Hydrochemical parameters governing the occurrence of estuarine and marginal estuarine ostracodes; an example from south-central Alasksa

Salinity is generally believed to be the primary hydrochemical parameter responsible for determining which ostracode species occur in a particular environment. The importance of solute composition to ostracode occurrence has either been ignored or viewed as being secondary to salinity. However, the transition zone between nonmarine and marine waters often coincides with major (ordinal, superfamilial) ostracode faunal boundaries and substantial solute compositional differences but small salinity differences. Studies of ostracode occurrences from south-central Alaska and other areas suggest that most ostracode species are restricted to waters containing particular solute types. Thus, marine ostracodes not only live in Na+ and Cldominated marine water, but when they are introduced to nonmarine environments, they can only survive in waters whose dominant ions are Na+ and C1-. Nonmarine taxa that live in fresh water dominated by Ca2+ and HCO3ions typically cannot live in water dominated by Na+ and Clions or ions characterizing other solute types. These solute-controlled ostracode occurrences frequently result in near mutual exclusion of marine and nonmarine taxa in estuaries with a cold and wet continental climate, because that climate often leads to nonmarine solutes dominated by Ca2+ and HCO3ions. Co-occurrence of marine and nonmarine ostracode species is common in areas where the continental climate is warm and/or dry because such climates often lead to the solute compatibility of marine and nonmarine waters. Salinity appears to be important for determining the abundance distribution of an ostracode species within a particular solute type, but it only secondarily determines occurrence. The estuarine taxa upon which this study is based are all new species and are described in this paper. We propose the following new species: Elofsonia susitnensis, Cytheromorpha knikensis, Cytherura tonsinensis and Leptocythere klutinensis.