Water conductivity in stream environments

Background.  Conductivity, a meaure of electrical current flow through a solution, is expressed in units of microSiemens (uS). Conductivity is the reciprocal of electrical resistance (ohms).  Because conductivity increases nearly linearly with increasing ion concentration, we can use conductivity measurements to estimate ion concentrations in solutions.  Seawater conductivity is approximately 5000 uS, campus tap water is about 110 uS, and distilled water is near 0 uS. College Grant streams have low conductivities (between 12 and 47 uS) and thus have low concentrations of dissolved ions.  Conductivity can tell us a lot about the nature of a stream. For example, low-conductivity streams typically have less groundwater input than high-conductivity streams, i.e., they are "precipitation-dominated."  Thus, they also have more dynamic flow and temperature regimes.  Conductivity also indicates the degree to which a watershed's bedrock and mineral soil resists erosion.  The Little Dead Diamond River has the highest conductivity value of the streams we regularly monitor (see Table) indicating that it has relatively high levels of groundwater input; this is also supported by cool summer water temperatures, numerous iron seeps, and easily detected groundwater input.

Measurement.  Water conductivity is measured by the diminution of electrical current passed through a water sample.  Higher concentrations of electrolytes in the sample solution permit a larger fraction of the current emitted by one probe to reach the sensor probe, and produce a higher the conductivity measurement (greater uS).  We measure water conductivity with a YSI model 85 handheld meter system equipped with a conductivity sensor.  Conductivity is influenced by temperature (uS can change by up to 3% per °C) so measurements are sometimes temperature compensated to correspond to uS at 25° C.  The YSI model 85 can provide either raw conductivity measurements or measurements compensated according to the effects of temperature on conductivity of KCl in solution. Calibration of the conductivity sensor with reference solutions of known conductivity is critical to obtaining good measurements. 

Biological relevance. Organisms in precipitation-dominated streams, indicated by low conductivity, must be able to withstand floods, dry downs, and scouring by anchor ice.  Many have adaptations to persist through stressful periods in a state of quiescence or to seek refuge in protected microhabitats.  Most animals in low-conductivity streams must also maintain high internal ion concentrations relative to those in the surrounding environment.  Conductivity is linearly related to osmolarity (mOsm / liter = 5.79 + 0.0734 uS for NaCl). Thus, the waters of the Diamond River watershed have ionic concentrations of only 6-10 mOsm / liter compared to 200-300 mOsm / liter for typical freshwater invertebrates.  Because these animals are extremely hyperosmotic to their environment, there is a tendency for uptake of water and loss of ions, especially across respiratory surfaces, which are necessarily permeable.  Therefore osmoregulation involves the elimination of water, the retention of ions, and active transport of ions from the external medium into the animal. Freshwater animals exhibit numerous structural and physiological adaptations to minimize the costs associated with osmoregulation.  Nonetheless, it has been estimated that osmoregulation accounts for >30% of the total energetic expenditures of some aquatic organisms.  It is possible, but untested, that the aquatic communities of the Diamond River watershed are strongly shaped by differential abilities among species for osmoregulation and that the energy budgets of residents are dominated by the costs of osmoregulation. 

Water conductivity (uS) at Second College Grant during 1997-99.

Stream	Low	High	Mean
Diamond River	14	47	34
Dead Diamond River	13	45	16
Little Dead Diamond River	23	46	32
Loomis Valley Brook	13	27	24
Horne Brook	13	26	20
Swift Diamond River	13	45	28
Alder Brook	14	33	27
Johnson Brook	14	34	30
Bennett Brook	22	38	33