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 m3 s 1) to a strong freshet in July (4870 m3 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 ( < 2%). The biological response to these physical forcings was an intense phytoplankton bloom in the western low salinity waters in July 1997 (O2sat. up to 206%, Chlorophyll-a (Chl-a) up to 7.64 Ag dm 3), a common feature in all years with mucilage. It determined a strong increase of dissolved organic nitrogen (DON, + 100%) and phosphorus (DOP, + 39%) in the most productive western waters of the basin, which constitute available material for mucilage formation. Nutrient content in the upper waters shifted in concomitance to mucilage appearance, from a PO4-scarce condition in spring (Si/N/P = 225:105:1), to a rather severe dissolved inorganic nitrogen (DIN) and SiO4-depletion in summer (Si/N/P = 21:13:1). This drastic change can stimulate exudate hyperproduction and/or modify bacteria degradation capacity of organic matter. As a consequence of degradation of sinking aggregates, in summer 1997 strong hypoxic conditions were reached in the deeper waters (down to 55% in most of the profile, down to 16% in the western area). Increasing AOU/DIN (up to 143) and AOU/PO4 (up to 2500) ratios indicated a progressive nitrogen and phosphorous impoverishment in aged aggregates.