Assessing the distribution of nocturnal chlorophyll-a in the Northwest Pacific Ocean using ocean color and fishery echosounder acoustic data

Phytoplankton plays a crucial role in material cycling and energy flow within marine ecosystems. Ocean color remote-sensing chlorophyll-a (Chl-a) data serves as the primary means for assessing phytoplankton in the ocean environment. Nevertheless, the determination of nocturnal Chl-a still relies heavily on in situ ocean surveys. Diel variations in Chl-a, particularly nocturnal Chl-a, can more accurately reflect the ecological processes of marine ecosystems. We aimed to probe the nocturnal Chl-a distribution in the Northwestern Pacific Ocean by examining the predator–prey dynamics (reflected by acoustic data) and phytoplankton (reflected by Chl-a data). Drawing on continuous acoustic data and observed data variation, we reprocessed the 200 kHz frequency band data. Furthermore, we derived the nocturnal Chl-a in the Northwest Pacific Ocean based on diel 200 kHz frequency band data and Phytoplankton Size Class (PSC) algorithm. The inversion model can effectively retrieve nocturnal Chl-a data, and the validation results demonstrate superior performance compared to remote sensing Chl-a data. Moreover, we discovered a robust correlation between the diurnal and nocturnal Chl-a data (R2 = 0.9988, bias = 0.9925, MAE = 1.0196), indicating the feasibility of directly deriving nocturnal Chl-a data from diurnal Chl-a. This study innovatively integrates the characteristics of acoustic data for day-night monitoring and satellite remote sensing for large-scale monitoring, enabling large-scale observations of nocturnal Chl-a. The results deepen our understanding of diel ecological changes in the Northwest Pacific Ocean, provide a foundation for studying marine productivity variations and carbon cycling processes in the area, and contribute to the improvement of the monitoring and management of fishery activities in the region.

Abstract licence: CC BY-NC

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