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 23 higher and estimated grazing rates were 1.83 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. In oligotrophic lakes and oceans, large algal populations frequently develop below the surface mixed layer, either in a sharp peak (deep chlorophyll maxima) or in a broad deep chlorophyll layer (DCL). Even though net primary production in these layers can account for up to 72% of the productivity on an areal basis (Moll and Stoermer 1982), we do not understand why these layers form. DCLs occur in thermally stratified waters where there is sufficient light and nutrients for net algal growth (Longhurst and Harrison 1989), and they are frequently composed of mobile species specialized to live in low-light regimes. In oligotrophic lakes, sufficient light penetrates into the metalimnion, but the source of the ‘‘new’’ nutrients (Dugdale and Goering 1967) that supply elements for algal growth in the DCLs is unclear. The predominant hypothesis, at least for the oceans, is that phytoplankton grow in DCLs because diffusive processes 1 Present address: Department of Zoology, Miami University, Pearson Hall Room 212, Oxford, Ohio 45056-1400 (pilati@muohio. edu.). 2 Corresponding author ([email protected]).