Shortly after strong rains or tropical storms, chlorophyll-a concentration (Chla), turbidity, and colored dissolved organic matter (CDOM) in the lagoon of New Caledonia may be tripled. This effect is visible as far as 50 km offshore. The Eastern Coast of New Caledonia is more impacted than the Western Coast because of its geomorphological structure. Higher frequency and intensity of the rains, and the dominance of highly erosive lateritic formations resulted from deep weathering of ultramafic rocks. At the deeper parts of the lagoon (> 20 m), increases of Chla, turbidity, and CDOM absorption can be detected by satellite imagery with their plumes extending off shore as a result of wind-driven circulation in the lagoon. Such increases agree with the oceanographic measurements of Chla, absorption, backscattering, turbidity and CDOM during extensive cruises. Satellite and in situ measurements allow tracing of particulates and dissolved matter in lagoon waters and up to the coral barrier reefs. The largest particles settle at a short distance from the coast influencing the sediments not far from the river outflows. Fine particles (< 10 μm), colloids and/or CDOM are exported to reefs where they can be beneficial through protection from high UV illumination (colloids) and feeding of the corals (CDOM). Phytoplankton and associated detritus, when not dominated by toxic filamentous cyanobacteria, can also feed coral reefs. Finally, the export of fine particles associated with colloids and/or CDOM can also contribute to the distribution of trace metals to the barrier reefs of the East Coast and offshore. The question whether this input of trace metals has an overall positive or negative effect on the coral reefs is still under debate.
Ocean color of tropical lagoons is dependent on bathymetry and bottom type, as well as input of coastal living and
mineral particles and chromophoric dissolved organic matter (CDOM). The New Caledonia lagoon lies in the
Southwestern Tropical Pacific around 21° 30’S and 166° 30’E, with a great marine biodiversity in UNESCO Heritage
coral reefs, benthic sea grass, and benthic communities. They are largely connected to the open ocean in the southern and
eastern parts, but only by narrow passes in the southwest part. The trophic state is linked to spatial variations in flushing
times. High run offs due to rain carrying abundant chromophoric dissolved organic matter (CDOM) and particle loads
may greatly impact the functioning of ecosystems while rivers and sewage effluents may induce localized impacts. Two
oceanographic cruises (CALIOPE 1 in 2011 and CALIOPE 2 in 2014) were carried out off the Eastern Coast of New
Caledonia during a calm dry period and during high winds, respectively. Multi- and hyper-spectral marine reflectance
was measured with a SIMBADA instrument and a TRIOS radiometer system, together with inherent optical properties
(total and CDOM absorption coefficients with a PSICAM, in situ absorption and scattering with an AC9, backscattering
with a Hydroscat-6). Fluorescence of CDOM (EEM/PARAFAC) was measured on collected 0.2 μm filtered samples. In
2014, Satlantic and FieldSpec hyper-spectral radiometers were available for in-water profiling of upwelling radiance and
downwelling irradiance and above-water reflectance measurements, respectively. Inherent and apparent optical data from
the two cruises are compared and used to estimate ocean color algorithms performance and evaluate a Linear Matrix
Inversion method, providing tools for remote sensing on this highly under-sampled coastal region of New Caledonia.
We propose a statistical algorithm to assess chlorophyll-a concentration ([chl-a]) using remote sensing reflectance (Rrs)
derived from MODerate Resolution Imaging Spectroradiometer (MODIS) data. This algorithm is a combination of two
models: one for low [chl-a] (oligotrophic waters) and one for high [chl-a]. A satellite pixel is classified as low or high [chla]
according to the Rrs ratio (488 and 555 nm channels). If a pixel is considered as a low [chl-a] pixel, a log-linear model
is applied; otherwise, a more sophisticated model (Support Vector Machine) is applied. The log-linear model was
developed thanks to supervised learning on Rrs and [chl-a] data from SeaBASS and more than 15 campaigns accomplished
from 2002 to 2010 around New Caledonia. Several models to assess high [chl-a] were also tested with statistical methods.
This novel approach outperforms the standard reflectance ratio approach. Compared with algorithms such as the current
NASA OC3, Root Mean Square Error is 30% lower in New Caledonian waters.
