Spatial variability in Sediment Oxygen Consumption under winter conditions in a lagoonal system in...

Spatial variability in Sediment Oxygen Consumption

under winter conditions in a lagoonal system in

New Caledonia (South Pacific)

Christian Grenza,*, Lionel Denisb, Guy Boucherc,Laurent Chauvaudd, Jacques Clavierd,Renaud Ficheze, Olivier Pringaulte

aUniversite de la Mediterranee, UMR CNRS 6535 LOB, Rue de la Batterie des Lions, 13007 Marseille, FrancebUniversite des Sciences et Technologies de Lille 1, Station Marine de Wimereux, UMR CNRS 8013 ELICO,

28, Avenue Foch, 62930 Wimereux, FrancecMuseum National Histoire Naturelle, ESA8044 CNRS, rue Cuvier 75231, Paris, France

dUniversite de Bretagne Occidentale, UMR CNRS 6539, LEMAR, Place Nicolas Copernic,

29280 Plouzane, Francee Institut de Recherche pour le Developpement, Centre IRD de Noumea, BP A5,

98848 Noumea Cedex, New Caledonia

Received 2 May 2002; received in revised form 25 July 2002; accepted 13 September 2002

Abstract

Sediment Oxygen Consumption (SOC) was investigated during a winter (Southern Hemisphere)

cruise in the southwest lagoon of New Caledonia. Oxygen fluxes were measured at 11 sampling

stations distributed along two coast to reef transects. Three different methods of flux measurements

were used: diver-operated benthic chambers, laboratory incubation of sediment cores and oxygen

microprofiles determinations. SOC values varied between 450 and 2250 Amol O2 m� 2 h� 1. The

level of agreement between the three techniques strongly varied as a function of sediment type. Most

of the SOC values from the grey sand zone in the middle part of the lagoon and the muddy bottoms

of the bays did not show significant differences. A central station presenting a dense seagrass bed

gave lower SOC determined by oxygen microprofiles compared to the two other methods. In coarse

carbonated sands from the back reef area, SOC measured by in situ benthic chambers were higher

than SOC measured by incubation techniques. This discrepancy could be explained by physical

disturbance of the sediments, macroscale variability in benthic communities or technical efficiency

of the sediment sampling device and probably by a combination of all three processes. Nevertheless,

0022-0981/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0022 -0981 (02 )00518 -X

* Corresponding author. Tel.: +33-491-041-641; fax: +33-491-041-635.

E-mail address: [email protected] (C. Grenz).

www.elsevier.com/locate/jembe

Journal of Experimental Marine Biology and Ecology

285–286 (2003) 33–47

for the other sediment types that represented 85% of the lagoon bottoms, the results from the three

techniques used for SOC determination were strongly convergent. Based on this assumption, it could

be stated that the oxygen fluxes were essentially driven by microbial activity compared to

biologically mediated vertical transport of solutes. The SOC values determined during this study

were in agreement with budgets previously calculated for the lagoon. Regardless of the back reef

area, spatial variability in SOC can be explained by the organic matter content of sediments which

clearly showed a coast to reef gradient with higher organic carbon and nitrogen contents in the

coastal sediments. The C/N ratios demonstrate the higher rate of freshly deposited organic matter

near the coast compared to more central stations in the lagoon.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Oxygen fluxes; Organic matter; Mineralisation; Coral reefs

1. Introduction

Although the oxygen uptake in marine sediments is mainly regulated by the net

deposition and degradability of organic matter (Graf et al., 1982), the mechanism and

dynamics of the oxygen uptake are complex. Moreover, in shallow marine systems, most

of the sedimenting organic matter is primarily mineralised by anaerobic microorganisms

and animals on the sea floor. Different methodologies for measuring sediment oxygen

uptake are commonly used even simultaneously depending on what process is investigated

(Rasmussen and Jørgensen, 1992; Archer and Devol, 1992; Reimers et al., 1992; Reay et

al., 1995).

