Learning Objectives

2.6.1 Outline what determines the salinity and acidity of soils and waters.

2.6.2 Explain why oxygen concentrations vary depending on elevation, the influence of water on diffusion, and biological consumption.

All organisms are bathed in a matrix of chemicals. Water is the primary chemical constituent of aquatic environments, along with variable amounts of dissolved salts and gases. Small differences in the concentrations of these dissolved chemicals can have important consequences for the functioning of aquatic organisms, as well as for terrestrial plants and microorganisms that are dependent on water and dissolved chemicals in the soil. Terrestrial organisms are immersed in a gaseous atmosphere that is relatively invariant, consisting primarily of nitrogen (78%), oxygen (20%), water vapor (1%), and argon (0.9%). The atmosphere also contains trace gases, including the greenhouse gases, which play a critical role in Earth's energy balance, and pollutants derived from human activities, which can have important effects on atmospheric chemistry. We will discuss the effects of air pollutants and greenhouse gases in Chapter 25. Here we briefly review three chemical variables that influence biological and ecological function: salinity, acidity, and the availability of oxygen.