Quantification of the phosphorus released by zooplankton in an oligotrophic lake (La Caldera, Spain): regulating factors and adjustment to theoretical models

Using an in situ approach, we have evaluated the phosphorusinputs from zooplankton in a high-mountain oligotrophic lake.Values of the specific gross release rate (SGRR) fluctuatedbetween 0.2 and 2.9 µg P mg^–1 dry weight h^–1,and were higher when the nauplii of Mixodiaptomus laciniatusdominated the zooplankton community. The rate of P recyclingby the zooplankton was high, reaching 1.6 µg P l^–1day^–1, and showed a highly consistent seasonal patternfrom one year to the next, with maxima in midsummer. Zooplanktonsize accounted for as much as 85% of the variance obtained inthe measurements of the specific rate of P release, while otherfactors, such as the quality or quantity of food, did not significantlyinfluence the SGRR changes. Among the models tested, only theone proposed by Peters (Int, Ver. Theor. Angew. Limnol. Verh.,19, 273–279, 1975) was useful for predictions in thissystem. The stoichiometric model of Hessen and Andersen (Arch.Hydrobiol., 35, 111–120, 1992), applied in this oligotrophicsystem, adequately predicted the phyto- and zooplankton dynamics,whereas the values of P release estimated using this model werefar higher than the excretion rates obtained experimentally.These differences were related to the type of egestion (formationof faecal pellets) of metazooplankton and to the relative importanceof the food resistance to digestion. We believe that in communitieswhere copepods constitute a substantial percentage of the zooplankton,an evaluation of the P release which is readily available solublereactive phosphate (SRP), total dissolved phosphate (TDP)] toalgae and bacteria would not fit the predictions of generalmodels of mass balance; under these circumstances, assimilationefficiency proves to be the key parameter for predicting thereadily available P (excreted P).