Sediment water fluxes of dissolved elements have been widely studied from shallow

water systems to deep-sea ocean bottoms since the early 1960s to 1970s. The papers of

Pamatmat (1966) and Pamatmat and Banse (1969) are among the first works reporting

oxygen flux measurements in shallow (5 m) and shelf sites in the Puget Sound,

respectively. The measures were performed using one of the first benthic chamber

devices. Smith et al. (1976) presented Sediment Oxygen Consumption (SOC) results

for a deep-sea station off Gray Head-Bermuda (5200 m), measured with a free vehicle

equipped with a benthic chamber system. A large variety of measures performed in

shallow water systems reports spatial and temporal variability in SOC (Denis, 1999;

Grenz et al., 2000). SOC measurements in tropical areas are less common than in

temperate systems and additional information is needed especially as those oligotrophic

systems are confronted with increasing anthropogenic inputs (Alongi, 1998; Lourey et al.,

2001). In situ benthic chambers have frequently been chosen to describe SOC in those

shallow systems. In the New Caledonia lagoon, Boucher et al. (1994a) and Clavier and

Garrigue (1999) presented a spatial study of SOC measured by diver-operated benthic

chambers.

The use of enclosure to measure sediment water exchange rates is common in most

ecosystems and major advances have been performed in the characterisation of the

hydrodynamic conditions inside the enclosures (Glud et al., 1995). But caution is

necessary when interpreting results, as already stated by Sundby et al. (1986) and Booij

et al. (1991).

C. Grenz et al. / J. Exp. Mar. Biol. Ecol. 285–286 (2003) 33–4734

As the accuracy of the technique used is still questioned, we performed a spatial survey

of SOC measurements obtained with three different techniques during the winter season in

the lagoon of New Caledonia around the city of Noumea. Our goal was (1) to compare

results between the three different technologies and to explain what causes the similarity or

discrepancies between the techniques used and (2) to calculate the lagoon-wide sediment

oxygen consumption under winter conditions.

2. Methods

2.1. Study site

New Caledonia is a French Territory located in the Pacific Ocean at 22jS and 165jE(Fig. 1). Most of the studies dealing with the coral reef lagoons in this archipelago have

been conducted around the city of Noumea in the southern part of the west coast of the

main island, which represents an area of 2000 km2 (Testau and Conand, 1983). The

bathymetry of this lagoon is very heterogeneous, especially due to the presence of various

Fig. 1. Location of the study site and the stations visited during Camecal 2 cruise in August 2001.

C. Grenz et al. / J. Exp. Mar. Biol. Ecol. 285–286 (2003) 33–47 35

geomorphological structures (Gabrie, 1998), which include shallow inner and barrier reefs,

sedimentary plain, and deep canyons connected to reef passes (Fig. 1). The mean depth is

21 m with deep canyons at 60 m. The southern part of the lagoon is largely connected to

the open ocean and consequently subject to significant inputs of oceanic water. A

biogeochemical modelling approach (Bujan et al., 2000) based on 3D hydrodynamic

modelling (Douillet, 1998; Douillet et al., 2001) yielded an average residence time of less

than 11 days for the whole 2000 km2 lagoon under trade wind conditions. Freshwater is

delivered to the SW lagoon by three main rivers (Pirogues, Coulee, Dumbea). River flow

rates are usually low, with a yearly average value for the Dumbea river of 5 m3 s� 1. Flow

rates may occasionally and very shortly rise to 300–350 m3 s� 1 due to tropical

depressions or cyclones occurring during the Austral summer wet season (December to

April), a typical feature associated with tropical or subtropical meteorological conditions.

A recent review on the state of the marine environment in New Caledonia (Labrosse et al.,

2000) concluded that the globally pristine environmental conditions could be locally

impacted by terrigeneous inputs from natural or anthropogenic land erosion and by urban

derived nutrient enrichment. The measurement of oxygen consumption was considered as

a suitable method to assess the impact of terrigeneous and anthropogenic inputs on

sediment metabolism.

A flux study was performed during a cruise onboard RVAlis between 16 and 23 August

2001. Eleven stations representing various environmental conditions and grossly distrib-

uted along two land–reef transects were visited (Fig. 1, Table 1). The stations were

considered as representative of the different sediment types identified in the area (Chardy

et al., 1988; Clavier and Garrigue, 1999), i.e. mud deposits near the coast and in the deeper

parts (canyons), grey sand bottoms in the middle part of the lagoon and white sands close

to the reef. One or two stations were sampled each day. The three different procedures for

O2 flux measurements were performed in dark in order to avoid technical problems when

simulating in situ varying light conditions, and used in most of the sediments apart for the

deepest stations (A11, M16, M03) where diving operations could not be performed for

safety reasons. The O2 profile estimates could not be used for the white sands because the

coarse grain size of the sediments was not compatible with the use of microprobes.

