Enzymatic indices of respiration and ammonia excretion: relationships to body size and food levels

Enzyme indices of zooplankton respiration, citrate synthase(CS), and ammonia excretion rate, glutamate dehydrogenase (GDH),were measured in cultures of Artemia franciscana (San FranciscoBay) feeding in a 25-fold range of concentration of the algaDunaliella salina. Enzyme activities per animal were regressedagainst body size (expressed as dry weight or protein) in theform of the allometric equation, log Y = log a + b logW. CSactivity was related to size with a scaling exponent (b) of1.133 in terms of dry weight and 0.864 in terms of protein.GDH scaling exponents were 1.283 and 0.978 for dry weight andprotein, respectively. In cases where these variables are measured,it is essential that the size structure of populations and communitiessampled is taken into account, otherwise false conclusions maybe drawn. Despite specific growth rate variation from 0.117to 1.188 day^–1, no differences in CS or GDH activity couldbe attributed to feeding level or growth rate effects. Suchenzymatic indices, which represent only maximum potential metabolicrates, may therefore be poor predictors of actual rates in vivo.