Seasonal and axial variations of net water circulation and turnover in the estuary of Bilbao

• Published in: Estuarine, coastal and shelf science Vol. 150; pp. 312 - 324
• Main Authors: Uriarte, Ibon, Villate, Fernando, Iriarte, Arantza, Duque, Johanna, Ameztoy, Iban
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier Ltd 01.10.2014; Elsevier
• Subjects: advection; Animal and plant ecology; Animal, plant and microbial ecology; Biological and medical sciences; box model; Brackish; Brackish water ecosystems; Discharge; Estuaries; estuary of Bilbao; Exchange; Flushing; flushing time; Freshwater; Freshwaters; Fundamental and applied biological sciences. Psychology; Harbours; Inflow; residence time; spatio-temporal variations; Synecology; Water circulation; Southern Europe; Europe; Spain; ANE, Spain, Pais Vasco, Vizcaya, Bilbao; ANE, Europe, Biscay Bay; advection; flushing time; box model; residence time; spatio-temporal variations; estuary of Bilbao; Brackish water environment; Residence time; Vizcaya; Estuaries; Spatial variation; Time variation; Advection; Box model; Seasonal variation; Turnover
• ISSN: 0272-7714; 1096-0015
• DOI: 10.1016/j.ecss.2014.04.007

A two-layer box model based on salinity and freshwater inflow data was developed and used to estimate net water circulation, contributions of gravitational circulation exchange and tide-driven exchange, and turnover times for the estuary of Bilbao, a small estuary of the Basque coast (Bay of Biscay). Average monthly estimations for the 2001–2010 period were made and related to river discharge and saltwater inflow. Seasonal variations of surface-layer outflows were strongly related to the river discharge regime, even in the lower estuary (inner Abra harbour). Bottom-layer salt-water inflow from the outer Abra was the main driver of bottom landward flow, vertical advection and surface-layer outflow in the inner Abra, but not in the channelized zone that extends from the inner limit of the Abra harbour to the tidal limit. Gravitational circulation exchange dominated in the entire estuary over the annual cycle. Tide-driven exchange proportionately increased in summer and showed the highest contribution (42%) in the lower estuary in August. Flushing and residence times increased in summer in relation with the decrease of freshwater discharge, although in the innermost zone of the estuary they were also high in winter due to the retention of freshwater at the inner estuary under extremely high discharge conditions. Flushing and residence time maxima of 21.5 and 28.6 days respectively were obtained for the entire studied zone in August. It is of note that turnover times differed largely between the upper (flushing time of 0.4–2.4 days) and bottom (flushing time of 2–10 days) layers in the channelized zone. Results supported intuitive conclusions drawn in previous studies about the spatio-temporal dynamics of dissolved oxygen, chlorophyll and zooplankton populations in the estuary of Bilbao, in relation to the effect of water circulation and turnover.