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W WATER SEE SEAWATER WATER CHEMISTRY PATRICIA M. SCHULTE University of British Columbia, Vancouver, Canada Water chemistry is the collective term used to describe the types and amounts of all of the substances present in a particular sample of water, including dissolved salts, minerals , gases, nutrients, and other chemicals. Along with temperature, desiccation, and wave action, water chemistry is one of the critical abiotic factors that affect the type and number of organisms living in the rocky intertidal. Four aspects of water chemistry have substantial effects on these organisms: () the salinity of the water, () the concentration of dissolved gases in the water, () the pH of the water, and () the amount of dissolved nutrients available to photosynthetic microorganisms, plants and algae—the primary producers that form the base of the food web in the rocky intertidal. WATER CHEMISTRY IN TIDEPOOLS At high tide the water chemistry of the rocky intertidal is similar to that of the surrounding seawater, but as the tide recedes, the water that is trapped in the rocky depressions that form tidepools can undergo very large changes in water chemistry, and these changes can have profound effects on tidepool life. The extent of change in water chemistry during the tidal cycle varies among tidepools, depending on their location within the intertidal zone and other physical characteristics such as their size, shape, and orientation with respect to the sun. As a result, even adjacent tidepools can have somewhat different water chemistries at low tide, varying in salinity, dissolved gases, pH, and nutrient availability. SALINITY As shown in Table , seawater contains a mixture of various dissolved and ionized salts. Six of these (chloride, sodium, sulfate, magnesium, calcium, and potassium) make up more than % of the dissolved salts in seawater, but there are actually more than different elements or compounds in a typical sample of seawater. Many of these trace elements are very important for biological reactions. For example, cobalt is a component in vitamin B, and selenium is needed for the synthesis of thyroid hormones. The relative proportions of the various dissolved substances in seawater are fairly constant among oceans, but the total concentration of salts may vary. The term salinity is used to describe the total concentration of salts in a particular sample of water, and TABLE 1 Composition of Seawater Concentration Percent of Constituent (mg/L) total solids chloride (Cl- ) 19,000 55% sodium (Na+ ) 10,500 30% sulfate (SO42- ) 2,700 8% magnesium (Mg2+ ) 1,350 4% calcium (Ca2+ ) 400 1% potassium (K+ ) 380 1% bicarbonate (HCO3 - ) 142 0.4% bromide (Br- ) 65 0.2% other solids 34 0.1% total dissolved solids 34,500 water (balance) 965,517 621 is equivalent to the total number of grams of salts per kilogram (or liter) of water. Seawater in offshore areas typically has a salinity of grams of salts per liter of water, whereas in near-shore areas with substantial freshwater input, salinity is often much lower. Biologists usually express salinity in units of parts per thousand (abbreviated ppt or ‰). Thus, offshore seawater is said to have a salinity of approximately ‰. Although generally similar to the salinity of the adjacent seawater, the salinity of a tidepool can vary with the tidal cycle, depending on the influence of three main factors: () the height of the pool on the shore, () the amount of sunshine during the low-tide period, and () the amount of freshwater input due to precipitation. The height of a pool on the shore, and its shape and orientation with respect to the tides, influence how long the pool is submerged each day. As a result, intertidal habitats are divided into various zones, including the subtidal , low intertidal, mid-intertidal, high intertidal, and splash zone, depending on how much of the tidal cycle they spend under water. Figure shows how salinity varies during the day in tidepools at different heights in the intertidal on a rainy day. Pools in the low intertidal zone are underwater for much of the tidal cycle, and salinity varies only slightly during the day. Pools in the high intertidal are underwater only during high tide, and on rainy days rain falling in the pool can lower the salinity during low tide. On sunny days, however, evaporation of water due to wind or sunshine can increase the salinity of the tidepool. Pools in the splash zone receive seawater input only through wave action, and on rainy days much of the water in these pools is freshwater from the rain. Thus the salinity...
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