Water Column Optical Properties of Pacific Coral Reefs Across Geomorphic Zones and in Comparison to Offshore Waters

Assessing Derawan Island's Coral Reefs over Two Decades: A Machine Learning Classification Perspective

This study aims to understand the dynamic changes in the coral reef habitats of Derawan Island over two decades (2003, 2011, and 2021) using advanced machine learning classification techniques. The motivation stems from the urgent need for accurate, detailed environmental monitoring to inform conservation strategies, particularly in ecologically sensitive areas like coral reefs. We employed non-parametric machine learning algorithms, including Random Forest (RF), Support Vector Machine (SVM), and Classification and Regression Tree (CART), to assess spatial and temporal changes in coral habitats. Our analysis utilized high-resolution data from Landsat 9, Landsat 7, Sentinel-2, and Multispectral Aerial Photos. The RF algorithm proved to be the most accurate, achieving an accuracy of 71.43% with Landsat 9, 73.68% with Sentinel-2, and 78.28% with Multispectral Aerial Photos. Our findings indicate that the classification accuracy is significantly influenced by the geographic resolution and the quality of the field and satellite/aerial image data. Over the two decades, there was a notable decrease in the coral reef area from 2003 to 2011, with a reduction to 16 hectares, followed by a slight increase in area but with more heterogeneous densities between 2011 and 2021. The study underscores the dynamic nature of coral reef habitats and the efficacy of machine learning in environmental monitoring. The insights gained highlight the importance of advanced analytical methods in guiding conservation efforts and understanding ecological changes over time.

Water optical properties over or around coral reefs in the coastal region: implications for coral reef remote sensing

Coral reefs inhabit clear oceanic-type waters to turbid coastal regimes. For shallow coastal water environments, the coral reefs from remote sensing will be substantially influenced by the complexity and heterogeneity of the optical properties of water. Through a set of bio-optical and chemical measurements in July 2019 around Xidao Island, Sanya, China, we explored spatial variations of water components' concentrations (including suspended solids, phytoplankton, and dissolved organic carbon) and the optical absorptions (i.e., particulate pigments, detritus, and colored dissolved organic matter) in waters over or around coral reefs; further analysis of their influences upon the remote sensing reflectance (Rrs(λ)) of water was performed. It was observed that the bio-optical and chemical properties of the waters over or around coral reefs were spatially and vertically heterogeneous for different sampling sites. As expected, the suspended solids dominated the optical properties of coral reefs waters in areas of Xidao Island, which evidently influenced the Rrs(λ), especially for the surface waters (Pearson r > 0.60, p < 0.01). In addition, the dissolved organic carbon concentrations exhibited significant relations to the Rrs(λ) both in surface and bottom water layers, whereas the colored dissolved organic matter showed a weak negative correlation with the Rrs(λ). These findings will support the mapping and monitoring benthic habitats with remote sensing imagery in coastal regions, especially when a removal of the influence by particulate sediments was available.

Bio-Optical Measurements Indicative of Biogeochemical Transformations of Ocean Waters by Coral Reefs

The bio-optical properties of coral reef waters were examined across coral reef ecosystems not influenced by land-derived run-off, in the Great Barrier Reef lagoon (Heron Island) and the Coral Sea (the Coringa-Herald and Lihou Reefs). The aim was to determine whether the absorption properties, the concentration-specific absorption properties, and the phytoplankton and non-algal pigmented particle (NAP) absorption concentrations varied from the ocean waters flushing onto the reef at high tide to those waters on the reef or flushing off the reef at low tide. The optical and biogeochemical properties of on-reef waters systematically differed from the surrounding ocean waters. The chl a concentration values varied up to 7-fold and the NAP concentrations up to 29-fold; for the reef samples, the chl a values were on average 2 to 3 times lower than for the oceans whilst the NAP values were slightly higher on the reefs. The spectral absorption values of the chl a, NAP, and colored dissolved organic matter (CDOM) varied up to 6-fold for reef waters and up to 15-fold for ocean waters. The spectral absorption for chl a was up to 3-fold lower on the reef waters, the absorption by the CDOM was up to 2-fold higher and the NAP absorption was 1.6-fold higher on the reef waters. The concentration-specific absorption coefficients for chl a and NAP varied up to 9-fold in reef waters and up to 30-fold in ocean waters. In the case of Heron Island and Coringa-Herald cays, this concentration-specific absorption was on average 1.3 to 1.7-fold higher for chl a and up to 2-fold lower for NAP on the reefs. The Lihou Reef measurements were more ambiguous between the reef waters and ocean waters due to the complex nature and size of this reef. Based on our results, the assumption that the optical properties of on-reef waters and the adjacent ocean waters are the same was shown to be invalid. Ocean waters flowing on to the reef are higher in phytoplankton, whilst waters on the reef or flowing off the reefs are higher in CDOM and NAP. We found differences in the pico,- nano-, and microplankton distributions as well as in the ratios of photosynthetic to photoprotective pigments. The variability in the bio-optical properties between the reef waters and adjacent ocean waters has implications for the estimations of sunlight absorption along the water column, the UV penetration depth, the temperature distributions, and the nutrient and carbon fluxes in coral reef ecosystems. As Earth observation algorithms require proper parameterization for the water column effects when estimating benthic cover, the actual optical properties need to be used. These results will improve the use of Earth observation to systematically map the differences in the water quality between reefs and the adjacent ocean.

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.

Coral Reef Mapping of UAV: A Comparison of Sun Glint Correction Methods

Although methods were proposed for eliminating sun glint effects from airborne and satellite images over coral reef environments, a method was not proposed previously for unmanned aerial vehicle (UAV) image data. De-glinting in UAV image analysis may improve coral distribution mapping accuracy result compared with an uncorrected image classification technique. The objective of this research was to determine accuracy of coral reef habitat classification maps based on glint correction methods proposed by Lyzenga et al., Joyce, Hedley et al., and Goodman et al.. The UAV imagery collected from the coral-dominated Pulau Bidong (Peninsular Malaysia) on 20 April 2016 was analyzed in this study. Images were pre-processed with the following two strategies: Strategy-1 was the glint removal technique applied to the whole image, while Strategy-2 used only the regions impacted by glint instead of the whole image. Accuracy measures for the glint corrected images showed that the method proposed by Lyzenga et al. following Strategy-2 could eliminate glints over the branching coral—Acropora (BC), tabulate coral—Acropora + Montipora (TC), patch coral (PC), coral rubble (R), and sand (S) with greater accuracy than the other four methods using Strategy-1. Tested in two different coral environments (Site-1: Pantai Pasir Cina and Site-2: Pantai Vietnam), the glint-removed UAV imagery produced reliable maps of coral habitat distribution with finer details. The proposed strategies can potentially be used to remove glint from UAV imagery and may improve usability of glint-affected imagery, for analyzing spatiotemporal changes of coral habitats from multi-temporal UAV imagery.