This document is Chapter 14 of the Volunteer Estuary Monitoring Manual, A Methods Manual, Second Edition, EPA-842-B-06-003. The full document be downloaded from: http://www.epa.gov/owow/estuaries/monitor/ Voluntary Estuary Monitoring Manual Chapter 14: Salinity March 2006 Chapter 1Salinity4 Because of its importance to estuarine ecosystems, salinity (the amount of dissolved salts in water) is commonly measured by volunteer monitoring programs. Photos (l to r): U.S. Environmental Protection Agency, R. Ohrel, The Ocean Conservancy, P. Bergstrom Unit Two: Physical Measures Chapter 14: Salinity Overview Because of its importance to estuarine ecosystems, salinity (the amount of dissolved salts in water) is commonly measured by volunteer monitoring programs. This chapter discusses the role of salinity in the estuarine environment and provides steps for measuring this water quality variable. 14-1 Volunteer Estuary Monitoring: A Methods Manual Chapter 14: Salinity Unit Two: Physical Measures About Salinity Salinity is simply a measure of the amount of mixing the two masses of water. The shape of salts dissolved in water. An estuary usually the estuary and the volume of river flow also exhibits a gradual change in salinity throughout influence this two-layer circulation. See its length, as fresh water entering the estuary Chapter 2 for more information. from tributaries mixes with seawater moving in from the ocean (Figure 14-1). Salinity is usually Role of Salinity in the Estuarine Ecosystem Salinity levels control, to a large degree, the Non-Tidal types of plants and animals that can live in Fresh Water different zones of the estuary. Freshwater Average (During species may be restricted to the upper reaches Annual Tidal Low Flow Salinity Tidal Limit Fresh Water Conditions) of the estuary, while marine species inhabit the < 0.5 ppt estuarine mouth. Some species tolerate only Oligohaline intermediate levels of salinity while broadly < 5.0 ppt adapted species can acclimate to any salinity Mesohaline ranging from fresh water to seawater. < 18.0 ppt Salinity measurements may also offer clues about estuarine areas that could become afflict­ Polyhaline ed by salinity-specific diseases. In the Chesapeake and Delaware bays, for example, pathogens infecting oysters are restricted to sec­ < 30.0 ppt Euhaline tions that fall within certain salinity levels. OCEAN (Marine) Drastic changes in salinity, such as those due to 0 drought or storms, can also greatly alter the expressed in parts per thousand (ppt) or /00. Figure 14-1. Estuarine numbers and types of animals and plants in salinity slowly increases The fresh water from rivers has a salinity of the estuary. as one moves away 0.5 ppt or less. Within the estuary, salinity lev­ from freshwater sources Another role played by saline water in an els are referred to as oligohaline (0.5-5.0 ppt), and toward the ocean. estuary involves flocculation of particles. mesohaline (5.0-18.0 ppt), or polyhaline (18.0­ Flocculation is the process of particles aggre­ 30.0 ppt). Near the connection with the open gating into larger clumps. The particles that sea, estuarine waters may be euhaline, where enter an estuary dissolved in the fresh water of salinity levels are the same as the ocean at more rivers collide with the salt water, and may floc­ than 30.0 ppt (Mitsch and Gosselink, 1986). culate or clump together and increase turbidity Generally, salinity increases with water depth (Figure 14-2). unless the estuarine water column is well Salinity is often an important factor when mixed. Salinity, along with water temperature, monitoring many key water quality variables. is the primary factor in determining the stratifi­ For example: cation of an estuary. When fresh and salt water meet, the two do not readily mix. Warm, fresh • Most dissolved oxygen meters require water is less dense than cold, salty water and knowledge of the salinity content in will overlie the wedge of seawater pushing in order to calibrate the meter properly. from the ocean. Storms, tides, and wind, how­ • If you are interested in converting the ever, can eliminate the layering caused by salin­ dissolved oxygen concentration (usually ity and temperature differences by thoroughly expressed as mg/l or parts per million) to 14-2 Volunteer Estuary Monitoring: A Methods Manual Unit Two: Physical Measures Chapter 14: Salinity percent saturation (amount of oxygen in the water compared to the maximum it could hold at that temperature), you must Fresh Water take salinity into account. As salinity increases, the amount of oxygen that water can hold decreases. • If you use a meter to measure pH, the Salt Water techniques are the same whether you are testing salt or fresh water. However, if you measure with an electronic colorimeter, you must use a correction factor (available from the Figure 14-2. Turbidity increases when fresh water meets with salt water. manufacturer) to compensate for the effects of salinity. ■ Sampling Considerations Salinity will fluctuate with movement of the chemical methods determine chlorinity (the tides, dilution