Detection of surface algae blooms using the Sentinel 2A: An algorithm of the best strip ratio for a freshwater lake

Abstract

Harmful Algal Blooms (HABs) are problematic algal blooms that cause toxicity and associated environmental impacts on freshwater, marine and coastal ecosystems. HABs produce strong toxins that pose a threat to humans and wildlife, with significant negative impacts i.e., food web vectoring, airborne toxic events, decay of algal blooms resulting in low oxygen or hypoxia and killing fish and birds. Measurement of algae concentrations has conventionally relied on direct water sampling for lab-based cell enumeration. These traditional approaches are extremely labor-intensive, tedious, and limited spatially and temporally. Remote sensing (RS) based methods are capable to handle these complications in inland and near-coastal waters (consistent revisit rate for well-structured time series analyses, regular and reliable observations over a large area). The Multispectral Instrument (MSI) onboard European Space Agency’s (ESA) Sentinel 2 satellite initiates a new era in high-to-moderate resolution (10, 20, 60 m) of earth observation data. Sentinel 2A (S2A) satellites launched in 2015 as a part of the ESA’s Copernicus program. S2A filter-based push-broom imager, measures the reflected solar spectral radiances in 13 spectral bands ranging from the visible-near infrared (VNIR) (0.4422-0.8640 μm) to the short-wave infrared (SWIR) (0.9432-2.1857 μm) bands. This study aims to develop a method to estimate Chlorophyll-a (Chl-a) concentration in freshwater lake waters using in situ data of Chl-a, water reflectance, and contemporaneous S2A imagery over the Kotmale reservoir Sri Lanka.