Factors contributing to hypoxia in a highly turbid, macrotidal estuary (the Gironde, France)

Abstract

Dissolved oxygen (DO) is a fundamental parameter of coastal water quality, as it is necessary to aquatic biota, and it provides an indication of organic matter decomposition in waters and their degree of eutrophication. We present here a 7 year time series of DO concentration and ancillary parameters (river discharge, water level, turbidity, temperature, and salinity) from the MAGEST high-frequency monitoring network, at four automated stations in the central and fluvial regions of the macrotidal Gironde Estuary, one of the largest European estuaries. The spatio-temporal variability of DO at different time scales was first related to the migration and position of the maximum turbidity zone in this extremely turbid estuary. Since 2005, the Gironde Estuary has recorded several borderline hypoxic situations (DO close to 2 mg L⁻¹) and a 7 day-long hypoxic event (DO < 2 mg L⁻¹) in July 2006. Summer hypoxia occurred exclusively in the fluvial, low salinity, and high turbidity sections of the estuary and was significantly more pronounced in front of the large urban area of Bordeaux (715 000 inhabitants). Detailed analysis of the data at the seasonal, neap-spring and semi-diurnal tidal time scales, reveals that hypoxia in this area occurred: (i) in the maximum turbidity zone; (ii) during the spring to neap tide transition; (iii) at highest water temperature; and (iv) at lowest river discharge; there was also evidence of an additional negative impact of sewage treatment plants of Bordeaux city. Enhancement of respiration by turbidity, temperature and inputs of domestic biodegradable organic matter and ammonia, versus renewal of waters and dispersion of reduced pollutants with the river discharge, appeared as the dominant antagonist processes that controlled the occurrence of summer hypoxia. In the context of long-term environmental changes (increase in temperature and population, decrease in summer river discharge), the occurrence of severe hypoxia could not be excluded in the next decades in the upstream reach of the Gironde Estuary.