Mucilaginous aggregates in the northern Adriatic in the period 1999 2002: Typology and distribution

Spatial and temporal distributions of different types of visible mucilaginous aggregates were investigated monthly by underwater video cameras in the norther Adriatic (NA) from June 1999 to July 2002. Small aggregates (flocs, macroflocs, and stringers) were observed in all seasons with higher values in autumn and winter. Larger aggregates (ribbons, cobwebs, clounds and false bottoms) formed only during late-spring and summer, particularly in 2000 and 2002, when the seasonal thermohaline stratification was marked. In fact, the process of mucilage formation took place mainly in the upper water column, above the main pycnocline. The spatical distribution shows that higher concentrations of small aggregates occur in the more productive coastal waters. The larger aggregates form or accumulate mainly in coastal waters in the southern part of the NA and in the central part of the Po River delta-Rovinja transect, where a gyre greatly influences the aggregates distribution. The fronts between low salinity coastal waters and high salinity wateres of souther origin play an important role in the accumulation and aggregation of the mucilage during spring and summer. Most of the aggregates accumulate in correspondence with strong pycnoclines with differences in density anamoly of 2 kg m-3 or higher. False bottoms formed in correspondence with strong haloclines, while cobwebs and ribbons also occurred when the halocline was less marked. Meteorological conditions greatly influenced the aggregate formation. Calm weather and weak winds favour aggregation processes, while strong north-easterly winds, causing intense mixing, result in the dispersion of the mucilaginour aggregates, as occurred during the bora event in July 2000.