Damaging UV radiation and invertebrate predation: conflicting selective pressures for zooplankton vertical distribution in the water column of low DOC lakes

Abstract

In nature most organisms have to manage conflicting demands of food gathering, predator avoidance, and finding a favorable abiotic environment (oxygen, temperature, etc.) in order to maximize their fitness. In the vertical water column of lakes with high solar ultraviolet radiation (UV) and invertebrate predators, zooplankton face two particularly strong and conflicting selective pressures. During daylight hours invertebrate predators often induce an upward vertical migration of zooplankton prey while potentially damaging UV forces a downward migration. We used 2.2 m long columns suspended vertically in a lake to conduct 2x2 factorial experiments to examine patterns of depth selection behavior by zooplankton in the presence and absence of both the invertebrate predator Chaoborus and UV. We hypothesized that Chaoborus and UV both affect the distribution of zooplankton and a combination of both factors would lead to a narrowing of depth distribution. We found that when Chaoborus were present zooplankton tended to be distributed at shallower depths in the columns, while in the presence of UV they exhibited a deeper distribution. Chaoborus themselves were always found near the bottom of the columns regardless of the UV treatment. Simultaneous exposure to predators and UV resulted in a peak of zooplankton (especially Daphnia catawba) distribution at intermediate depths. In a significant number of cases, depth range was narrowed in response to Chaoborus, UV, or both.

UV‐enhanced fish predation and the differential migration of zooplankton to UV radiation and fish

“…Lake Giles D. catawba are substantially less pigmented than L. minutus (Persaud et al 2007). Although decreasing their visibility to visual predators, this lack of pigmentation increases susceptibility to damaging solar radiation, which can evoke downward daytime migrations (Leech and Williamson 2001;Boeing et al 2004;Cooke et al 2008). Lake Giles D. catawba are known to be more sensitive to UVR compared to the rotifers and copepods (Leech and Williamson 2000) and are typically least abundant in the epilimnion during the day (i.e., ,25% of the population, PCLP database unpubl.).…”

“…Chaoborus typically evoke an upward migration in Daphnia (Dodson 1988;Boeing et al 2004), and in the presence of both fish and Chaoborus kairomones, Daphnia demonstrate signs of stress, reducing swimming behaviors overall (Weber and van Noordwijk 2002). However, during the present study, Chaoborus were absent from the surface waters at the time the columns were filled, suggesting that Chaoborus kairomone levels were below detection.…”

“…Some studies suggest that not all fish kairomones are perceived as an equal threat (Weber 2003), so repeating the experiment with kairomones from other fish species may elicit a different response. Additionally, some Daphnia clones either lack a behavioral response (Boeing et al 2004) or swim upwards (Spaak and Boersma 2001;Boeing et al 2006) in the presence of fish kairomones. Recent studies indicate that both predation and UVR risk assessment in Daphnia varies with body size, such that small daphnids (,0.9 mm) remain in the surface waters whereas larger daphnids (.0.9 mm) migrate to deeper depths (Hansson and Hylander 2009).…”

“…Column and experimental design-The design of the experimental columns is described in detail in Boeing et al (2004) and Cooke et al (2008). Briefly, columns (8 cm 3 8 cm 3 2.2 m long, total volume 5 14 liters) were constructed of two acrylics with sharp long wave pass cutoffs: OP-2, 50% transmittance at 410 nm (UVR2 treatment) and OP-4, 50% transmittance at 272 nm (UVR+ treatment) (CYRO Industries).…”

“…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;Ylö nen 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.…”