waters brought by
the Guyana Current.
Keywords Marine sediments � Organic carbon �Nitrogen � Phosphorus � Organic matter
Introduction
Several investigations of coastal sediments have shown
that these ecosystems are extremely fertile with high
organic productivity, the product of coastal dynamics such
as upwelling and continental inputs from rivers, which can
provide a large supply of nutrients. On the north eastern
coast of Venezuela, upwelling phenomena occur due to the
action of trade winds during the first months of the year.
This affects the hydrodynamics of the entire area and
contributes to fertility as a result of increased concentra-
tions of nutrients in the surface layer, resulting in a short
time lag to phytoplankton growth. During the rainy season,
nutrient inputs from the Orinoco and Amazon rivers that
reach this region due to sea water circulation, promote high
organic productivity.
The marine sediment is defined as an aggregate of untold
numbers of insoluble particles of unconsolidated material,
which have been transported to the bottom of the oceans and
seas by various transport agents. These sediments are the
ultimate repository of most of the waste generated by man,
M. C. Martinez-Soto (&) � G. Martınez
Department of Oceanography, Instituto Oceanografico de
Venezuela, Universidad de Oriente, Avenida Universidad,
Cerro Colorado, Cumana 6101, Venezuela
e-mail: [email protected]
G. Martınez
e-mail: [email protected]
Present Address:M. C. Martinez-Soto
IMEDEA (Mediterranean Institute of Advanced Studies)
(UIB-CSIC), Miquel Marquez, #21, 07190 Esporles,
Balearic Islands, Spain
e-mail: [email protected]
123
Environ Earth Sci (2012) 65:429–439
DOI 10.1007/s12665-011-1319-y
and can thus be used as sensitive indicators for monitoring
the spatial and temporal distribution of pollutants (Balls
et al. 1997; Kishe and Machiwa 2003). They also provide
information on the geochemical changes that occur over
time in these environments and can be used to establish
baseline levels in a particular area (Dassenakis et al. 1997;
Rubio et al. 2000; Tuncer et al. 2001).
Thus, the characterization of sediments is of some
importance in oceanography and geochemistry and may
help to understand certain phenomena such as the distri-
bution of contaminants and their relationship to the geo-
chemical and hydrodynamic characteristics of each marine
region (Heling et al. 1990). The nature, size distribution
and some physicochemical characteristics of marine sedi-
ments may help understand the current system, the baseline
redox condition, the activity of microorganisms and the
nature of the sedimentary deposits (Bonilla et al. 2003).
The marine area of the Paria Peninsula (PP) represents a
small portion of the southeastern Caribbean Sea. It is
bounded by Margarita Island to the west, to the east by the
waters of Trinidad and Tobago, to the south by the north-
east coast of the PP and to the north by the Caribbean Sea.
The PP is entirely within the continental shelf; thus the
waters are relatively shallow throughout the study area. A
typical phenomenon of this area is the influx of water from
the Atlantic sector, which flows through the passage
between Trinidad and Tobago. The Gulf of Paria (GP) is a
shallow inland sea between the east coast of Venezuela and
the island of Trinidad. The GP covers an area of about
160 km from east to west and 45 km from north to south.
Average depth is around 20 m and reaches a maximum of
30 m. The waters of adjacent rivers (Orinoco, San Juan,
etc.), the Caribbean Sea and the Atlantic Ocean, driven by
a branch of the Guyana Current, all converge in the GP, so
that it can be considered as a true mixture of watersheds.
This mixture of waters has a particular influence on the
behavior of the hydrographic variables in this ecosystem.
The environmental characteristics of the PP and GP are
the result of water flow and sediments brought by the
Orinoco River, the ocean currents that move along the east
coast of South America that transport a huge quantity of
sediment from the Amazon River, the action of tides,
waves and the current regime of the continental shelf and
the coastal upwelling regime determined by the action of
the winds. All these factors affect the physical and chem-
ical processes of the sediments. Thus, the organic matter
present has different sources and conditions of transport,
sedimentation and preservation. The study and character-
ization of C, N and P in the surface sediments in the PP and
GP are therefore essential.
