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

Light availability regulated by particulate organic matter affects coral assemblages on a turbid fringing reef

Recently, increasing evidence suggests that reef-building corals exposed to elevated suspended solids (SS) are largely structured by changes in underwater light availability (ULA). However, there are few direct and quantitative observations in situ support for this hypothesis; in particular, the contribution of SS to the diffuse attenuation coefficient of the photosynthetically active radiation (K_d-PAR) variations is not yet fully understood. Here, we investigated the variations in ULA, the structure of coral assemblages, and the concentration and composition of SS on the Luhuitou fringing reef, Sanya, China. Light attenuation was rapid (K_d-PAR: 0.60 ± 0.39 m^-1) resulting in a shallow euphotic depth (Z_eu-PAR) (<11 m). Benthic PAR showed significant positive correlations with branching and corymbose corals (e.g. Acropora spp.), while massive and encrusting species (e.g. Porites spp.) dominated the coral communities and showed no significant correlations with PAR. These results indicate that the depth range available for coral growth is shallow and the tolerance to low-light stress differs among coral species. Notably, K_d-PAR showed no significant correlations with the grain size fractions of SS, whereas significant positive correlations were found with its organic fraction content, demonstrating that the light attenuation of SS is mainly regulated by particulate organic matter (POM). Intriguingly, our isotopic evidence revealed that POM concentration contributed the most to changes in K_d-PAR, with its source being slightly less important. Combined, our results highlight ULA regulated by POM is an important factor in contributing to changes in coral assemblages on inshore turbid reefs, and reducing the input of terrestrial materials, especially POM, is an effective measure to alleviate the low-light stress on sensitive coral species.

Evaluating semi-analytical algorithms for estimating inherent optical properties in the South China Sea

Using large amounts of bio-optical data collected in the South China Sea (SCS) from 2003 to 2016, this study checks the consistency between well-known semi-analytical algorithms (SAAs)-the quasi-analytical algorithm (QAA) and the default generalized inherent optical property (GIOP-DC)-in retrieving the non-water absorption coefficient (a_nw(λ)), phytoplankton absorption coefficient (a_ph(λ)) and particulate backscattering coefficient (b_bp(λ)) from remote-sensing reflectance (R_rs(λ)) data at 412, 443, 490, 531, and 555 nm. The samples from the SCS are further separated into oligotrophic and mesotrophic water types for the comparison of the SAAs. Several findings are made: First, the values of an_w(λ) derived from the two SAAs deliver similar performance, with R² values ranging from 0.74 to 0.85 and 0.74 to 0.87, implying absolute percent error differences (APDs) from 37.93% to 74.88% and from 32.32% to 71.75% for the QAA and GIOP-DC, respectively. The QAA shows a value of R² between 0.64 and 0.91 and APDs between 43.57% to 83.53%, while the GIOP-DC yields R² between 0.76 to 0.89 and APDs between 44.65% to 79.46% when estimating a_ph(λ). The values of b_bp(λ) derived from the QAA are closer to the in-situ b_bp(λ) values, as indicated by the low values of the normalized centered root-mean-square deviation and normalized standard deviation, which are close to one. Second, a regionally tuned estimation of a_ph(λ) is proposed and recommended for the SCS. This consistency check of inherent optical properties products from SAAs can serve as reference for algorithm selection for further applications, including primary production, carbon, and biogeochemical models of the SCS, and can provide guidance for improving a_ph(λ) estimation.