Sensitivity of reef-relevant ocean color phenomena to satellite data resolution

Satellite ocean color measurements are a valuable tool for evaluating water quality parameters relevant to coral reef habitats. However, low earth orbiting satellites offer a limited number of observations for detecting episodic, extreme events to which coral reefs are sensitive, and the spatial resolution of most sensors is too coarse for fine scale processes in optically shallow nearshore environments. Here, we assess whether high-resolution satellite ocean color measurements from the first geostationary satellite ocean color sensor (GOCI) can improve our understanding of coral reef habitat conditions and monitoring capabilities for potential reef changes. Using ten years (2011–2021) of GOCI ocean color measured eight times per day at a spatial resolution of 500 m around the Okinawa Prefecture region, we found that the high-resolution grid significantly increased retention of coastal areas where waters were otherwise masked at coarser resolutions (i.e., 4 km) to avoid optical reflectance in shallow waters. Contrary to expectation, we found that the often highly correlated variables chlorophyll-a and Kd490 did not notably decouple at higher spatiotemporal resolutions. However, the ability to detect episodic, extreme chlorophyll blooms increased significantly with increasing spatiotemporal resolution and the locations of these events became much more refined at higher resolutions. Finally, the high-resolution chlorophyll-a data captured up to 770 episodic events that were missed by the commonly used 4-km 8-day resolution data in any given grid cell around Okinawa across the 10-year period. High-resolution ocean color data can therefore enable us to assign more reliable risk to coral reef tracts that could be affected by frequent episodic events, and allows us to assess the persistence of such events in waters much closer to coastal habitat.