According to Gomez (1996), the waters of the Caribbean
are generally poor with a few moderately fertile areas, such
as those close to northeastern Venezuela. Regional
enrichment is commonly associated with the upwelling of
subsurface waters, an annual hydrographic phenomenon
that occurs during the first few months of the year. How-
ever, the possibility of the existence of factors that promote
water fertility throughout the year has been raised. The
great South American rivers (Orinoco and Amazon) and
internal waves bring nutrients, thus producing eutrophica-
tion of the waters of coastal lagoons such as La Restinga
(Margarita Island) from May to November, when upwell-
ing becomes less intense or ceases altogether. Monente
(1997) attributes changes in the composition of the surface
waters of the Caribbean to variations in the flow of the
Orinoco and Amazon rivers throughout the year, as well as
efficient geochemical processes that operate between the
mouths of these rivers and the Caribbean Sea. Other pro-
cesses that occur in upwelling zones near the coast of
Venezuela have already been mentioned. Regardless of the
importance or likely importance of the above mentioned
phenomena, there are others that also contribute signifi-
cantly to the enrichment of these waters. East of 63� west
longitude there is an important upwelling zone close to the
coast as well as downwelling, causing rearrangement of the
surface layers in the first 100 m. This process is not con-
tinuous and is interspersed with waters of continental ori-
gin. These two phenomena contribute significantly to the
enrichment of surface water bodies. They are not contin-
uous, however, but rather occur intermittently throughout
the year in a pulse-like fashion.
This research is to assess the size distribution of surface
sediments in the region, the distribution of C, P and total N
in the silt and clay fractions of sediments in the region, the
linking of the contents of C, P and total N size distribution
of sediments in the region, establish relationships between
C, P and total N content in the silt and clay fractions of
sediments in the region and the assessment of whether
there are relationships between content and distribution of
organic compounds present.
The results obtained from this project improve the
knowledge of the biogeochemical cycles of carbon species,
nitrogen and phosphorus in tropical areas, which are
associated with both the minerals that form part of the
sedimentary matrix and the organic fraction present. On the
other hand, this study will reveal the behavior of organic
matter north of the PP and the GP, which is of economic
importance to the country for its great fishing potential and
gas potential, thus making estimates of the wealth of the
area for hydrocarbon generation.
Materials and methods
Sediments were sampled at 44 stations set up throughout
the study area, 24 in the PP and 20 in the GP, during an
430 Environ Earth Sci (2012) 65:429–439
123
oceanographic campaign conducted aboard the research
vessel ‘‘Guaiqueri II’’ in October 2005 (Fig. 1). The posi-
tion of the vessel was determined at each station using
differential GPS corrections transmitted by satellite, which
guarantees a location error of less than a meter. Sampling
was performed using a Petersen dredge; subsamples were
then taken, which were processed and stored until analysis.
The separation of size fractions of sediments was carried
out in two stages: first, the gravel and sand fractions were
separated from the mud fraction (silt and clay) by wet
sieving after drying and weighing the samples. Secondly,
the silts were separated from the clays using a modification
of the pipette method without dispersant. This procedure is
based on the rate of sedimentation of grains over different
time intervals, according to Stokes’ Law (Roa and Berthois
1975). Once separated, the two fractions were subjected to
mild heating to evaporate most of the water and then left at
ambient temperature. The fractions were then quantified by
weighing and classified using ternary diagrams.
Corg was determined by first applying an acid attack
with HCL 10% to the samples in order to eliminate car-
bonates. The samples were then dry-combusted at a tem-
perature of 1,490�C, and it was assumed that the carbon
measured was associated solely with the organic matter in
the sediment that had not reacted with the acid. Total N and
P were determined using the method described by Val-
derrama (1981), based on the simultaneous oxidation of
nitrogen and phosphorus compounds in the samples and the
resulting solution used for both analyses. The phosphorus
content was determined following the method of Murphy
and Riley (1962). Nitrogen content was ascertained by
passing a 4-ml aliquot through a Technicon autoanalyzer II,
with a Scientific Instruments detector AC-100, which
reduced NO3–NO2, and recorded the resulting concentra-
tions. Total N content was calculated stoichiometrically
from the nitrite concentration recorded. To demonstrate the
efficiency and accuracy of the techniques used to determine
CT, Corg, NT, Pbio and PT, they were tested in
quadruplicate in a sediment sample. The results are shown
in Table 1, showing that the methods used for detection of
these parameters are reliable and repeatable.