In recent years great progress has been made in global mapping of phytoplankton from space. Two main trends have
emerged, the recognition of phytoplankton functional types (PFT) based on reflectance normalized to chlorophyll-a
concentration, and the recognition of phytoplankton size class (PSC) based on the relationship between cell size and
chlorophyll-a concentration. However, PFTs and PSCs are not decorrelated, and one approach can complement the other
in a recognition task. In this paper, we explore the recognition of several dominant PFTs by combining reflectance
anomalies, chlorophyll-a concentration and other environmental parameters, such as sea surface temperature and wind
speed. Remote sensing pixels are labeled thanks to coincident in-situ pigment data from GeP&CO, NOMAD and
MAREDAT datasets, covering various oceanographic environments. The recognition is made with a supervised Support
Vector Machine classifier trained on the labeled pixels. This algorithm enables a non-linear separation of the classes in the
input space and is especially adapted for small training datasets as available here. Moreover, it provides a class probability
estimate, allowing one to enhance the robustness of the classification results through the choice of a minimum probability
threshold. A greedy feature selection associated to a 10-fold cross-validation procedure is applied to select the most
discriminative input features and evaluate the classification performance. The best classifiers are finally applied on daily
remote sensing datasets (SeaWIFS, MODISA) and the resulting dominant PFT maps are compared with other studies.
Several conclusions are drawn: (1) the feature selection highlights the weight of temperature, chlorophyll-a and wind speed
variables in phytoplankton recognition; (2) the classifiers show good results and dominant PFT maps in agreement with
phytoplankton distribution knowledge; (3) classification on MODISA data seems to perform better than on SeaWIFS data,
(4) the probability threshold screens correctly the areas of smallest confidence such as the interclass regions.
Hyperspectral remote sensing reflectance (Rrs) was measured by a TriOS radiometer system along the East Coast of
New Caledonia during the R/V Alis 03-13 October 2011 CALIOPE cruise. The TriOS system consists of radiance and
irradiance sensors measuring in the spectral range 320-950 nm, at a spectral resolution of about 10 nm (sampled by every
3.3 nm), and within a 7-degree field-of-view for the radiance sensor. The method developed by Froidefond and Ouillon
(2005) was used to determine Rrs, i.e., the radiance sensor was mounted on a small raft to measure upwelling radiance
just below the surface, and Rrs was calculated by normalizing water-leaving radiance with downward solar irradiance
measured on the ship deck. Inherent Optical Properties (IOPs), i.e., absorption coefficients of phytoplankton and
detritus+dissolved substances (aph and adg, respectively), and particulate backscattering coefficient (bbp) were estimated
from the hyperspectral Rrs data by applying linear matrix inversion (Hoge and Lyon, 1996). The IOP inversion
algorithm was adapted to MODIS data and applied to Level 1b imagery at 500 m resolution to demonstrate the feasibility
of regular IOP monitoring from space in the study area. Local characteristics of the IOP spectra were used for the
candidate spectra in the algorithm. The estimated MODIS Rrs and IOPs were evaluated using TriOS Rrs and in-situ IOP
measurements obtained concomitantly during the cruise.
Coastal ocean-color estimation needs to retrieve not only molecular and aerosol scattering (ρa), but also high spatial
resolution sea-surface reflectance (ρa) because ρg has fine temporal and spatial scales due to variable winds and air-sea
stability caused by the coastal geographical structure. Murakami and Frouin 2008 showed a possibility of ρg correction
by using near infrared (NIR) and shortwave infrared (SWIR) channels of MODIS 500m observations. This study
investigated the correction of the atmospheric and sea-surface reflectance on the southwest of New Caledonia lagoon
using AVNIR-2 which has 10-m resolution but doesn't have SWIR. After corrections of gas absorption and molecule
scattering, we estimated ρa+ρg and water-leaving reflectance iteratively through IOPs retrieved from visible bands.
Spectral slope of ρa+ρg
was assumed uniform within our small target area (60km×40km). We tested sensitivity to several
possible IOP spectra (total absorption of particle and dissolved matter and back-scattering coefficients) with comparison
to in-situ IOP measurements. The AVNIR-2 derived remote sensing reflectance agreed well to the MODIS one (rootmean
square difference / average of Rrs 443nm was 43%), and AVNIR-2 IOPs agreed well to in-situ IOP measurements
(correlation coefficients more than 0.9) when we used the IOP spectra modeled by in-situ measurements around the New
Caledonia. Chlorophyll-a (Chla) calculated by the AVNIR-2 IOPs showed better agreement to in-situ Chla in the lagoon
areas where traditional blue/green algorithms overestimated.
