Biomass, feeding and production of Noctiluca scintillans in the Seto Inland Sea, Japan

Abstract

The abundance and biomass of the large heterotrophic dinoflagellate Noctiluca scintillans, together with the changes in its potential prey items, were monitored in the Seto Inland Sea. Japan, during summer 1997 (17 July-11 August). Growth and grazing rates of N.scintillans fed natural plankton populations were also measured eight and seven times, respectively, during the survey period. The abundance and biomass of N.scintillans averaged over the water column (19 m) were m the range 1-345 cells l-1 (temporal average = 93 cells l-1) and 0.1-49.6 μg C l-1 (temporal average = 13.8 μg C l-1; three times higher than that of calanoid copepods during the same period). Noctiluca scintillans populations followed the changes in phytoplankton: N. scintillans biomass was increasing during the period of diatom blooms and was at a plateau or decreasing during periods of low chlorophyll a. The growth rates of N. scintillans (μ) were also consistent with the wax and wane of the N. scintillans population: N. scintillans showed highest growth rates during diatom blooms. A simple relationship between μ and chlorophyll a concentration was established, and the production of N. scintillans was estimated using this relationship and the measured biomass. The estimated production averaged over the water column was in the range <0.1-5.2 μg C l-1 day-1 (temporal average = 1.4 μg C l-1 day-1; 64% of the production of calanoid copepods during the same period). Diatom clearance rates by N. scintillans were in the range 0.10-0.35 ml cell-1 day-1, and the phytoplankton population clearance by N. scintillans was <12% day-1. Thus, although the feeding pressure of N. scintillans on phytoplankton standing stock was low, N. scintillans was an important member of the mesozooplankton in terms of biomass and production in the Seto Inland Sea during summer.