The parameters bathymetry, temperature, dissolved
oxygen and fluorescence indicators in the bottom waters
were taken from the Environmental Baseline Mariscal
Sucre Project: Abiotic Components. The Final Report,
Volume II, developed by Senior et al. (2006) CAMUD-
OCA, Universidad de Oriente, Venezuela, which provides
data from an integrated study of the environmental char-
acteristics of the coastal marine environment in the northern
PP continental shelf and the northwest sector of the GP.
Results and discussion
Bathymetry, temperature, dissolved oxygen
and fluorescence indicators in the bottom waters
The underwater depth of the ocean floor commonly controls
the textural distribution of sediments and the geochemistry
of benthic micro- and macro-nutrients in coastal and oceanic
environments. Figure 2 shows the bathymetric character of
the region. Within the study area, the shallowest sediments
were in the GP, specifically in the coastal zone where they
were only 4.1 m deep, while maximum sediment depths
were recorded for the PP, northeast of the coast, where they
reached 144.4 m deep opposite Boca Dragon.
Bottom water temperatures in the study area (Fig. 2) are
influenced by Atlantic waters flowing through the passage
between Trinidad and Tobago and those coming from
within the GP. These waters show a temperature range of
between 21.38 and 30.94�C with colder water entering from
the northeast to the PP and warmer waters located in the GP,
due to its lower depth and inputs from the Orinoco River.
Dissolved oxygen in water comes from many sources,
the main one being oxygen absorbed from the atmosphere.
The movement of the waves allows water to absorb more
Fig. 1 Study area in the coastal
marine region north and south
of the Paria Peninsula,
Venezuela
Environ Earth Sci (2012) 65:429–439 431
123
oxygen. Physical, chemical and biological processes
induce the exchange of oxygen across the air-ocean inter-
face (Redfield 1942). Figure 3 shows the distribution of
dissolved oxygen in the bottom waters of the PP. Oxygen
concentrations lower than 2.75 ml/l were found in the
waters towards the central sector, which then increase to a
peak of above 3.25 ml/l in the northeastern sector. In the
GP, oxygen concentrations were below 2 ml/l in areas
furthest from the shore. These low values are due to pro-
cesses such as heterotrophic respiration and the bacterial
oxidation of organic matter.
Phytoplankton fluorescence is attributed to the emission
of the energy absorbed by the chlorophyll a of photosyn-
thetic pigments (Ostrowska et al. 2000). The fluorescence
index (FI) is used as an indicator of phytoplankton bio-
mass. Many authors have shown that the FI is well corre-
lated with chlorophyll concentration in water bodies. The
bottom waters of the northern PP continental shelf had
higher FI values towards the west (Fig. 3) and were higher
than 1.5 units between the towns of Carupano and Morro
de Puerto Santo, suggesting that there was considerable
biological activity in the waters of this region at the time of
sampling. To the east, the bottom waters showed low
phytoplankton activity, possibly because of a higher pro-
portion of suspended matter due to tidal influences at Boca
del Dragon.
The waters of the GP are considered to be some of the
least productive along the northeastern Venezuelan coast
Table 1 Analysis of accuracy
for certain speciesCT (%) Corg (%) NT (mg/kg) PT (mg/kg) Pbio (mg/kg)
PMSL(2)11
1 2.89 1.64 190.73 399.99 24.14
2 2.99 1.62 187.05 404.58 23.72
3 2.89 1.62 187.22 386.12 23.34
4 2.89 1.60 189.67 398.89 23.18
Mean 2.89 1.62 189.17 397.39 23.84
SD 0.01 0.09 2.38 7.91 0.39
CV (%) 0.46 0.87 1.26 1.99 1.65
Fig. 2 a Bathymetry of the
study area, b bottom water
temperature in the study area
(Data source: Abiotic
Components. Final Report,
Volume II)
432 Environ Earth Sci (2012) 65:429–439
123
(Benitez and Okuda 1976). The differences between values
reported at different times of the year may be attributed to
seasonal variations in nutrient levels and the turbidity of
the water (Moigis and Bonilla 1988). In the GP, the highest
values (higher than 1.5 units) were recorded towards the
west of the study area (Fig. 6), suggesting significant
phytoplankton activity compared with the other sampling
sites. The rest of the Gulf showed low biological activity,
with rates below 0.5 units although there was a slight
increase in the area of Puerto de Hierro, west of Macuro,
where values were slightly above 0.5 units.
