Dissolved oxygen in the rehabilitation phase of an estuary: influence of sewage pollution abatement and hydro-climatic factors.

Seasonal and inter-annual variations of dissolved oxygen (DO) along the estuary of Bilbao were investigated from 1998 to 2008, during its rehabilitation phase from pollution, to determine whether anthropogenic or natural forcings or both govern DO dynamics and hypoxia. Both seasonal and inter-annual variations of DO were best explained by hydro-climatic factors, sewage pollution and phytoplankton dynamics in the inner, intermediate and outer estuary respectively. The most remarkable intra-decadal improvement in DO occurred in the halocline layer of the intermediate estuary, where the factor that best explained these changes was sewage pollution abatement. However, in the estuarine hotspot for hypoxia, i.e. inner estuary bottom waters, no parallel response to sewage pollution abatement was observed and hydro-climatic factors were the main drivers of inter-annual DO variations. Differences in the degree of stratification and flushing accounted for this differential response of DO to anthropogenic and climate-related forcings at both axial and vertical scales.