Dynamics of the oceanographic properties during mucilage appearance in the Northern Adriatic Sea: analysis of the 1997 event in comparison to earlier events

The dynamics of the oceanographic properties in Northern Adriatic Sea during the 1997 mucilage event is reported. These observations, compared to a longer time series (1985–1998), indicate that processes on short temporal scales and abrupt changes of the physical and biogeochemical conditions in the basin have a great effect on mucilage events. The seawater warming during the years with the mucilage was delayed in May and June compared to those without aggregate. Since the same typical summer values were afterward reached in both cases, a more abrupt warming is supposed in the period preceding mucilage events. During 1997, the Po River regime had an abrupt change from very low flow rates in April–May (526–985 m 3 s −1) to a strong freshet in July (4870 m 3 s −1). This river pulse, coupled to the scarce intrusion of the high salinity Middle Adriatic Deep Water (MAdDW), determined the large retention of low salinity waters in the Northern Adriatic Sea from July to September. In 1997, as well as during the events in 1989, 1991, the freshwater content in the basin was two times higher (4–5%) with respect to the years without aggregates from 1992 to 1998 (