Jeff Bowman         

Senior Thesis

 

 

 

 

What is salinity?

Salinity can be defined several ways, depending on how it is measured.  In general terms salinity refers to the amount of charged, dissolved particles present in a volume of water.  These charged particles are salts, and in their undissolved form include NaCl, NaSO4, KCl, and MgSO4 among many others. 

 

When salt dissolves in water the metal ion (Na, K, and Mg from the above examples), called a cation, retains a positive charge.  The rest of the molecule, called an anion, retains a negative charge.  In cases where the dominant anion has a charge of -1 the salinity can be measured by the conductivity of the water.  This is the technique most commonly used in marine environments, where the dominant anion is Cl which has a charge of -1.  Salinity measured in this manner can be reported by conductivity, or converted to the practical salinity scale (PSS).  PSS is simply a ratio of measured conductivity against the conductivity of a standard.

 

Inland saline environments often present a more complex group of ions with varying charges.  As a result conductivity does not correlate well with salinity.  In the case of very saline (hypersaline) lakes, the conductivity of the brine exceeds the current of the measuring device.  In this case conductivity will always underestimate the amount of salt present.

 

Because of this the only way to accurately measure salinity in these environments is to analyze each ion independently, an expensive and time consuming operation.  A less accurate, but still reliable solution is to evaporate all the water from a sample and weigh the amount of material that remains, called the total dissolved solids (TDS).  As an alternative the specific gravity of the water can be measured, though this is less common.  The most accurate means of reporting salinity is to report absolute salinity, an index derived by dividing TDS by specific gravity.  This reduces some of the error inherent in measuring out and weighing volumes of water by hand.

 

All of this raises some questions about the physiological responses of microorganisms to salinity.  Exactly which ion is the study organism responding to as salinity increases?  Since Na+ is the most widely distributed constituent of salinity, my experiments have focused on changes in the concentration of NaCl.
Salt (primarily NaCl) remaining after evaporating 10 mL of water from the North Great Salt Lake, approximate salinity 275 g L-1.

Salt (primarily NaCl) remaining after evaporating 10 mL of water from the North Great Salt Lake, approximate salinity 275 g L-1.