Recognising cyanobacterial blooms based on their optical signature: a modelling study

Mass populations of cyanobacteria are increasingly attracting the attention of environment agencies, water authorities, and human and animal health organizations, since they can present a range of water quality and treatment problems as well as hazards to human and animal health. The problem is especially severe in the Baltic Sea where cyanobacterial blooms occur every summer covering areas of more than 100000 km2. We studied optical properties of several phytoplankton species (including cyanobacteria) present in the Baltic Sea region. The measurements results were used in a bio-optical model together with optical properties of other phytoplankton species from literature. Our results show that cyanobacteria have a characteristic double feature (peak at 650 nm and phycocyanin absorption feature near 630 nm) in their reflectance spectra which can be detected by remote sensing instruments. Our estimation for the open Baltic Sea waters shows that concentration of chlorophyll has to be 8–10 mg m–3 before the double feature becomes detectable by remote sensing instruments which spectral resolution is 10 nm and signal-to-noise-ratio is 1000:1. Therefore, it is highly unlikely that remote sensing can be used for early warning of emerging potentially harmful blooms as chlorophyll concentrations higher than 4 mg m–3 qualify as blooms here.