Abstract
The Eastern Lagoon of New Caledonia (ELNC) is a semi-enclosed system surrounded by an extensive coral reef barrier. The system has been suffering impacts from climate variability and anthropogenic activities, including mining exploitation. Satellite monitoring is thus an essential tool to detect such changes. The present study aimed to assess the bio-optical variability of the ELNC and examine the applicability of ocean colour algorithms, using in situ bio-optical and radiometric data, collected during the March 2014 CALIOPE 2 cruise. The chlorophyll a concentration (Chla) varied from 0.13–0.72 mg·m−3, and the coastal stations were spectrally dominated by non-algal particles (NAP) and coloured dissolved organic matter (CDOM) (>80% of the total non-water absorption at 443 nm), due to the contribution of allochthonous sources. The phytoplankton specific absorption was generally lower (mean, 0.049 m2·mg Chla−1) than typical values observed for the corresponding Chla range, as well as the spectral slopes of the absorption of CDOM plus NAP (adg) (mean, 0.016 nm−1) and of the particle backscattering coefficient (bbp) (mean, 0.07 nm−1). The remote sensing reflectance obtained using two in-water approaches and modelled from Inherent Optical Properties (IOPs) showed less than 20% relative percent differences (RPD). Chla estimates were highly biased for the empirical (OC4 and OC3) and semi-analytical (GSM, QAA, GIOP, LMI) algorithms, especially at the coastal stations. Excluding these stations, the GSM01 yielded the best retrievals with 35–40% RPD. adg(443) was well retrieved by all algorithms with ~18% RPD, and bbp(443) with ~40% RPD. Turbidity algorithms also performed reasonably well (30% RPD), showing the capacity and usefulness of the derived products to monitor the water quality of the ELNC, provided accurate atmospheric correction of the satellite data. Regionally tuned algorithms may potentially improve the Chla retrievals, but better parameterization schemes that consider the spatiotemporal variability of the specific IOPs are still needed.
Highlights
Tropical lagoons and coral reefs are among the marine ecosystems with highest biodiversity that provide to man various services such as tourism, fishery and coastal protection
The biophysical parameters of the surface waters had a relatively small range of variability, with water temperature varying from 26.2–28.1 ◦C, salinity between 35.2–34.7 and turbidity between 0.12–1.09 Nephelometric Turbidity Units (NTU)
The East Lagoon of New Caledonia, a tropical semi-enclosed lagoon surrounded by a coral reef barrier, has oligo-mesotrophic waters containing a variety of optically active constituents, i.e., phytoplankton, coloured dissolved organic matter, and organic and inorganic non-algal particles
Summary
Tropical lagoons and coral reefs are among the marine ecosystems with highest biodiversity that provide to man various services such as tourism, fishery and coastal protection. These environments are highly vulnerable to natural and anthropogenic changes. The Eastern Lagoon of New Caledonia (ELNC) (Southwest Pacific Ocean) is a semi-enclosed tropical system, surrounded by an extensive coral reef barrier (~300 km), with high biodiversity, and declared as part of the UNESCO World Heritage List since 2008 [7]. Monitoring programs are essential to understand the processes affecting the water quality and coral reef ecosystem of the ELNC, to detect changes timelessly, and to improve protection programs
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