UV-enhanced fish predation and the differential migration of zooplankton in response to UV radiation and fish

The intensity and spectral composition of visible light are known to influence fish predation on zooplankton. However, in clear-water systems, ultraviolet (UV) radiation (UVR) may also influence fish predation, directly through UV-enhanced foraging, indirectly through alterations in predator–prey overlap, or in a combination of the two. Here we test the hypothesis that UVR facilitates fish predation on zooplankton in an oligotrophic lake. Experiments were conducted in 2.2-m-long UV-transparent or UV-blocking columns suspended in the epilimnion. Zooplankton consumption by young-of-the-year (YOY) largemouth bass (Micropterus salmoides) was compared in the presence and absence of UVR, and the vertical distributions of prey were quantified across light and fish treatments. In addition, a separate experiment was conducted to examine zooplankton vertical responses to UV exposure vs. fish kairomones. Overall, YOY predation on zooplankton was higher in the presence than in the absence of UVR, particularly on diaptomid copepods. This result was only partially explained by zooplankton migratory behaviors, suggesting UV-enhanced searching capabilities in YOY bass. Furthermore, diaptomids displayed a stronger vertical behavioral response to fish kairomones than to UVR whereas Daphnia exhibited a stronger response to UVR. Fish predation on zooplankton is most often assessed in terms of the quantity and spectral distribution of visible light (400–700 nm) present in the water column. Both light intensity and spectral composition influence the reaction distance and predation rate of fish predators. The visible light environment can also alter prey selection by fish, affecting the size distribution and species composition of zooplankton communities (Brooks and Dodson 1965). As a consequence, visible light is recognized as the primary proximate cue that zooplankton use to assess predation risk and initiate diel vertical migrations out of the surface waters during the day (Forward 1988; Lampert 1989). However, in clear, ultraviolet (UV)-transparent systems, UV radiation (UVR) may also enhance fish predation on zooplankton, both directly and indirectly (Leech and Johnsen 2003). UVR extends deep into the water column of fresh waters with low concentrations of dissolved organic carbon (DOC, ,1–2 mg L21). Within the UV spectrum, UV-A radiation (320–400 nm) penetrates to greater depths than UV-B (290–320 nm) (Morris et al. 1995). For example, in small glacial lakes with DOC concentrations less than 1 mg L21, 320-nm UV-B can reach depths in excess of 10 m and 380nm UV-A can extend to twice these depths (Williamson et al. 1996). Increased exposure to UVR, particularly UV-B, can lead to deleterious effects on growth, reproduction, and survival of many aquatic organisms, including young-ofthe-year (YOY) fish and their zooplankton prey (Leech and Williamson 2000; Olson et al. 2006). Negative phototactic migrations in response to ambient UVR levels have been observed in both zooplankton (Leech and Williamson 2001; Alonso et al. 2004; Boeing et al. 2004) and fish (Kelly and Bothwell 2002; Ylönen et al. 2005; Holtby and Bothwell 2008), suggesting that behavioral avoidance of UVR has the potential to influence interactions between predator and prey in time and space. In addition, although some wavelengths of UVR are potentially damaging to aquatic organisms, others are potentially beneficial. Numerous species of freshwater and marine fishes as well as invertebrates possess UV photoreceptors (reviewed in Leech and Johnsen 2003). In many freshwater fishes, UV photoreceptors are present only during early life history stages, coinciding with planktivory in the surface waters, and have a peak absorbance in the UV-A range (350–370 nm) (Loew et al. 1993; Leech and Johnsen 2003). Navigation (Hawryshyn 1992) as well as recognition and communication between conspecifics, predators, and prey (Losey et al. 1999; Siebeck 2004) are augmented by UV perception in some species. Studies have also shown that UV vision may enhance prey contrast, increasing the foraging efficiency of YOY fish (Loew et al. 1993; Browman et al. 1994), yet others have detected no beneficial effects of UVR on foraging (Leech and Johnsen 2006; Holtby and Bothwell 2008). In the present study, we tested the hypothesis that UVR facilitates fish predation in high UV systems using an enclosure approach and examined the potential differences in UVR facilitation of feeding on different zooplankton species. Field experiments were conducted in 2.2-m-long columns that either transmitted or blocked UVR such that increased fish predation might be a consequence of UVR avoidance behaviors, an enhancement in UV-searching capabilities, or a combination of these factors. An Limnology limn-54-04-32.3d 24/4/09 14:39:42 1152 Cust # 08-167 * Corresponding author: dmleech@email.unc. edu Limnol. Oceanogr. , 54(4), 2009, 1152–1161 E 2009, by the American Society of Limnology and Oceanography, Inc.