Southwest Lagoon Of New Caledonia Research Articles

Description of Palleronia rufa sp. nov., a biofilm-forming and AHL-producing Rhodobacteraceae, reclassification of Hwanghaeicola aestuarii as Palleronia aestuarii comb. nov., Maribius pontilimi as Palleronia pontilimi comb. nov., Maribius salinus as Palleronia salina comb. nov., Maribius pelagius as Palleronia pelagia comb. nov. and emended description of the genus Palleronia

Strain MOLA 401T was isolated from marine waters in the southwest lagoon of New Caledonia and was shown previously to produce an unusual diversity of quorum sensing signaling molecules. This strain was Gram-negative, formed non-motile cocci and colonies were caramel. Optimum growth conditions were 30°C, pH 8 and 3% NaCl (w/v). Based on 16S rRNA gene sequence analysis, this strain was found to be closely related to Pseudomaribius aestuariivivens NBRC 113039T (96.9% of similarity), Maribius pontilimi DSM 104950T (96.4% of similarity) and Palleronia marisminoris LMG 22959T (96.3% of similarity), belonging to the Roseobacter group within the family Rhodobacteraceae. As its closest relatives, strain MOLA 401T is able to form a biofilm on polystyrene, supporting the view of Roseobacter group strains as prolific surface colonizers. An in-depth genomic study allowed us to affiliate strain MOLA 401T as a new species of genus Palleronia and to reaffiliate some of its closest relatives in this genus. Consequently, we describe strain MOLA 401T (DSM 106827T=CIP 111607T=BBCC 401T) for which we propose the name Palleronia rufa sp. nov. We also propose to emend the description of the genus Palleronia and to reclassify Maribius and Hwanghaeicola species as Palleronia species.

Tracking trace elements into complex coral reef trophic networks

The integration, accumulation and transfer of trace elements across the main tropic levels of many food webs are poorly documented. This is notably the case for the complex trophic webs of coral reef ecosystems. Our results show that in the south-west lagoon of New Caledonia both abiotic (i.e. sediments) and biotic (i.e. primary producers, consumers and predators) compartments are contaminated by trace elements. However, our analyses revealed different contamination patterns from the sources of organic matter to the predators. The trophic levels involved in the sedimentary benthic food web (S-BFW, based on the sedimentary organic matter) and to a lesser extent in the reef benthic food web (R-BFW, based on algal turf) were mainly contaminated by trace elements that originate from mining activities like Ni and associated trace elements (Co, Cr, Fe, and Mn). Trace elements linked to agro-industrial (As, Hg, and Zn) and urban (Ag, Cd, Cu, Pb, Se, and V) activities were also integrated into the S-BFW, but preferentially into the R-BFW, and to a lesser extent into the detrital benthic food web (D-BFW, supplied by sea-grass plants). Most of the trace elements were biodiminished with increasing trophic levels along food webs. However, a marked biomagnification was observed for Hg, and suspected for Se and Zn. These results provide important baseline information to better interpret trace element contamination in the different organisms and trophic levels in a highly diversified coral reef lagoon.

Monthly variability of self-recruitment for a coral reef damselfish

Understanding the dynamics of marine populations is critical to managing marine systems effectively and requires information on patterns of population dispersal and connectivity that are still poorly known. We used transgenerational marking to study larval dispersal of the humbug damselfish, Dascyllus aruanus, in the patchy reef seascape of the southwest Lagoon of New Caledonia (SWL), southwest tropical pacific. The adult population of a patch reef located in the central part of the SWL was injected repeatedly with an enriched 137Ba solution to ensure mass production of marked larvae over two successive reproductive seasons. Multiple cohorts of newly settled larvae were sampled, and their otolith core was analyzed by laser ablation inductively coupled plasma mass spectrometry to assess the seasonal and interannual variability of self-recruitment at the central reef. Connectivity between this reef and ten neighboring reefs was also estimated. Analysis of >1200 settlers indicated that self-recruitment varied significantly between months (ranging from 0 to 68 %) and years (21 % in 2011 and 0 % in 2012). However, variable self-recruitment did not always correspond to variable numbers of self-recruits. Therefore, whereas self-recruitment is undoubtedly a good indication of the degree of population openness, it may not indicate local population persistence. Finally, being the first self-recruitment study to include such a large number of settlers, our study reveals that the threshold used to determine marked individuals significantly affects perceived self-recruitment and connectivity rates and, therefore, must be carefully chosen.

