Modelling the impact of a La Niña event on a South West Pacific Lagoon

In view of increasing environmental awareness and biodiversity conservation, understanding the main forcing mechanism driving biogeochemical cycles in coral reefs and lagoon coastal areas is a priority. La Ni?a events cause unbalanced situations in the Equatorial Pacific and result in enhanced precipitation in South West Pacific coastal areas. We investigated the impact of heavy rainfalls during the 2008 La Ni?a event on the New Caledonia lagoon using a 3D coupled on-line hydrodynamic-biogeochemical model. Simulations and data showed that the whole lagoon was impacted by river inputs and stronger hydrodynamics, enhancing chlorophyll-a concentration by a factor between 1.7 and 1.9. The coupled model provided new insights into plume transport, highlighting that eastern plumes can be advected northwards or can reach the South West Lagoon, depending on the balance between regional, tide-induced, and wind-induced surface currents. It also provided a synoptic view of lagoon biogeochemical-hydrodynamic response, when remote sensing data are not available due to cloud coverage.     

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, Faure et al., 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.     

Developing erosion models for integrated coastal zone management: A case study of The New Caledonia west coast

The tropical climate and human pressures (mining industry, forest fires) cause significant sediment inputs into the New Caledonia lagoon and are a major cause of degradation of the fringing reefs. The erosion process is spatially characterized on the west coast of New Caledonia to assess potential sediment inputs in the marine area. This paper describes the methodologies that are used to map soil sensitivity to erosion using remote sensing and a geographic information system tool. A cognitive approach, multi-criteria evaluation model and Universal Soil Loss Equation are implemented. This article compares the relevance of each model in order to spatialize and quantify potential erosion at catchment basin scale. These types of studies provide valuable results for focusing on areas subject to erosion and serve as a decision-making tool for the minimization of lagoon vulnerability to the natural and human dynamics on the level of the catchment basins.     

Detection of ciguatoxin-like and paralysing toxins in Trichodesmium spp. from New Caledonia lagoon

Marine pelagic cyanobacteria Trichodesmium are widespread in the New Caledonia lagoon. Blooms of these Oscillatoriales are suspected to be a potential source of toxins in the ciguatera food chain and were previously reported to contain certain types of paralysing toxins. In the present study, toxicity experiments were conducted on lipid- and water-soluble extracts of freeze-dried samples of these cyanobacteria. Lipid-soluble fractions revealed a ciguatoxin-like activity in both in vivo (mouse bioassay) and in vitro (mouse neuroblastoma cells assay and receptor binding assay using tritiated brevetoxin-3) assays. The water-soluble fractions tested on mice exhibited neurotoxicity with paralytic symptoms. These toxicities have also been observed with benthic filamentous cyanobacteria within the Oscillatoriales order, also collected in New Caledonia. This study provides an unprecedented evidence of the toxicity of Trichodesmium species from the New Caledonia lagoon. This survey also demonstrates the possible role of these cyanobacteria in ciguatera fish poisoning.     

Funding in integrated coastal zone management partnerships

In countries where integrated coastal zone management (ICZM) is not enshrined in statute, a voluntary approach has often been taken. This review examines issues relating to funding in ICZM partnerships, particularly the local coastal fora (LCF) network in the UK. It highlights the lack of sustainable long-term funding mechanisms for organisations with the core aim of sustainable management of coastal resources. The relative difficulty of raising funds for core costs as opposed to projects is addressed, as is the lack of business/industry involvement. The 'hamster wheel syndrome' of fundraising is noted, where staff spend too much time trying to raise money, as opposed to developing ICZM. The resultant impact on the staff and the operation of the LCF is considered. Examples beyond the UK are considered, as are the problems associated with 'pump-prime' funding and implementation. Is the lack of formality in ICZM the crux of the funding problem?     

Coastal upwelling,circulation and heat balance around New Caledonia’s barrier reef

An outstanding characteristic of New Caledonia upwelling is that most events appear limited to the southern half of the western barrier reef. This north–south difference cannot be explained by alongshore variability of the projected wind stress and no strong evidence for alternative explanations has been proposed. A major objective of this paper is to provide the first dynamical analysis of New Caledonia upwelling and its regional environment, based on numerical simulations. Coastal upwelling around New Caledonia is shown to be modulated by a system of geostrophic currents interacting with the island mass. Upwelling velocities are weaker than expected from the two-dimensional Ekman theory, as Ekman divergence is balanced by “coastal geostrophic convergence”. The cooling effect of upwelling is also attenuated by alongshore transport of warm water by the Alis current, reminiscent of the Leeuwin current off Western Australia. Nevertheless, coastal upwelling can locally modify the large-scale surface water heat budget, dominated by meridional advection warming and surface cooling. The upwelled waters appear to be mostly of western origin and are transported below the surface by the Subtropical Counter Current before upwelling off New Caledonia. This appears in sharp contrast with the eastern barrier reef where the general warming by meridional advection of tropical surface waters is accentuated by the vigorous western boundary type Vauban current.     

