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.

More than 90% of the global ocean dissolved organic carbon (DOC) is refractory, has an average age of 4000-6000 years and a lifespan from months to millennia. The fraction of dissolved organic matter (DOM) that is resistant to degradation is a long-term buffer in the global carbon cycle but its chemical composition, structure, and biochemical formation and degradation mechanisms are still unresolved. We have compiled the most comprehensive molecular dataset of 197 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses from solid-phase extracted marine DOM covering two major oceans, the Atlantic sector of the Southern Ocean and the East Atlantic Ocean (ranging from 50°N to 70°S). Molecular trends and radiocarbon dating of 34 DOM samples (comprising D 14 C values from À229& to À495&) were combined to model an integrated degradation rate for bulk DOC resulting in a predicted age of >24 ka for the most persistent DOM fraction. First order kinetic degradation rates for 1557 mass peaks indicate that numerous DOM molecules cycle on timescales much longer than the turnover of the bulk DOC pool (estimated residence times of up to~100 ka) and the range of validity of radiocarbon dating. Changes in elemental composition were determined by assigning molecular formulae to the detected mass peaks. The combination of residence times with molecular information enabled modelling of the average elemental composition of the slowest degrading fraction of the DOM pool. In our dataset, a group of 361 molecular formulae represented the most stable composition in the oceanic environment ("island of stability"). These most persistent compounds encompass only a narrow range of the molecular elemental ratios H/C (average of 1.17 ± 0.13), and O/C (average of 0.52 ± 0.10) and molecular masses (360 ± 28 and 497 ± 51 Da). In the Weddell Sea DOC concentrations in the surface waters were low (46.3 ± 3.3 lM) while the organic radiocarbon was significantly more depleted than that of the East Atlantic, representing average surface water DOM ages of 4920 ± 180 a. These results are in accordance with a highly degraded DOM in the Weddell Sea surface water as also shown by the molecular degradation index I DEG obtained from FT-ICR MS data. Further, we identified 339 molecular formulae which probably contribute to an increased DOC concentration in the Southern Ocean and potentially reflect an accumulation or enhanced sequestration of refractory DOC in the Weddell Sea. These results will contribute to a better understanding of the persistent nature of marine DOM and its role as an oceanic carbon buffer in a changing climate.

Although sulfur is an essential element for marine primary production and critical for climate processes, little is known about the oceanic pool of nonvolatile dissolved organic sulfur (DOS). We present a basin-scale distribution of solid-phase extractable DOS in the East Atlantic Ocean and the Atlantic sector of the Southern Ocean. Although molar DOS versus dissolved organic nitrogen (DON) ratios of 0.11 ± 0.024 in Atlantic surface water resembled phytoplankton stoichiometry (sulfur/nitrogen~0.08), increasing dissolved organic carbon (DOC) versus DOS ratios and decreasing methionine-S yield demonstrated selective DOS removal and active involvement in marine biogeochemical cycles. Based on stoichiometric estimates, the minimum global inventory of marine DOS is 6.7 petagrams of sulfur, exceeding all other marine organic sulfur reservoirs by an order of magnitude.

Abstract. Dissolved organic matter (DOM) was extracted by solid-phase extraction (SPE) from 137 water samples from different climate zones and different depths along an eastern Atlantic Ocean transect. The extracts were analyzed with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) with electrospray ionization (ESI). 14 C analyses were performed on subsamples of the SPE-DOM. In addition, the amount of dissolved organic carbon was determined for all water and SPE-DOM samples as well as the yield of amino sugars for selected samples.Linear correlations were observed between the magnitudes of 43 % of the FT-ICR mass peaks and the extract 14 C values.Decreasing SPE-DOM 14 C values went along with a shift in the molecular composition to higher average masses (m/z) and lower hydrogen/carbon (H/C) ratios. The correlation was used to model the SPE-DOM 14 C distribution for all 137 samples. Based on single mass peaks, a degradation index (I DEG ) was developed to compare the degradation state of marine SPE-DOM samples analyzed with FT-ICR MS. A correlation between 14 C, I DEG , DOC values and amino sugar yield supports that SPE-DOM analyzed with FT-ICR MS reflects trends of bulk DOM. DOM weighted normalized mass peak magnitudes were used to compare aged and recent SPE-DOM on a semi-quantitative molecular basis. The magnitude comparison showed a continuum of different degradation rates for the detected compounds. A high proportion of the compounds should persist, possibly modified by partial degradation, in the course of thermohaline circulation. Prokaryotic (bacterial) production, transformation and accumulation of this very stable DOM occur primarily in the upper ocean. This DOM is an important contribution to very old DOM, showing that production and degradation are dynamic processes.

Carbon cycling in the coastal zone affects global carbon budgets and is critical for understanding the urgent issues of hypoxia, acidification, and tidal wetland loss. However, there are no regional carbon budgets spanning the three main ecosystems in coastal waters: tidal wetlands, estuaries, and shelf waters. Here we construct such a budget for eastern North America using historical data, empirical models, remote sensing algorithms, and process‐based models. Considering the net fluxes of total carbon at the domain boundaries, 59 ± 12% (± 2 standard errors) of the carbon entering is from rivers and 41 ± 12% is from the atmosphere, while 80 ± 9% of the carbon leaving is exported to the open ocean and 20 ± 9% is buried. Net lateral carbon transfers between the three main ecosystem types are comparable to fluxes at the domain boundaries. Each ecosystem type contributes substantially to exchange with the atmosphere, with CO2 uptake split evenly between tidal wetlands and shelf waters, and estuarine CO2 outgassing offsetting half of the uptake. Similarly, burial is about equal in tidal wetlands and shelf waters, while estuaries play a smaller but still substantial role. The importance of tidal wetlands and estuaries in the overall budget is remarkable given that they, respectively, make up only 2.4 and 8.9% of the study domain area. This study shows that coastal carbon budgets should explicitly include tidal wetlands, estuaries, shelf waters, and the linkages between them; ignoring any of them may produce a biased picture of coastal carbon cycling.