Kinetic examination of nitrogen release by zooplankters’

Kinetics of nitrogen release by Daphnia mugna (0.2-0.4 mg dry wt) and Daphnia pulex (ca. 0.03 mg dry wt) were followed by measuring ammonium (plus primary amines) in water flowing past individual animals. Culture media water was pumped slowly (6 ml *h-l) through an incubation chamber (volume 0.05 ml) containing a daphnid and then either mixed with o-phthalaldehyde reagent for continuous analysis of nitrogen compounds or passed into a sample loop of an ammonium analyzer for measurements of ammonium release at discrete intervals. Within the resolution of the technique (3 min), nutrient regeneration appeared continuous rather than pulsed. Highest rates of ammonium release, 44 nmol. (mg dry wt)-l* h-’ (SE = 5, N = B), were typically observed immediately after the animals were removed from food. Regeneration rates for D. manna gradually decreased during the first hour to a mean steady state rate of 11 nmol. (mg dry wt)“.h -1 (SE = 2, N = 8). The release of nitrogen and phosphorus by zooplankton provides nutrients for phytoplankton in marine and freshwaters. Many investigators have measured nitrogen and phosphorus release in laboratory and field experiments, but the kinetics of nutrient release are not yet well defined. For example, it is not clear whether zooplankters release nutrients in pulses or continuously. This is important in estimating the persistence of concentrated nutrient patches which may be microenvironments for enhanced nutrient uptake by phytoplankton (cf. Goldman et al. 1979; Lehman 1980). Factors controlling rates of nutrient release are also not well defined. Although recently fed animals release nitrogen and phosphorus at higher rates than unfed ones (Corner et al. 1965; Hargrave and l GLERL Contribution 244. Geen 1968; Marshall and Orr 1961; Conover and Mayzaud 1976), the extent to which food quantity and quality affect nutrient release has been masked by previous experimental designs. For example, because phytoplankton, added as food, assimilate nutrients released by the animals before the increased nutrient concentrations can be measured, investigators have incubated animals without food before measuring nutrient accumulation. Also, because relatively large incubation chambers were used, experiments have often been run for more than 12 h to accumulate sufficient nutrients for measurement. The shortest incubation times have been about 2 h. The retention time of food in the animal’s gut is long (~1 h) in the absence of an external food source (Schindler 1968; Geller 1975; Lampert 1977), and food in the gut is partly digested during the experiment. Thus, nutrient release rates obtained