Microbial food web structure of the planktonic ecosystem in the neritic area of Sagami Bay, Japan: seasonal variability in chlorophyll a < 20 µm, bacteria, heterotrophic nanoflagellates and microzooplankton

Abstract

Seasonal variations in physicochemical properties (i.e. temperature, salinity, dissolved inorganic nutrient concentration), chlorophyll a (Chl-a) concentration, abundance and biomass of the main microbial food web components, i.e. bacteria (BA), heterotrophic nanoflagellates (HNF) and microzooplankton (MZ: i.e. naked ciliates, tintinnids, heterotrophic dinoflagellates and copepod nauplii), were investigated at a neritic survey station in Sagami Bay, Kanagawa, Japan, from April 2006 to April 2009. These abiotic/biotic variables varied seasonally in an essentially similar way during the 3 year period. Abundance and biomass of BA, HNF and MZ revealed seasonal variations: they were low throughout the water column (0–50 m depth) in winter and were high from spring through summer/autumn, especially in the upper layer (0 to 10–30 m depth) in spring–summer. The year-on-year variations in abundance of BA, HNF and MZ are correlated weakly with the temporal variations in physicochemical properties and Chl-a (total, < 20 µm) concentration, which showed no significant difference between the annually mean values. BA, HNF and MZ abundance and Chl-a < 20 µm showed a nearly similar pattern of temporal variations and significantly positive correlations between them, indicating that the abundance variations of these heterotrophs would be affected by their prey concentration and abundance (i.e. bottom-up effect). Additionally, remarkable declines and low levels of prey abundance and biomass were associated with high abundances and/or high estimated feeding impacts of their possible feeders/predators, suggesting that the temporal variations in abundance and biomass of prey organisms would be induced by effective top-down effect.