Mapping groundwater discharge to a coastal lagoon using combined spatial airborne thermal imaging,radon (222Rn) and multiple physicochemical variables

Coastal lagoons are significant wetland environments found on coastlines throughout the world. Groundwater seepage may be a key component of lagoon water balances, though only a few studies have investigated large (>100 km²) coastal lagoons. In this study, we combined airborne thermal infrared imagery with continuous measurements of radon (²²²Rn—a natural groundwater tracer), conductivity, water temperature and dissolved oxygen to map groundwater seepage to a large coastal lagoon in New Zealand. We found evidence of seepage along the margins of the lagoon but not away from the margins. Our findings confirmed previously known seepage zones and identified new potential locations of groundwater inflow. Both point source and diffuse seepage occurred on the western and northwestern margins of the lagoon and parallel to the barrier between the lagoon and sea. These observations imply geologic controls on seepage. The combination of remote sensing and in-situ radon measurements allowed us to effectively map groundwater discharge areas across the entire lagoon. Combined, broad-scale qualitative methods built confidence in our interpretation of groundwater discharge locations in a large, dynamic coastal lagoon.     

Reactivity of transparent exopolymeric particles: A key parameter of trace metal cycling in the lagoon of Nouméa, New Caledonia

Seawater samples were collected in the lagoon of Nouméa (New Caledonia) along two transects from coastal bays to the oligotrophic barrier reef. Dissolved (<0.2 μm) and sub-micrometer (0.2–0.8 μm) concentrations of chromium, nickel and zinc were measured. Concentrations in the 0.2–0.8 μm size range were correlated with transparent exopolymeric particles (TEP) turnover rates, suggesting that a TEP pool rapidly cycling in the system is more reactive toward metals. The role of TEP reactivity in the transfer efficiency of metals from dissolved to particulate phases was estimated through variations of the metal partitioning coefficient K between sub-micrometer TEP and solution, as a function of the water mass residence time. The log K (6.0–8.0) increased from high to low residence time, suggesting that TEP reactivity increases when water masses renew rapidly. This suggests that hydrodynamics control TEP reactivity toward metals. The characteristics (adsorption vs. complexation) of TEP-metal association are discussed.     

Spatial and seasonal variability of sediment oxygen consumption and nutrient fluxes at the sediment water interface in a sub-tropical lagoon (New Caledonia)

In order to quantify the spatial and seasonal variations of sediment oxygen consumption and nutrient fluxes, we performed a spatial survey in the south west lagoon of New Caledonia during the two major seasons (dry and wet) based on a network of 11 sampling stations. Stations were selected along two barrier reef to land transects representing most types of sediments encountered in the lagoon. Fluxes were measured using ex-situ sediment incubations and compared to sediment characteristics. Sediment oxygen consumption (SOC) varied between 500 and 2000 μmol m⁻² h⁻¹, depending on season and stations. Nutrient effluxes from sediment were highly variable with highest fluxes measured in muddy sediments near the coast. Inter-sample variability was as high as seasonal differences so that no seasonally driven temperature effect could be observed on benthic nutrient fluxes in our temperature range. Nutrient fluxes, generally directed from the sediment to the water column, varied between −5.0 and 70.0 μmol m⁻² h⁻¹ for ammonia and between −2.5 and +12.5 μmol m⁻² h⁻¹ for PO₄ and NO₂₊₃. SOC and nutrient fluxes were compared to pelagic primary production rates in order to highlight the tight coupling existing between the benthic and pelagic compartments in this shallow tropical lagoon. Under specific occasions of low pelagic productivity, oxygen sediment consumption and related carbon and nutrient fluxes could balance nearly all net primary production in the lagoon. These biogeochemical estimates point to the functional importance of sediment biogeochemistry in the lagoon of New Caledonia.     

