Features of the Comacchio Ecosystem Transformed during Persistent Bloom of Picocyanobacteria

The big Comacchio lagoons (NW Adriatic coast) have been experiencing a super-dense, long-term bloom of picocyanobacteria (PCB) since 1985. This bloom has caused profound transformations in their ecosystem and has resulted in the loss of valuable fish and clam resources. The composition, density and seasonal dynamics of this bloom and its impact on the ecosystem were studied between spring and autumn 1995 and in the summer of 2001. The density of the blooming phytoplankton assemblages varied from 8 to 35 x 10(6) cells ml(-1), and biomass from 20 to 40 g m(-3), thus demonstrating a quasi-stable status since 1993. The share of pico + nanocyanobacterial fraction of the total phytoplankton biomass was 98% in spring to early summer, and 92% in autumn. The remaining 0.2-5% comprised eukaryotic nano-algae. All components of the animal food web were found to be drastically depleted in the lagoons. Their biomass range in the open lagoons areas were: 2 to 20 mg m(-3) of microzooplankton, 10 to 40 mg m(-3) mesozooplankton, and 5 to 12 g m(-2) macrozoobenthos. The share of filtering fauna in the whole ecosystem metabolism was less than 2%. The malfunction of the deteriorated animal food web resulted in the hyper-accumulation of non-grazed microplankton biomass and detritus in the water column and in bottom sediments. The sequence brought the acceleration of sulfide production and the accumulation of acid soluble sulfides in the upper sediment layer of up to 700-800 mg S dm(-3) of wet silt. The results of this study provide evidence that the state of ecological collapse continues in the lagoons since 1993.