The use of acoustic doppler current profiler acoustic backscatter to develop a regional zooplankton indicator for temperate marine ecosystems

Zooplankton are important prey in ocean food webs, linking primary producers to higher trophic levels; understanding their biomass is important to understanding ecosystem state and managing commercial fisheries accordingly. Due to expense and necessary processing time, there are limited indicators of zooplankton that can be produced timely enough to be useful for real-time management needs. We analyzed and compared an 18-year dataset of zooplankton biomass of 21 different species/taxa groups from net samples and acoustic backscatter from Acoustic Doppler Current Profilers (ADCPs) deployed on underwater gliders in the California Current System. Results show positive linear relationships across multiple spatial and temporal scales with greater than 50% of the variance of the total zooplankton biomass explained by the acoustic backscatter when samples were collected within 24 h and 1 km of each other. Copepod species usually dominated the total net haul biomass and there were similar relationships between copepod biomass and a) acoustic backscatter and b) total net biomass. An analysis of cross-shelf transects of the acoustic backscatter data was used to create a Glider Acoustic Backscatter Index (GAZI). The indicator shows consistent low values during El Niño periods and consistently high values during La Niña periods, conforming to our understanding of general productivity in the California Current during El Niño-Southern Oscillation (ENSO) events. These results provide a zooplankton indicator that can be rapidly produced on a sub-monthly time frame and a pathway to potentially using autonomous underwater gliders with ADCPs to monitor zooplankton in other oceanic environments.