Water Quality: Salinity

This month, we are talking about a water quality parameter that really doesn’t tell us much about the health of the water but is still an important piece of data to collect; salinity. Salinity is the amount of salt in the water and is typically measured in parts per thousand (ppt) or practical salinity units (psu). Parts per thousand is a reference to the total grams of salt per kg of water, with the average ocean salinity being 35 grams of salt per kilogram of water, which equates to 35 ppt. Practical salinity units are basically the same but also account for temperature, conductivity, and pressure, making this unit easier to use for calculations but harder to measure precisely.

Salinity, while not a parameter to measure the health of an ecosystem, is still an important factor to consider and measure when studying any aquatic system. Many plants and animals have specific tolerance ranges for the amount of salt in the water, so knowing the salinity can help us understand what animals might be present in that area. Salinity can also fluctuate due to environmental factors like freshwater input (which includes rainfall), evaporation and temperature. Across the world’s oceans, proximity to land and glaciers, as well as solar intensity (and therefore evaporation) can shift the salinity of the water and surface ocean salinities range from 32 to 37 ppt. Because salinity affects the density of water (water with more salt is more dense than water with less salt), ocean currents below the surface are driven by salinity.

We even see these salinity driven currents in the Chesapeake Bay, through tidal currents and freshwater flow. Ocean water pushes up into the Bay as the tide comes in and this water tends to be closer to the bottom since it is more dense. Fresher water tends to flow southward, down tributaries and out to the ocean, and this water tends to be higher up in the water column (at the surface). Some animals will use these currents to move around the Chesapeake Bay, including larval blue crabs as they return to the Bay from spending a month or so on the Continental Shelf.

Anytime someone is collecting water quality data, they are typically including a salinity reading as part of that work. Here at Easton Point Park, salinity varies but it can help provide valuable information. Following major rain events, surface salinity here can reach as low as 5 ppt, while the average salinity is around 10 ppt. During drought periods, the salinity can reach as high as 16 ppt. When major rain events occur, we also typically see lower turbidity (more stuff in the water due to runoff) and may see increased levels of E. coli bacteria. If we aren’t sure there was a major rain event, the salinity can help us understand whether or not a large amount of freshwater entered the system. Sometimes it’s hard to tell because storms around here can be patchy – during a storm event at the beginning of June, Unionville got 5 inches of rain and St. Michaels got 0. That same week, Easton Point failed the Swimmable ShoreRivers E. coli screening at a level we haven’t seen before, 5794 CFU. This is more than 50 times the safe limit of 104 CFU recommended by the EPA. This number, along with low salinity readings, shows that even though rainfall was patchy, a significant amount of freshwater runoff entered the Tred Avon watershed following that storm.

A refractometer

When it comes to measuring salinity, there are two ways to do it. One, which provides salinity in ppt, simply measures the density of water, ie. how much stuff is dissolved in it. For this, we can use relatively simple methods including a refractometer or a hydrometer. For a refractometer, we place a few drops of water on a screen, look through the device, and quickly get a measurement of how much salt is dissolved in the water. Meanwhile, a hydrometer measures the density of water and converts it to salinity using a scale. More complex and sensitive devices actually measure the electrical conductivity of water by passing an electrical current between two electrodes. The amount and make up of the salt in the water between those two currents is then read and converted into salinity in psu. YSIs and other electronic salinity measuring devices use this method.

Here in the Chesapeake Bay, there are no specific standards for salinity in the Chesapeake Bay Watershed Agreement or any other water quality guidelines because it is not readily influenced by human activity, nor is a specific measure of salinity a sign of an impaired ecosystem. 

Rather, estuaries like the Chesapeake Bay are broken into several different regions based on average salinity and these regions help us understand the ecosystems and animals that thrive in a given area. 

In the image below, you can see the standard zones from the ocean (most salty) to non-tidal freshwater (no salt). In the Chesapeake Bay, these zones can help explain the distribution of important species such as submerged aquatic vegetation (SAV). Eelgrass prefers more salt, so it is typically seen in the polyhaline zone. Widgeon grass, which is seen here in Talbot County, has a wide distribution because it can tolerate a wide range of salinity. Redhead grass prefers moderate salinity to fresh water, so it is likely to be seen in the mesohaline and oligohaline regions of the Chesapeake Bay. Wild celery and water star grass are freshwater SAV species, so they are more likely be found in the tidal fresh regions.

We see similar distributions for animals as well. As adults, blue crabs can tolerate a wide range of salinity, so they can be found throughout the Chesapeake Bay. However, as larval crabs, they need to be in full salt water, so pregnant females head to the mouth of the Chesapeake Bay to produce and hatch their egg masses. Another species of crab that can be found in the Chesapeake is the Lady Crab but they prefer polyhaline or euhaline water so you’ll only see them around the mouth of the Chesapeake Bay. This pattern is true for many other creatures as well, which helps to highlight why salinity is important.

General salinity zones in a estuary system

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