Variability in the bottom-up and top-down controls of bacteria on trophic and temporal scales in the middle Adriatic Sea

Variability in the bottom-up (BU) and top-down (TD) regulation of bacteria was analysed on trophic and temporal (seasonal and inter-annual) scales in the middle Adriatic Sea during 1997-2006 using 3 empirical models. The analyses showed the tendency for bacteria to be TD controlled in oligotrophic open sea stations, and BU controlled in more eutrophic coastal stations. However, temporal variability in BU and TD controls was much stronger, with periods of both strong BU and strong TD controls being observed at all studied stations, independently of their trophic status. Decomposition of the time series was performed to identify seasonal and inter-annual changes in the relative importance of the BU and TD controls of bacteria. At all stations, BU control dominated during colder periods of the year, whereas TD control dominated during warmer periods. Nonseasonal fluctuations in the relative importance of BU and TD controls of bacteria pointed to a few periods when one or the other type of control was very strong. These periods coincided with some specific meteorological and hydrographic conditions - the strong influence of North Adriatic Dense Water in 1997, the strong Levantine Intermediate Water ingression in 2004, and the extremely warm winter and the Po River runoff in 2000-2001.