Wind-induced variability in larval retention in a coral reef system: A biophysical modelling study in the South-West Lagoon of New Caledonia

In the present work, a biophysical dispersal model is used to understand the role of the physical environment in determining reef fish larval dispersal patterns in the South-West Lagoon of New Caledonia. We focus on a reef fish species, the humbug damselfish Dascyllus aruanus, to investigate seasonal variability of simulated larval retention at the scale of a reef patch and at the scale of the lagoon, and to explore links between larval retention and wind variability. The model shows that retention exhibits considerable temporal variability and periodically reaches values much larger than anticipated. Non-zero larval settlement occurs over a large part of the lagoon. Nevertheless, settlement values decrease quickly away from the natal reef and mean dispersal distances are of order 25–35km. Cross-correlation analyses indicate that weather conditions characterized by strong south east trade winds lead to low retention rates at both local (reef) and regional (lagoon) scales. By contrast, subtropical weather conditions characterized by weak winds result in high retention rates. These results suggest that large-scale weather regimes can be used as proxies for larval retention of the humbug damselfish in the South-West Lagoon of New Caledonia. Nevertheless, relatively small mean dispersal distances suggest that metapopulation dynamics occur on relatively small spatial scales.

Benthic pelagic coupling in a shallow oligotrophic ecosystem: Importance of microphytobenthos and physical forcing

In coral reef lagoons, microphytobenthos (MPB) primary production can be of the same order of magnitude as coral or macroalgal production. As they are situated in oligotrophic waters, sediments receive low amount of allochtonous nutrients, and their high metabolism cannot be achieved without an efficient nutrient recycling. In oligotrophic environments nutrient exchange between sediments and the water column is considered to be low. However, physical forcing that disturbs the sediment interface may accelerate nutrient exchange. The objectives of this work were to: (i) characterize the benthic trophic status and associated nutrient diffusion fluxes as a function of terrestrial and anthropogenic influence and (ii) investigate the impact of physical forcing on the exchanges at the interface and the response of the water column. For that purpose, a multidisciplinary study was performed in the southwest lagoon of New Caledonia combining field survey, laboratory experiments and mathematical modelling. The field survey indicated that most of the lagoon sediments were autotrophic and presented negligible nutrient fluxes. This suggests an apparent decoupling between the sediment and the water column as long as diffusion is the dominant nutrient exchange mode process. This was confirmed by experimental ex situ incubations that showed bacterial production in the water column was not affected by the presence of sediment. The modelling approach showed that physical forcing, such as sediment erosion or wave driven pore water advection, might enhance the coupling between the sediment and the water column through the release of nutrients and organic matter leading to the stimulation of the bacterioplankton and phytoplankton compartments.

Subtidal microphytobenthos: effects of inorganic and organic compound supplies on migration, production, and respiration in a tropical coastal environment

Microphytobenthos (MPB) is an important primary producer in coastal ecosystems. In oligotrophic environments, its activity may be controlled by the availability of organic or inorganic compounds but also by its migration behavior. The objective of this study was to determine, in MPB-colonized subtidal sediments, the consequences of short-term enrichments (< 24 h) of organic (alanine, glutamate, and glucose) and inorganic (ammonium, phosphate) compounds on MPB vertical migration and metabolisms, net production (NP), areal gross production (AGP), and community respiration (R). Two contrasting stations located in the southwest lagoon of New Caledonia were investigated: 1 under strong anthropogenic influence and 1 under more oceanic influence. Both stations were dominated by epipelic diatoms. Differences in net primary production were explained by diurnal variation of MPB biomass at the sediment surface, showing the importance of MPB migration in the functioning of these subtidal environments. However, a stimulation or inhibition of MPB migration did not necessarily impact the net primary production of the system; this strongly depends upon the interactions between the autotrophic and heterotrophic compartments, the latter being controlled by the environmental conditions. For the station under low anthropogenic influence, AGP and R were both significantly stimulated by alanine, glucose, and ammonium, and significantly inhibited by phosphate. The similar responses of AGP and R to enrichments suggest that autotrophs and heterotrophs were tightly coupled. Conversely, in the station under strong anthropogenic influence, AGP and R responded differently. Addition of ammonium inhibited AGP without having an impact on R, whereas addition of phosphate inhibited R whilst having no measurable effect on AGP. In this station, the coupling between autotrophs and heterotrophs was weakened, suggesting that the carbon demand of the heterotrophic compartment is probably sustained by the supplies of allochthonous organic matter rather than by exudates from the autotrophic compartment.