by precipitation, and mixing of chloride concentration), which is closely the water by wind. There are also seasonal related to salinity. differences in salinity. Conductivity Season and Weather Conductivity is a measure of the ability of Environmental conditions vary with the water to pass an electrical current. seasons, and salinity levels can reflect those Conductivity in water is affected by the variations. During wet weather periods and presence of inorganic dissolved solids such as during the spring thaw in colder regions, more chloride, nitrate, sulfate, and phosphate fresh water enters the estuary, so salinity is anions (ions that carry a negative charge) or lower at these times. On the other hand, dry sodium, magnesium, calcium, iron, and weather periods mean less fresh water aluminum cations (ions that carry a positive entering the estuary, so higher salinity levels charge). As the concentration of salts in the may be found. Another way the seasons water increases, electrical conductivity rises— influence an estuary’s salinity involves the the greater the salinity, the higher the mixing of fresh water and salt water. Seasonal conductivity. Organic compounds like oil, storms help mix estuarine waters and serve to phenol, alcohol, and sugar do not conduct decrease the vertical salinity and temperature electrical current very well and, therefore, gradients in the estuary. have a low conductivity when in water. Conductivity is also affected by temperature: Choosing a Sampling Method the warmer the water, the higher the conductivity. For this reason, conductivity is Salinity can be measured either by physical extrapolated to a standard temperature (25°C). or chemical methods. Physical methods use Conductivity is measured with a probe and conductivity, density, or refractivity. The a meter. Conductivity meters require physical methods are quicker and more temperature correction and accurate convenient than the chemical methods. The 14-3 Volunteer Estuary Monitoring: A Methods Manual Chapter 14: Salinity Unit Two: Physical Measures Figure 14-3. calibration can be difficult. The cost of these specific gravity. The volunteer can calculate Hydrometer. meters ranges from $500 to $1,500. Voltage is salinity by measuring a water sample’s Salt water is 1.0 00 .005 denser than applied between two electrodes in a probe specific gravity. This is done with a .010 fresh water and immersed in the sample water. The drop in hydrometer (Figure 14-3). .015 has a greater voltage caused by the resistance of the water Hydrometers are a fairly simple and .020 specific gravity. is used to calculate the conductivity per inexpensive means of obtaining salinity. .025 Volunteers can .030 calculate salinity centimeter. The meter converts the probe Specific gravity hydrometers cost from $13 to .035 by measuring a measurement to micromhos per centimeter $25 (although professional sets can cost much water sample’s .040 and displays the result for the user. more), and a hydrometer jar costs about $13 specific gravity .045 Some conductivity meters can also be used to $15, although you can also use a large, .050 with a hydrometer. to test for total dissolved solids and salinity. clear jar that is deep enough to float the .055 .060 The total dissolved solids concentration in hydrometer. .065 milligrams per liter (mg/l) can also be Because hydrometers measure specific calculated by multiplying the conductivity gravity, the presence of suspended solids can 1.0 75 result by a factor between 0.55 and 0.9, which raise hydrometer readings and result in a is empirically determined (see APHA, 1998). salinity measurement that is higher than the Meters should also measure temperature true value. This has especially been shown in Specific Gravity and automatically compensate for temperature low salinity areas (Bergstrom, 1997; in the conductivity reading. Conductivity can Bergstrom, 2002). Volunteer programs, be measured in the field or the lab. In most therefore, should consider their accuracy and cases, it is probably better if the samples are precision requirements before electing to use collected in the field and taken to a lab for a hydrometer. testing. In this way, several teams of volunteers can collect samples simulta­ Refractivity neously. If it is important to test in the field, Refractometers (Figure 14-4) are used to meters designed for field use can be obtained measure substances dissolved in water, using for around the same cost mentioned above. the principle of light refraction through liquids. The more dissolved solids in water, Density the slower light travels through it. As water becomes saltier, its Refractometers measure the change in the 100 weight increases although its direction of light as it passes from air into 90 volume does not measurably water. Salinity and temperature both affect 80 rise. Since salt water is denser the index. 70 than fresh water, this change Refractometers use a scale to quantify the 60 of weight results in a greater effect that dissolved solids in water have on Salinity Specific 50 40 Gravity light.