Hypoxia has become a growing concern in coastal regions throughout the world . A watermass is hypoxic when the concentration of dissolved oxygen is so low as to cause stress for aquatic organisms. In general this happens when the oxygen concentration is 2 mg/l (65 µM) and less. At this level, many fish species can not survive, and the benthic community structure undergoes significant modifications.
During a recent cruise to the Lower St. Lawrence Estuary, sporadic measurements of dissolved oxygen revealed concentrations of 65 µM and less in the bottom 50 meters of the water column. A rough estimate suggests that the area bathed in water of such low O2 concentrations may cover more than 1000 km2. This observation is cause for concern because of the effects low oxygen will have on benthic and epibenthic fauna, and on nutrient release by sediments and subsequent effects on primary production. We may also be observing a trend of decreasing oxygen concentrations that could ultimately lead to complete anoxia in the deep water, which would be nothing less than an environmental disaster.
Bottom water hypoxia in the Laurentian Trough is not a seasonal phenomenon because the water column is permanently stratified and the bottom water is isolated from the atmosphere. This distinguishes the St. Lawrence from shallow environments such as the Gulf of Mexico shelf where hypoxia is seasonal. In these areas, hypoxia develops in the bottom water during spring and summer, but the conditions return to oxic when vertical mixing of the water column by strong winds in fall and winter replenishes the deep water oxygen. In the St. Lawrence, oxygen in the deep water can not be replenished by mixing. The oxygen concentration at a given location is determined uniquely by the oxygen concentration in the water that flows from the Atlantic Ocean along the bottom towards the head of the Laurentian Trough and by the local rate of oxygen consumption. There are no local sources of oxygen. The bottom water oxygen concentration in the Cabot Strait region is about 60% of saturation; it is now down to 15% of saturation in the Estuary.
HYPOTHESES AND QUESTIONS
We hypothesize that, as in the case of the Gulf of Mexico, the low oxygen concentrations are at least in part a response to an increased loading of nitrogen nutrients. Unlike freshwater environments, where phosphorus usually is the limiting nutrient, ammonia and nitrate can limit primary production in coastal marine areas. Alternatively, it could be a response to other factors including changes in fresh water runoff, increased terrestrial organic matter input, change in the supply and/or properties of bottom waters supplied from the Atlantic Ocean. Therefore, the existence of low oxygen concentrations in the bottom waters raises a number of questions, including: