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Abstract
We present a new method for the detection of coral bleaching using satellite time-series data. While the detection of coral bleaching from satellite imagery is difficult due to the low signal-to-noise ratio of benthic reflectance, we overcame this difficulty using three approaches: 1) specialized pre-processing developed for Planet Dove satellites, 2) a time-series approach for determining baseline reflectance statistics, and 3) a regional filter based on a preexisting map of live coral. The time-series was divided into a baseline period (April-July 2019), when no coral bleaching was known to have taken place, and a bleaching period (August 2019-present), when the bleaching was known to have occurred based on field data. The identification of the bleaching period allowed the computation of a Standardized Bottom Reflectance (SBR) for each region. SBR transforms the weekly bottom reflectance into a value relative to the baseline reflectance distribution statistics, increasing the sensitivity to bleaching detection. We tested three scales of the temporal smoothing of the SBR (weekly, cumulative average, and three-week moving average). Our field verification of coral bleaching throughout the main Hawaiian Islands showed that the cumulative average and three-week moving average smoothing detected the highest proportion of coral bleaching locations, correctly identifying 11 and 10 out of 18 locations, respectively. However, the three-week moving average provided a better sensitivity in coral bleaching detection, with a performance increase of at least one standard deviation, which helps define the confidence level of a detected bleaching event.
Highlights
Coral reefs are some of the most diverse and valuable ecosystems on the planet
We introduce a coral bleaching probability index—the Standardized Bottom Reflectance (SBR)—to measure the coral bleaching probability based on multi-temporal statistics obtained from the baseline period Planet Dove observations to help in defining subsequent reflectance anomalies
The SBR threshold of 1 is the initial criterion we used for coral bleaching detection, because when the SBRs are outside one standard deviation of the historical average, the signal is anomalous and detectable
Summary
Coral reefs are some of the most diverse and valuable ecosystems on the planet. They provide key ecosystem services such as fisheries, tourism, and cultural practices [1,2]. Coral reefs are experiencing rapid changes from increased bleaching events due to the changing climate. Local scales (10–1000 m2) has been reported for almost a century [3]. In recent years coral bleaching, along with marine ecosystem change, is becoming a global-scale problem [4,5,6,7]. The Intergovernmental Panel on Climate Change (IPCC) estimates that 90–99% of global coral reefs will be affected by ocean warming and acidification by 2050 [8]
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