Bio-Optical Characterization and Ocean Colour Inversion in the Eastern Lagoon of New Caledonia, South Tropical Pacific

Abstract

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 m 2 ·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 (a dg ) (mean, 0.016 nm −1 ) and of the particle backscattering coefficient (b bp ) (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. a dg (443) was well retrieved by all algorithms with~18% RPD, and b bp (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.

Impact of Contrasted Weather Conditions on CDOM Absorption/Fluorescence and Biogeochemistry in the Eastern Lagoon of New Caledonia

“…For all cruises, ocean color from reflectance measurements exhibited a classical increasing gradient from Bay to offshore waters in the Eastern lagoon at both wavelengths. This corroborates the finding of a higher transparency of waters offshore, as described for CAL2 in Favareto et al (2018) (Supplementary Figure S2). R rs (390) was lower in March 2016 (CAL3) in relation with high CDOM absorption in the UV domain, and R rs (440) was lower in March 2014 (CAL2) in relation with particulate material, living and non-living (absorption in the blue).…”

“…Neither wind, nor rain disturbed the general decreasing patterns of CDOM or particulate absorption from bays to offshore. Surprisingly, no drastic changes in any component of absorption were observed after rain compared with after wind (Favareto et al, 2018). At the opposite, backscattering of particles was enhanced similarly after rain as after wind (Favareto et al, 2018).…”

“…Surprisingly, no drastic changes in any component of absorption were observed after rain compared with after wind (Favareto et al, 2018). At the opposite, backscattering of particles was enhanced similarly after rain as after wind (Favareto et al, 2018). CDOM slopes (0.0143-0.0246 nm −1 ) were slightly higher than during the mesocosm experiment VAHINE (0.015-0.019 nm −1 ; Tedetti et al, 2016) which could reflect a higher influence of photobleaching.…”

“…Hyperspectral remote sensing reflectance [R rs (λ)] spectra were derived from measurements by a TriOS system mounted in a surface frame of upwelling radiance, L u (0 − ,λ) (FOV = 7 • ), and downwelling irradiance, E d (0 + ,λ), at λ = 320 to 950 nm with λ ∼10 nm (sampled by every 3.3 nm). Above surface R rs was calculated as described in Dupouy et al (2010); Murakami and Dupouy (2013), and Favareto et al (2018).…”

“…In order to identify the main factors driving CDOM composition and biogeochemistry in the Eastern Lagoon of New Caledonia (ELNC), three CALedonian Inherent Optical Properties of the East coast (CALIOPE) cruises were conducted during the last decade, allowing one to perform measurements under a wide range of environmental conditions. Tropical storms with strong rain or wind events can negatively alter the lagoons balance by massive inflow of turbid fresh water that may extend even further offshore, beyond the lagoon (Dupouy et al, 2014(Dupouy et al, , 2018Favareto et al, 2018;Martias et al, 2018).…”