RUNOFF AND STRATIFICATION The circulation and mixing of the ocean can be impacted by runoff from the land especially in areas that receive large amounts of runoff, like the St. Lawrence Estuary. Layering called stratification can form in the water column because waters with different densities don’t mix very easily [See box: Stratification and the cold intermediate layer]. Freshwater runoff from the land can increase stratification, since fresher water is less dense than saltier water. Along with tides and wind, runoff drives the circulation within the St. Lawrence Estuary and, to a lesser extent, in the whole Gulf of St. Lawrence.
West Quaco, New Brunswick. Credit: Tim Foster.
Ocean stratification can affect the way nutrients are mixed into surface waters. A change in nutrient mixing influences phytoplankton growth and blooms. This, in turn, impacts ecosystem productivity.
The amount of stratification in the water column is measured by looking at the difference in density between water at the surface and water at a depth of 50 metres. Long-term trends are reported for three locations: Station 27 (a site off St. John’s, Newfoundland and Labrador), Rimouski Station in the St. Lawrence Estuary, and the Scotian Shelf. Year to year, stratification at the Rimouski Station is strongly related to the seasonal average runoff of the St. Lawrence River (Figure 8).
STATUS AND TRENDS •
Freshwater runoff into the St. Lawrence Estuary decreased between the early 1970s and 2001. This was followed by an upwards trend between 2001 and 2011, and has been fairly stable to 2016. Stratification in the St. Lawrence Estuary is tied to seasonal freshwater runoff and has followed a similar pattern.
•
At Station 27, stratification increased from below-average values in the mid-1960s to a record high in 1995. Since then, it has been following a mostly decreasing trend.
•
Since 1948, there has been an increase in the average stratification on the Scotian Shelf. This change is due mainly to a decrease in the surface density (76% of the total density change). This decrease is caused equally by warming and the addition of freshwater to the surface water.
CANADA’S OCEANS NOW: ATLANTIC ECOSYSTEMS
2018
STRATIFICATION TRENDS
Figure 8 : Water column stratification measured as the difference between the density of water at the surface and water at 50m depth. The estimate for the St. Lawrence estuary is based on fresh water runoff. A large difference in density, such as shown here in the St. Lawrence Estuary, means more stratification and potentially inhibited mixing. On the Scotian Shelf and Newfoundland and Labrador Shelves the difference is lower. There is lower stratification in these areas.
10
