WWW.WA S .ORG • WORLD AQUACULTURE • JUNE 2022 21 ( C O N T I N U E D O N P A G E 2 2 ) TABLE 1. Current satellite sensors and the HAB parameters monitored. Sat e l l i t e Sensor s HAB- re lat ed Var iabl es Moni tored MODIS – Moderate Resolution Imaging Spectro-Radiometer SST, turbidity and ocean color to estimate algal biomass from chlorophyll pigments MERIS – Medium Resolution Image Spectrometer SST, turbidity and ocean color to estimate algal biomass from chlorophyll pigments (higher spatial resolution than MODIS) VIIRS – NOAA Visible Infrared Imaging Radiometer Suite SST, turbidity and ocean color to estimate algal biomass from chlorophyll pigments OLCI – Ocean and Land Color Imager Chlorophyll a and SST GOCI – Geostationary Ocean Color Imager SST, turbidity, and ocean color to estimate algal biomass from chlorophyll pigments SeaWiFS – Sea-Viewing Wide Field-of-View Sensor Ocean color to estimate algal biomass from chlorophyll pigments (no longer in orbit) MSI – Multispectral Imager Turbidity, SST, and ocean color to estimate algal biomass from chlorophyll pigments OLI – Operational Land Imager SST, turbidity, and ocean color to estimate algal biomass from chlorophyll pigments that are typically not possible with in situ measurement. Such technologies are important for offshore aquaculture because they can help minimize the need for on-site personnel and vessel-based monitoring, both financially burdensome. In contrast, remote sensing technologies send data directly to an operator onshore, who can monitor conditions in real time. As such, remote sensing is central in the development, implementation and control of HABmanagement strategies for offshore aquaculture. Remotely sensed data used for these purposes can be collected indirectly with satellite sensors in near-real time and directly with in-situ sensors. The use of remote sensing technologies that can help monitor and predict HABs near offshore aquaculture facilities is an emerging field of research and could help provide valuable insight on bloom dynamics. This technology-based review aims to discuss current remote sensing technologies used to monitor HABs in offshore aquaculture facilities. In addition, we review advancements in predictive early warning systems (EWS) and mitigation strategies built on data from remote sensing technologies. Satellite Sensor Measurements Used for HAB Detection Satellites are the most widely used remote sensing platforms globally and can be used for long-term spatial and temporal monitoring of HAB trends. Satellite images provide information on the current location and size of a HAB and aid in bloom tracking over time. Satellites used for these purposes include Sentinel, Envisat, Landsat, Orbview, Terra, Pegasus and Aqua, among others. Data collected from these satellites are publicly available. Specialized sensors outfitted on satellites can indirectly measure environmental parameters used to detect HAB presence before they become visible to the naked eye (Fig. 1). HAB-specific environmental parameters monitored include chlorophyll a levels (detected via ocean color), sea surface temperatures (SST, detected via reflectance data) and turbidity levels (detected via suspended particulate matter concentration, SPM). Table 1 is an exhaustive list of these sensors and their applications to HAB detection/monitoring. New sensors are continuously evolving. For example, researchers from the United States Geological Survey (USGS) are currently working on a technique called SMASH (spectral mixture analysis of surveillance of HABs) to determine the type of microbe in a bloom from a satellite image. This could help determine whether a bloom is made up of dangerous, toxinproducing species, which is crucial information for aquaculture producers in the development of effective management options. Chlorophyll a. Satellite sensors that monitor ocean color can detect increases in phytoplankton density using fluorescence as a proxy for increased chlorophyll a levels. Chlorophyll a levels can also be used as a bioindicator of nutrient concentration (Fig. 2). Chlorophyll a data can also be used in a time-series analysis to help with the accuracy of predicting HAB occurrence temporally by indicating how concentrations change over time. Additionally, chlorophyll a levels can be used to create a red tide (a specific type of HAB) index models using satellite-derived data (Lee et al. 2021, Sakuno et al. 2019). However, chlorophyll a sensors can be impacted by cloud cover and are most valuable for detecting highdensity blooms; smaller, less dense blooms are more difficult to detect. SeaSurfaceTemperature. Phytoplankton and HAB growth and productivity are directly correlated with SST. Warmer temperatures prevent water frommixing, allowing phytoplankton to grow denser and faster (Izadi et al. 2021). Temperature sensors aboard satellites can measure the magnitude of energy reflecting from the sea surface at different wavelengths to determine when SST levels are within a range of concern for potential bloom occurrence. Turbidity. Satellites can also indirectly determine turbidity by measuring the suspended particulate matter concentration (SPM)
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