IS NITRATE AN OLIGOTROPHICATION FACTOR IN THE VENICE LAGOON

Approximately 30% of the area in the Venice lagoon ecosystem shows a p henomena of seasonal instability. Hypertrophic conditions caused by in creased anthropic stress, particularly during the years 1987-1991, hav e progressively favoured massive springtime blooms (up to 20 kg/m(2), 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/m(2), on a wet weight basis. of Ulv a laetevirens. Phosphorus measured in macroalgal tissue shows a very l ow concentration. This seems to indicate a low concentration of phosph orus in the water and not a great quantity of this element for the mac roalgae. Since the phosphorus availability in the water comes from sed iments (almost 50%), it is therefore possible to explain this conditio n as low release of phosphorus from the sediments. Phosphorus and nitr ogen fluxes at the water-sediment interface, denitrification, and sulf ate reduction probably all show a high nitrification and high nitrate uptake in the sediments. Because of this, the nitrate could play the i mportant 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. (C) 1997 Elsevier Science B.V.