Modelling the spatial and temporal variability of the SW lagoon of New Caledonia II: Realistic 3D simulations compared with in situ data

Coral reef lagoons are under the growing influence of anthropogenic activities, leading to increasing loads of nutrients and various contaminants. Modelling approaches are a useful tool for studying such a complex coastal environment. In this study, we carried out the development of a three-dimensional coupled hydrodynamical-biogeochemical model of the south-west lagoon of New Caledonia. The biogeochemical model presented in Faure et al. (2006, 2010) was dynamically coupled with a hydrodynamical model (MARS3D) in order to study the short-term variability of the ecosystem. Two simulations (in winter and summer) were then performed from measured initial conditions using realistic wind and irradiance conditions and river inputs. Examinations of the biogeochemical response to these transient meteorological conditions were presented and compared with temporal field data corresponding to the considered periods. Results highlighted the ecosystem functioning, based on the balance of hydrodynamical and biogeochemical processes. Influence of urban and terrigeneous inputs were limited to the coastal zone. The model accurately reproduced the measured Chl.a and bacterial production, highlighting the improvement made on the biogeochemical model. However, the underestimation of some variables in model outputs, in particular nutrients, led us to focus on different inputs, such as sediment inputs which were not taken into account or properly estimated. Moreover, the role of boundary waters appeared crucial and suggested a calibration effort. Last, the final aim of our modelling study will help the development of a useful tool for studying the key processes of the ecosystem of the south-west lagoon of New Caledonia, as well as the examination of the biogeochemical response under different scenarios.

Circulation and suspended sediment transport in a coral reef lagoon: The south-west lagoon of New Caledonia

The south-west lagoon of New Caledonia is a wide semi-open coral reef lagoon bounded by an intertidal barrier reef and bisected by numerous deep inlets. This paper synthesizes findings from the 2000–2008 French National Program EC2CO-PNEC relative to the circulation and the transport of suspended particles in this lagoon. Numerical model development (hydrodynamic, fine suspended sediment transport, wind-wave, small-scale atmospheric circulation) allowed the determination of circulation patterns in the lagoon and the charting of residence time, the later of which has been recently used in a series of ecological studies. Topical studies based on field measurements permitted the parameterisation of wave set-up induced by the swell breaking on the reef barrier and the validation of a wind-wave model in a fetch-limited environment. The analysis of spatial and temporal variability of suspended matter concentration over short and long time-scales, the measurement of grain size distribution and the density of suspended matter (1.27kgl−1), and the estimation of erodibility of heterogeneous (sand/mud, terrigenous/biogenic) soft bottoms was also conducted. Aggregates were shown to be more abundant near or around reefs and a possible biological influence on this aggregation is discussed. Optical measurements enabled the quantification of suspended matter either in situ (monochromatic measurements) or remotely (surface spectral measurements and satellite observations) and provided indirect calibration and validation of a suspended sediment transport model. The processes that warrant further investigation in order to improve our knowledge of circulation and suspended sediment transport in the New Caledonia lagoon as well as in other coral reef areas are discussed, as are the relevance and reliability of the numerical models for this endeavour.

