Abstract
The red tide bloom-forming dinoflagellate Margalefidinium polykrikoides is well known for its harmful effects on marine organisms, and for killing fish in aquaculture cages via gill clogging at a high cell abundance. To minimize the damage caused by red tide blooms, it is essential to understand their detailed spatial distribution with high accuracy. Airborne hyperspectral imagery (HSI) is useful for quantifying red tide cell abundance because it provides substantial information on optical features related to red tide species. However, because published red tide indexes were developed for multichannel ocean color sensors, there are some limitations to applying them directly to HSI. In this study, we propose a new index for quantifying M. polykrikoides blooms along the south coast of Korea and generate a M. polykrikoides cell abundance map using HSI. A new index for estimating cell abundance was proposed using the pairs of M. polykrikoides cell abundances and in situ spectra. After optimization of the published red tide indexes and band correlation analyses, the green-to-fluorescence ratio (GFR) index was proposed based on red tide spectral characteristics. The GFR index was computed from the green (524 and 583 nm) and fluorescence wavelength bands (666 and 698 nm) and converted into red tide cell abundance using a second-order polynomial regression model. The newly proposed GFR index showed the best performance, with a coefficient of determination (R2) of 0.52, root mean squared error (RMSE) of 877.98 cells mL−1, and mean bias error (MBE) of −18.42 cells mL−1, when applied to atmospherically corrected HSI. The M. polykrikoides cell abundance map generated from the GFR index provides precise spatial distribution information and allowed us to estimate a wide range of cell abundance up to 5000 cells mL−1. This study indicates the potential of the GFR index for quantifying M. polykrikoides cell abundance from HSI with a reasonably high level of accuracy.
Highlights
Margalefidinium polykrikoides is a major red tide species that causes massive annual harmful algal blooms (HABs) in Korean coastal waters, which have become a large-scale chronic issue, affecting broad areas, since 1995 [1,2]
It is essential to understand the spatiotemporal distribution of these blooms based on quantification of red tide cell abundance to minimize damage caused by red tide blooms
Higher wavelengths were associated with lower R2, and the lowest value was found for wavelengths >700 nm
Summary
Margalefidinium polykrikoides is a major red tide species that causes massive annual harmful algal blooms (HABs) in Korean coastal waters, which have become a large-scale chronic issue, affecting broad areas, since 1995 [1,2]. M. polykrikoides blooms have frequently occurred along the south coast of Korea, where fish farms are concentrated. These blooms have caused large-scale mortality of finfish and have resulted in significant economic losses in many countries. Contact with M. polykrikoides bloom water with cell abundances greater than 1000 cells mL−1 generally leads to rapid mortality in fish (i.e., hours) and shellfish (i.e., days) [3,4,5,6]. Cell abundance is a critical factor affecting the survival or mortality of fish in marine aquaculture cages. It is essential to understand the spatiotemporal distribution of these blooms based on quantification of red tide cell abundance to minimize damage caused by red tide blooms
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