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CADDIS Volume 2: Sources, Stressors & Responses

Ionic Strength

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Authors: C.R. Ziegler, G.W. Suter II, B.J. Kefford, K.A. Schofield, G.J. Pond

Ionic strength is the concentration of ionic charge in solution. Increased ionic strength and changes in ionic composition may lead to shifts in community composition and function based on factors such as taxa-specific preferences and adaptations. Measurements of electrical conductivity, salinity, and total dissolved solids (TDS) are often used to represent the ionic strength of water and generally increase with increasing ion content (see the Ways to Measure tab). This module provides advice for deciding whether or not to include increased ionic strength as a candidate cause of biological impairment.

Because ionic strength issues include a broad range of potential freshwater problems, we are forced to generalize about overall effects. There will be exceptions to these generalizations, especially in terms of taxa-specific reactions to various ion-specific stresses. Nevertheless, this stressor module introduces a common language and identifies some of the more widespread ion-related issues. Ultimately, causal assessors may need to dig deeper into site-specific characteristics and relevant literature.

Ionic compounds are natural constituents of both inland and marine systems, and are not harmful unless levels exceed or fall below the tolerance range of aquatic organisms. Indeed, some constituents of ionic compounds are essential elements, necessary for the survival of aquatic organisms. Salts are ionic compounds composed of cations (positive charge) and anions (negative charge). Common salt ions include:

Generalization of species richness versus salinity, redrawn and adapted from Remane (1971) and Wetzel (2001).
Figure 1. Generalization of species richness versus salinity. Fresh, inland surface waters usually have salinities below 0.5 parts per thousand (ppt), whereas the salinity of ocean water is approximately 35 ppt.
Redrawn and adapted from Remane (1971) and Wetzel (2001).
  • Cations:    Na+    Ca2+    Mg2+    K+
  • Anions:    Cl-    HCO3-    CO32-    SO42-

Ionic strength varies naturally across aquatic ecosystems (Figure 1), and aquatic organisms generally prefer waters with specific ionic strength ranges, containing specific ions. When these parameters are changed, biota may be adversely affected. In recent years, several states (e.g., Florida, West Virginia) have adopted criteria which address the importance of ionic strength in determining water quality.

Ionic strength may significantly impact freshwaters through interactions with other stressors, and it may be difficult to distinguish among proximate stressors and interacting stressors. Potential interactions include:

  • Increased salinity may foster or hinder uptake of toxic substances by organisms (Bidwell and Gorrie 2006, Zalizniak et al. 2006, Environment Canada and Health Canada 2001).
  • Salts impact soil structure, potentially decreasing soil permeability (Rengassamy 2002), increasing runoff volume, and increasing soil erosion (Environment Canada and Health Canada 2001).
  • Salt solubility increases as temperature increases.

Salinity, along with temperature and altitude, also affects the solubility of oxygen in water. Freshwater (salinity generally < 0.5 parts per thousand) at 22°C and sea level can hold approximately 8.7 mg/L of dissolved oxygen (DO). Ocean water (salinity ≈ 35 parts per thousand) can hold only about 7.1 mg/L of DO at saturation. DO saturation might change by 6% over a 10 parts per thousand difference in salinity; therefore, with freshwater ecosystems generally falling below 0.5 parts per thousand, the associated change in DO over the spectrum of freshwater salinity levels is negligible (Stickney 1979).

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Checklist of sources, site evidence and biological effects

Simplified generic conceptual model for ionic strength
Figure 2. A simple conceptual diagram illustrating causal pathways, from sources to impairments, related to ionic strength. Click on the diagram to go to the Conceptual Diagrams tab and view a larger version.

This module addresses ionic strength as a proximate stressor; concentration of hydrogen ions (pH) and heavy metal ions (e.g., lead and zinc), while related to ionic strength and composition, are discussed in separate modules.

Ionic strength should be listed as a candidate cause when potential human sources and activities, site observations, or observed biological effects support portions of the source-to-impairment pathways in the conceptual diagram for ionic strength (Figure 2). This diagram and some of the other information also may be useful in Step 3: Evaluate Data from the Case.

The checklist below will help you identify key data and information useful for determining whether to include ionic strength among your candidate causes. The list is intended to guide you in collecting evidence to support, weaken, or eliminate ionic strength as a candidate cause. For more information on specific sources and activities, site evidence, and biological effects listed in the checklist, click on checklist headings or go to the When to List tab of this module.

Consider listing ionic strength as a candidate cause when the following sources and activities, site evidence, and biological effects are present:

Sources and Activities
  • Road salt
  • Land cover alteration, leading to dryland salinity
  • Water withdrawal
  • Irrigation
  • Combustion wastes
  • Resource exploration and extraction (i.e., mining activities)
  • Sewage and industrial waste discharges
Site Evidence
  • Signs of snow disposal
  • Crystalline deposits
  • Mineral precipitates
  • Loss of vegetation
  • Presence of salt-tolerant plants
  • Decreased productivity of aquatic vegetation
Biological Effects
  • Unlike other stressors, “rules of thumb” for ionic strength are not listed here. See the When to List tab for common examples of potential biological effects due to changes in ionic strength.

Consider contributing, modifying, and related factors as candidate causes when ionic strength is selected as a candidate cause:

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