Texture
According to the results, the surface sediments of the PP
and the GP have a mainly sandy loam texture. However, in
the central area of the PP and the central-coastal area of the
GP, the sediments have a sandy-silt texture (Fig. 4), with
the GP sediments having a higher proportion of fine grains
(silt and clay). This reflects the sediment dynamics pre-
valent in this coastal marine region, which promote the
deposition of predominantly fine-grained sediments, due to
the geomorphology of the area and the flocculation of clays
that occur when seawater is mixed with fresh water from
the Orinoco River.
Organic carbon (Corg)
The Corg content of the silt fractions varied between 0.18
and 4.08%, with an average of 1.53% (SD 0.09%). From
Fig. 5, it can be observed that the highest values were
located in the westernmost part of the PP, while the lowest
figures were located mostly in the GP. The high percent-
ages of Corg obtained in the surface sediments are the
product of the organic material (OM) contribution of
continental origin (Demaison and Moore 1980; Tissot and
Welte 1984). Marine OM inputs generated in situ also
contribute to organic productivity in the PP. Moreover, low
oxygenation conditions must exist to preserve this material
in the sediment.
High values of Corg in the PP were found in shallower
areas with high dissolved oxygen and FI values and coin-
cide with an upwelling area, implying high primary pro-
ductivity. This allows the inference that the conditions are
necessary for the production and accumulation of OM. In
contrast, despite the contribution of terrestrial OM from the
Orinoco Delta and that brought by the Guyana Current
from the Amazon, Corg values were low in the GP.
According to Benitez and Okuda (1976), the waters of the
GP should be considered some of the least productive
along the northeastern coast of Venezuela. This is because
Fig. 3 a Dissolved oxygen in
the bottom waters of the study
area, b fluorescence indices of
the bottom waters of the study
area (Data source: Abiotic
Components. Final Report,
Volume II)
Environ Earth Sci (2012) 65:429–439 433
123
this region is very shallow and influenced by different
currents and water masses, which tend to cause mixing
conditions and therefore oxidizing conditions unfavorable
for the preservation of OM in the sediments. The high
dissolved oxygen values found in the GP, and turbidity due
to the suspension of fine material in the water column
prevents the passage of sunlight, thus limiting photosyn-
thetic processes and consequently the production of OM.
The oscillation of the tides in the GP holds the fine material
in suspension, which is then exported out to sea.
In the clay fraction Corg ranged between 0.65 and
5.99%, with an average of 1.64% (SD 0.09%). As for the
silt fraction, higher percentages were found in the western
sector of the area studied and the lowest values were
recorded from the GP. These distributions appear to be
associated with coastal upwelling, especially in the west-
ernmost sector, where a high fluorescence rate and high
levels of dissolved oxygen were observed, indicating
phytoplankton primary productivity. Although, in general,
the silt contained the highest percentages of OC, in rare
Fig. 4 Spatial distribution of
the texture of the surface
sediments in the study area
Fig. 5 a Spatial distribution of
Corg in the silt fraction of
surface sediments in the study
area, b spatial distribution of
Corg in the clay fraction of
surface sediments in the study
area
434 Environ Earth Sci (2012) 65:429–439
123
cases, the clays accumulated a higher proportion of organic
matter. This was observed mostly for coastal samples, from
both the PP and the GP. Generally, microorganism activity
increases in coastal areas, so that the carbon originating in
these regions is generally associated with very fine grain
sizes. In contrast, in the open ocean other sources of carbon
are present, which together with the transport and remin-
eralization of the OM in the water column, can produce
carbon associated with slightly larger particle sizes, or
particles that are not deposited in the sediment.
