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

Abstract

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.

Preferential accumulation of carotenoids rather than of mycosporine-like amino acids in copepods from high altitude Himalayan lakes

“…For example, avoidance of depths with high UVR during daytime is an effective strategy that is particularly relevant to Daphnia species (Leech & Williamson, 2001), but it has also been observed in copepods (Alonso et al, 2004). In fact, in clear and fishless alpine lakes, UVR has been proposed to be the primary determinant explaining the vertical distribution of zooplankton (Kessler et al, 2008). However, avoidance may not be enough to obtain full protection in shallow ponds and lakes where, in addition, wind-driven mixing can recurrently bring planktonic organisms to the surface where UV irradiance levels are highest.…”

Implications of nitrogen‐rich glacial meltwater for phytoplankton diversity and productivity in alpine lakes

“…Bags were randomly loaded into racks and incubated for 7 d at a depth of 7 m in Beauty Lake. This depth was selected because it was well below the depth to which damaging ultraviolet-B radiation penetrates in this lake (2.5 m) but still above the depth of 10% PAR (, 9 m; Kessler et al 2008), keeping it well within the photic zone. We confirmed these light conditions with the BIC prior to incubation.…”

“…The depth to which 1% of photosynthetically active radiation (PAR) remains (Z 1%PAR ) was previously measured for some of the lakes in July and August of multiple years (Kessler et al 2008;Rose et al 2009;Saros et al 2010). For those not previously measured, the Z 1%PAR was determined with a Biospherical Instruments Cosine (BIC) submersible profiling radiometer (Biospherical Instruments).…”

Effects of the proximal factors on the diel vertical migration of zooplankton in a plateau meso-eutrophic Lake Erhai, China

“…Studies have demonstrated pronounced differences in the behavioural responses of zooplankton to UV radiation. Copepods and rotifers exhibit a greater UV tolerance than cladocerans, which often show strong UV avoidance of surface waters (Leech and Williamson, 2000;Leech et al, 2005b;Kessler et al, 2008).…”