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 objective of this study was to expand the spatial scale of previous experiments on the effects of ultraviolet radiation (UVR) on diel vertical migration (DVM) by freshwater zooplankton. We conducted an in situ mesocosm experiment in highly UVR transparent Lake Giles, Pennsylvania, in which we imposed two treatments: ambient UVR and UVR-shielded. Mesocosms (3440 L, 0.74 m diameter, 8 m deep) were large enough to include a spatial refuge from UVR and permit relatively large-scale DVM. Daphnia catawba adopted a significantly deeper distribution during the day in the ambient UVR treatment compared to the UVR-shielded treatment, but effects of UVR were absent at night. In contrast, DVM by Leptodiaptomus minutus was unaffected by the UVR treatment. In both treatments, Leptodiaptomus minutus were most abundant at the bottom of the mesocosms during the day and exhibited a more uniform distribution across depths at night. These results suggest that UVR, along with temperature, algal resources, and predators, may affect zooplankton DVM in aquatic ecosystems.

[1] The assimilation of elements by an organism is dependent on both the environmental availability of the element and the processes of the organism. For some elements, organisms have a challenging time discriminating between nearly identical chemical analogs, for example, calcium and strontium. We tested the hypothesis that in environments where a desired element is scarce, the organism will assimilate a chemically similar analog at an increased rate. Populations of Daphnia pulicaria were manipulated using a microcosm in situ experiment and the results of that experiment tested with a field survey. Experimental results indicated a correlation between higher environmental calcium concentration and lower [Sr]/[Ca] ratios (R 2 = 0.91, p < 0.05), suggesting that Daphnia in high calcium environments will assimilate more calcium relative to strontium. Field survey results across eight lakes confirmed that as lake calcium concentration increased, the value of [Sr]/[Ca] between the organism and the lake water decreased (R 2 = 0.60, p < 0.05). Measurement of the elemental ratio of major and trace element analogs within organisms compared to their environments may be a useful tool for measuring the relative bioavailability of the major element, and provide insight into elemental limitation in other calcifying aquatic invertebrates.Citation: Peters, S. C., R. Lockwood, C. E. Williamson, and J. E. Saros (2008), Using elemental ratios of calcium and strontium to track calcium availability in the freshwater zooplankton