Measurements of dissolved oxygen in an impounded stratified estuary

Like many impounded stratified estuaries, the Lagan (in Belfast, Northern Ireland) suffers water quality problems from time to time. These problems date back to 1937, when the first weir was constructed to allow navigation within the estuary. With the replacement of the original weir in 1994 by the Lagan Weir, a half tide-barrage, water quality issues were addressed. Provision was made for flushing of saline water that would otherwise become trapped at the base of the barrage, and artificial aeration facilities were installed with the intention of improving dissolved oxygen in the upper reaches. Despite these measures, the semi-tidal nature of this impounded estuary continues to allow the formation of a highly stratified flow regime. This prevails during the summer months when the weather is warm and the freshwater flow in the river is very low. Low dissolved oxygen levels in the lower dense seawater layer result, due to a high sediment oxygen demand within the estuary and reduced tidal mixing. This paper outlines the effects of an artificial hydraulic regime on dissolved oxygen levels and density stratification in the impounded estuarine waters of the River Lagan. Measurements of dissolved oxygen and salinity are presented for a number of different aerator operating regimes under low freshwater flow conditions. The results suggest that the aerators serve to improve water quality by mixing of the fresh and saline waters, but uptake of oxygen by dissolution remains unproven. The discussion is confined to the nature of the different aerator regimes and their influence on the distribution of dissolved oxygen and salinity within the estuarine waters.