engleski

Inter-annual variability in the bottom-up and top-down control of bacteria in the middle Adriatic Sea

Variability in the bottom-up (BU) and top-down (TD) regulation of bacteria was analysed on non-seasonal (inter-annual) scales in the coastal and open middle Adriatic Sea during one decade (1997-2006). Decomposition of time series was performed to eliminate seasonal variability and to isolate non-seasonal (inter-annual) changes in the relative importance of the BU and TD control of bacteria. In order to examine the regulation of bacteria by substrate availability (bottom-up control, BU) and by predation (top-down control, TD), data were analyzed according to empirical model based on comparative data analyses. The analyses showed the tendency for bacteria to be TD controlled in oligotrophic open sea, and BU controlled in more eutrophic coastal sea. However, temporal variability in the BU and TD control was much stronger, and the periods of both strong BU and strong TD control were found at all studied stations, independently of their trophic status. Non-seasonal fluctuations in relative importance of the BU and TD control of bacteria pointed to a few periods when one or another type of control was very strong. In the coastal sea, the period of the strong TD control of bacteria (established in 1997) was characterized by marked increase of dissolved inorganic nitrogen and bacterial abundance, followed by an increase in HNF abundance. On the other hand, bacterial abundance was also strongly coupled with nitrogen during both periods of strong BU control (2002 and 2004) but, opposite to 1997, the response of HNF predators failed to occur. In the open sea, similar fluctuations in relative importance of the BU and TD control of bacteria were found in both surface and bottom layers. The periods of strong BU or TD control coincided with some specific meteorological and hydrographical conditions (strong influence of North Adriatic Dense Water in 1997, strong Levantine Intermediate Water ingression in 2004 or extremely warm winter and the Po River runoff in 2000/01).

top-down control; bottom-up control; Adriatic Sea

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