PH, the CO_2 system and freshwater science

摘要

Appreciation of pH as an ecological index has varied considerably over its past history, influenced by perceptions of chemical rigour, ease or difficulty of measurement, and multiple chemical and biological correlations. These factors, and especiallythe last, are considered in relation to the extensive range of pH in inland waters. Emphasis is placed upon the role of the CO_2 system, the components of which are subject to biological metabolism (photosynthesis, respiration) and are extensively determined by products from rocks (e.g. limestone) and soils. Titration alkalinity, or acid neutralising capacity, is a most valuable summarising and reference measure. For this, and CO_2 variables, potentiometric Gran titration opens new possibilities - including the definition of negative alkalinity (acidity). The relationship of pH and titration alkalinity is close and semi-logarithmic for waters in equilibrium with atmospheric CO_2. Very high pH, above 10, can develop from the photosynthetic depletion ofCO_2 and by the evaporative concentration of bicarbonate-carbonate waters in closed basins. Very low pH, below 4.5, results from the introduction of strong acids by volcanic emissions, pyrite oxidation, 'acid rain' and cation exchange; here the CO_2 system lacks influence, biological diversity is reduced, and ionic aluminium often exerts toxic biological effects. Situations of pH excursion are discussed and illustrated; they operate over day-night, seasonal and long-term time-scales. A summer rise of pHis widespread in productive near-surface waters. There is also a seasonal pH rise in the anoxic deep water of many lakes, as a consequence of the interaction of acid-base and oxidation-reduction systems. These can be regarded as two 'master systems' ofenvironmental chemistry and - dating from pioneer studies of wetland soils and waters - of much freshwater ecology.