Marine Organic Aggregates in the Northern Adriatic, Their Physical Properties, Microstructure and Biological Origins

Marine organic aggregates of different sizes are common in oceans and coastal seas and usually attain higher abundance and mass in nearshore regions. In the northern Adriatic visible marine snow and large flocks reaching the size of several centimetres can be observed intermittently throughout the year and are recognised as a part of usual plankton dynamics. Extensive mucilage aggregates develop sporadically and can grow to several meters in size and affect large areas (up to 10,000 km2). This phenomenon appears to be unique to the northern Adriatic and has been known for over 250 years. Analysis of the recurrence pattern indicated that the secular hazard rate of mucilage events did not vary notably over the last 120 years and the calculation of its periodicity revealed an average cycle of 5.74 years (Vollenweider et al. 1995). Since 1872 researchers reported on ten mucilage events spreading over a large part of the northern Adriatic and fourteen more localised ones. In the last decade large mucilage masses appeared in the summers of 1988, 1989, 1991, and 1997 and have caused problems to touristic and fisheries activities. We have followed this phenomenon in the summer of 1997 and compared the results to those obtained in 1991.