Spatial and temporal distribution of zooplankton related to the environmental conditions in the coral reef lagoon of New Caledonia, Southwest Pacific

The distribution of zooplanktonic prey of fish larvae was examined in three bays and two lagoonal stations in the Southwest lagoon of New Caledonia. Water column conditions were characterized by increasing chlorophyll a and particulate organic matter (POM) concentrations from the lagoon to the estuarine bay. The mean zooplankton settled volume and total density were significantly higher in the estuarine bay, reaching 35.1mLm−3 and 3.5×105 individualsm−3, respectively. The total zooplankton density also progressively increased along the sampling period. The composition of assemblages differed between the lagoon and the bays, and was similar in the three bays. Wind speed, surface temperature, chlorophyll a and POM explained these variations, as revealed by a co-inertia analysis (COIA). The prey preferred by fish larvae, i.e. small crustaceans and small copepods, were more abundant in bays. Sheltered bays, most influenced by terrigenous inputs, are likely to provide the best feeding conditions.

Modelling the spatial and temporal variability of the SW lagoon of New Caledonia I: A new biogeochemical model based on microbial loop recycling

This work is an extension and improved version of the biogeochemical model of the South-West lagoon of New Caledonia, presented by Bujan et al. (2000) and Pinazo et al. (2004). This new ecological model was developed to include an explicit description of the microbial loop. Additional variables included bacterial production and dissolved organic matter and a better description of organic matter recycling. A particular effort was made to calibrate parameters of the model for the studied area, using representative field measurements and experiments. The biogeochemical model described the nitrogen and carbon cycles relating the variable stoichiometry of the elements in each biological compartment.Several lagoon surveys demonstrated that, on average, the water column is nearly homogenous. We chose therefore to present in this paper non dimensional model outputs in order to study the behaviour of the new model. The addition of a microbial loop modified the simulated functioning of the lagoon and the fluxes of carbon and nitrogen between the different compartments: it allowed a better description of the recycling of organic matter, recognized as important processes in oligotrophic ecosystems like in the SW lagoon of NC. A sensitivity analysis was performed in order to identify the most sensitive parameters and variables of the model. The different results emphasised the importance of the dissolved inorganic and organic compartment.Preliminary comparisons with field data showed that the model reproduced realistic values. However, the next important step of this work was to dynamically couple this new biogeochemical model in a 3D hydrodynamical model in order: (1) to perform a realistic validation with in situ data (2) to achieve an analysis of the spatial and temporal variability of the ecosystem. This study is presented in the companion paper (Faure et al., 2010).

Hydrography and plankton temporal variabilities at different time scales in the southwest lagoon of New Caledonia: A review

The New Caledonia SW lagoon is wide (5–20 nautical miles) and semi-closed. It is influenced by both the open ocean and the high island within a meteorological context subject to seasonal, inter-annual and longer term variations. The short-term variability (>1day) of meteorological, hydrographical and planktonic parameters is illustrated by a 5-month long time series and is linked to local or remote wind, and precipitation. Seasonal and inter-annual variabilities, inferred from a 10-year long station by spectral analysis, appear clearly for all parameters. Seasonality is the main scale of variability as the island lies near the tropic of Capricorn. Inter-annual variability of a 3–4year periodicity is poorly related to the Southern oscillation index (an equatorial climatic index), stressing the need for a separate tropical index. Long term trends appear on several parameters but their reliability depends on the length of the records. Considering only the longest records (1958–2005), surface temperature appears to have increased since the end of the 1960s in Noumea area. Finally, as a result of greater terrestrial influence, shallower depths, and longer water turnover times close to shore, the temporal variability amplitude decreases from the shore to the barrier reef.

Chlorophylls and Phycoerythrins as Markers of Environmental Forcings Including Cyclone Erica Effect (March 2003) on Phytoplankton in the Southwest Lagoon of New Caledonia and Oceanic Adjacent Area

Spatio-temporal variations of chlorophylls and phycoerythrins, inferred by spectrofluorometric methods, were studied from April 2002 to June 2003 in the southwest lagoon and oceanic waters of New Caledonia. Trade winds blew 75% of the time and appeared as the main factor influencing surface Tchla (sum of monovinyl- and divinyl-chlorophyll a) variations in the ocean, near the barrier reef. Lagoon and oceanic waters differed in the composition of picoplanktonic cyanobacteria with a relative dominance of Prochlorococcus and high-phycourobilin Synechococcus in the ocean, and a relative dominance of high-phycoerythrobilin Synechococcus in the lagoon. Main pigment variations in the lagoon were associated with cyclone Erica in March 2003 and showed a 5-6 fold Tchla increase around Nouméa. The cyclone stimulated mainly diatom growth as indicated by the high chlorophyll (c1+c2)/chlorophyll a ratio and by the lowest values for the other pigment ratios. The relative importance of divinyl-chlorophyll a concentration and fluorescence excitation spectra of phycoerythrins appeared as useful tools for characterizing lagoon-ocean exchanges.

