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Salinity

Marine Science  Feild Station

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Salinity can be defined as the concentration of dissolved anions (primarily salts) in a body of water.

Salinity is most commonly expressed in parts per thousand, or ppt. This is based on the number of grams of salt per liter of water.

 

15 g salt/ 1 L water = 15 ppt

The world’s oceans have fairly consistent salinity values ranging from 32 – 35 ppt. In estuarine systems where the majority of your research and educational activities will take place salinity plays a major role in determining species distribution. Along with temperature and secchi disk depth, salinity is the most common measurement taken by field scientists working in tidal waters.

Recalling the basic definition of an estuary, based on the mixing of salt water (ocean influence) and fresh water (river/land influence), it reasons that salinity values in estuarine systems vary by location. As a general trend, locations closer to the saltwater influence (ocean via an inlet) will exhibit higher salinity values than those toward the inland areas where rivers and landmass runoff contribute fresh water to the system (Figure 1).

There are other factors that affect salinity values in estuaries:

Precipitation vs. evaporation
Precipitation decreases salinity, evaporation increases salinity. In well mixed systems such as the Mullica River Great Bay estuary evaporation has little affect on salinity values. Evaporation can have much larger effects on systems with less flushing action (of seawater) and areas of the marsh where salt ponds are formed on the marsh surface. Many of these areas may only see saltwater influence a few times a month so in between these periods of higher tides there can be significant evaporation, leading to increased salinity values that can reach over 40 ppt.

Tidal influence
In this part of the world we experience diurnal tides, meaning we have 2 high tides and two low tides per 24 hour (approximate) time period. These tides are further influenced by moon phases. See below.

Type of tide Moon phase Affect
Spring tides new and full moon higher highs, lower lows, more current flow and water exchange
Neap tides 1st and 2nd quarter lower highs, higher lows, less current flow and water exchange

Flood tides increase salinity in a given location; ebb tides decrease salinity in a given location (Figures 2 and 3).



An excellent way to observe the effects of tides on salinity values is to examine data from a stationary monitoring station. At the Marine Science and Environmental Field Station water parameter data, including tide level and salinity, is collected every 30 minutes. Figure 4 is a graph from a two week period:

The following observations from this graph are important to consider:

  • salinity ranges from a low of ~ 10 ppt to a high of ~20 ppt
  • depth range reflects tide level; the instrument is fixed 1 meter from the bottom
  • salinity increases and decreases accordingly with the ebb and flood tides
  • high and low tide values (heights) vary with local conditions and the moon phase

How do we measure salinity?

Salinity is typically reported in units of parts per thousand (ppt). Seawater generally ranges from ocean values of 32 - 34 ppt to values of zero or near zero at upriver sites. Seawater can be measured by taking a known volume of water and evaporating the water off. Evaporation of seawater leave behind the soluble salts, which can then be weighed. See example 1.

Example 1. 1 liter of seawater is evaporated, leaving behind 26 g of salts salinity = 26 ppt
However, it is certainly not convenient to determine salinity via this method. The two most common methods for determining salinity are through the use of a refractomer or a salinometer.

Refractometers
Refractometers work on the basis of the refraction of light through a prism. These are handheld instruments that accept a small droplet of sample water that light travels through - the refractive index of light through that prism is a variable of the number of negative ions (dissolved salts) present. The value is displayed visually as a distinction between a light blue and a clear area against a visible scale. See below.

Salinometers
Salinometers are monitoring instruments that are common for the measurement of salinity data. A more collective term for these instruments today is multi-parameter water quality instruments, as most units are capable of measuring up to 6 or more parameters simultaneously.

The principle of their operation for salinity measurement lies in their ability to electrically measure the conductance of water. Conductance is the ease by which electricity flows through a medium and is enhanced by the presence of negative ions (soluble salts). The more salts present in a water sample the higher the conductivity. Two small electrodes are present on the probe end (placed into the water) and the time required for a small charge of electricity to flow from electrode A to electrode B is calculated, providing the instrument with the conductivity of the water. The conductivity is processed to provide users with a salinity value in ppt. See below.

Salinity with depth
Salinity and temperature both affect the density of water. Colder and more saline water is denser than warmer, less saline waters. This increasing density of colder, saltier water is the basis for ocean circulation - in the polar regions where the surface water is cooled and salinity increases due to the formation of sea ice (which extrudes the salt as it freezes) water sinks, being continually replaced by surface waters. In a well mixed system where energy (currents and wave action) is relatively high there may not be much variation of salinity with depth. However, in low energy or especially deep areas within an estuary salinity generally increase up to a few ppt with depth (from surface to bottom).