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INTRODUCTION Regardless of source, natural seawaters (= NSWs) contain various ions in essentially the same proportions [A3, A5]. NSWs differ principally in total ionic content and, therefore, in osmolarity, salinity, and conductivity. Salinity is defined as total grams of solid per kilogram of seawater, after complete oxidation of organic matter and conversion of halides and carbonate to chloride and oxide, respectively [A3]. Constancy of ionic proportions permits determination of salinity from Cl- measurements alone, and conductivity is also commonly used to measure salinity [A5]. Average salinity of the oceans is reported as 34.7 o/oo (34.7 parts per thousand by weight [A3]), and salinity of "Woods Hole" NSW as 31 o/oo [A1]. COMPOSITION AND USE OF NSWs Although nearly all of the elements are present in NSWs, >99% of the salinity of NSWs is attributable to only six major ions ([A3, A4]; Table 1). NSWs also contain miscellaneous particulates, which are normally removed by filtration prior to experimental use. Initial filtration can be accomplished by passage through glass wool (or synthetic fiber substitutes) and clean sea sand [B13]. Filter paper has also been employed [A9]. For studies of development and regeneration, filtration through ~0.2µm polycarbonate filters is recommended [B2, B13]. Natural pH buffering of NSWs is provided by bicarbonate; however, some researchers have adjusted the natural pH or added other buffers to NSWs for special purposes [A7]. Dilution of filtered NSWs with deionized water yields seawaters of varying salinities and osmolarities for experimental use [A8]. |
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