The Eastern Lagoon of New Caledonia (ELNC) is a semi-enclosed system surrounded by 21 an extensive coral reef barrier. The system has been suffering impacts from climate variability and 22 anthropogenic activities, including mining exploitation, and satellite monitoring is essential to 23 detect such changes. The present study aimed to assess the bio-optical variability of the ELNC 24 during the March 2014 CALIOPE 2 cruise, and examine the applicability of ocean colour 25 algorithms. The ELNC waters were characterized as oligo-mesotrophic, with spectral dominance of 26 non-algal particles (NAP) and colored dissolved organic matter (CDOM) at the coastal stations, 27 and generally low specific optical properties. In situ radiometry, obtained using two in-water 28 approaches and compared to forward modelled remote sensing reflectance (Rrs), showed less than 29 ~20% Relative Percent Differences (RPD). Chlorophyll-a concentration estimates were highly 30 biased for both the empirical, OC4 and OC3, and semi-analytical algorithms at the coastal stations, 31 with NAP and CDOM spectral dominance. Excluding these stations, the GSM01 yielded the best 32 retrievals with 35-40% RPD. NAP plus CDOM absorption was well retrieved by all algorithms with 33 ~18% RPD, and the marine particle backscattering coefficient at 443 nm with ~40% RPD. The linear 34 matrix inversion regional algorithm has the potential to improve IOP estimates, but better 35 parameterization schemes that consider the spatiotemporal variability of the specific IOPs are still 36 needed. Water turbidity algorithms also performed reasonably well (30% RPD), making the use of 37 ocean colour satellites promising for monitoring this highly vulnerable system. 38