Calculations of hydrodynamic time parameters in a semi-opened coastal zone using a 3D hydrodynamic model

Hydrodynamic time parameters (HTs) in a semi-opened aquatic ecosystem are synthetic indicators offering the opportunity to bring out the links between its physical functioning and its biology. The generic term “residence time” is frequently used through literature to mention HTs resulting in various calculation methods. This article presents different computing methods relying on the use of a 3D numerical hydrodynamic model and the HTs to which they give access. Several large-scale (water exchange time, average water export time, e-flushing time) and local time parameters (export time, flushing lag, local e-flushing time) are defined. The applications presented are carried out within the south-west lagoon of New Caledonia (SLNC), on three embedded control volumes. The definition of the control volume is more important for the values of local HTs than for their comparative distribution. The comparison of the global hydrodynamic time scales applied to a control volume provides information on the mixing processes inside the control volume.     

A combined modelling and geochemical study of the fate of terrigenous inputs from mixed natural and mining sources in a coral reef lagoon (New Caledonia)

Open-cast mining for Ni, Cr and Co was conducted in the south-west part of New Caledonia during the 20th century. Abandoned mining and prospecting sites were severely affected by erosion, resulting in an increase in the load of terrigenous particles transported to the coral reef lagoon. This article assesses the impact of a typical small catchment area (La Coulée River, 85 km2 watershed) on two bays (Boulari and Sainte Marie) located near Noumea, New Caledonia's main city. This multi-disciplinary study combines geochemical, sedimentological, and hydrodynamic approaches. Ni and Cr concentrations contained in the geochemical matrix phase of the pelitic fraction were determined. The study of the geochemical signatures together with sedimentological data and 3D numerical simulations of the transport of non-settling particles throughout the lagoon demonstrated that terrigenous inputs from the Coulée River were mainly transported and deposited along the shoreline, reaching areas as distant as Sainte Marie Bay. Although quantitatively low (about 3% of the pelite mass of the bay sediments), the terrigenous inputs in Sainte Marie Bay originating from the Coulée River could be traced. The metal content in suspended matter was over 7000 mg kg(-1) for Ni and 4200 mg kg(-1) for Cr.     

Integrating Geotechnical and Interferometric SAR Measurements for Secondary Compressibility Characterization of Coastal Soils

Persistent scatterer interferometry (PSI) provides a new perspective to monitor the movements of coastal structures due to long-term consolidation using satellite-borne remote sensors. The method has the advantages of detecting the displacements at a very high spatial (from 1 to a few meters) and temporal (from 10 to 30?days) resolution. Cost-effective monitoring of complex and large (some kilometer long) structures can be done over long time (up to 10?years) intervals and at large scales (tens times tens km²) of investigation. Here, these measurements are integrated with geotechnical, site-specific measurements to characterize in a unique framework the long-term compressibility of coastal soils over large areas. The approach is tested on the 60-km-long coastland of the Venice Lagoon, Italy. An accurate quantification of the movements of coastal infrastructures at the Venice coastland is carried out by PSI using ENVISAT ASAR and TerraSAR-X images acquired from April 2003 to December 2009 and from March 2008 to January 2009, respectively. Several nearshore and offshore structures were constructed over the decades to protect Venice and its coastal environment from sea storms and high tides. Long jetties were built at the lagoon inlets since the end of the 18th century, significantly reinforced between 1994 and 1997, and finally reshaped since 2003 in the framework of the MOSE construction (i.e., the project of mobile barriers for the temporarily closure of the lagoon to the sea). The measured displacements range from a few mm/year for the structures older than 10?years to 50–70?mm/year for those realized a few years ago. The PSI measurements are combined with the outcome of a detailed geomechanical characterization of the lagoon subsoil obtained by a field-scale experiment started at the end of 2002 and monitored to 2008. The use of the stress-strain properties derived from the trial embankment and the actual lithostratigraphy below the coastal structures, which is available from several piezocone profiles and boreholes, allows for the computation of secondary compression (consolidation) rates that match very well the PSI-derived movements. The results provide important information on the potential of using PSI to characterizing geotechnical properties (magnitude and distribution) of coastal deposits, as well as to estimate the expected time-dependent geomechanical response of coastal structures or other large constructions.     

Population and trophic dynamics of Trichodesmium thiebautii in the SE lagoon of New Caledonia. Comparison with T. erythraeum in the SW lagoon

While Trichodesmium erythraeum is prevalent in the semi-closed SW lagoon of New Caledonia, T. thiebautii is dominant in the more open SE lagoon. This led to the comparison of the two species from the results of two high-frequency surveys focused on T. erythraeum (Rodier and Le Borgne, 2008) and T. thiebautii (the present paper). (1) Environmental conditions and triggers of the blooms are the same: calm weather and/or temperature >26 °C, and temporary nutrient inputs are required for both; (2) growth rates under favorable conditions are similar (0.14–0.27 d⁻¹) but (3) T. thiebautii has lower net ascent rates along the water column during blooming events, due to distinct buoyancy capacities and the resulting vertical distributions; (4) carbon and dinitrogen fixation rates are not significantly different and contribution of Trichodesmium spp. represent less than 35% and 5% of the total phytoplankton carbon and nitrogen requirements, respectively. It is concluded the two species can be mixed in ecological studies, except for their vertical distributions during the bloom periods.     

