REDEKE (1933) defines brackish water as a mixture of sea water s.str. and fresh water, and some authors distinguish estuaries from brackish water. From the biological point of view, however, all waters of moderate salinity can be regarded as brackish, as REMANE (1958) points out. The terms, poikilohaline and homoiohaline, could be used to divide brackish waters into two types, in a sense somewhat modified from that of DAHL (1956). Thus, brackish waters of unstable and variable salinity, temporally but not spatially, are referred to as poikilohaline while those of temporally stable salinity are termed homoiohaline. In poikilohaline brackish waters, the productivity does not always appear to coincide with increase of nutrients, because the unstability of salinity may result in limiting the immigration of both the marine and the freshwater euryhaline forms. It is generally believed that life began in the sea and then migrated landwards. Most of the migration may have begun by entering brackish waters. There, homoiosmosity may first have been required of immigrating organisms. The biological significance of brackish waters may be here, and this is also the principal motive of the present review.
