Organic carbon, phosphorus and nitrogen in surface sediments of the marine-coastal region north and...

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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


Present Address:M. C. Martinez-Soto

IMEDEA (Mediterranean Institute of Advanced Studies)

(UIB-CSIC), Miquel Marquez, #21, 07190 Esporles,

Balearic Islands, Spain


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)


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)


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


area


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


123


phosphorus in the silt fraction of


area, b spatial distribution of

phosphorus in the clay fraction

of surface sediments in the

study area


nitrogen in the silt fraction of


area. b spatial distribution of

nitrogen in the clay–silt

fractions of surface sediments in

the study area


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


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


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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