Importance of Zooplankton for the Persistence of a Deep Chlorophyll Layer: A Limnocorral Experiment

A variety of interacting physical, chemical, and biological hypotheses have been proposed to explain the formation of deep chlorophyll layers (DCL). We used an experiment to test the importance of zooplankton grazing and nutrient transport as factors maintaining the DCL. In oligotrophic Yellow Belly Lake (Sawtooth Mountains, central Idaho), which has a DCL, we compared changes in the chlorophyll profiles in 17-m-deep limnocorrals with and without crustacean zooplankton. 15N ammonia and rhodamine dye were added to the epilimnion or metalimnion of the corrals to measure nutrient transport and diffusivity. In the limnocorrals with zooplankton, epilimnetic macrozooplankton biomass was 2× higher and estimated grazing rates were 1.8× higher than those in the metalimnion. After 11 d, chlorophyll levels in the zooplankton treatment declined 72% in the epilimnion but only 53% in the metalimnion, leading to the maintenance of the DCL. In the treatment without zooplankton, the epilimnetic chlorophyll increased 11% and the metalimnetic algal levels decreased 41%, resulting in the formation of an epilimnetic chlorophyll maxima. Biologically mediated movement of 15N from the epilimnion and metalimnion was downward, into either the metalimnion or the hypolimnion. Turbulent movement measured with rhodamine was high in the limnocorrals, and presumably 15N also moved into adjoining strata through this process. Grazing, however, coupled with a downward movement of nutrients via sedimentation into the lower strata appears to explain the persistence of the DCL.