SEASONAL SWITCHING BETWEEN RELATIVE IMPORTANCE OF BOTTOM-UP AND TOP-DOWN CONTROL OF BACTERIAL AND HETEROTROPHIC NANOFLAGELLATE ABUNDANCE

Seasonal dynamics of bacterial and heterotrophic nanoflagellate (HNF) species assemblages were analysed in Kastela Bay (middle Adriatic Sea) . Dominant patterns identified were: (1) during summer and autumn bact erial abundance was mainly controlled by HNF grazing (top-down), where as HNF abundance was controlled by bacterial abundance (bottom-up); (2 ) during winter and spring the coupling between bacteria and HNF was v ery weak, and bacterial abundance was mainly controlled by resources s upply (bottom-up), whereas HNF abundance was controlled by microzoopla nkton grazing (top-down); (3) throughout the year, both bacterial and HNF species assemblages alternated with two periods of stable abundanc e, first with high and second with low values; (4) top-down effect was dominant in bacterial switching from stable abundance with high value s to stable abundance with low values, whereas bottom-up model dominat ed in inverse process; and vice versa for HNF.