Abstract
Ocean color remote sensing significantly contributes to our understanding of phytoplankton distribution and abundance and primary productivity in the Southern Ocean (SO). However, the current SO in situ optical database is still insufficient and unevenly distributed. This limits the ability to produce robust and accurate measurements of satellite-based chlorophyll. Based on data collected on cruises around the Antarctica Peninsula (AP) on January 2014 and 2016, this research intends to enhance our knowledge of SO water and atmospheric optical characteristics and address satellite algorithm deficiency of ocean color products. We collected high resolution in situ water leaving reflectance (±1 nm band resolution), simultaneous in situ chlorophyll-a concentrations and satellite (MODIS and VIIRS) water leaving reflectance. Field samples show that clouds have a great impact on the visible green bands and are difficult to detect because NASA protocols apply the NIR band as a cloud contamination threshold. When compared to global case I water, water around the AP has lower water leaving reflectance and a narrower blue-green band ratio, which explains chlorophyll-a underestimation in high chlorophyll-a regions and overestimation in low chlorophyll-a regions. VIIRS shows higher spatial coverage and detection accuracy than MODIS. After coefficient improvement, VIIRS is able to predict chlorophyll a with 53% accuracy.
Highlights
The Southern Ocean (SO) contains relatively high seasonal levels of phytoplankton biomass in its coastal waters due to the complex processes of ice melt and intense seasonal light availability
Through the collection of hyper-spectral downwelling irradiance and water leaving reflectance (Rrs) around the West Antarctica Peninsula (WAP), an area which lacks a sufficient in situ validation datasets for satellite remote sensing, we evaluate the chlorophyll-a algorithm estimations from three aspects: first, the downwelling irradiance total signal was collected just above the sea surface and was used to derive atmospheric spectral information such as particle scattering and gas and aerosol absorption; second, the hyper-spectral Rrs (±1 nm) were used to determine detailed bio-optical characteristics of WAP waters; our data were collected after launching of VIIRS
Atmospheric correction is important for water-leaving reflectance because the atmosphere comprises over 90% of the total water leaving signal [25]
Summary
The Southern Ocean (SO) contains relatively high seasonal levels of phytoplankton biomass in its coastal waters due to the complex processes of ice melt and intense seasonal light availability. Seasonal blooms in this region play a significant role in driving global biogeochemistry cycling [1,2]. Satellite imagery can provide high spatial and temporal coverage of the SO in the southern spring and summer (from October to March), and is useful for understanding the patterns and variability of phytoplankton biomass across broader temporal and spatial scales This is an advantage over in situ environmental detection [3,4], where severe weather can make routine and comprehensive sampling difficult. The OC2v2 algorithm, when applied to CZSC and SeaWiFS imagery, underestimated chlorophyll-a (0.7–43 mg/m3 ) concentrations by 60%
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