Abundance, Biomass, Production and Trophic Roles of Micro- and Net-Zooplankton in Ise Bay, Central Japan, in Winter

Abstract

We investigated the geographical variations in abundance and biomass of the major taxonomic groups of micro- and net-zooplankton along a transect through Ise Bay, central Japan, and neighboring Pacific Ocean in February 1995. The results were used to estimate their secondary and tertiary production rates and assess their trophic roles in this eutrophic embayment in winter. Ise Bay nourished a much higher biomass of both micro- and net-zooplankton (mean: 3.79 and 13.9 mg C m⁻³, respectively) than the offshore area (mean: 0.76 and 4.47 mg C m⁻³, respectively). In the bay, tintinnid ciliates, naked ciliates and copepod nauplii accounted for, on average, 69, 18 and 13% of the microzooplankton biomass, respectively. Of net-zooplankton biomass, copepods (i.e. Acartia, Calanus, Centropages, Microsetella and Paracalanus) formed the majority (mean: 63%). Average secondary production rates of micro- and net-zooplankton in the bay were 1.19 and 1.87 mg C m⁻³d⁻¹ (or 23.1 and 36.4 mg C m⁻²d⁻¹), respectively, and average tertiary production rate of net-zooplankton was 0.75 mg C m⁻³d⁻¹ (or 14.6 mg C m⁻²d⁻¹). Available data approximated average phytoplankton primary production rate as 1000 mg C m⁻²d⁻¹ during our study period. The transfer efficiency from primary production to zooplankton secondary production was 6.0%, and the efficiency from secondary production to tertiary production was 25%. The amount of food required to support the zooplankton secondary production corresponded to 18% of the phytoplankton primary production or only 1.7% of the phytoplankton biomass, demonstrating that the grazing impact of herbivorous zooplankton was minor in Ise Bay in winter.