Dynamics of phosphorus in the Venice lagoon during a picocyanobacteria bloom

Abstract

The lagoon of Venice experienced dense blooms of picocyanobacteria in the summer of 2001. The dynamics of soluble reactive inorganic phosphorus (DIP) was studied in these blooms in the lagoon. DIP uptake and regeneration rates were measured in situ using a 32 P-radiotracer. DIP content remained high even during the bloom (0.2-0.7 μM), at P tot concentrations from 4 to 8 μM. The DIP uptake rate by microplankton varied between 20 and 73 nM min -1 , depending mainly on the bloom density. The DIP residence time varied from 5 to 20 min, whereas in the waters of the coastal Adriatic, where picocyanobacteria were also present, it was 2.5 h. The DIP regeneration rate varied from 18 to 59 nM min -1 . The picocyanobacterial bloom assemblage was able to consume and to store in its biomass up to 10 μM of phosphorus in the samples of lagoon water enriched with DIP at up to 17 μM. The results demonstrate the capability of picocyanobacterial blooms to recycle DIP rapidly and to accumulate high stocks of phosphorus in their biomass. This feature of their physiology enables assemblages of picocyanobacteria to produce dense blooms in shallow marine basins experiencing the impact of eutrophication.