Total phosphorus and nitrogen
The concentrations of TP in the sediments of the study area
ranged between 0.02 and 0.21%, averaging 0.04% (SD
0.001%) for the silt fraction and between 0.02 and 0.23%,
with an average of 0.04% (SD 0.001%) for the clay frac-
tion. The spatial distributions of TP in both fractions are
shown in Fig. 6. The lowest concentrations of TP were
found in the GP and to the central and eastern sectors of the
PP, but then increased gradually towards the western sector
of the PP, where the highest values were found, thus
coinciding with the Corg distribution. The spatial distri-
bution of this element appears to be influenced by ocean
currents and coastal upwelling patterns. The production,
distribution and sedimentation of OM in the study area is
affected by the discharge of the Orinoco River that impacts
directly to the GP through numerous rivers and creeks, and
the contributions of the Amazon River brought by the
Guyana Current.
Unlike Corg, however, TP shows a higher affinity for
the clay fraction. It is known that surface water P is
depleted due to its assimilation by phytoplankton and
incorporation into organic matter that aggregates into lar-
ger particles and then settles. In general, this particulate
matter is remineralized at depth. In addition, zooplankton
produce particles during feeding and excretion that settle to
the deeper layers of the ocean leading to greater propor-
tions of TP in the finer fractions of sediments. De La Lanza
and Caceres (1994) indicated that up to 60% of ortho-
phosphates can be removed from the water by their
adsorption in sediments, which could explain their high
concentrations in coastal areas. These same authors con-
cluded that, in general, the high concentrations of TP
present in sediments reflect the ability or particular char-
acteristics of the sediments to retain phosphorus, both
indigenous and allochthonous, which in turn promotes
abiotic processes. For Lai and Che Lam (2008) studied the
release and retention of P in eutrophic sediments of the Mai
Po marshes, and found that the enrichment of nutrients in
the sediment is a potential source of P to the overlying
water and that most of the TP sediment is composed of
labile inorganic phosphate.
TN concentrations in the silt fraction of sediments in the
PP and GP varied between 0.01 and 0.12%, with an aver-
age of 0.03% (SD 0.001%) and in the clay fraction between
0.02 and 0.58%, with an average of 0.13% (SD 0.001%).
The lowest TN values were recorded from the GP and in
the central and eastern part of the PP, showing a gradual
increase towards the western sector of the PP, where the
highest values were found, thus coinciding with the dis-
tribution of Corg in silt and TP in both fractions (Fig. 7).
There is a small area in the center of the GP with increased
concentrations of TN in the silt fraction, possibly due to
local prevailing currents, generating an accumulation
effect. This effect was not observed in the clay fraction, but
a similar distribution of OC in this fraction was observed in
a central coastal area of the PP. In both cases the nitrogen
and carbon probably originate from a common source from
the coastal margin, such as runoff containing these ele-
ments. The nitrogen could have both an indigenous (phy-
toplankton production) and an allochthonous origin (runoff
and sewage, among others).
As for TP, this element shows a higher affinity for fine
textured sediment and is found in a greater proportion in
the clay fraction. The main source of nitrogen in marine
sediments is known to be organic matter released by the
action of decomposing microorganisms (Pellerin et al.
2004). The organic matter that accumulates in the form of
particles is mineralized by the microbial flora with the lib-
eration of ammonia. Nitrogen may be retained by the clays
present in the sediment by cation exchange; thus the
affinity for the fine-grained fraction is due to the proportion
of clay minerals present. However, the proportion of fixed
ammonium is dependent on the amount of this ion in
solution, the type of clay present and the presence of other
ions in the pores of the sediment (Pellerin et al. 2004). In
sediments, the processes of N fixation and P availability are
influenced by a variety of environmental factors including
water depth, the redox state of sediments, benthic primary
production, pH and temperature (Joye and Hollibaugh
1995; An and Joye 2001).
