Field-based method for evaluating the annual maximum specific conductivity tolerated by freshwater invertebrates
A field-based model was developed to identify the annual maximum specific conductivity (SC) that may be tolerated by salt-intolerant genera and that may be used as an acute benchmark or ambient water quality criterion for SC. This approach used only water chemistry. The results for the chemical method from a case example in Central Appalachia were compared with a model of paired chemical and biological data from the same ecoregion. The chemical method estimated the 90th centile of SC observations at sites in streams that attained a specified stream annual average SC. The specified annual average was the SC estimated to extirpate 5% of genera in streams. The paired chemical-biological method identified the maximum SC that occurred in a stream during the lifetime of observed salt-intolerant genera having a univoltine phenology with spring emergence. The analysis used observations of the 5% most salt-intolerant genera in the region and the maximum of 6−12 SC measurements taken in the preceding 12 months. Salt-intolerant genera were rarely observed at locations exceeding the threshold calculated using chemical method thus corroborating the results of the chemical method. The method using only SC (water chemistry) data is practical because it is less expensive to sample SC compared to sampling SC and biological samples.