What is salinity?
Salinity is a measure of the total amount of dissolved material, or the salt content, in water. Salinity is the number of grams (g) of material in 1000g of water. For example, if seawater has 35g of salt in 1000g of water, it has a salinity of 35‰ (parts per thousand). Practical Salinity Units (PSU) are often used to describe salinity: a salinity of 35‰ equals 35 PSU. (Because salinity is measured as "grams per grams" many oceanographers prefer a "unitless" description: "35" rather than "35 PSU" or "35‰".)
Gulf of Maine salinity
Freshwater sources into the Gulf of Maine help drive the salinities found there. Freshwater enters the gulf as river flow from more than 60 rivers along the coastline and as runoff from rain and melting snow. However, the main source of lower salinity water into the Gulf is Scotian shelf surface waters. Saltwater enters the gulf over Browns Bank and through the Northwest Channel.
The profiles below show how salinity can vary seasonally in the gulf. Throughout the year, the highest salinities are near the bottom of the gulf. This is because relatively dense, high salinity water sinks below that of lower salinity. In the autumn profile, notice the plume of freshwater at the surface (i.e., dark blue at upper left). Autumn brings cold fronts and rain to the Gulf of Maine; this freshwater influx decreases the surface salinity along the coast.
If we had to figure out the salinity of seawater by weight, it would take a LOT of water. It is more efficient to measure seawater's electrical conductivity, which increases with salinity. Every hour, GoMOOS buoys measure conductivity at several depths. These data, plus temperature information, are used to compute salinity as Practical Salinity Units (PSU).
Along with temperature and pressure, salinity affects the density of seawater: more saline water has a higher density. Thus higher salinity water will sink below that with lower salinity (assuming temperatures are equal). In the absence of mixing, salinity will generally increase with depth. The halocline is a region where salinity increases sharply with depth.
Animals and plants that live in seawater have special adaptations that allow them to balance the level of salt in their bodies. Some organisms, such as salmon and eels, are able to tolerate freshwater and saltwater.
Salinity also affects the freezing point of water as dissolved salts inhibit ice crystal formation. Water with a salinity of 35 PSU has a freezing point of -1.9 °C (28.6°F). This means that ocean water can be colder than 0°C (32°F) without forming ice! This is why folks in Maine salt their roads in winter.
Beyond the Gulf: Why is ocean water salty?
- Rivers and land run-off pick up minerals (salts) and carry them to the ocean. As the water evaporates from the ocean, the salts are left behind.
- Undersea chemical processes contribute to the "saltiness" of the ocean. Some dissolved minerals (e.g., calcium, manganese) reach high enough concentrations to deposit on the sea bottom. On the other hand, underwater volcanoes and hydrothermal sea vents can cause ocean floor minerals to dissolve, releasing them into seawater.
- Some marine life (e.g., shrimp, diatoms) influence the ocean's salt concentration by extracting certain minerals from seawater to help build their bodies.
What are typical salinity values and patterns in our oceans?
Sea surface salinity in the open ocean generally ranges between 32 and 38 PSU. It is related to the amount of freshwater that is added or removed. At ocean centers, salinity is high because the trade winds promote evaporation, there is little rain, and there is no freshwater input that far from land. Along the equator, salinity is slightly lower because there is more rain to offset the high evaporation. Coastal areas often have lower salinities due to freshwater runoff and river input; conversely, the dry air blowing off land can increase evaporation and thus salinity. Surface salinity at high latitudes varies seasonally: it is high during the winter when ice is forming (leaving the salt behind) and low during the summer when ice is melting. The movement of water by currents can also affect salinity patterns.
- Conkling, P.W. (ed.). (1995). From Cape Cod to the Bay of Fundy: An Environmental Atlas of the Gulf of Maine. MIT Press.
- deCharon, A., and M. Sieracki. (1999). Ship Mates: Explore the Gulf of Maine as Oceanographers Do. URL: http://www.bigelow.org/shipmates.
- Lalli, C. M. and Parsons, T. R. (1993). Biological Oceanography - An Introduction. Butterworth-Heinemann.
- Swenson, H. USGS Publication: Why is the Ocean Salty? URL: http://www.palomar.edu/oceanography/salty_ocean.htm.