Monitoring harmful algal bloom in Baja California using in-situ measurement and satellite remote sensing

Coastal waters are a key source of income and nutrition for communities all over the world. In Baja California, Mexico, there have been hypoxia-driven mortalities of benthic habitat in coastal water. While the hypoxia occurred, harmful algal blooms (HABs, red tides) events were observed in this area. We conducted in-situ measurements when red tides bloomed in coastal water in Baja California in 2018. Rhodomonas sp. was most dominant with 1,650-33,000 cells/ml of density. Ceratium furca, Pseudo-nitzschia pungens, and Thalassiosira sp. were rarely present. Dissolved oxygen ranged 6-14 mg/l at 1-m depth and 0.3-9.8 mg/l at 1-m above from the bottom. Red tides have known that it has bimodal peaks over visible wavelengths. The sea water with red tide in this area also showed clear bimodal peaks, which were converted to red tide index (RTI). RTI shows a positive exponential relationship with in-situ red tide density. We derived red tide density estimation algorithm applicable to satellite imagery. Utilizing this algorithm to optical satellite imagery, it has become possible to monitor red tides density distribution over onshore to offshore. This approach may grant a predictive tool to minimize the impact of red tides on coastal fishery and industries and further advance a better understanding of local HABs spatial and temporal characteristics is required to capitalize on natural refuges and inform adaptive management strategies.