C/N ratio
The C/N ratio averaged 23.67 with a minimum of 10.65
and a maximum of 67.78 (SD 3.74%). Overall, depending
on the authors and their approach, these values are within
acceptable ranges as reported in the literature (Talbot and
Laerdal 2000; Tyson 1995; Meyers 1994; Hecky et al.
1993; Hedges 1992; Romankevich 1984; Healey and
Hendzel 1980). Table 2 summarizes the ratios of C/N
given by various authors according to various criteria and
sedimentary environments. Figure 8 shows the significant
correlation between C and N in the sediments in the study
area, R2 = 0.80. However, it can be observed that the GP
Environ Earth Sci (2012) 65:429–439 435
123
Fig. 6 a Spatial distribution of
phosphorus in the silt fraction of
surface sediments in the study
area, b spatial distribution of
phosphorus in the clay fraction
of surface sediments in the
study area
Fig. 7 a Spatial distribution of
nitrogen in the silt fraction of
surface sediments in the study
area. b spatial distribution of
nitrogen in the clay–silt
fractions of surface sediments in
the study area
436 Environ Earth Sci (2012) 65:429–439
123
samples group separately from those of the PP. As men-
tioned previously, there are a number of factors that make
the GP sediments different from those of the PP in terms of
composition, texture, and distribution. The GP is charac-
terized by a mix of waters with different properties and
compositions, originating from continental, transitional and
marine sources; this makes the lack of a relationship
marked by C/N in the GP. In the PP, however, in spite of
the influence of some currents, the behavior of the C/N
ratio is that expected for a marine environment, with its
variations of dissolved oxygen, temperature and other
physicochemical parameters depending on depth and the
influence of coastal upwelling.
The spatial distribution of the C/N ratio (Fig. 8) in the
surface sediments in the PP is fairly uniform with values
not exceeding 30 units, whereas in the GP the distribution
is highly variable, with some areas having values exceed-
ing 50 and others where the values are as low as those of
the PP. It can be argued that even with values near 30, the
OM in the PP sediments is a result of marine primary
productivity, especially in the westernmost sector where
values of less than 20 coincide with the most productive
zone in the area due to coastal upwelling processes. In the
GP, the higher variability may indicate different OM
sources. For instance, values exceeding 55 units are due to
the presence of OM from higher plants discharged by the
Orinoco River and the waters from the Amazon brought by
the Guyana Current. The mean, 23.67 (SD 3.74%) is
slightly more than twice 10, which according to Bonilla
et al. (1995) indicates that the geochemical balance
between the sedimentation and decomposition of organic
matter in marine sediments has been reached.
On the other hand, nitrogen is more easily degraded than
carbon, thus the importance of the accumulation of OM,
reflecting intense biological activity, with organic decom-
position and nitrogen release into the water predominating.
This process could be the cause of the high C/N values
recorded for most of the region, as well as mixing with
continental OM. Obviously, the values of this ratio are
related to sediment texture and organic production in situ.
The influence of OM of autochthonous origin predominates
in the PP, whereas in the GP there is a mix of both sources
and different types of processes. Increased biological
activity and temperature increase the metabolism of
organic nitrogen; NH4? diffuses into the water; and dis-
solved oxygen penetrates into the sediment, this release
being higher than that required by the phytoplankton. This
consumption of oxygen could lead to an overestimation of
the depletion of organic carbon (Raine and Patching 1980)
and indicate reduction (Bonilla 1993; Bonilla et al. 1995).
Conclusions
High Corg, P and N values associated with shallower
depths in the western area of the PP indicate a zone of
substantial primary productivity that can be attributed to
coastal upwelling processes and the influence of the waters
of the Orinoco and Amazon rivers brought by ocean
currents.
