Effects of water color on predation regimes and zooplankton assemblages in freshwater lakes

Abstract

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...

Indirect effect of environmental factors on interactions between microbial and classical food webs in freshwater ecosystems

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