Isohalines are lines (or contours) that join points of equal salinity in an aquatic system. Isohaline position refers to the distance (kilometres) of a near-bottom isohaline (usually 2 ppt) from the mouth of a coastal waterway (Figure 1).
Figure 1. Schematic illustration showing the 30, 10, 2 and 1 ppt isohalines in vertical section and how the 2 ppt isohaline position is measured.
In the San Francisco Estuary, the 2 ppt near-bottom isohaline (X2):
Although the 2 ppt isohaline has not been trialled as a habitat indicator in Australian coastal waterways, it is highly likely that isohalines in the range from 1 - 5 ppt also have physical and ecological meaning here. This is because important changes in the ionic structure of water occur in the salinity range from 1 - 5 ppt in a seawater-meteoric water mixing system, and there is evidence that these changes influence the species of crustaceans found in salt lakes . These changes, which may also affect the species composition of estuarine organisms, include:
The position of near-bottom isohalines change in response to the amount of freshwater inflow and thus can be used as a guide to policy makers and managers for setting environmental flow standards .
Isohaline position probably has widespread application as a habitat indicator in coastal waterways .
Salinity distribution should be monitored continuously from at least six stations spaced roughly 5 km apart .
Although the 2 ppt isohaline position is a good indicator of an estuarine communities response to net freshwater flow, other hydraulic features can have effects on resource levels and population abundances that are independent of X2.
Lynda Radke, Geoscience Australia