English: Annual mean sea surface salinity from...

English: Annual mean sea surface salinity from the World Ocean Atlas 2005. Salinity here is in practical salinity units (PSU). It is plotted here using a Mollweide projection (using MATLAB and the M_Map package). (Photo credit: Wikipedia)

We all know that there is a difference between freshwater and saltwater. We also know that the amount of salt in saltwater can vary from area to area. Now you’re probably wondering why knowing and understanding it even matters, right? The salt in the water does a lot more than just taste bad when you accidentally  swallow some when a wave smacks you in the face when you are swimming at the beach. It does much more than some people believe in fact. The salt content of water can affect the weather and climate of a region, and it determines what species live in an area.

Salinity: The Big Picture

In its broadest sense, salinity is the concentration  of salt ions in seawater (NOAA). This is measured most commonly in parts per thousand or ppt, but it has recently been measured in practical salinity units or psu (NSIDC 2015). It can be calculated as the amount of salt (in grams) dissolved in 1,000 grams of seawater (Ocean Salinity). There are several different devices to measure salinity such as refractometers, hydrometers, and conductivity probes  just to name a few (Reefkeeping 2008). There will be more on some of these devices in a later article.

About 86% of global evaporation and 78% of the global precipitation occur over the ocean (NASA 2013). Ocean surface salinity is very important for understanding how freshwater input and output can affect the ocean dynamics (NASA 2013). By tracking ocean surface salinity,  we can directly monitor variations in the water cycle such as: land run off, sea ice freezing and melting,  and evaporation and precipitation over the oceans (NASA, 2013). The ocean’s surface currents in the upper part are wind driven, but deep below the surface circulation is primarily driven by changes in sea water density which are mostly determined by salinity and temperature (NASA 2013).  

The ocean stores more heat in the top three meters than the entire atmosphere (NASA 2013).  Therefore,  density controlled circulation is key to transporting heat in the ocean and maintaining the Earth’s climate. Studies suggest that seawater is becoming fresher in higher latitudes and tropical areas that are dominated by rain, while in subtropical high evaporation region waters are getting saltier (NASA 2013). These changes in the water cycle could significantly impact not only ocean circulation but also the climate in which we live (NASA 2013).  For example, the Arabian Sea has been measured to be much saltier than the neighboring Bay of Bengal (NASA 2013). One big difference is the Bay of Bengal gets hit with intense monsoon rains and receives even more freshwater from rivers flowing into it (NASA 2013). Another example is that there is a large patch of very saline water across the North Atlantic, this area is the saltiest anywhere in the open ocean (NASA 2013). It can be compared to a desert because there is very little rainfall and an abundance of evaporation (NASA 2013).

Salinity variations are one of the main drivers of ocean circulation (NASA 2013).  In cold polar region changes in salinity affect ocean density more than changes in temperature (NSIDC 2015).  When salt is ejected into the ocean and sea ice forms, the water salinity increases (NSIDC 2015).  Since saltwater is heavier the density of the water increases and the water thing the exchange of salt between sea ice and the ocean influences ocean circulation across hundreds of kilometers (NSIDC 2015 ). Freshwater freezes at 0℃, but the freezing point of saltwater varies (NSIDC 2015). For every 5 ppt increase in salinity the freezing point of water decreases by 0.28℃ (NSIDC 2015).  In cold regions, changes in salinity affect ocean density more than changes in temperature (NSIDC 2015). So as sea ice forms and the salt levels increase, the water becomes heavier which causes changes in the ocean circulation for hundreds of kilometers (NSIDC 2015).

Salinity on a smaller scale

A diagram of an estuary showing how the salinity changes from one end to the other. Image courtesy of Toxipedia.

For this section we are going to look at how it can affect smaller areas, like estuaries. In estuaries salinity levels are generally highest near the mouth of the estuary where the ocean water comes into the estuary (NOAA 2008). The lowest is at the head of the estuary where the fresh water flows in (NOAA 2008).   The actual salinity varies throughout the title cycles and during seasonal changes (NOAA 2008).  Salinity levels decrease during the spring months when snow is melting and the rains because the freshwater runs into the rivers (NOAA 2008). These levels increase during the summer months when higher temperatures increase the  levels of evaporation in the estuary (NOAA 2008).  The organisms that live in estuaries have different tolerances and responses to these dramatic salinity changes (NOAA 2008). Many bottom dwelling animals like oysters, crabs, and polychaete worms, can tolerate some changes in salinity but when those levels get outside their tolerance range it can negatively impact their growth, reproduction, and their survival (NOAA 2008).  It also affect other chemical conditions within the estuary (NOAA 2008).  For example, too much or too little salt can change the amount of oxygen that can be dissolved in the water which can negatively impact the organism living there (NOAA 2008).

 

Let’s go even smaller

Now we are going to break down a little further since some people do have salt water aquariums. From my personal experience in caring for saltwater tank for my freshman Ichthyology class and taking care of freshwater tank currently; saltwater ones may be more diverse and way more fun, but they are also a lot more work if you don’t take care of it correctly especially when it comes to the salinity. If the salinity is off even a bit it can cause massive algae blooms and even kill what you have in your tank depending on what you have in it. Too high or too low of a salt levels can cause a lot of stress on your tank. So regularly checking your salinity with your refractometer and adjusting with more brine water or freshwater as needed, you can keep your tank happy.

Conclusion

As you can see, salinity is not just how salty the water is. It has a lot more components and is much more important than that. Salinity is so important that to keep track of, in 2011 aboard the Argentine spacecraft Aquarius,  NASA put its first satellite instruments that specifically studies the salt content of the ocean’s surface waters (NASA 2013). The sensors on the satellite detect microwave emissivity of the top 1 to 2 centimeters of the ocean’s surface, a physical property that varies depending on the temperature and saltiness (NASA 2013).  This instrument collects data in a 386  kilometer  wide survey  designed  to obtain complete reading of global salinity every seven days (NASA 2013). It has already greatly increased our knowledge and understanding of how our oceans and climate are so intertwined. Salinity can affect things on a large scale and a small one, and that’s your skinny on salinity.

 

References

Gilbert, Steven. Estuary. Toxipedia. 2014

“Salinity”. National Aeronautics and Space Administration. 2013.

“Salinity”. NOAA Ocean Service Education. 2008.

“Salinity versus Depth”. Ocean Salinity. ND.
“Salinity and Brine”. National Snow and Ice Data Center: All About Sea Ice.  2015.