The St. Lawrence is a marine ecosystem—it is a favourable environment for a multitude of living organisms adapted to living in seawater at this latitude. Here we find life in all shapes and sizes—plants and animals from the smallest microscopic organisms to the biggest creatures on the planet, as shown by the blue whales that come to the St. Lawrence each summer to feed on krill. In seawater, different chemical and physical reactions and processes occur that support biological activity. As part of the monitoring program, certain basic parameters are routinely measured that allow us to evaluate the quality of the St. Lawrence environment. Here is the list of variables for which data are available on this site.
Because of the estuarine character of the St. Lawrence, salinity is of key importance. It allows the characterization of water masses as a function of the degree of mixing of the fresh and salt water. Seawater is made up of about 50 salts, the most common of which is sodium chloride, the same salt that we use as a seasoning. Salinity is calculated by measuring the conductivity of the water. Since this measurement is a ratio, it is dimensionless (unitless), although one will frequently see salinity recorded in units of “psu” to indicate “practical salinity unit,” making reference to the Practical Salinity Scale 1978 (PSS78).
Seawater extends up the St. Lawrence as far as Ile d’Orléans, the same point at which oceanographers consider the estuary to begin. The freshwater flowing downstream creates a salinity gradient in the estuary’s surface waters toward the ocean. In the figure showing the salinity profiles, we note that the surface salinity is about 26 during the summer at the Rimouski station, which is located in the maritime estuary, while the salinity is above 30 at the Anticosti Gyre station, which is located in the Gulf of St. Lawrence. Water that is more saline contains more salts and so is denser and heavier. That is why the deep waters of the estuary and Gulf salinities near 34, which is comparable to water from the Atlantic Ocean.
Temperature exerts a major control on the distribution and activities of marine organisms. Its effects include the regulation of biological and chemical processes of cellular metabolism, the succession of species in the phytoplankton community, and the migration patterns of fish. For example, a more rapid growth rate in living organisms is generally associated with higher temperatures. Temperature is expressed in degrees Celsius (°C).
The temperature of a water mass is in part responsible for the degree of stratification of the water column. If we look at the water from the surface to the bottom at the Anticosti Gyre station in the summer, we see three distinct layers of water as defined by their temperatures. The surface layer is characterized by relatively warm temperatures between 5 and 15°C over a depth of about 20 m (A); the cold intermediate layer has temperatures between -1 and 2°C over a depth of about 100 m (B); and finally, the deep layer of more than 200 m (C) in thickness is characterized by water temperatures of about 5°C, which is similar to the temperature of water in the Northwest Atlantic. Water temperature changes with the seasons. Each winter, a new cold layer develops with the formation of ice and the inflow of cold water from the Labrador Shelf. The winter surface temperatures, near -1°C, merge with the cold intermediate layer. In the spring, the fresher water in the surface layer that comes from the melting ice and snow is warmed by the sun. The surface layer thus becomes more stable and better defined, reaching a depth of 20 to 30 metres. This surface layer is made up of warmer and less saline water that literally floats on top of the cold intermediate layer, which remains isolated and persists throughout the year. The phytoplankton is found in this surface layer.
Oxygen is essential for life and one of the basic elements involved in the chemical reactions that govern most biological processes in aquatic ecosystems. It regulates the distribution of living organisms, affecting, for example, fish migrations. Oxygen is an atmospheric gas that crosses the atmosphere–ocean boundary and dissolves in seawater. Marine organisms are capable of using oxygen in this dissolved form. Oxygen is also produced when marine plants, mainly phytoplankton, undergo photosynthesis. The amount of oxygen in the water is often expressed as the volume of dissolved oxygen per volume of water: millilitres of oxygen per litre of water (mL/L).
Because of the mixing action of wind and waves, the top 50 metres or so of the St. Lawrence Gulf and estuary are saturated with respect to dissolved oxygen. From the surface toward the bottom, the vertical profile of the dissolved oxygen concentration at the Anticosti Gyre station reaches a minimum at about 225 metres of depth. This minimum represents the lowest oxygen concentrations to which ground fish are exposed and is thus of particular interest. In general, the concentration of dissolved oxygen in the deep water decreases westward of Cabot Strait toward the head of the Laurentian Channel, as the “age” of the waters moving up the channel increases.
Chorophyll a is a plant pigment that is essential for photosynthesis. The measurement of the amount of this pigment gives an index of the abundance of marine plants, mainly made up of microscopic algae known as phytoplankton. Primary production by plants supports the food web and determines an ecosystem’s productivity. Chlorophyll a is expressed as the amount of pigment per volume of water: milligrams of chlorophyll a per litre of water (mg/m3).
On this graph showing the quantity of chlorophyll a measured from May through October 2003 at the Rimouski station, the green, yellow, and red patches indicate high concentrations of chlorophyll a. These high amounts resulted from the spring bloom that began around May 10th, became very intense near the end of May with concentrations above 20 mg/m3, and continued through the end of June.
During the spring ice-melt in the St. Lawrence, the nutrient-rich surface layer warms up and solar radiation extends deeper into the water column. At this time, conditions are favourable for photosynthesis and for the propagation of phytoplankton—the minute flora that float with the currents in the top 30 metres of the water column. There is an explosion in the number of phytoplankton cells, a phenomenon known as the spring bloom. During a bloom, the rate of cell division can reach one division per day; at this rate, a single cell can give rise to about one million cells in three weeks. Within a few weeks, small aggregates of phytoplankton become dense blooms spreading over several square kilometres with thicknesses reaching tens of metres.
On graph A, which shows the changes over time in the amount of nitrogen in the form of nitrate at the Rimouski station, we can see high concentrations between 18 and 20 µmol/L in the surface water at the beginning of May. The amount of nitrogen decreases considerably with the start of the spring phytoplankton bloom (graph B). The blue areas seen near the surface on graph A correspond to very low concentrations of nitrogen, nearly 0.0 µmol/L. The spring phytoplankton bloom exhausted the reserves of nitrogen in the surface layer so that very little phytoplankton growth was possible during the summer. The strong autumn winds mix the water column, bringing enough nitrogen to the surface to enable another bloom. This explains the bloom observed at the end of September at the Rimouski station.