Time trend of PCB concentrations in surface sediments from a hypertrophic, macroalgae populated area of the lagoon of Venice.

The concentrations of polychlorinated biphenyls have been determined over a period of 1 year in surface sediments sampled monthly and semi-monthly in the lagoon of Venice. At the sampling station, due to the poor water circulation and an over-abundance of nutrients, macroalgae thrive in spring-summer and standing crops greater than 12 kg m-2 were determined. As a consequence of rapid algal decomposition, sediment PCB concentrations were observed to increase. In particular, on occasions when anoxia occurred at the end of July and all the biomass had decayed, PCB concentrations increased by more than one order of magnitude. In March, when macroalgae started to grow, concentrations were 13 ng g-1 (dry wt); in July-August they exceeded 100 ng g-1. The sediment PCB concentrations were restored to the previous values after algal biomass became negligible, in November. Concentrations of PCBs in young algae fronds were 27 +/- 12 ng g-1, and 121 +/- 4 ng g-1 in older fronds.