Diffuse attenuation coefficients (&) for solar UV radiation (UVR) (305, 320, 340, 380 nm, and PAR) were measured in the mixed layer of 65 lake sites in Alaska, Colorado, and Pennsylvania and the Bariloche region of Argentina. Integrated mixed layer samples of lake water were concurrently collected, and a multivariate approach was used to model Kd with a number of optical and chemical variables.Substantial variation in transparency was observed among lakes. Attenuation depths (zlYO) for UV-B radiation ranged from several centimeters to > 10 m. In some shallow, low DOC (dissolved organic C) lakes typical of high elevation ecosystems, substantial fluxes of UVR penetrated the entire water column. In deeper lakes with low DOC concentrations, high fluxes of UVR were found in a significant proportion of the mixed layer. Much of the among-lake variation in & (87-96%) was explained by differences in DOC concentration, which strongly influenced dissolved absorbance. On average, dissolved absorbance accounted for between 33% (for PAR) and 68% (for 305 nm) of Kd measured in situ. Throughout the solar UV-A and UV-B range, Kd was best estimated with a univariate power model based solely on DOC concentration. Models are also presented that relate absorption coefficients to Kd. These models can be used with archival DOC or color data to provide approximate estimates of UV transparency of lakes.

Increases in terrestrially-derived dissolved organic matter (DOM) have led to the browning of inland waters across regions of northeastern North America and Europe. Short-term experimental and comparative studies highlight the important ecological consequences of browning. These range from transparency-induced increases in thermal stratification and oxygen (O2) depletion to changes in pelagic food web structure and alteration of the important role of inland waters in the global carbon cycle. However, multi-decadal studies that document the net ecological consequences of long-term browning are lacking. Here we show that browning over a 27 year period in two lakes of differing transparency resulted in fundamental changes in vertical habitat gradients and food web structure, and that these responses were stronger in the more transparent lake. Surface water temperatures increased by 2–3 °C in both lakes in the absence of any changes in air temperature. Water transparency to ultraviolet (UV) radiation showed a fivefold decrease in the more transparent lake. The primary zooplankton grazers decreased, and in the more transparent lake were largely replaced by a two trophic level zooplankton community. These findings provide new insights into the net effects of the complex and contrasting mechanisms that underlie the ecosystem consequences of browning.

We experimentally tested the hypothesis that accumulations of dietary compounds such as carotenoids or UVabsorbing mycosporine-like amino acids (MAAs) protect against natural levels of ultraviolet radiation (UVR). A calanoid copepod, Leptodiaptomus minutus, was collected from a relatively UV-transparent lake in Pennsylvania where levels of copepod MAAs and carotenoids vary during the year (MAAs high/carotenoids low in summer). Animals raised in the laboratory under different diet/UVR treatments accumulated MAAs from an MAA-producing dinoflagellate but not from a cryptomonad that lacks them. The acquisition efficiency increased under exposure to UVR-supplemented photosynthetically active radiation (PAR, 400-700 nm), yielding MAA concentrations up to 0.7% dry weight compared with only 0.3% under unsupplemented PAR. Proportions of individual MAAs differed between the animals and their diet. Shorter wavelength absorbing palythine and shinorine ( max 320 and 334 nm, respectively) were disproportionately accumulated over usujirene and palythene ( max ca. 359 nm). Carotenoids accumulated under UVR exposure (to 1% dry weight) when dietary MAAs were not available. Tolerance of ultraviolet-B (UV-B) radiation was assessed as LE 50 s (UV exposure giving 50% mortality after 5 d) following 12-h acute exposure to artificial UV-B radiation. LE 50 s increased 2.5-fold for UV-acclimated, MAA-rich animals, but only 1.5-fold for UV-acclimated, carotenoid-rich animals. Compared with carotenoids, MAAs offer this copepod a more effective photoprotection strategy, potentially as important as photorepair of DNA damage, to promote tolerance of natural levels of UV-B radiation.

The water columns of lakes and oceans provide a diverse habitat gradient in which light, temperature, food, and predation risk all change with depth. Many planktonic organis:ns exhibit diel vertical migrations (DVM) in response to daily oscillations in many of these variables. DVM theory often assumes that surface waters are more food-rich than deeper, subsurface layers and proceeds IO try to explain why zooplankton migrate out of these beneficial surface layers during the day. Here, we test the assumption that food is best in surface waters by feeding two common crustacean zooplankton with natural epilimnetic and metalimnetic food assemblages from a eutrophic lake and examining their egg production rates. Both Diaptomus and Daphnia showed greater reproductive rates in the metalimnetic water and significant food limitation in the epilimnetic water. Mass-specific ingestion rates were approximately three times higher in the metalimnion than in the epilimnion. In spite of the poorer food in the surface waters, these two crustaceans migrated into the epilimnion at night. These observations are contrary to the assumption that food is best in the surface water, and a review of the literature suggests that food frequently is not best in surface waters. The upward migrations at night are best explained by the warmer temperatures and reduced predation risk in the surface waters at night.Diel vertical migration of zooplankton in the world's lakes and oceans is one of the earth's most massive animal migrations. The patterns of these migrations among water bodies and among seasons differ in both their timing and amplitude. These migrations influence the population and community ecology of zooplankton, the trophic dynamics of aquatic food webs, and the vertical transport of nutrients in the water column.Many hypotheses have been proposed to explain the adaptive significance of vertical migration. Of these, the three that have received the most attention are the metabolic-demographic hypothesis that organisms derive a Acknowledgments

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