The interannual variability of phytoplankton appeared
very high everywhere, being abundance trends quite
heterogeneous in time and space (Fig. 2). Qualitatively,
phytoplankton was mainly composed of diatoms (mean values ranged from 52% to 74% of total), except at
Marghera, where microflagellates prevailed (avg¼52%
of total; Fig. 2). Other classes were found occasionally:
coccolithophorids, typical of marine environments, were
mainly observed at Lido, but always in very small percentages
(max 1%). Chlorophyceans and euglenophyceans,
typical freshwater forms, were found in stations
influenced by inputs from the innermost areas. The
phytoplankton populations of Palude were typically
both marine and brackish, confirming the transitional nature of this area.
During summer, S. Giuliano and Palude had diatom blooms in both 1997 and 1998.
At S. Giuliano in June 1997, 92% of the phytoplankton was composed of Chaetoceros cfr. compressus (21106 cell
dm3) and Cylindrotheca closterium (29106 cell dm3), whereas, in June 1998, 70% of the total was
represented by Thalassiosira sp. (22106 cell dm3).
In Palude, in August–September 1997 and 1998, a Nitzschia frustulum bloom was observed, with abundances
respectively of 30106 and 15106 cell dm3.