Changes in the northern Adriatic ecosystem and the hypertrophic appearance of gelatinous aggregates

Biweekly to monthly measurements of a large number of physical, chemical, and biological parameters and visual observations (by scuba divers and underwater video cameras) were performed at 80 stations in the northern Adriatic in the framework of an ‘Alpe-Adria’ research of a phenomenon of gelatinous aggregate hypertrophy. This phenomenon was observed in the entire investigated region during the summers of 1988, 1989, and 1991 and only in the Kvarner areas in 1990. Results for some parameters were compared with available historical data series collected since 1966. Some qualitative changes in the phytoplankton communities (increased diatom contribution, decreased diversity, different dominant species) were evident during the eighties compared with the seventies. However, chlorophyl a and nutrient concentrations remained approximately at the same level. The dynamics of the Po river discharge (and nutrient inputs) during the spring, which was the critical period for aggregate formation, was different during the eighties from that in the preceding decade. An hypothesis is developed which relates the appearance of large quantities of gelatinous material to modifications of the environmental conditions (climatic, hydrology, and oceanographic).