Abstract
Respiration rates for the cladocerans <it>Bythotrephes cederstroemi</it> and <it>Daphnia middendorffiana</it> were measured over a broad range of temperatures. The data were fit to the model <it>f</it>(<IMG SRC="/math/deg.gif">K)<it>W</it><sup>b</sup> where<it>f</it>(°K) is a formulation based on enzyme activity as a function of temperature. The enzyme kinetic model better described the biological observations of respiration as a function of temperature than standard empirical models. The results indicate that<it>Bythotrephes</it> has characteristics of a stenotherm and that the arctic <it>Daphnia middendorffiana</it> has characteristics of a eurytherm based on the linear region of Arrhenius plots (where enzymes are fully active). Above 23°C,<it>Bythotrephes</it> was limited by inactivation of respiratory enzymes. Also, below 14°C, enzyme inactivation became significant and not fully compensated for. <it>Daphnia middendorffiana</it> has a temperature region from 10 to 26°C in which the respiratory process was linear on an Arrhenius plot. Temperature-dependent and weight-specific respiration was described for each species with a formulation over a complete range of environmental temperatures. The formulation for temperature inactivation is applicable to all enzyme processes and should be used in aquatic studies requiring a broad temperature range.
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