The C/N ratio clearly distinguishes the OM sources
between the two sectors of the study area: of marine origin
for the PP and a mixture of marine and continental origin
for the GP, indicating the influence of the Orinoco and
Table 2 Ratios of C/N proposed by various authors according to different criteria
C/N ratio Criteria
Meyers (1994) \10 Algae
10–20 Vascular land plants
[20 Mix
Hecky et al. (1993) \8.3 Algal growth in an environment where the N is not a limiting
8.3–14.6 Moderate N limitation
[14.6 Important limitation of N
Talbot and
Laerdal (2000)
\13.3 Algal growth in an environment lacustrine where the N is not a limiting
13.3–9.6 Moderate N limitation, in a lacustrine environment
[19.6 Important limitation of N, in a lacustrine environment
Hedges (1992) [10 Highly degraded planktonic remains in marine sediments or plankton mix fresh vascular plant debris
10–13 Nitrogen-rich organic matter associated with fine-grained
15–85 Plant organic matter transported from the continent that presents itself as highly
oxidized humic substances and low-nitrogen
Romankevich (1984) 11.8–33.3 Carbonate sediments, in which nitrogen-containing compounds are not preferably preserved
as particulate matter, which implies that the degradation of these compounds is accelerated
Environ Earth Sci (2012) 65:429–439 437
123
Amazon rivers in these areas. It also indicates the presence
of two types of sedimentation environment: one slightly
anoxic, thereby conserving the OM in the sediment, and the
other anoxic, not permitting its conservation. In addition,
the C/N ratio reveals a deficit of nitrogenous elements,
limiting the productivity of the area. Productivity in the PP,
however, is not limited by TN deficiency due to a steady
supply of this nutrient to the area from the contribution of
the rivers, as mentioned earlier.
The silt and clay fractions differ in their content of C
and N, whereby apparently the less degraded or more dif-
ficult to degrade N-rich organic compounds accumulate
preferentially in the clay fraction, generating a relative
enrichment of nitrogen compounds in this fraction and a
relative enrichment of Corg in the silt fraction.
Considerable differences were found in the distribution
and content of Corg, TN and TP in the sediments of the PP
and GP, with higher concentrations in the PP, possibly due
to different geomorphological conditions as well as hyd-
rochemical and river inputs, between the two environ-
ments. Ocean currents and the sediment source may be the
most important factors that control the composition, texture
and spatial distribution of the sediments and elements
considered in this study.
According to Gomez (1996), the the fertility of the
waters off northeastern Venezuela are caused by: (1)
upwelling, a hydrographic phenomenon that occurs during
the first months of each year, (2) the contributions of
dissolved and particulate organic matter from the great
South American rivers, particularly the Amazon in the
first few months and the Orinoco during the second half
of the year, (3) Coastal lagoons and other coastal water
bodies that enrich the adjacent sea, especially from May
to November and (4) the local supply of nutrients from
the erosion of the numerous islands and islets, headlands
and cliffs on the continental shelf by internal waves.
According to Odriozola (2004), a surface layer of low
salinity observed during each cruise of the SIMBIOS-
Orinoco indicates that both the GP and the southeastern
Caribbean (SEC) are under the influence of freshwater
inputs during the wet and dry seasons. According to this
author, in the GP, this surface layer is a direct result of
the discharge from the Orinoco River plume, while in the
SEC it could be due to the discharges from both the
Orinoco and the Amazon rivers, transported to the SEC
by the Guyana Current. However, the similarity of the
silicate-salt mixture found by Odriozola (2004) to that
reported by Froelich et al. (1978) for the Caribbean,
suggests that water from the Orinoco River plume, rather
than the Amazon, dominates this region. This finding
supports the observations made by Muller-Karger et al.
(1988) using satellite imagery.
Fig. 8 a Relationship between
Corg and TN in the surface
sediments in the study area.
b Spatial distribution of the C/N
in surface sediments in the study
area
438 Environ Earth Sci (2012) 65:429–439
123
Acknowledgments The authors thank PDVSA CAMUDOCA for
their financial support of the second phase of the project ‘‘Baseline
Environmental Study, Mariscal Sucre’’ and to the Research Council
of the Universidad de Oriente for financing the project ‘‘Geochemistry
of superficial sediments of the coastal-marine region of the north and
south continental shelf of the Paria Peninsula, Sucre State, Venezu-
ela’’ (CI-02-030700-1404/08); and thanks to the Institute of Earth
Sciences and the Oceanographic Institute of Venezuela for their
institutional support of this study.
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