Table 1

Sampling location, depth and sediment types of the stations

Dates Station South East Depths (m) Sediment type Flux measurementsa

16/08/2001 N04 22j17,22 166j27,76 10 muds BC–INC–PROF

17/08/2001 M42 22j23,42 166j24,76 12 white sands BC–INC

17/08/2001 A11 22j28,17 166j30,46 58 canyon muds INC–PROF

18/08/2001 D34 22j13,55 166j23,07 18 muds BC–INC–PROF

19/08/2001 M43 22j20,44 166j14,73 11 white sands BC–INC

19/08/2001 M16 22j18,04 166j15,78 52 canyon muds INC–PROF

20/08/2001 M17 22j17,61 166j17,33 13 grey sands BC–INC–PROF

21/08/2001 M03 22j15,42 166j19,36 24 muds INC–PROF


22/08/2001 M41 22j22,89 166j25,24 20 grey sands BC–INC–PROF


a BC: In situ benthic chambers, INC: laboratory incubations, PROF: profile determinations.


2.2. Benthic chamber procedure

In situ incubations were carried out using three diver-operated benthic chambers

(Boucher and Boucher-Rodoni 1985, 1988; Boucher et al., 1994a,b). These clear acrylic

hemispheres fastened to 0.2 m2 PVC bases were darkened with a black plastic cover.

Internal volume of incubation water varied between 48 and 63 l depending on the depth to

which the bases were driven into the sediments. A submersible pump connected to a

waterproof battery provided water circulation. Oxygen concentrations were recorded using

the polarographic probes of three YSI oxymeters refitted in a waterproof container and

connected to a datalogger. O2 concentrations were measured every 10 s and the mean

logged every minute. Water samples were taken at the end of the incubations by means of

a 60-ml syringe for O2 Winkler titration and probe validation. Incubation duration varied

between 2 h 30 min and 3 h 30 min. Oxygen fluxes were calculated by regressing O2

concentrations against time (least square regression) and expressed as Amol m� 2 h� 1.

2.3. Sediment sampling and incubation procedure

Sediments were sampled with a sediment multicorer Mark VI (Bowers & Connelly,

Barnett et al., 1984) using Perspex cores (i.d.: 15 cm, length: 50 cm). Three or four sediment

cores were collected at each station, containing around 30 cm of sediment and a clear

overlying water. For each core retrieval, bottom water samples (ca 20 l) were taken with a

Niskin bottle approximately 1 m above the sediment. The samples were gently transferred

in an inflatable reserve tank excluding bubbles, in the same conditions as the incubated

cores and were used both as bottom water control and as replacement water during the core

incubation (Fig. 2). A Seabird CTD recorded bottom water temperature and salinity.

The cores and bottom water were transported to the laboratory within 1 or 2 h at field

temperature and in darkness. The cores were sealed with tops equipped with magnetic

stirrers and gas-tight sampling ports, and placed together with the reserve tank, in dark

refrigerated cabinets at in situ temperature. As previously described in Cowan and Boynton

(1996) and detailed in Denis et al. (2001), the cores were incubated over periods of 5–15 h

during the (presumed) linear uptake phase of oxygen. During core incubations five to seven

overlying water samples were taken using a plastic syringe (ca. 120 ml) at 2-h intervals, in

each core and in the reserve tank. Oxygen was immediately analysed by Winkler micro-

titration (ca. 16 ml). Fluxes were calculated as slopes of linear regressions of oxygen

concentration against time. Aerial fluxes were calculated by multiplying the rates of

concentration change with the water height in each core tube. In spite of the low sampling

volume with respect to overlying water volume, the correction for water replacement (with

bottom water from the reserve tank) was systematically applied (Denis, 1999).

