Development of a coral and competitive alga-related index using historical multi-spectral satellite imagery to assess ecological status of coral reefs

Understanding the characteristics of the growth zones of live corals and competitive algae, including turf algae and macroalgae, is crucial for assessing the degradation of coral reef ecosystems. However, identifying live corals and competitive algae in multispectral satellite images is challenging because different objects can have similar spectra. To address this, we used two satellite images acquired at different times (Landsat thematic mapper (TM), Landsat operational land imager (OLI), or Sentinel-2 multi-spectral instrument (MSI)) to assess the growth zone characteristics of live corals and competitive algae. This assessment leveraged the seasonal dieback of competitive algae and the relative stability of live-coral growth zones over a short period. Specifically, we developed a normalized red–green difference index (NRGI) to segment live-coral-or-competitive-alga growth zones in satellite images. By comparing the segmentation results from an image captured during a period with few competitive algae and another image captured during a period with lush competitive algae, we estimated the growth zone areas of the live corals and competitive algae. Finally, we calculated the ratio of the competitive-alga growth zone area to the live-coral growth zone area (RCL). Experiments on eight typical coral islands and reefs in the South China Sea (SCS) from 1995 to 2022 revealed that: (1) the identification accuracies of live-coral-or-competitive-alga growth zones reached 80.3 % and 92.6 % during periods with few competitive algae (January to March) and lush competitive algae (April to October), respectively; (2) the RCL was well correlated with the coral-macroalgae encounter rate (an ecological index indicating the pressure of the competitive algae on the live corals) (r = 0.79, P<0.05); and (3) the trends in the growth zones of competitive algae and live corals, along with the RCL, were consistent with major ecological events in the SCS, such as coral bleaching, outbreak of Acanthaster planci, and black band disease. (4) Moreover, a time-lagged correlation was observed between heat stress and the RCL. In summary, the proposed approach is simple, effective, and feasible. The RCL is a valuable indicator of the status of coral reef ecosystems, highlighting the pressure of competitive algae on live corals and the degradation of coral reef ecosystems. This method introduces a novel application of multispectral satellite images for assessing coral reef ecosystems and has significant potential for future coral reef ecosystem monitoring.