Are sea snakes pertinent bio-indicators for coral reefs? a comparison between species and sites

Classical sampling methods often miss important components of coral reef biodiversity, notably organisms that remain sheltered within the coral matrix. Recent studies using sea kraits (sea snakes) as bio-indicators suggest that the guild of predators represented by anguilliform fish (Congridae, Muraenidae, Ophichthidae, henceforth “eels” for simplicity) were far more abundant and diverse than previously suspected. In the current study, eel diversity (similarity and species richness indices) estimated via sea snake sampling (SSS) was compared among six areas of one of the main oceanic biodiversity hotspot of the Pacific Ocean (southwest lagoon of New Caledonia). Based on the eel diversity in the snakes’ diet, the results obtained in six areas, in two snake species, and using different estimates (ANOSIM, Shannon index…) were consistent, suggesting that SSS provided robust information. Analyses also suggested subtle, albeit significant, differences in the eel assemblages among islets. Such spatial differences are discussed in light of local management practices. As SSS is easy to use, cost-effective, and provides the best picture of eel assemblages to date, it can be employed to monitor the eel assemblages in addition to the snakes themselves in many areas of the Indo-Pacific Ocean, thereby providing an index of the top predator biodiversity of many coral reefs.

Barium and molybdenum records in bivalve shells: Geochemical proxies for phytoplankton dynamics in coastal environments?

Barium : calcium and molybdenum : calcium ratios were investigated in shells of the tropical scallop Comptopallium radula. Three juvenile specimens were harvested alive in the southwest lagoon of New Caledonia after a 1‐yr hydrological survey. Calcite samples representing a few hours of biomineralization were laser‐ablated along the maximal growth axis and analyzed for Ba and Mo content with an inductively coupled plasma mass spectrometer. Absolute dates of shell precipitation assigned on the basis of periodic formation of shell growth patterns led to the accurate reconstruction of ontogenetic variations of elemental ratios with subweekly resolution. Interindividual variability of Ba : Ca and Mo: Ca time series was low, indicating an environmental control on the incorporation of these elements within shells. Both profiles were characterized by a background level punctuated by sharp peaks. The ingestion of diatoms enriched in Ba (adsorbed on iron oxyhydroxides associated with the frustules) is the most likely cause of the formation of Ba : Ca peaks. Some contribution of diatom‐associated barite is also possible. In every instance, Ba : Ca would possibly be a proxy for the timing and magnitude of diatom blooms. Among all the theories that could be advanced to explain the occurrence of Mo: Ca peaks, the most plausible appears to be the ingestion of phytoplankton cells grown on NO3−, and therefore containing high levels of Mo required for the activity of nitrate reductase. If this is so, then Mo: Ca could be a new proxy for nitrate uptake by phytoplankton in coastal ecosystems, helping to reconstruct the balance between new and regenerated production in paleoenvironments.

A “quick and clean” photographic method for the description of coral reef habitats

