Dissolved Oxygen in a Temperate Estuary: the Influence of Hydro-climatic Factors and Eutrophication at Seasonal and Inter-annual Time Scales

Dissolved oxygen dynamics in estuarine and coastal environments are complex and highly variable, which highlight the need to compile information from many different types of estuaries. Small estuaries where euhaline habitats dominate are particularly ill represented in the literature. As a contribution to fill this gap, a study on dissolved oxygen dynamics was conducted in the small estuary of Urdaibai (inner Bay of Biscay). Spatial and temporal variations in the percentage saturation of dissolved oxygen (DO-saturation) along the salinity gradient of the estuary and the role of hydro-climatic factors and eutrophication as drivers of those variations at seasonal and inter-annual time scales were analyzed (period 1998-2008). DO-saturation showed an inner to outer estuary increasing gradient. DO-saturation below the salinity gradient layer was either significantly higher than or not significantly different from that in the salinity gradient layer. DO-saturation showed summer minima, but hypoxia was rarely observed. At the outermost estuary, seasonal variations of DO were small and it is hypothesized that DO dynamics were governed mainly by tidal exchange and turbulence. In the intermediate and inner estuary, seasonal variations of DO-saturation were best explained by river discharge, and to a lesser extent by chlorophyll a and temperature, each factor gaining relevance in different periods of the year. In intermediate and inner zones, river discharge exerted a positive effect on DO-saturation, likely via an increase in the renewal rate of DO. At the inter-annual time scale, unlike at the seasonal scale, temperature did not show a significant negative relationship with DO-saturation.