Northern temperate lakes often have high water color because of high concentrations of dissolved organic carbon (DOC). Altered light, temperature, and oxygen profiles in these brown-water lakes should reduce the foraging abilities of planktivorous fish and reduce predation on zooplankton and invertebrate predators such as Chaoborus. Additionally, reduced diurnal vertical migration should limit exposure to cold temperatures and increase zooplankton growth rates. We hypothesized that, with increasing water color, Chaoborus would become more important, and this change would be followed by a shift in the zooplankton assemblage toward larger species. To test this hypothesis, we carried out a 2 ϫ 2 ϫ 2 factorial enclosure study to examine the effects of high and low color and the presence and absence of both fish and Chaoborus on zooplankton assemblages. We also analyzed the zooplankton community structure of two lakes with similar morphometry and fish composition but very different water color. Both studies showed that, in highly colored water, Chaoborus was more abundant and the zooplankton community shifted from small species, such as Bosmina and small copepods, to large species, such as Daphnia and Holopedium. Concurrently, the food web structure changed from top-down control to intraguild predation. Because not only the physical habitat differs between clear-and brown-water lakes, but also the predation regimes and food web structure, we conclude that brown-water lakes are a distinct lake type.The humic compounds of DOC are the major factor controlling water color in lakes (Thurman 1985;Pace and Cole 2002). DOC concentrations vary from 0.5 to 20 mg C L Ϫ1 , enough to span the range from crystal clear to darkly teacolored waters. Elevated levels of DOC (Ͼ5 mg L Ϫ1 ) are typical for many northern temperate lakes. A study including more than 1,000 Finnish lakes revealed that Ͼ80% had DOC concentrations Ͼ5 mg L Ϫ1 (Kortelainen 1993). The proportion of lakes with elevated DOC in the northeastern United States and Ontario, Canada, is about 40% (Carter at al. 1980; EMAP 1996).Attenuation of light by DOC could be detrimental to fish planktivory in two ways: low light could directly interfere with visual foraging by planktivorous fishes, and DOC could 1 To whom correspondence should be addressed. Björn Wissel, Coastal Ecology Institute, Louisiana State University, 1243 ECE Building, Baton Rouge, Louisiana 70803 (bwissel@lsu.edu).2 Present address: University of Washington, Joint Institute for the Study of Atmosphere and Oceans, Seattle, Washington 98195.3 Present address: Department of Biology, Laurentian University, Sudbury, Ontario P3E 2C6, Canada. AcknowledgmentsWe thank Joelle Young, Emily Hyfield, Amanda Logue, Shannon Allen, Zoraida Quiñones, Kenyatta Esters, and Michelle Menard for assistance in the field and laboratory. Howard Riessen, Norm Yan, Peter Dillon, and Mark Ridgway were helpful with the experimental design and in locating field sites. We are grateful to the Ontario Ministry of Natural Resources...

Zooplankton accumulate several groups of photoprotective compounds to shield against damaging ultraviolet radiation (UV). One of these groups, the carotenoids, makes the animals more conspicuous to visually hunting predators, whereas others, such as the mycosporine-like amino acids (MAAs) may not. The blend of photoprotective compounds is therefore important for the UV defense but also for the ability to escape predation through crypsis. Here we assess laboratory and field data from different latitudes to examine how UV, predation threat, and pigment availability (in food) affects the mixture of UV-protective compounds in copepods. Overall, the blend of MAAs and carotenoids was partly explained by the availability of MAAs in the food, the UV-threat, and the presence of predators. Copepods upregulated their MAA content when UV threat was increasing (i.e., if MAAs were abundant in food), and in field data this accumulation only occurred at high levels of predation threat. If MAAs were scarce, copepods instead compensated with higher carotenoid accumulation. However, when there was a high predation threat this carotenoid compensatory effect was disadvantageous, and low concentrations of both MAAs and carotenoids at high UV-threat resulted in lower reproduction. In all, these results showed that carotenoids and MAAs are complementary substances, i.e., one is high when the other is low, and copepods are, hence, able to adjust their blend of different UV-protective compounds to optimize their defenses to the threats of UV and predation. These defense systems may buffer against direct food-web interactions and help the zooplankton to survive in environments with high UV threat.

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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.