EFFECTS OF DECREASING WATER DISCHARGE AS A CONSEQUENCE OF A CIRCUMVALATION CANAL CONSTRUCTION ON SALINITY, NUTRIENTS AND MACROPHYTES IN A COASTAL LAGOON.  <B>EFFECTS OF DECREASING WATER DISCHARGE AS A CONSEQUENCE OF A CIRCUMVALATION CANAL CONSTRUCTION ON SALINITY, NUTRIENTS AND MACROPHYTES IN A COASTAL LAGOON.</B><P>Margarita Menendez1, Ester Gimenez2, Rafael de Cid2 and Elisenda Fores2 </B><I><P> 1. Department of Ecology. University of Barcelona. Av.Diagonal, 645.08028 Barcelona. Spain<BR> 2. Parc Natural del Delta de l'Ebre. Placa 20 Maig, 2. Deltebre. Tarragona. Spain.</I><P> Abstract<P> Encanyissada lagoon is a small (5.5 Km2) and shallow (80 cm average depth, 100 cm maximum depth) eutrophic coastal lagoon. It receives drainage water from irrigated lowland ricefields from April to October. Because of the high nutrient contents of the water coming from ricefields (mainly nitrogen) due to the fertilizers applied, environmental conditions of the water have deteriorated during the last two decades. The lack of submersed macrophytes and the huge development of phytoplankton, which include Cyanobacteriae in late summer, characterize the system. In 1992 a circumvalation canal who regulate freshwater discharge from ricefields was constructed. The effect of this construction was the reduction of freshwater inputs to the lagoon. Conductivity and nutrients concentrations were measured in 1994 and were compared with results of previous years. A decrease in ammonium concentration was observed. Nitrate, nitrite and soluble reactive phosphorus did not change. Total phytoplankton chlorophyll decreased in 1994 compared with previous years. As a result of this phytoplankton decrease a colonization and growth of rooted macrophytes was observed, mainly Ruppia cirrhosa (Petagna) Grande, Potamogeton pectinatus L., Najas marina L., Myriophyllum spicatum L. and Ceratophyllum demersum L.<P>