Nutrients (organic C, P, N, Si) in the eutrophic River Loire (France) and its estuary

Abstract The Loire estuary has been surveyed from 1982 to 1985 by 13 isochronous longitudinal profiles realized at low tide. Nutrient (SiO 2 , NO 3 − , NH 4 + , PO 3− 4 , particulate organic carbon or POC) patterns are very variable depending on the season, the estuarine section [river, upper-inner estuary, upstream of the fresh-water-saline-water interphase FSI, the lower-inner estuary characterized by the high turbidity zone (HTZ), the outer estuary] and the river discharge. Biological processes are dominant. In the eutrophied River Loire (summer pigment > 100 μg l −1 ), the high algal productivity (algal POC > 3 mg l −1 ) results in severe depletion of SiO 2 , PO 4 3− , NO 3 − . The enormous biomass (55 000 ton algal POC/year) is degraded in the HTZ where bacterial activity is intense. As a result, there is generally a regeneration of dissolved SiO 2 and PO 4 3− , a marked NH 4 + maximum, while NO 3 − is conservative or depleted when the HTZ is nearly anoxic. Other processes can be considered including pollution from fertilizer plans (PO 4 3− , NH 4 + ) and from a hydrothermal power plant (NH 4 + ). In the less turbid outer estuary, nutrients are generally conservative. Major variations of concentrations are observed in the lowest chlorinity section (Cl − − ) and also upstream the FSI, defined here as a 100% increase in Cl − . Nutrient inputs to the ocean are not significantly modified for SiO 2 and NO 2 − , but are increased by 70% and 180% for PO 4 3− and NH 4 + and depleted by 60% for POC. Odd hydrological events, especially some floods, may perturbate or even mask the usual seasonal pattern observed in profiles.