2.4. Microsensor measurements and profile interpretations

Microprofiles of oxygen were performed on cores (i.d.: 5 cm, length: 15 cm)

subsampled from the cores retrieved as described above. The subcores were placed in a

flow cell system maintained at field temperature by a thermostated water bath (Fig. 2). A

computer-driven micromanipulator placed on top of the sediment water interface allowed


small scale profiling with the O2 microelectrode (probe diameter 50 Am, vertical resolution

100 Am). Oxygen concentrations were measured with a Clark-type O2 microelectrode

(Revsbech, 1989). The response time was less than 1 s and the stirring sensitivity was

around 1%. Two-point calibrations were performed using the procedure described by

Revsbech and Jørgensen (1986). Oxygen partial pressures were converted to concen-

trations as a function of salinity and temperature (Garcia and Gordon, 1992). From

triplicate steady-state profiles performed in the dark, depth distribution of oxygen

respiration was calculated using the numerical model described by Berg et al. (1998).

This procedure is based on a series of least square fits to the measured steady-state oxygen

profiles, assuming an increasing number of production and consumption zones. The fits

are compared by statistical F-testing, so that the simplest production–consumption

profiles results which reproduces the measured concentration depth distribution within

the chosen statistical accuracy (more details in Berg et al., 1998). For this procedure,

porosity in the top millimetres (i.e. 0–5 mm) was assumed to be equal to 0.9. The

Fig. 2. Laboratory incubation devices for the O2 profile measurements (A) and the core incubation technique (B),

and examples of oxygen consumption versus time in a benthic chambers (C) and during four core incubations (D)

obtained for station M03.


sediment diffusion coefficient, Ds, was calculated from the diffusion coefficient in water

D0 (Broeker and Peng, 1974) and the sediment porosity, /, according to Ullman and Aller

(1982), i.e. Ds =/2D0. Areal rates of oxygen uptake were then obtained by multiplying the

calculated volumetric rates by the thickness of the reaction zone.

Subsamples (i.d.: 2.7 cm) of 1 cm thick slices of sediments were analysed for porosity,

organic carbon and nitrogen content. Porosity was calculated using water content (differ-

ence between wet and freeze-dry weight) and assuming a bulk density of 2.65 g cm� 3

(Berner, 1980). Organic matter content was calculated by the difference between total dry-

weight and ash-weight of sediments. Organic carbon and nitrogen contents were analysed

using a LECO NA 2100 analyser.

3. Results

3.1. Sediment characteristics

Sediment porosity and organic matter content showed a broad variability with depths

and locations (Fig. 3). Porosity varied between around 0.4 and 0.8 with lower values

measured at M41, M42, M43 and M16 stations with small vertical gradients. These

stations correspond to the more offshore stations near the barrier reef in the white sand

zone. For the rest of the stations we observed that the porosity in the top layer increased

toward the coast, together with a reinforcement of the vertical gradients. The highest

porosity values (0.82) were measured in Dumbea and Sainte Marie Bay. Organic matter

contents in sediment showed similar patterns with no clear vertical gradient and low values

around 1% DW for stations M41, M42, M43 and M16. Stations N40, A11 and N28

showed a stronger vertical variability and higher values between 2% and 4% DW in the

top layers. Again stations M03, M17, D34 and N04 showed the highest values and

strongest vertical gradients in organic matter contents, the maximum values being

measured at station N04.

Fig. 3. Sediments profiles in porosity and organic matter contents for the Noumea stations.


Organic carbon and nitrogen analyses confirmed the spatial variability observed in

organic matter contents (Fig. 4). The lowest sediment organic carbon and nitrogen

concentrations were measured in the white sand stations near the barrier reef. Sediment

carbon concentrations ranged around 0 to 0.5% DW and nitrogen concentrations were

below 0.04% DW. The central lagoon stations showed higher concentrations in organic

carbon (0.5% to 1.5% DW) and nitrogen (0.04% to 0.12% DW). The highest concen-

trations were measured in the bays with values from 1.5% to 2.5% DWand 0.12% to 0.2%

DW for organic carbon and nitrogen, respectively, in the top sediment layer. The

relationships between sediment organic matter and organic carbon and nitrogen were

statistically significant ( p < 0.001, r= 0.92 for carbon and 0.56 for nitrogen, 165 analyses).

Considering the linear regression slopes for organic matter versus carbon and nitrogen,

organic carbon and nitrogen contributed to 40% and 2% of the organic matter pool of the

sediments, respectively. An average molar C/N ratio of 15 was calculated by combining

the whole data set. Carbon and nitrogen contents in white sand sediments were to low for

an accurate calculation of the C/N ratio. The sediment in the central lagoon stations

showed a C/N ratio between 20 and 30, whereas the more coastal stations showed a ratio

between 6 and 15.

