In situ growth and mortality of mesozooplankton during the austral fall and winter in Marguerite Bay and its vicinity

Abstract

Abstract Zooplankton play a critical role in the austral winter ecosystem of the Southern Ocean, transporting biomass from lower to higher trophic level organisms through their consumption and growth, and ultimately determining the survival of marine animals such as sea birds and mammals. Previous studies have provided evidence that krill utilize zooplankton through austral winter when the primary production is negligible. Though they are different, all survival strategies produce the same result, a decrease in total zooplankton biomass. It is difficult to examine the total zooplankton biomass balance and process rates from traditional net tow samples because of the spatial variation in zooplankton distribution and the lack of applicable mathematical methods to estimate population process rates. This study focuses on short-term (overnight) loss rates of mesozooplankton associated with adult krill grazing in austral fall and the long-term (winter) body growth and removal rates of mesozooplankton in austral winter based on measurements of zooplankton size structures obtained using an Optical Plankton Counter (OPC) mounted on a Multiple Opening and Closing Nets and Environmental Sensing System (MOCNESS). In order to target an adult Euphausia superba aggregation for a short period, a 153 kHz Vessel-Mounted (VM) Acoustic Doppler Current Profiler (ADCP) was used for the acoustic backscattering. The in situ process rates of growth and mortality are estimated based on the biomass balance and biomass spectrum theory. The overnight in situ grazing by adult E. superba on mesozooplankton is examined based on the in situ measurements of mesozooplankton biovolume within the water where an adult E. superba aggregation remained during a short study period in late April. The measurements show that approximately 16% of the existing mesozooplankton biovolume was removed in 4 h, which leads to an average specific daily grazing rate of 0.96 day - 1 assuming a daily feeding period of 16 h from dusk to dawn. From the amount of biovolume removed and the abundance of adult euphausiids of approximately 3 individuals m - 3 estimated from the volume backscattering strength and the size of adult euphausiids, the daily ration for an adult euphausiid is approximately equal to 7% body biovolume or biomass day - 1 . The total mesozooplankton biovolume decreased at a specific rate of 0.017 day - 1 over the austral winter 2002, while the specific individual body growth was approximately 0.045 day - 1 . The sum of these two rates leads to the specific mortality rate equal to approximately 0.07 day - 1 for the winter. These rates lead to a system efficiency of 67% for recycling the biomass during the winter. The measurements indicate that approximately 90% of biovolume (biomass) was removed in the period between the fall cruise in April–May and the winter cruise in August–September 2002.