Long-term changes in heterotrophic prokaryotes abundance and growth characteristics in the northern Adriatic Sea

During the 1990–2008 period heterotrophic prokaryotes abundance (HP) in the northern Adriatic showed a substantial decrease after 2003. Before 2003 the HP annual cycle was characterized by minimal values during winter (100–134 · 107 cell l− 1, on average) and a gradual increase toward summer (up to 212 · 107 cell l− 1, on average). After 2003 HP was low during the most part of the year (27–63 · 107 cell l− 1, on average), with somewhat higher values only at the end of summer and in April (up to 80 · 107 cell l− 1 in average). The overall higher Leu/TdR ratio after 2003 showed that prokaryotes were increasing their cellular biomass but more slowly dividing than before. There were indications that changed hydrographic conditions, reduced substrate supply and quality changes might be the most responsible factors for the observed HP decrease and change in their growth characteristics. As a consequence of lower HP heterotrophic flagellates abundance (HF) after 2003 (average 0.57 · 106 cell l− 1) generally decreased by about three times. Weakened coupling between HP and HF confirmed the minor role of grazing pressure in controlling HP after 2003. This study shows the direct response of the microbial community to meteorological and subsequent hydrographical condition changes that occurred in the northern Adriatic.