Water as a Chemical Environment

Water is a solvent capable of dissolving an extraordinarily large number of substances, providing a medium from which aquatic organisms can acquire substances. Water also brings into contact solutes, which may chemically react to produce new substances while lowering reactant concentrations. This means that ecosystem productivity and ecological structure are profoundly dependent on a substance’s capacity for dissolving in water, as well as factors that remove and add it to solution. The five major classes of reactions that occur in freshwaters are acid–base reactions, redox reactions, photochemical reactions, complexation reactions, and adsorption–desorption reactions. The concentration of a solute in the water column of a surface water at any point in time is the net result of dynamic processes, defined as the difference between inputs from all sources and all outputs including transport processes and chemical reactions. Several gases are of interest because of their metabolic roles (e.g., O2, CO2, and N2), their effect on pH (CO2), or their role in climate change (e.g., the greenhouse gases CO2, CH4, N2O). Similar to nongaseous solutes, dissolved gas concentrations in lakes are affected by a host of internal processes that produce and consume gases.