Water

Water availability is a critical factor for organisms, unsurprisingly since life on Earth originated in water. In deserts, water is so scarce that only species with special adaptations can survive there; the productivity and geographic distribution of plants in general depend heavily on water availability. For organisms living in water bodies -- oceans, lakes and rivers -- the chemical composition and pH of the water matter greatly, and one especially important variable is salinity, the concentration of dissolved salts, usually measured in parts per thousand (ppt). Fresh water in streams, lakes and rivers typically has a salinity below 5 ppt, while seas and oceans run 30 to 35 ppt, and some hypersaline lagoons can reach up to 100 ppt. Species tolerant of a wide range of salinities are called euryhaline, while those confined to a narrow salinity range are called stenohaline. Because of the …