Remotely sensed short-term changes in noctilucent algae blooms in the Bohai Sea

Abstract

ABSTRACT Short-term changes in red tide characteristics are very helpful in monitoring the larger change progress and associations with environmental factors. We conducted an experiment to collect data on short-term changes in noctilucent algae blooms in the Bohai Sea, on 14 June 2015. The performance of three classic indices (OC2, OC3G and YOC) for noctilucent algae surveying was compared based on the measured remote sensing reflectance (Rrs (λ)) and Chl_a (chlorophyll a). OC2 was finally selected here. We established two thresholds: α (Rrs (745)/ Rrs (865)) and ε (L c(745)/ L c(865)), based on the measured reflectance and the Geostationary Ocean Colour Imager (GOCI) images corrected by Rayleigh scattering, respectively. These two thresholds were subsequently used in Management Unit Mathematics Models (MUMM), which were embedded in the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Data Analysis System (SeaDAS), for atmospheric correction of the selected GOCI images. Based on the correlation between phytoplankton cell density and Chl_a, we defined the threshold (6.5 μg L−1) of Chl_a concentration as a proxy of noctilucent algae blooms. Finally, we determined the temporal and spatial variations of the noctilucent algae bloom waters in the Bohai Sea, on 14 June 2015. The changing pattern demonstrated a general acknowledgement of phototropic vertical migration of the algae over the course of the day. Our results also demonstrated the utility of GOCI images, with MUMM atmospheric correction for short-time monitoring of noctilucent algae blooms.