THE PHYTOPLANKTON COMPOSITION AND POPULATION ENRICHMENT IN GELATINOUS MACROAGGREGATES IN THE NORTHERN ADRIATIC DURING THE SUMMER OF 1989

Macroaggregates and samples of the surrounding seawater were collected at 10-m depths on a transect along a trophic gradient in the northern Adriatic. Autotrophic micro-, nano- and picoplankton were enumerated by epifluorescence microscopy. Phytoplankton cell densities in the gelatinous "macroaggregates" were up to four orders of magnitude higher than in the surrounding seawater. Microplankton exhibited the highest enrichment factors relative to nano- and picoplankton (20- and 12-fold, respectively). The degree of enrichment was higher at the less eutrophic eastern side, because the organism abundances in the seawater were lower and because the populations in the "macroaggregates" were more abundant. "Macroaggregates" exhibited a temporal succession of species similar to the surrounding seawater, with an indication of a time-lag for the dominants in the surrounding seawater, to attain dominance in the "macroaggregates". However the diatoms Nitzschia longissima and Nitzschia closterium maintained a prominent contribution to the "macroaggregate" microplankton throughout the study, irrespective of their density in the surrounding seawater, suggesting that they were embedded in the gelatinous matrix earlier in the succession and protected from grazers maintained their growth within the "macroaggregate". Significant correlations between enrichment factors and "macroaggregate" microplankton cell densities, combined with poor correlations with seawater cell densities, suggest that once organisms are embedded they become less dependent on contemporary processes in the surrounding seawater.