In-situ growth and mortality of mesozooplankton
during the austral fall and winter in
Meng Zhou, Yiwu Zhu, and Jay O. Peterson
Zooplankton play a critical role in the austral winter ecosystem of the Southern Ocean, transporting biomass between organisms through their consumption and growth, and ultimately determining the survival of krill. Previous studies have provided evidence of various ways krill utilize zooplankton through austral winter when the primary production is negligible. Though they are different, all survival strategies will produce the same result, a decrease in total zooplankton biomass. The objectives of this study are to examine the grazing rate of adult krill on mesozooplankton, and the removal rates of mesozooplankton in austral winter.
It is difficult to examine the total zooplankton biomass balance and process rates from traditional net tow samples because of the spatial variation of species compositions, the amount of labor needed to obtain zooplankton species-stage based estimates of biomass, and the lack of applicable mathematical methods to estimate population process rates. This study is based on the measurements of zooplankton size structures obtained using an Optical Plankton Counter (OPC) mounted on a Multiple Opening and Closing Nets and Environmental Sampling System (MOCNESS), and a 153 kHz Vessel Mounted (VM) Acoustic Doppler Current Profiler (ADCP). The in situ process rates of growth and mortality are estimated based on the biomass spectrum theory. The total mesozooplankton biovolume (biomass) decreased at a specific rate of 0.02 day-1 over the austral winter 2002, while the specific individual body growth was approximately 0.02 day-1 and the specific mortality rate was approximately 0.04 day-1. These rates lead to a system efficiency of 50% for recycling the biomass. The measurements indicate that approximately 82% of biovolume (biomass) was removed in the period between the April-May fall cruise and the August-September winter cruise in 2002. The in situ grazing of adult Euphausia superba Dana on mesozooplankton is examined based on the in situ measurements of mesozooplankton biovolume within the water where adult Euphausia superba Dana aggregations remained during the short study period. The measurements show that approximately 16% of the existing mesozooplankton biovolume was grazed in 4 hours, which leads to an average specific daily rate of 0.96 day-1 assuming a daily feeding period of 16 hours. Though the grazing rate by adult euphausiids is approximately 25 times higher than the seasonal mortality rate of 0.04 day-1, the amount of biovolume removed by adult euphausiids is approximately enough to support ten adult euphausiids m-3 at a daily respiration rate of 3% of the body biomass.