Climate warming in North America is likely to be accompanied by changes in other environmental stresses such as UV-B radiation. We apply an empirical model to available DOC (dissolved organic C) data to estimate the depths to which 1% of surface UV-B and UV-A radiation penetrate for several major regions of North America. UV attenuation depths are also estimated from DOC data collected from treatment and reference basins during the experimental acidification of Little Rock Lake, Wisconsin. In some regions of North America 25% of the lakes have 1% attenuation depths for UV-B radiation on the order of 4 m or more (western and northwestern U.S., Newfoundland). In other regions, 75% of the lakes have 1% attenuation depths for UV-B shallower than 0.5 m (Florida, upper midwestern U.S., northwestern Ontario, Quebec, and Nova Scotia). Attenuation depths for UV-A radiation are -2.5 times as deep as those for UV-B. Experimental acidification approximately doubled the estimated 1% attenuation depths for UV radiation in Little Rock Lake.The strong dependence of 1% attenuation depth on DOC below the l-2 mg liter-l DOC range suggests that UV attenuation in low DOC lakes is highly sensitive to even very small changes in DOC. We conclude that changes in climate, lake hydrology, acid deposition, and other environmental factors that alter DOC concentrations in lakes may be more important than stratospheric ozone depletion in controlling future UV environments in lakes.

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Population growth rates of eight species of planktonic rotifers were assessed for five to seven food concentrations using daily renewed batch cultures. The food concentration for which population growth rate was zero (the threshold food level) varied by a factor of 17 among species. The log of threshold food concentration was positively and significantly related to the log of body mass. Similarly, a strong positive log—log relationship was found between rotifer body mass and the food concentration supporting one—half the maximum population growth rate (°max/2); this food concentration varied by a factor of 35 among species. There was a positive relationship between rotifer body mass and maximum population growth rate. Because the smallest species have the lowest threshold food levels and the food levels necessary for them to attain °max/2 are lowest, they appear well adapted to living in food—poor environments. Large species appear to be restricted to food—rich environments but may thrive there because of their high reproductive potential. These conclusions are consistent with observations on species distribution and community structure of rotifers in nature.

Recent studies have emphasized the need for understanding the accumulation and fate of metal contaminants at different trophic levels and across a broad spectrum of lake types. To address both issues, metal concentrations (Hg, Zn, Cd, As, and Pb) were measured in the water, two size fractions of zooplankton, and fish from 20 lakes in contaminated to pristine watersheds in the northeastern United States. Our goals were to examine links between watershed characteristics and aqueous metal levels in lakes and relationships between aqueous concentrations, metal burdens in different plankton groups and in fish. Two pairs of metals, (1) Hg and Zn and (2) As and Pb, exhibited strong similarities both in the factors that predict their concentrations in water and in the patterns of accumulation in particular trophic levels. Aqueous concentrations of Hg and Zn were highest in cool water lakes, whereas As and Pb were highest in more eutrophic lakes in agricultural areas. Aqueous Cd concentrations were closely correlated with the land-use variables, percentage of agricultural land, and road densities. Similarly, Hg and Zn both biomagnified from small plankton (45-202 m) to macrozooplankton (Ͼ202 m) and from macrozooplankton to fish. In contrast, bioaccumulation of both As and Pb diminished with increasing trophic level. Although aqueous metal and zooplankton metal levels were not significant predictors of As and Pb levels in fish, metal levels in zooplankton were predictive of Hg and Zn in fish, suggesting that sources of bioaccumulation differ for different metals. Our findings demonstrate the importance of investigating upper and lower trophic levels separately, to fully understand metal transfer pathways in aquatic food webs.Metals transferred through aquatic food webs to fish, humans, and other piscivorous animals are of environmental and human health concern. High levels of Hg in fish from apparently pristine lakes have resulted in the adoption of conservative fish consumption advisories in many states (Håkanson et al. 1988;DiFranco et al. 1995;USEPA 1997;Yeardley et al. 1998). However, pathways of metal movement from land to water and then through aquatic food webs are not well understood. This makes it difficult to extrapolate findings from single systems to other lakes or to account for the significant variation in metal levels found in the fish from different lakes within the same geographic regions. Our broad goal is to elucidate factors across a variety of lake types that determine metal levels in water, in fish, and in the zooplankton, which are the primary dietary conduit of metal from water to pelagic feeding fish.Although knowledge of metal movement in freshwater systems has grown significantly in recent years, there are still large gaps in our understanding. For example, most past studies of metals focus on a few taxa, single metals, or transfer mechanisms in a small portion of the food web (Prahalad

A general method for establishing cultures of planktonic rotifers from natural waters is described. Algal isolates are tested for their suitability as food for rotifers collected from the same source and season. Rotifer clones displaying the highest reproductive rates under the given culture conditions are selected for the final culture system. This procedure yielded indefinite cultures for the eulimnetic species, Asplanchna priodonta, A. herricki, Polyarthra major, and Synchaeta pectinata, and appears to have a good probability of success when the diet of a species is unknown. Algal isolates from the same source as the rotifers produced better results than algae obtained from commercial sources. Cryptomonad species produced the most consistent positive responses in growth and reproduction of rotifers tested.Key words: rotifers, zooplankton culture, Asplanchna, Polyarthra, Synchaeta, Cryptomonas