Approximately 30% of the area in the Venice lagoon ecosystem shows a phenomena of seasonal instability. Hypertrophic conditions caused by increased anthropic stress, particularly during the yearn 1987-1991, have progressively favoured massive springtime blooms (up to 20 kg/m2, on a wet weight basis) of some species of macroalgae (Ulva laetevirens). During the years 1993-1994 this phenomena has not occurred, as the same area was covered by only 3 kg/m2, on a wet weight basis, of Ulva laetevirens Phosphorus measured in macroalgal tissue shows a very low concentration. This seems to indicate a low concentration of phosphorus in the water and not a great quantity of this element for the macroalgae. Since the phosphorus availability in the water comes from sediments (almost 50%). it is therefore possible to explain this condition as low release of phosphorus from the sediments. Phosphorus and nitrogen fluxes at the water-sediment interface, denitrification, and sulfate reduction probably all show a high nitrification and high nitrate uptake in the sediments. Because of this, the nitrate could play the important role of the oxidant to immobilise the phosphorus. The present work discusses the role of nitrate as an oligotrophication factor and as the immobilisation of phosphorus in sediment which is the probable cause of the limited growth of macroalgae Ulva laetevirens.

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