Castle Lake Limnological Research Station

1. An in situ experiment was carried out in a high-mountain oligotrophic lake (La Caldera) to quantify the specific release rates of ammonium and phosphorus in different phases of the seasonal succession of plankton. The zooplankton community was dominated by the calanoid copepod Mixodiaptomus laciniatus. 2. The rates of release of N and P (from non-detectable to 3.58 mu gN mg(-1) DW h(-1) and from 0.26 to 2.29 mu g P mg(-1) DW h(-1), respectively) and the N : P released ratio (not detectable to 4.77) fell within the range typical of oligotrophic lakes dominated by copepodite assemblages. 3. The values of the zooplankton N : P ratio (5 : 1 to 10 : 1 by weight) were lower than those established for other species of calanoids, and followed a well-established pattern of seasonal variation from year to year with higher values after ice thaw and lower values as individual size increased (ontogenetic development). 4. The elemental composition of the zooplankton depended on individual size, while the released N : P ratio was inversely related to the N : P ratio of the food. This pattern conforms to the predictions of the homeostasis theory of Sterner (1990). 5. A feedback regulation was established between the stoichiometric composition of the zooplankton, their food and the released N : P ratio that can help explain changes in pelagic community structure during the ice-free period.