Sels minéraux et cycles de marées dans un estuaire de Bretagne nord (Dourduff, France)

Abstract

Short-term variations in nutrient concentrations of water-masses have been studied in a Brittany estuary over several tidal cycles, during winter and summer. NH inf4 sup+ , NO inf2 su− , NO inf3 sup− , SiO2 and PO inf4 su3− have been measured at a fixed station. The Dourduff estuary is characterized by a very low river discharge (80 l · s−1 during the minimum water runoff and 1 000 l · s−1 during the maximum) and an important tidal range (9 m at spring tides). SiO2 and NO inf3 sup+ concentrations are directly related to freshwater flow whereas PO inf4 su3− is partially adsorbed by seston in the turbid ebb waters. NH inf4 sup+ concentration seems to be, in part, dependent upon sediment resuspension: late ebb and onset of flood periods liberate NH inf4 sup+ into the overlying water column. Nutrient concentrations are also related to seasons. Nutrient fluxes are insignificant or negative during summer periods, so the estuary imports nutrients for its own regulation whereas during winter periods it exports NO inf3 sup− and SiO2 (ca 50 kg NO inf3 sup− and ca 200 g SiO2 during a single spring tide). The NO3: PO4 ratio is always above 15:1 and can reach 300:1; moreover this ratio fluctuates during the tidal cycle. This imbalance originates in terrestrial discharges of nitrogen compounds.