Document: ALB-3-74-23

Effects of zooplankton on the development of a deep chlorophyll layer and nutrient transport: A ¹⁵N limnocorral experiment.

PILATI, A.* and W.A.WURTSBAUGH

Utah State University, Logan UT 84322-5210 USA ¹

Abstract:
Deep chlorophyll layers (DCL) are dominant features of many oligotrophic lakes and oceans and often contribute over 50% of water column primary production. A variety of competing physical, chemical, and biological hypotheses have been proposed to explain their formation. Here we describe an experiment that tested the importance of zooplankton grazing as a factor driving the development of the DCL. In oligotrophic Yellow Belly Lake (Sawtooth Mountains, central Idaho) we compared changes in the chlorophyll profiles, zooplankton-mediated nutrient transport, and eddy diffusivity in 6 limnocorrals (1-m diameter, 17-m deep). ¹⁵N ammonia and rhodamine dye were added to the epilimnion or metalimnion of the corrals in a factorial design in the presence or absence of zooplankton. In the limnocorrals with zooplankton, epilimnetic zooplankton biomass was 2X higher, and estimated grazing rates 3X higher, than in the metalimnion. After 11 days, chlorophyll levels in the zooplankton treatment declined 72% in the epilimnion, but only 53% in the metalimnion, leading to the development of a DCL. In the no-zooplankton treatment the epilimnetic chlorophyll increased 11%, and the metalimnetic algal levels decreased 41%, resulting in the formation of an epilimnetic chlorophyll layer. Measurements of zooplankton distribution in the lake and limnocorrals, and modeled grazing rates are consistent with the observed changes. Biologically-mediated movement of ¹⁵N from the epilimnion and metalimnion was downward, either into the metalimnion or hypolimnion. Eddy diffusivity was high in the limnocorrals therefore rhodamine, and presumably ¹⁵N, moved into adjoining strata, indicating the importance of this process. Grazing, however, coupled with a downward movement of nutrients (¹⁵N) by fecal material and/or algal sedimentation, appears to explain the development of a metalimnetic chlorophyll maxima.

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This abstract is being presented at: 2:00 PM in session:
Oral Session #47: Zooplankton Ecology.