Oxygen

The amount of dissolved oxygen in seawater is important for the health of marine organisms. In deep water, mixing from the surface waters can replace oxygen. When there is little mixing, dissolved oxygen can be depleted by the respiration of organisms and the breakdown of organic matter. When oxygen levels are too low, the condition is called hypoxia. When oxygen levels fall below 30% of the maximum they can hold, it is considered severely hypoxic. This can have serious effects on ecosystems. Changes in climate can contribute to hypoxia occurring. So too can the input of organic matter from algal blooms caused by high levels of nutrients. This has been a problem in the Gulf of St. Lawrence (Figure 7).

Hypoxia can dramatically affect marine life and ecosystems. It can slow growth and reduce reproductive success among ocean species. It can also impact the way species are distributed, as most species leave an area before hypoxia can kill them. With severe hypoxia, species that cannot move fast enough out of the affected area can suffer high mortality rates.

Dissolved oxygen is regularly measured throughout the water column as part of oceanographic surveys done within the region.

DISSOLVED OXYGEN, ACIDITY (pH) AND TEMPERATURE IN THE LOWER ST. LAWRENCE ESTUARY (300m)

Figure 7 : Average dissolved oxygen, pH (acidity) and temperature measurements for the lower St. Lawrence Estuary at approximately 300 metres. Values of dissolved oxygen below 30% saturation are considered extremely hypoxic.