3.2. Sediment Oxygen Consumption

Sediment Oxygen Consumption (SOC) measured by the three different methods are

given in Table 2 and plotted in Fig. 5. SOC values ranged from 450 to 2250 Amol O2

m� 2 h� 1. The level of agreement between the three techniques strongly varied as a

function of sediment type. Most of the SOC values from the grey sand zone in the middle

part of the lagoon and the muddy bottoms of the bays did not show significant

differences ( p>0.05). At station M17, SOC measured by the incubation technique and

Fig. 4. Sediment profiles in organic carbon and nitrogen at the Noumea stations.


the benthic chamber device agreed, whereas the microelectrode profile technique

provided significantly lower values (pair-wise t-test, p < 0.01). In the deep canyons

(Stations A11 and M16), where the diver-operated in situ benthic chambers could not be

used, results from sediment core incubations and oxygen microprofiles strongly agreed.

The highest discrepancy between in situ and ex situ techniques was observed at stations

M43 and M42 which corresponded to coarse carbonated sands from the back reef area.

The incubation of sediment cores gave SOC values around 600 Amol O2 m� 2 h� 1,

whereas the benthic chamber measurements gave values of 1400 and 2200 Amol O2 m� 2

h� 1 for stations M43 and M42, respectively.

Fig. 5. Comparison between the three different measurement techniques of Sediment Oxygen Respiration used at

the 11 stations in Noumea lagoon (BC: in situ benthic chambers, INC: laboratory incubations, PROF: profile

determinations).

Table 2

Sediment oxygen consumption measured at the different location during the RVAllis cruise in Noumea lagoon

Stations Lab incubations Benthic chambers Profile determinations

INC INC-STD BC BC-STD PROF PROF-STD

M43 584.9 154.5 (4) 1382.3 116.4 (3)

M42 601 266.2 (2) 2248 31.1 (2)

A11 550.3 234.8 (3) 410 57.8 (3)

M16 622.2 122.2 (4) 459.9 40.2 (3)

M17 1570.2 134.3 (4) 1652 231.0 (3) 940.3 105.2 (3)

M41 950.3 351.8 (4) 998 203.9 (3) 1090.3 136.1 (3)

M03 922 140.4 (4) 1211.7 116.7 (3)

N40 988.6 96.5 (4) 1175 339.4 (3) 1027.2 39.9 (3)

N28 1337.6 521.0 (4) 1438.7 418.1 (3) 1180.3 40.2 (3)

D34 731 123.2 (4) 1041 199.4 (2) 769.1 52.8 (3)

N04 1041.3 531.5 (4) 1193.1 95.0 (3)

Units: Amol m� 2 h� 1, STD: standard deviations, number of replicates in parentheses.


4. Discussion

4.1. Comparison between the techniques

Our results demonstrate that the three techniques used gave similar SOC estimates for

most of the studied sediments. Earlier findings (e.g. Sundby et al., 1986; Glud et al., 1994;

Per Hall, pers. comm.) established that profile determinations of SOC as well as nutrient

fluxes yielded lower values than flux measurement based on incubations either in or ex

situ. A few shallow water flux measurements in marine environments confirmed that

biological transport processes can enhance benthic fluxes by as much as up to a factor of

10 over those supported by vertical molecular diffusion (Klump and Martens, 1981;

Rutgers Van Der Loeff, 1981; McCaffrey et al., 1980).

The differences observed were attributed to the benthic macrofaunal impact in terms of

bioturbation (biopumping), of the presence of biogenic structures that increase the surface

of the sediment water interface due to the presence of burrows and of the metabolic

activity of the infauna (Christensen et al., 1984). Conversely, Roy et al. (2002) showed that

biogenic surface structures induced less than 10% difference between one-dimensional and

three-dimensional diffusive flux calculations but at smaller, centimetre- to millimetre-

scales.

