The Depletion of Carbon Dioxide from Lake Water by Phytoplankton

It has long been known that dense populations of phytoplankton tend to deplete the carbon dioxide present in natural waters, with an associated raising of pH. The ecological significance of these reactions, as possible growth-limiting or retarding factors, is far from clear. This situation is remarkable in view of the intensive study given to the relationship between rates of photosynthesis and carbon dioxide supply in some unicellular algae, the numerous measurements of carbon assimilation by phytoplankton in nature, and the attention bestowed on pH as an ecological factor. Recently a controversy has developed around the relative importance of carbon and phosphorus supply in the enrichment ('eutrophication') of inland waters (e.g. Kuentzel 1969, 1971; King 1970; Kerr, Paris Kerr et al. 1972; Schindler 1971; Schindler et al. 1973; Goldmann et al. 1972; Goldman, Oswald & Jenkins 1974), partly sustained by a shortage of detailed information on the dynamics and availability of inorganic carbon in freshwaters. In particular, the exchange of carbon dioxide between atmosphere and water is rarely estimated, the interrelations of various forms of carbon dioxide are rarely assessed, analytical methods are usually unspecific, and the relevant physiological responses of characteristic common species of freshwater phytoplankton are largely unknown. The present work developed from observations on depletions of carbon dioxide and upward pH shifts, occasionally very pronounced, in two English lake waters. Examples are described in relation to the algal crops involved and the stratification of environmental factors. The general limnological background of these environmental events, including details of diurnal changes, will be described more fully elsewhere. Further analysis is based upon systematic laboratory measurements of photosynthetic activity in relation to episodes of algal production in nature, extended to experimental manipulations of media, population density, and time-sequences of carbon dioxide consumption. As the physiological work is based upon natural populations of known history, it is hoped that ecological inferences can be drawn without undue extrapolation between the properties of different species and different media.