Seasonal Switching Between Relative Importance of Bottom-Up and Top-Down Control of Bacterial and Heterotrophic Nanoflagellate Abundance

Abstract

Seasonal dynamics of bacterial and heterotrophic nanoflagellate (HNF) species assemblages were analysed in Kaštela Bay (middle Adriatic Sea). Dominant patterns identified were: (1) during summer and autumn bacterial abundance was mainly controlled by HNF grazing (top-down), whereas HNF abundance was controlled by bacterial abundance (bottom-up); (2) during winter and spring the coupling between bacteria and HNF was very weak, and bacterial abundance was mainly controlled by resources supply (bottom-up), whereas HNF abundance was controlled by micro-zooplankton grazing (top-down); (3) throughout the year, both bacterial and HNF species assemblages alternated with two periods of stable abundance, first with high and second with low values; (4) top-down effect was dominant in bacterial switching from stable abundance with high values to stable abundance with low values, whereas bottom-up model dominated in inverse process; and vice versa for HNF.