Estimates of Whole Lake Metabolism: Hypolimnetic Oxygen Deficits and Carbon Dioxide Accumulation

Abstract

Measurements of the supply of organic matter to aquatic ecosystems are complex and require an elaborate research program continuing over at least a year. The inputs from the products of photosynthesis of autotrophic phytoplankton and of littoral flora must be evaluated, as well as the inputs from allochthonous organic matter entering the aquatic ecosystem from the atmosphere and from the drainage basin.

Assuming that the allochthonous inputs of organic matter are small in relation to those synthesized within a lake, and that the lake is sufficiently large to stratify thermally, the autotrophic productivity can be estimated indirectly from long-term changes in either hypolimnetic oxygen deficits or accumulations of dissolved inorganic carbon (DIC).The assumption is that organic matter, which was synthesized in the trophogenic zone, sinks into the hypolimnetic zone and decomposes there. Changes in oxygen or DIC concentrations of the hypolimnetic water reflect the rates of loading of organic matter and of decomposition. However, the relationships are not so tidy in nature. Several major reactions in the development of an anaerobic hypolimnion are related to the decomposition of organic matter [see Wetzel (1983, 1999) and Schindler (1985)]. These reactions are based on a simplified molecule for organic matter of planktonic material:*

The procedures given in this exercise are appropriate for lakes where methanogenesis is the primary decomposition pathway, and the dominant anion in the hypolimnion is bicarbonate. Other confounding problems include, for example, decomposition in the epilimnion, turbulent exchange between the stratified water layers, and photosynthesis in the hypolimnion. Nonetheless, a general, direct relationship does exist between autotrophic productivity and hypolimnetic changes. The advantages of these approaches are that they provide whole ecosystem values and integrate diverse processes of production and decomposition of organic matter. The approaches treated here obviously are not applicable to lakes that do not stratify or to those that are stratified permanently (i.e., meromictic lakes).