Abstract
The presence‐absence of macroalgae and/or phytoplankton in some areas of the Venice lagoon and the factors controlling the prevailing of one over the other were tested by monitoring two critical sites over March‐December 1990. One, Sacca Sessola [B1], was the lagoon area more densely populated by macroalgae, vice versa the other, namely Fusina [D] displayed only phytoplankton blooms. For this purpose macroalgae and phytoplankton biomass variations were studied directly in field. In addition, macroalgal biomass growth was studied by resuspending pre‐determined biomass amounts into net cages placed at different depths. These semi‐natural methods were not able to reproduce the real algal growth, however allowed to identify the main causes of phytoplankton blooms and macroalgal absence at Fusina.