Incubation techniques are based on bi-dimensional exchange rates, whereas profile

measurements are strictly based on one-dimensional diffusion-driven exchange rates. In

our case, the difference between incubations and profile determination showed only one

case of discrepancy for station M17 where the diffusive estimated SOC is 30% lower than

the incubation derived SOC. In that case, the contribution of biological advection to the

transport of solutes across the sediment–water interface could be significant when

compared to molecular diffusion. This station located in the central part of the lagoon

and close to a small island presented particular microscale variability with a Hadule

uninervis seagrass bed (Clavier and Garrigue, 1999) and embedded detritic leaves deeper

in the sediments. Organic matter determined for this station showed high sediment carbon

and nitrogen contents, in the range of the values observed for the near-shore inner bays.

This spatial variability was probably not integrated in the profile determination of SOC,

whereas the incubations performed on 0.2 and 0.01 m2 sediment surface area for benthic

chamber and core incubation, respectively, took into account the presence of these

structures.

The second major result corresponds to the difference observed between the benthic

chamber and the core incubations at the two white sand stations. These stations are

characterised by coarse sandy sediment. It is well known that in permeable sediments,

especially at high flow velocities, advective pore water transport may enhance the benthic

exchange rates (Vanderborght et al., 1977). Thus, pore water transport may be due to

percolation of the seabed by wave action (Rutgers Van Der Loeff, 1981) or influenced by

biogenic microtopography (Webster, 1992), termed ‘bioroughness’ according to Huettel

and Gust (1992). Moreover, the presence of a Diffusive Boundary Layer (DBL) can in

some cases be an important diffusion barrier which limits the exchange of solutes across

the sediment water interface (Boudreau and Guinasso, 1982). Again the thickness of the

DBL is regulated along with water viscosity, by water flow velocity and sediment


roughness. Booij et al. (1991) showed rapid responses of the oxygen distribution in sandy

sediments to changes in the free-flow velocity. The time scales of the responses

experienced in their study were within seconds to minutes. In our case, ex and in situ

incubation measurements were performed on the same stations and the stirring devices of

the water phase over the incubated sediments for both the BC and the INC techniques

served only to mix the overlaying water, not to mimic external flow conditions. So one

explanation could be the overall physical environment around the benthic chambers that

impacted the in situ measures by an increase in SOC compared to the lower core

incubation SOC measurements. However, sediment coring in those back reef carbonated

coarse sediments might also be responsible for a major bias. Sampling with a multicorer

gave only half-filled sediment tubes at M43 and even at M42 the cores had to be sampled

by a diver because of the limited thickness of the sediment layer overlying carbonate

hardgrounds. In such circumstances, core retrieval can largely deteriorate fine structures at

the sediment–water interface like biofilms while the use of in situ benthic chambers would

generate far less disturbances. Microbenthic algae exist in high density in these sediments

where chlorophyll a concentrations are double those found in the muddy grey sands

(Clavier and Garrigue, 1999). Microphytobenthic and associated microbial metabolism

strongly concur to the measured respiration rates and the sediment reworking due to core

sampling would affect the vertical structure of the whole system and modify its cycling

efficiency. It is also important to note that the high SOC measured with the benthic

chambers in our experience were in accordance with previous results presented by

Boucher and Clavier (1990), Boucher et al. (1994a) and Clavier and Garrigue (1999).

For instance, Boucher and Clavier (1990) showed that for this lagoon, sediment

Chlorophyll and ATP pool were dominant over macro-fauna and -flora in the control of

SOC. Finally, macro-heterogeneity due to large infauna could also explain the observed

difference between the two techniques. No direct faunal determination were made in our

sediments incubated for flux measurements but spatial variability in macrobenthic

communities has been studied in a previous survey (Chardy et al., 1988) and showed

noticeable differences in macrobenthos communities between back reef and coastal

sediments. Boucher (1997) already mentioned that in back-reef white-sands, the mean

density of meiobenthos was nearly twice that of the central and coastal mud locations. As

stated by Christensen et al. (1984), the diameter of the sediment area experienced has to

take into account a part of the spatial heterogeneity imputed to macrobenthic and smaller

organisms in order to obtain an accurate measurement of fluxes. For instance, Glud et al.

(1998) showed that solute exchange rates obtained in Arctic sediments and calculated from

in situ flux data were consistently higher than the corresponding rates determined from

incubation data. They ascribed the observed differences to exclusion of fauna due to the

difference between the sediment area sampled with a corer and the surface area covered in

situ by the chamber.

