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.

1. While studies of phytoplankton and terrestrial plant communities have increasingly emphasised the use of functional traits in ecological research, few have yet to apply this approach to zooplankton communities. 2. This study reviews laboratory and observational studies on zooplankton feeding and life history and provides a series of functional trait tables for the North American freshwater zooplankton. Qualitative and quantitative trait tables highlight areas where data were more scarce and point to which types of studies could fill in gaps in our knowledge of zooplankton niches. 3. Data were most complete for the Cladocera across most traits, while feeding information for cyclopoids was most sparse. Qualitative data that distinguished congeneric species were lacking for most groups. 4. A regional community dendrogram for common north-eastern North American zooplankton species was generated and shows that taxonomic differences between species do not capture fully functional differences based on the traits of body length, habitat, trophic group and feeding type. 5. The data collected here, combined with readily measurable species attributes, can be used to generate a multivariate measure of the functional niche of each species found in a community. Armed with this information, functional relationships that are useful for ecological studies of lake ecosystems can be more easily conducted.

Dissolved nitrogen (N) as urea ([NH 2 ] 2 CO), nitrate (NO { 3 ), and ammonium (NH z 4 ) was added to naturally phosphorus (P)-rich lake water (up to 175 mg P L 21 ) to test the hypotheses that pollution of hypereutrophic lakes with N increases total algal abundance, alters community composition, and favors toxic cyanobacteria that do not fix atmospheric N 2 . Monthly experiments were conducted in triplicate in polymictic Wascana Lake, Saskatchewan, Canada, during July, August, and September 2008 using large (. 3140 liters) enclosures. Addition of all forms of N added at 6 mg N L 21 increased total algal abundance (as chlorophyll a) by up to 350% relative to controls during August and September, when soluble reactive P (SRP) was . 50 mg P L 21 and dissolved N : P was , 20 : 1 by mass. In particular, NH z 4 and urea favored non-heterocystous cyanobacteria and chlorophytes and NO { 3 , urea promoted chlorophytes, some cyanobacteria, and transient blooms of siliceous algae, whereas N 2 -fixing cyanobacteria and dinoflagellates exhibited little response to added N. Added N also increased microcystin production by up to 13-fold in August and September, although the magnitude of response varied with N species and predominant algal taxon (Planktothrix agardhii, Microcystis spp.). These findings demonstrate that pollution with N intensifies eutrophication and algal toxicity in lakes with elevated concentrations of SRP and low N : P, and that the magnitude of these effects depends on the chemical form, and hence source, of N.

Urea is the most abundant nitrogen (N) fertilizer used on agricultural soils, yet its effects on adjacent aquatic ecosystems are largely unknown. Here 21-d, 3000-liter mesocosm experiments were conducted monthly in a hypereutrophic lake during July-September 2007 to quantify how addition of urea might affect phytoplankton abundance, gross community composition, and algal toxicity in a phosphorus (P) -rich lake. Repeated measures analysis of variance demonstrated that addition of sufficient urea to increase ratios of soluble N : P from , 15 : 1 to . 24 : 1 (by mass) also increased algal biomass (as Chlorophyll a) and microcystin concentrations 200-400%, as non-N 2 -fixing but toxic cyanobacteria (Microcystis, Planktothrix) and less harmful chlorophytes (Micractinium, Oocystis) replaced colonial N 2 -fixing cyanobacteria (Anabaena, Aphanizomenon). No significant effects of urea amendment were recorded for trials in which N : P ratios were elevated at the start of the experiment, or in which ambient light levels were reduced to 25 mmol quanta m 22 s 21 , although preliminary evidence suggests that urea addition stimulated growth of heterotrophic bacteria irrespective of light regime. Development of toxic non-N 2 -fixing cyanobacteria by N pollution of P-rich lakes is consistent with findings from whole-lake experiments and paleolimnological studies of deep lakes, and suggests that the fertilization needed to feed 3 billion more people by 2050 may create conditions in which future water quality in P-replete regions is degraded further by urea export from farms and cities.

Six hard-water lakes were sampled May-August for 14 yr in a 52,000 km 2 catchment to identify the mechanisms that regulate the spatial and temporal variability of net atmospheric exchange of CO 2 of lakes on the Northern Great Plains. Annual mean daily fluxes ranged from 2100 to .200 mmol C m 22 d 21 , while pCO 2 values varied between 0.3 and 5500 Pa. We observed periods of net CO 2 uptake (1995, 2000) and release (1998, 2006) resulting in synchronous variations in net CO 2 flux among lakes. Furthermore, pCO 2 , pH, and chemical enhancement of CO 2 influx all varied coherently among sites. Interannual variation in net CO 2 flux and pCO 2 was correlated strongly with pH, correlated weakly with other physical and chemical conditions, and was uncorrelated to algal biomass, productivity, or ecosystem respiration. In contrast, spatial variability of water-column pCO 2 was correlated negatively to concentrations of soluble reactive phosphorus, total dissolved nitrogen, pH, and gross primary productivity, suggesting an important role of lake metabolism at large spatial scales. Finally, comparison with an additional 20 saline lakes demonstrated that changes in mean annual pH, pCO 2 , and CO 2 flux during 2002-2007 were coherent in diverse lakes within a region of .100,000 km 2 and suggest that climatic control of pH and pCO 2 had an unexpectedly great effect on net CO 2 flux through productive hard-water lake ecosystems.