The use of scuba-based photo/video methods for characterizing coral reef habitats has gained increased popularity within the last decade, but few work examined the potentiality of surface photography to provide accurate, reliable habitat profiles in contrasted habitats. Photo transects were thus conducted by snorkeling in contrasted reef biotopes (reef flat, reef crest, sandy bottom) from the south-west lagoon of New Caledonia, to develop and test a “quick and clean” approach suitable for addressing monitoring as well as research-oriented programs. Pictures were taken by a snorkeler from the surface over twelve (20 × 1 m) reef crest/reef flat/soft bottoms transects using a standard 8 Mpixel digital-camera with underwater housing. Habitats were characterized from percent covers for 15 categories of local habitat variables related to sediment type and substrate coverage. Exhaustive area analyses using computer-assisted manual digitalizing were used to provide reliable habitat profiles from the digital pictures. Results were subsequently compared with surface estimates derived from random stratified point count techniques, for numbers of points comprised between 1 and 99 per m 2. Sampling-based randomization techniques allowed us to provide robust, reliable statistical estimates of accuracy and precision over 1000 randomized bootstrap replicates per transect. Results emphasized high accuracy and precision at transect scale whatever the reef biotopes considered, with maximum deviations from reference values of ∼ 1 percent cover in almost all cases and associated variances < 0.001. From a practical point of view, using a 9 points/m 2 ratio clearly provided reliable, quantitative descriptions of our reef transects (maximal errors < 1.5 percent cover with 95% confidence level). Cost-effectiveness is high, with 15-30 minutes/transect from field data collection (< 10 min) to computation of final percent covers (10-20 min). The method outlined in this paper thus combines high statistical efficiency and logistical ease, and could be used to address more functional perspectives.

Optical Algorithms at Satellite Wavelengths for Total Suspended Matter in Tropical Coastal Waters.

Is it possible to derive accurately Total Suspended Matter concentration or its proxy, turbidity, from remote sensing data in tropical coastal lagoon waters? To investigate this question, hyperspectral remote sensing reflectance, turbidity and chlorophyll pigment concentration were measured in three coral reef lagoons. The three sites enabled us to get data over very diverse environments: oligotrophic and sediment-poor waters in the southwest lagoon of New Caledonia, eutrophic waters in the Cienfuegos Bay (Cuba), and sediment-rich waters in the Laucala Bay (Fiji). In this paper, optical algorithms for turbidity are presented per site based on 113 stations in New Caledonia, 24 stations in Cuba and 56 stations in Fiji. Empirical algorithms are tested at satellite wavebands useful to coastal applications. Global algorithms are also derived for the merged data set (193 stations). The performances of global and local regression algorithms are compared. The best one-band algorithms on all the measurements are obtained at 681 nm using either a polynomial or a power model. The best two-band algorithms are obtained with R412/R620, R443/R670 and R510/R681. Two three-band algorithms based on Rrs620.Rrs681/Rrs412 and Rrs620.Rrs681/Rrs510 also give fair regression statistics. Finally, we propose a global algorithm based on one or three bands: turbidity is first calculated from Rrs681 and then, if < 1 FTU, it is recalculated using an algorithm based on Rrs620.Rrs681/Rrs412. On our data set, this algorithm is suitable for the 0.2-25 FTU turbidity range and for the three sites sampled (mean bias: 3.6 %, rms: 35%, mean quadratic error: 1.4 FTU). This shows that defining global empirical turbidity algorithms in tropical coastal waters is at reach.

Effect of solar ultraviolet radiation on bacterio- and phytoplankton activity in a large coral reef lagoon (southwest New Caledonia)

Measurements of underwater solar ultraviolet radiation (UV-R, 280 to 400 nm), particu- late (PPP) and dissolved (DPP) primary and bacterial (BP) production allowed us to infer the respec- tive contribution of UV-B (280 to 315 nm) and UV-A (315 to 400 nm) in algal-bacterial interactions in the southwest lagoon of New Caledonia. Differences in and between activities were assessed by in situ exposure of plankton to full sun radiation and shielded for UV-B or UV-R. We found a coherent response of phytoplankton and bacteria in total UV-R inhibition. The deepest limit of UV-R influence was 8 m, and inhibition increased with oligotrophy to reach 55% for phytoplankton and 75% for bac- teria, at 1 m depth. UV-B contributed up to 50% to the inhibition, especially near the surface and at the oligotrophic station. The UV-A effect on PPP was stronger than that of UV-B; however, bacteria were more sensitive to UV-B than were phytoplankton. When considering UV-A and UV-B sepa- rately, the inhibition response was linear without threshold. However, a threshold of 0.002 for PPP and 0.004 for BP was obtained when considering the ratio UV-B:UV-A. The rate of PPP normalized to chlorophyll concentration (PP chl ) is proposed to reconcile the large range of inhibition encountered in the literature. A significant linear relationship predicted a PPP inhibition of 60% in surface water with no significant effect on excretion for cells characterized by a low PP chl , whereas PPP inhibition should be limited for cells characterized by a PP chl higher than 18 mg C (mg chl) -1 h -1 , even if excre-