Despite the difference observed at station M17 and for the white sand stations, the SOC

measures were strongly convergent for 8 of the 11 visited stations. Thus, although

biological exchange processes can contribute, they apparently are responsible for only a

small portion of the total benthic oxygen flux for most of the soft bottom sediments of

Noumea lagoon. Oxygen fluxes can then be mostly considered as gradient-driven

exchanges between the sediment and the water column.


4.2. SOC and implications for net ecosystem metabolism

The strategy experienced in this study was to define SOC values in different part of the

Noumea lagoon along coast to reef transects representing various environmental con-

ditions. The results demonstrated that SOC is spatially variable mainly depending on the

sediment characteristics of the stations visited. The two back reef stations with carbonated

sand showed the highest SOC values when considering the benthic chamber measure-

ments. The muddy canyons showed the lowest SOC values whatever the technique used.

In between, SOC values increased toward the coast reaching a maximum in the bay subject

to significant wastewater inputs. Based on a lagoon-wide scale, we estimated total SOC by

taking into account the area covered by the three major sediment types as described by

Clavier and Garrigue (1999), i.e. white sands: 15%, grey sand bottoms: 35%, and mud

deposits including the canyons: 35%. The lagoon-wide calculated mean SOC ranged

between 830 and 1120 Amol O2 m� 2 h� 1 (F 240 SD) depending on the technique. From

a compilation of 65 references of SOC measurements including all the different techniques

used, i.e. benthic chambers, incubation of sediment cores and profile determinations in

coastal sites (depths < 200 m) as reported in Denis (1999), we calculated a mean SOC of

1680F 680 Amol O2 m� 2 h� 1. The mean SOC calculated for the New Caledonia lagoon

therefore lies in the lower range of the existing data confirming the strong oligotrophic

status of those environments.

Based on a carbon respiration quotient of 1.14 determined by Clavier and Garrigue

(1999), benthic respiration gives an annual sediment CO2 production of 8.3 and 11.3 mol

C m� 2 year� 1. This range is in accordance with a previous estimate of the mean benthic

respiration rate, e.g. 13.68 mol C m� 2 year� 1, also published by Clavier and Garrigue

(1999) for the same lagoon. Our measurements took place during the cool season and it

can be assumed that during the hot season, benthic respiration increases significantly. By

utilising core incubations and oxygen microprofile techniques, new data were provided for

deep canyons that represent 17% of the lagoon bottoms. The high SOC values measured

near the coast as in the bays are related to the proportional high content in organic carbon

and nitrogen in the upper layers of the sediments. Moreover, the C/N ratio in the coastal

stations were lower than in the central stations, showing an input of more freshly deposited

organic matter. Conversely, higher C/N ratios in the middle lagoon demonstrate the

presence of more degraded organic matter poorly fuelling the benthic metabolism. This

result demonstrates that the impact of terrestrial inputs on sediments have a significant

local effect but are restricted to the most coastal sites of the lagoon. Moreover, the low

SOC values associated with low sediment organic carbon content in the deep canyons

showed that, in this coral reef system, no direct transfer of land originating organic

material occurred through the canyons unlike in other continental shelf systems (Durrieu

de Madron, 1994; Monaco et al., 1999).

In summary, this study demonstrated that the use of the three different methods to

measure SOC gave similar results apart for the white sand sites located in the back reef

area. For these later sites, It can be hypothesised that physical disturbance of the

sediments, presumed macroscale variability in benthic communities, technical efficiency

of the sediment coring technique, and probably a combination of all three processes could

explain the discrepancy in white sand flux measurements. For the other lagoon sediments,


which represented 85% of the area, the oxygen profile determination of SOC gave results

similar to those obtained by in situ and ex situ incubation measurements, showing that the

oxygen fluxes seemed to be specifically driven by microbial activity compared to

biologically mediated vertical transport of solutes. The SOC determined for the cool

season with the three techniques was further consistent with previous budgets calculated

for the lagoon.

Acknowledgements

This work received financial support from the French National Programme National en

Environnement Cotier (PNEC), the Centre National de la Recherche Scientifique (CNRS-

INSU), the Institut de Recherche pour le Developpement (IRD), the Centre d’Oceanologie

de Marseille (COM) and the University of Brest (UBO). We are very thankful to the

technical staff of IRD Noumea for nutrient analyses and to the R.V. Alis crew for its

support in fieldwork. Two anonymous reviewers are thanked for their constructive

comments on the manuscript. [RW]

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