THE INFLUENCE OF WATER COLUMN STRATIFICATION ON DISSOLVED OXYGEN LEVELS IN COASTAL AND SHELF WATERS AROUND IRELAND

In temperate coastal and shallow shelf waters oxygen depletion near the seabed can occur naturally as a result of seasonal water column stratification. The presence of stratification limits the downward mixing of well-oxygenated surface water with the result that oxygen near the bottom can become depleted as a result of biological consumption. In this paper we show that in coastal and shelf waters around Ireland a strong positive correlation (r = 0.81) exists between bottom layer oxygen under-saturation and the presence or absence of stratification. A predictive empirical model based on this correlation together with an analysis of the deviation in bottom oxygen levels from a reference value of 1.00% saturation was used to map the bottom distribution of oxygen in these waters. The model was also used to estimate the daily rate of oxygen consumption in the bottom layer during the summer stratification period (approximately 120 days) on the north-western Irish shelf. This was found to be 0.25 percentage saturation points (or 0.022mg l(-1) O-2) per day. These findings provided the basis for a discussion on the development of oxygen- and nitrogen-based environmental quality standards and the identification of potentially oxygen vulnerable areas in waters around Ireland. Finally, this work demonstrates that the likelihood of large-scale hypoxia occurring in Irish waters as a result of anthropogenic nutrient inputs is very low.