Inter-annual variability of wet season freshwater plume extent into the Great Barrier Reef lagoon based on satellite coastal ocean colour observations

Riverine freshwater plumes are the major transport mechanism for nutrients, sediments and pollutants into the Great Barrier Reef (GBR) lagoon and connect the land with the receiving coastal and marine waters. Knowledge of the variability of the freshwater extent into the GBR lagoon is relevant for marine park management to develop strategies for improving ecosystem health and risk assessments. In this study, freshwater extent has been estimated for the entire GBR lagoon area from daily satellite observations of the Moderate Resolution Imaging Spectroradiometer (MODIS) between 2002 and 2010. To enable a reliable mapping of freshwater plumes we applied a physics-based coastal ocean colour algorithm, that simultaneously retrieves chlorophyll-a, non-algal particulate matter and coloured dissolved organic matter (CDOM), from which we used CDOM as a surrogate for salinity (S) for mapping the freshwater extent.     

The role of benthic macrophytes and their associated macroinvertebrate community in coastal lagoon resistance to eutrophication

Eutrophication is widely recognised as one of the major menaces to coastal environments, particularly enclosed bays and lagoons. Although there is a general understanding of the consequences of eutrophication in these systems, there is a lack of sufficient knowledge concerning biotic feedbacks that influence eutrophication patterns and the resistance capacity of coastal environments.In this paper, the isotope ratios of main producers and consumers of a Mediterranean lagoon were examined in order to elucidate the fate of anthropogenic inputs from the main watercourse flowing into the lagoon.The results of the study of stable isotope data in the Mar Menor lagoon reflected that the whole benthic community plays an important role as a natural ‘filter’ that removes excess nutrients from the water column and stores them in the sediments, thereby enhancing lagoon resistance to eutrophication.     

A 3D hydrodynamic fate and transport model for herbicides in Sacca di Goro coastal lagoon (Northern Adriatic)

Sacca di Goro is a shallow coastal microtidal lagoon with a surface area of 26 km2, and an average depth of about 1.5m. Fresh water pollutant loads from Po River branches and several drainage canals lead to anthropogenic eutrophication, frequent summer anoxia crises and chemical contamination. Such events not only affect the lagoon ecosystem but also cause serious economic losses, the lagoon being the second largest producer of clams in Italy. The present work aims at using a fate model coupled with COHERENS 3D hydrodynamic model to simulate and to explain the spatial distribution and temporal variations of s-triazines herbicides in the Sacca di Goro lagoon. The simulation results of spatial and temporal dynamic behaviour of atrazine, simazine and terbuthylazine have been compared with experimental data obtained during an annual monitoring programme.     

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 35.1 mL m⁻³ and 3.5 × 10⁵ individuals m⁻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.     

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 (>1 day) 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 3–4 year 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.     

Benthic community response to habitat variation: A case of study from a natural protected area,the Celestun coastal lagoon

Little information currently exists on spatial and temporal benthic community variations in tropical coastal lagoons. Here, the benthic community response to habitat variation in the Celestun coastal lagoon, northwest Yucatan peninsula, was seasonally examined during the 1994–1995 climatic cycle into a grid of 12 sampling sites distributed along the salinity gradient of the lagoon. Habitat variation was assessed through physical factors associated both to the water column (e.g. salinity) and the bottom sediment (e.g. sand, silt and clay fractions). The benthic community response was assessed through species diversity measures and abundance. Under the influence of climatic seasonality, variations in habitat conditions followed by changes in the benthic community characteristics were expected. Results from two-way ANOVAs showed that for the period of study, Celestun lagoon was more heterogeneous along the spatial axis of variability than along the temporal one. Multiple regression analysis showed that salinity was spatially the main factor influencing the benthic community characteristics. Temporally, the sediment characteristics were observed to exert significant effects on the species diversity characteristics but not on abundance. Other variables assessed (dissolved oxygen, pH, temperature and water column transparency) exhibited no significant covariance with species diversity and abundance. Since generated from historical data, these results have the potential to be useful as a benchmark to the establishment of monitoring programs in the light of the increasing anthropogenic pressure on the natural resources of the lagoon and surrounding coastal area.