We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.

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.

The current prevailing theory of diel vertical migration (DVM) of zooplankton is focused largely on two biotic drivers: food and predation. Yet recent evidence suggests that abiotic drivers such as damaging ultraviolet (UV) radiation and temperature are also important. Here we integrate current knowledge on the effects of abiotic factors on DVM with the current biologically based paradigm to develop a more comprehensive framework for understanding DVM in zooplankton. We focus on ''normal'' (down during the day, up at night) DVM of holoplanktonic, primarily herbivorous zooplankton. This new transparency-regulator hypothesis differentiates between structural drivers, such as temperature and food, that vary little over a 24-h period and dynamic drivers, such as damaging UV radiation and visual predation, that show strong variation over a 24-h period. This hypothesis emphasizes the central role of water transparency in regulating these major drivers of DVM. In less transparent systems, temperature and food are often optimal in the surface waters, visual predators are abundant, and UV radiation levels are low. In contrast, in more transparent systems, vertical thermal gradients tend to be more gradual, food quality and quantity are higher in deeper waters, and visual predator abundance is often lower and damaging UV radiation higher in the surface waters. This transparency-regulator hypothesis provides a more versatile theoretical framework to explain variation in DVM across waters of differing transparency. This hypothesis also enables clearer predictions of how the wide range of ongoing transparency-altering local, regional, and global environmental changes can be expected to influence DVM patterns in both inland and oceanic waters of the world.Diel vertical migrations (DVM) of zooplankton in the world's lakes and oceans comprise some of the most widespread and massive migrations of animals on Earth. These striking migrations across strong vertical habitat gradients have inspired numerous ecological and evolutionary studies addressing mechanisms of habitat selection and consequences for species interactions. These migrations also have important implications for water quality and fisheries production as well as biogeochemical cycling. Natural and anthropogenic environmental changes, such as shifting local land use patterns and regional to global climate change, are altering water transparency and have the potential to affect DVM in lakes and oceans worldwide. The purpose of this article is to provide a common theoretical framework for understanding variation in the drivers of DVM across transparency gradients with a particular emphasis on recent advances in our understanding of the role of ultraviolet radiation (UV).Many environmental factors are recognized as providing both important proximate cues as well as ultimate consequences of adaptive significance for DVM, including light, temperature, food availability, and predation pressures (Table 1; Lampert 1989;Ringelberg 1993;Hays 2003). In spite of the wides...

As the lowest point in the surrounding landscape, lakes act as sensors in the landscape to provide insights into the response of both terrestrial and aquatic ecosystems to climate change. Here a novel suite of climate forcing optical indices (CFOI) from lakes across North America is found to respond to changes in air temperature, precipitation, and solar radiation at timescales ranging from a single storm event to seasonal changes to longerterm interdecadal trends with regression r 2 values ranging from 0.73 to 0.89. These indices are based on two optical metrics of dissolved organic carbon (DOC) quality: DOC specific absorbance (a* 320 ) and spectral slope (S 275-295 ), where the ratio a* 320 to S 275-295 gives a composite climate forcing index. These indices of DOC quality are more responsive to climate forcing than is DOC concentration. A similar relationship between the component indices a* 320 and S 275-295 is observed across a wide range of lake types. A conceptual model is used to examine the similarities and differences in DOC-related mechanisms and ecological consequences due to increased temperature vs. precipitation. While both warmer and wetter conditions increase thermal stratification, these two types of climate forcing will have opposite effects on water transparency as well as many ecological consequences, including oxygen depletion, the balance between autotrophy and heterotrophy, and depth distributions of phytoplankton and zooplankton.

Climate change and variation in atmospheric ozone are influencing the intensity of ultraviolet radiation (UVR) reaching ecosystems. Changing UVR regimes, in turn, may alter epidemics of infectious disease. This possibility hinges on the sensitivity of epidemiologically relevant traits of host and parasite to UVR. We address this issue using a planktonic system (a zooplankton host, Daphnia dentifera, and its virulent fungal parasite, Metschnikowia bicuspidata). Controlled laboratory experiments, coupled with in situ field incubations of spores, revealed that quite low levels of UVR (as well as longer wavelength light) sharply reduced the infectivity of fungal spores but did not affect host susceptibility to infection. The parasiteÕs sensitivity to solar radiation may underlie patterns in a lake survey: higher penetration of solar radiation into lakes correlated with smaller epidemics that started later in autumn (as incident sunlight declined). Thus, solar radiation, by diminishing infectivity of the parasite, may potently reduce disease.