Phytoplankton-bacterioplankton coupling in a subtropical South Pacific coral reef lagoon

Tropical coastal ecosystems are often ignored in carbon cycling budgets even though it is now recognised that these ecosystems can be important in terms of coastal carbon cycling. We pre- sent results from a series of incubations conducted during a 10 d cruise in the southwest lagoon of New Caledonia, southwestern Pacific, aimed at examining phytoplankton-bacterioplankton cou- pling. We measured primary production, including dissolved (DPP) and particulate (PPP) primary production, and bacterial production (BP, 3 H-thymidine method) along a gradient. Regardless of inor- ganic nutrient concentration, we failed to see a tight coupling over the short term (4 h incubation) between net DPP and BP. A negative relationship between BP and the photosynthetic response to light was observed, leading to a decreased ΔBP:DPP ratio with increasing photon flux density. How- ever, when all the data were compiled, BP was correlated with DPP and total primary production. Assuming bacterial growth efficiencies from 1 to 10%, bacterial carbon demand (BCD) was higher than the supply of DPP in the most oligotrophic sites (up to 400% of DPP). This implies a weaker link- age between bacteria and phytoplankton than previously observed in other oligotrophic sites and requires external subsidies of dissolved organic carbon. In the coastal sites, characterised by higher inorganic and organic matter concentrations, DPP was estimated as being sufficient to support BCD, and bacteria and phytoplankton are considered to be strongly linked. These results were in contrast to previously published results for temperate and polar coastal regions and highlight the differences of these often understudied tropical systems.

Spatial predictability of juvenile fish species richness and abundance in a coral reef environment

Juvenile reef fish communities represent an essential component of coral reef ecosystems in the current focus of fish population dynamics and coral reef resilience. Juvenile fish survival depends on habitat characteristics and is, following settlement, the first determinant of the number of individuals within adult populations. The goal of this study was to provide methods for mapping juvenile fish species richness and abundance into spatial domains suitable for micro and meso-scale analysis and management decisions. Generalized Linear Models predicting juvenile fish species richness and abundance were developed according to spatial and temporal environmental variables measured from 10 m up to 10 km in the southwest lagoon of New Caledonia. The statistical model was further spatially generalized using a 1.5-m resolution, independently created, remotely sensed, habitat map. This procedure revealed that : (1) spatial factors at 10 to 100-m scale explained up to 71% of variability in juvenile species richness, (2) a small improvement (75%) was gained when a combination of environmental variables at different spatial and temporal scales was used and (3) the coupling of remotely sensed data, geographical information system tools and point-based ecological data showed that the highest species richness and abundance were predicted along a narrow margin overlapping the coral reef flat and adjacent seagrass beds. Spatially explicit models of species distribution may be relevant for the management of reef communities when strong relationships exist between faunistic and environmental variables and when models are built at appropriate scales.

Relevance of various formulations of phytoplankton chlorophyll a:carbon ratio in a 3D marine ecosystem model

Numerous experimental studies showed that the phytoplankton Chl a -to-Carbon ratio ( Chl a :C) is highly variable, whereas most of the marine ecosystem models use a constant ratio. In this work, we tested three different formulations for computing the modelled Chl a in a 3D coupled hydrodynamical–biogeochemical model of the Southwest lagoon of New Caledonia. The first formulation considers a constant Chl a :C ratio. In the second one, Chl a is a diagnostic variable related to the variable phytoplankton nitrogen-to-carbon ratio. In the last formulation, Chl a is a state variable of the model, which is dynamically simulated. Results showed important differences between the formulations, the first leading to overestimate the Chl a concentration in low nutrients conditions. Thus, this study strengthens the importance of the Chl a modelling in a coupled model in order to better estimate a crucial variable for validation of ecosystem models. To cite this article: V. Faure et al., C. R. Biologies 329 (2006).