The major part of the New Caledonia (NC) lagoon was classified as UNESCO Natural Site of Humanity Patrimony. Indeed, 22 175 km2 of tropical coral lagoon area exhibit high biodiversity. The NC lagoon is semi enclosed and connected to the Coral Sea through a barrier reef segmented by narrow passes. The environment is oligotrophic, due to important flush during trade winds events, and bathymetry is highly variable.
In order to predict eutrophication events, we used an extension of a 3D coupled physical-biogeochemical model recently developed on NC south western lagoon. The model is based on the Nitrogen and Carbon cycles, relating the variable stoechiometry of the elements in each biological compartment. The ecological model was developed to include an explicit description of the microbial loop. The resulting coupled model, forced by tide, wind, light, temperature and freshwater inputs, was used to calculate phytoplankton biomass, bacterial production, dissolved organic matter concentrations and nutrient recycling.
Here we present results issued from the 3D coupled model ECO3M_LAGOON (biogeochemical, LOPB-IRD) and MARS3D (regional physical model, IFREMER-IRD) describing spatial and temporal interactions between water motion and biology, on larger domain including reef barrier and water exchanges through ocean-lagoon interface.
To validate physical processes in the lagoon we used in situ data collected during field cruise (ValHyBio 2008, La Niña episode). Surface chlorophyll concentrations are compared with water color data from ValHyBio cruise and satellite data (MODIS/MERIS) corrected from bathymetry effects.
We present data collected as part of ValHyBio- VALidation HYperspectral of a BIOgeochemical model in
the South Western Tropical Lagoon of New Caledonia, a PNTS-sponsored program dedicated to chlorophyll satellite
imaging and validation as affected by bathymetry. The specific goals of ValHyBio are to: - examine time-dependent
oceanic reflectance in relation to dynamic surface processes, - construct field/satellite reflectance-based chlorophyll
models, - investigate the feasibility of inverting the model to yield surface chlorophyll and turbidity, - validate the
biogeochemical model with field/satellite observations. In situ bio-optical parameters include absorption coefficients
by CDOM and particles, Secchi disk depth, backscattering coefficient, pigment concentration, suspended matter
concentration, and K_dPAR. They are measured every month at 5 stations, of contrasted bathymetry and bottom
reflectance, as well as at a reference station situated 4 miles offshore, and on a station over coral reefs. Remote sensing
reflectance is calculated from the absorption and backscattering coefficients and compared with satellite data.
SeaWIFS and MODIS AQUA match-ups collected over the period 1997-2010 (ValHySat-VALidation HYperspectral
SATellite database) are used. Satellite retrievals are examined as a function of bathymetry. The feasibility of a longterm
monitoring program of optical water retrieval with satellite remote sensing technique is examined in the frame of
the GOPS (South Pacific Integrated Observatory).
No map of the sea floor is available yet on the whole lagoon of New Caledonia. We tried to validate a method
to map it with MeRIS images on the south western part of the lagoon. The non-linear effect of water column
light attenuation can then be corrected to obtain the absolute reflectance of the seabed. Light attenuation by
the water column can be determined by comparing the radiance of standard features on the seabed at different
depth. Bathymetry can also be determined by measuring the relative reflectance of the seabed in green and
red light spectral bands. Once the effect of attenuation has been removed, a supervised classification can be
applied in order to obtain the location of each item on the sea floor. Validations are operated with ground
measurements of depth, spectral profiles and some available maps.
Inherent optical properties (IOPs) and remote sensing reflectance were measured in the southern part of the lagoon of New
Caledonia during the VALHYBIO cruise (March-April 2008). The goal was to validate satellite chlorophyll data from
MODIS and MERIS and to validate simulations of surface chlorophyll by a biogeochemical model. Physical parameters were
collected from a Seabird CTD. Particulate and detritus absorption were measured with the filter pad technique.
Backscattering was measured with a Hydroscat-6. Mapping of IOPs and Rrs were done for the whole southern lagoon area
and compared with pigment maps. The cruise provided a description of the IOPs in different water types including bays, open
ocean waters, mid-shelf lagoon, and above reefs. With respect to climatology, the heavy rainfall episode of March-April 2008
resulted in a large increase in chlorophyll-a concentration (by a factor of 3) attributed to increased nutrient availability from
land drainage. Low backscattering ratios characterized the chlorophyll-rich plumes associated with the nutrient increase. The
data are useful for the development of a specific algorithm for chlorophyll concentration retrieval by satellite in all
oligotrophic lagoons during dry and wet seasons.
