Sea dynamics impacts on the macroaggregates: A case study of the 1997 mucilage event in the northern Adriatic

We analysed the 1997 summer mucilage event in the northern Adriatic and showed that macroaggregates were formed in the upper layers of the water column in central parts of large gyres and were dispersed by currents which seem to be of geostrophic origin. Beside Cylindrotheca closterium, an already established regular opportunistic species of the mucilage phenomenon, originating from the sediment, we detected another benthic species, Thalassionema nitzschioides, of sporadic character. Both were present in scarce amounts in the water column at the time of sampling first macroaggregates, but reached high abundances after/close to the mucilage event ending. The first, and the most intense surface heat loss episode, on 18 July (heat loss of around 240 W m−2, during episode of NW wind), in time of first macroaggregates formation, introduced C. closterium from sediment into the water column and enabled dispersion of T. nitzschioides, present in central region both in water and in first macroaggregate samples, eastwards and westwards. The later, less intense surface heat loss episodes, between end of July and mid of August (30-160 W m−2, induced mostly by NE winds) were efficient (1) in tearing already formed macroaggregates, enabling in that way release of opportunistic species from a macroaggregate into the surrounding water column, and (2) in transporting lower salinity waters of specific species populations with macroaggregates form west to east, extending the event to large area. We hypothesise, first, that summer surface heat loss episodes play a role in sustaining mucilage events by introduction of C. closterium from the sediment and/or decaying macroaggregates in the water column. Second, that geostrophic currents are important in spreading of various types of aggregates, including marine snow, over the wider marine areas.