There is no horohalinicum

Species abundance declines to a minimum (the Artenminimum) between 5 and 8‰, not only in estuaries, but in all bodies of brackish water. Khlebovich (1968) examined published hydrochemical data for estuaries and concluded that sharp changes in the ionic composition of seawater diluted with fresh water occur at salinities below 5 to 8‰. He further argued that these ionic changes constitute a physico-chemical barrier between marine and freshwater faunas. Kinne (1971) gave the name “horohalinicum” to the segment of the salinity gradient between 8 and 5‰. We have re-examined the data used by Khlebovich (1968) and found that, in fact, while the ionic composition of diluted seawater changesslightly between 8 and 5‰, the changes in ionic ratios below 2‰ are much larger. Thus, the proposed physico-chemical barrier does not exist between 8 and 5‰; it cannot then explain the Artenminimum; and there is no basis for the horohalinicum concept of Kinne (1971). Two ecological explanations for the occurrence of the Artenminimum—a species-area effect and the stability-time hypothesis—are discussed and found to be inconsistent with published data on species distributions in brackish waters. The low species diversity of brackish water may be explained, in part, by two factors: few animals evolve those physiological mechanisms required for life in the variable habitat; and these species, which are very eurytopic, have low rates of speciation.