A workshop on fisheries was held in Noumea on November 21, 2008 to address remote-sensing applications to fisheries adapted to the particular needs and problems of Western and Central Pacific Island countries. During the workshop, presentations and discussions covered various topics related to remote sensing of coastal and open ocean waters and its applications to fisheries. Participants were introduced to remote sensing of ocean colour and its significance vis-à-vis the marine food web. Applications to fisheries included improvements of fisheries operations to increase efficiency of fishing effort, assessment of fish stocks health, growth and recruitment, and ecosystem dynamics. A project on the Societal Applications in Fisheries & Aquaculture using Remote Sensing Imagery (SAFARI) and a global Network for marine ecosystem management (ChloroGIN) were also presented. The particular issues arising in the use of remote sensing for fisheries in the tropical island regimes were reviewed and recommendations on the use of remote sensing in the context of fisheries were presented.
Trichodesmium, a major colonial cyanobacterial nitrogen fixer, forms large blooms in tropical oligotrophic oceans and
enhances CO2 sequestration by the ocean due to its ability to fix dissolved dinitrogen, however, its detection by
satellite has not yet been successful in the South Western Tropical Pacific. Here, an algorithm has been developed for
discriminating radiance anomalies observed in SeaWiFS imagery over the summertime Tropical Pacific. Its validation
used 70 in situ observations of Trichodesmium accumulations for the period 1997-2004. The fraction of pixels
identified as Trichodesmium in the region 5°S-25°S
160°E-190°E is low (between 0.1 and 0.5%) but is about 100 times
higher than previous algorithms indicate. The algorithm reproduces the observed seasonal and inter-annual variability
of Trichodesmium blooms in the SWTP.
Tropical oligotrophic coral reef lagoons are areas of high biodiversity. Chlorophyll concentration, a proxy for
phytoplankton biomass and primary production, is useful to monitor the carbon balance in the context of the climate
change and to validate simulations by coupled biogeochemical models. Chlorophyll monitoring by Aqua/MODIS is
examined on the large tropical oligo- to mesotrophic lagoon of New Caledonia (23,900 km2). The classical OC3
algorithm developed for MODIS can only be applied in deep waters. In shallow water, when the water is clear with a
weak attenuation, the bottom reflectance influences the surface reflectance and then induces an error in the chlorophyll
determination. Here, a new OC3-type polynom, relating satellite reflectance ratios and chlorophyll, was determined from
bio-optical data collected during a cruise (Valhybio) on the R/V Alis in the frame of the Programme National de
Télédétection Spatiale. From the 22th of March to the 9th of April, data were collected during two surveys of the same
network. A total of 170 in situ bio-optical measurements in the South Western and South lagoons of New Caledonia
were obtained, within a 2 weeks interval (70 non-cloudy match-ups). Four Modis images were acquired during this
cruise with moderate to good atmospheric conditions. The new polynom gives a RMS of 14.8% and a MNB of - 9% and
gives a better representation of the "true" water column chlorophyll concentration of the New Caledonia lagoon.
Bio-optical spectral properties were determined on fresh suspensions of Trichodesmium spp. collected in a tropical lagoon and put in seawater tanks (total chlorophyll concentrations range between 0.1 and 3.8 mg m-3). The spectrum of the backscattering coefficient was a hyperbolic function with a slope of 1.2, often showing troughs at 440, 550 and 676 nm, due to absorption peaks of chlorophyll and phycoerythrin. The absorption spectrum computed with a specific beta correction for Trichodesmium, showed a blue to red ratio (B/R) equivalent to the one of a single colony (B/R=2), and also showed the double peak of mycosporine-like amino acids (MAA's, 330 and 360 nm). The CDOM absorption spectrum showed minor MAA peaks when cyanobacterial concentrations were above 1 mg Chl a m-3. The chlorophyll a-specific backscattering and absorption coefficients at 442 nm were respectively 0.0126 m2 (mg.chl a)-1 and 0.027 m2 (mg chl a)-1. Suspensions in tanks exhibited a high backscattering ratio at 660 nm (bb=bbp/bp). The above-water reflectance spectrum clearly showed troughs at the wavelength of the pigment absorption peaks. Datasets of Trichodesmium normalized absorption, backscattering and reflectance spectra will allow its detection with future hyperspectral ocean colour sensors.
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