Vertical distribution of zooplankton in subalpine and alpine lakes: Ultraviolet radiation, fish predation, and the transparency-gradient hypothesis

The transparency-gradient hypothesis argues that ultraviolet radiation (UV) is a primary determinant of the vertical distribution of zooplankton in transparent lakes with fewer fish, while fish predation is the primary driver in less transparent lakes where fish are more abundant. We measured vertical profiles of UV, photosynthetically active radiation (PAR, essentially visible light used as a proxy for fish predation), temperature, pH, conductivity, chlorophyll a (Chl a), and zooplankton in seven subalpine and alpine lakes and examined the distribution and abundance of the major zooplankton groups relative to environmental variables using a canonical correspondence analysis (CCA) and multiple regression. Pigment concentrations and UV tolerance of representative species were examined experimentally. The CCA revealed that conductivity, PAR, and UV were most related to zooplankton distribution and abundance. The cladoceran Daphnia was associated with high PAR, while cyclopoids and nauplii were associated with low PAR. In contrast, the vertical distribution of calanoids was positively related to UV, while Holopedium was negatively associated with UV. The regressions revealed that UV consistently explained more of the variance in zooplankton vertical distribution than did either PAR or chlorophyll. Calanoids had high concentrations of photoprotective compounds and a relatively high UV tolerance compared to Daphnia. The positive association of Daphnia with visible light (PAR) and the importance of UV in overall zooplankton vertical distribution support the transparency-gradient hypothesis, indicating that UV is more important than fish predation in controlling the vertical distribution of zooplankton in more transparent lakes. The vertical distribution of zooplankton is mediated by a wide array of environmental factors, including abiotic factors such as light and temperature, as well as biotic factors such as food and predators (Lampert 1989). The relative importance of these various factors is likely to vary among lakes as well as among zooplankton species and even clones within a single species. There are also strong interactions among environmental factors. For example, the response of Daphnia to variation in food concentrations in the water column is modified by temperature gradients (Kessler and Lampert 2004; Winder et al. 2004). The relative importance of behavioral vs. pigmentation responses may also vary with taxon and latitude of a given zooplankton population (Hansson et al. 2007). Similarly, chemical kairomones released by fish stimulate stronger negative phototaxis in Daphnia collected from lakes with high fish predation pressures than in Daphnia collected from lakes with low predation pressures (De Meester 1993). An in situ experimental study similarly demonstrated a stronger negative phototaxis to solar ultraviolet (UV) radiation in a highly transparent lake than in a low transparency lake (Leech et al. 2005a). This has led to the hypothesis that fish predation is more important in stimulating downward migration of zooplankton in less transparent lakes, while UV is more important in highly transparent lakes where fish populations are low (Leech et al. 2005a). Here, we refer to this as the ‘‘transparencygradient’’ hypothesis. Alpine lakes are an extreme habitat where zooplankton must cope with very low average temperatures, low concentrations of food, and high fluxes of solar UV radiation (Sommaruga 2001). These factors combine to form vertical habitat gradients in alpine lakes that influence the vertical distribution of zooplankton (Winder et al. 2004). The high flux of solar radiation inhibits primary productivity near the surface of those lakes and may cause the maximal phytoplankton abundance to be located in deeper strata (Sommaruga 2001), resulting in the development of a deep-water chlorophyll maximum (DCM). The combination of high UV exposure, a DCM with a steep temperature gradient, and cold hypolimnetic temperatures in alpine lakes is thus likely to have an unusually strong influence on the vertical distribution of zooplankton. Visual predators such as fish are widely recognized as important determinant of the vertical distribution of 1 Current Address: Institut für Umwelt Analyse, Umweltberatung und Gutachten GmbH, Röhrenstrasse 27, 06749 Bitterfeld, Germany. 2 To whom correspondence should be addressed: Present address: Department of Zoology, Miami University, Oxford, Ohio 45056 ([email protected]).