As the permafrost region warms, its large organic carbon pool will be increasingly vulnerable to decomposition, combustion, and hydrologic export. Models predict that some portion of this release will be offset by increased production of Arctic and boreal biomass; however, the lack of robust estimates of net carbon balance increases the risk of further overshooting international emissions targets. Precise empirical or model-based assessments of the critical factors driving carbon balance are unlikely in the near future, so to address this gap, we present estimates from 98 permafrost-region experts of the response of biomass, wildfire, and hydrologic carbon flux to climate change. Results suggest that contrary to model projections, total permafrost-region biomass could decrease due to water stress and disturbance, factors that are not adequately incorporated in current models. Assessments indicate that end-of-the-century organic carbon release from Arctic rivers and collapsing coastlines could increase by 75% while carbon loss via burning could increase four-fold. Experts identified water balance, shifts in vegetation community, and permafrost degradation as the key sources of uncertainty in predicting future system response. In combination with previous findings, results suggest the permafrost region will become a carbon source to the atmosphere by 2100 regardless of warming scenario but that 65%-85% of permafrost carbon release can still be avoided if human emissions are actively reduced.

In nearly a dozen open‐ocean fertilization experiments conducted by more than 100 researchers from nearly 20 countries, adding iron at the sea surface has led to distinct increases in photosynthesis rates and biomass. These experiments confirmed the hypothesis proposed by the late John Martin [Martin, 1990] that dissolved iron concentration is a key variable that controls phytoplankton processes in ocean surface waters However, the measurement of dissolved iron concentration in seawater remains a difficult task [Bruland and Rue, 2001] with significant interlaboratory differences apparent at times. The availability of a seawater reference solution with well‐known dissolved iron (Fe) concentrations similar to open‐ocean values, which could be used for the calibration of equipment or other tasks, would greatly alleviate these problems [National Research Council (NRC), 2002[.

Sources, abundance, isotopic compositions, and export fluxes of dissolved inorganic carbon (DIC), dissolved and colloidal organic carbon (DOC and COC), and particulate organic carbon (POC), and their response to hydrologic regimes were examined through monthly sampling from the Lower Mississippi River during was the most abundant carbon species, followed by POC and DOC. Concentration and δ 13 C of DIC decreased with increasing river discharge, while those of DOC remained fairly stable. COC comprised 61 ± 3% of the bulk DOC with similar δ 13 C abundances but higher percentages of hydrophobic organic acids than DOC, suggesting its aromatic and diagenetically younger status. POC showed peak concentrations during medium flooding events and at the rising limb of large flooding events. While δ 13 C-POC increased, δ 15 N of particulate nitrogen decreased with increasing discharge. Overall, the differences in δ 13 C between DOC or DIC and POC show an inverse correlation with river discharge. The higher input of soil organic matter and respired CO 2 during wet seasons was likely the main driver for the convergence of δ 13 C between DIC and DOC or POC, whereas enhanced in situ primary production and respiration during dry seasons might be responsible for their isotopic divergence. Carbon export fluxes from the Mississippi River were estimated to be 13.6 Tg C yr À1 for DIC, 1.88 Tg C yr À1 for DOC, and 2.30 Tg C yr À1 for POC during 2006-2008. The discharge-normalized DIC yield decreased during wet seasons, while those of POC and DOC increased and remained constant, respectively, implying variable responses in carbon export to the increasing discharge.As one of the world's largest rivers, the Mississippi River drains 41% of the contiguous United States with an increasing trend of water discharge over the past century from 494 to 578 km 3 yr À1 [Milliman, 1991;Raymond et al., 2008]. Water chemistry in the Mississippi River has been heavily influenced by agricultural activities (58% cropland coverage in the entire basin) and the presence of hydraulic infrastructures (dams and flood control levees) on the primary tributaries and the main river channel. Along with the enormous annual river discharge, the Mississippi River delivers large amounts of terrestrially derived materials, including nutrients and carbon, to the Gulf of Mexico [Bianchi et al.

Nitrogen fixation is critical for the biological productivity of the ocean, but clear mechanistic controls on this process remain elusive. Here, we investigate the abundance, activity, and drivers of nitrogen-fixing diazotrophs across the tropical western North Pacific. We find a basin-scale coherence of diazotroph abundances and N 2 fixation rates with the supply ratio of iron:nitrogen to the upper ocean. Across a threshold of increasing supply ratios, the abundance of nifH genes and N 2 fixation rates increased, phosphate concentrations decreased, and bioassay experiments demonstrated evidence for N 2 fixation switching from iron to phosphate limitation. In the northern South China Sea, supply ratios were hypothesized to fall around this critical threshold and bioassay experiments suggested colimitation by both iron and phosphate. Our results provide evidence for iron:nitrogen supply ratios being the most important factor in regulating the distribution of N 2 fixation across the tropical ocean.

Climate and environmental changes are having profound impacts on Arctic river basins, but the biogeochemical response remains poorly understood. To examine the effect of ice formation on temporal variations in composition and fluxes of carbon and nutrient species, monthly water and particulate samples collected from the lower Yukon River between July 2004 and September 2005 were measured for concentrations of organic and inorganic C, N, and P, dissolved silicate (Si(OH) 4 ), and stable isotope composition (dD and d 18 O). All organic carbon and nutrient species had the highest concentration during spring freshet and the lowest during the winter season under the ice, indicating dominant sources from snowmelt and flushing of soils in the drainage basin. In contrast, inorganic species such as dissolved inorganic carbon (DIC) and Si(OH) 4 had the highest concentrations in winter and the lowest during spring freshet, suggesting dilution during snowmelt and sources from groundwater and leaching/weathering of mineral layer. The contrasting relation with discharge between organic, such as dissolved organic carbon (DOC), and inorganic, such as DIC and Si(OH) 4 , indicates hydrological control of solute concentration but different sources and transport mechanisms for organic and inorganic carbon and nutrient species. Concentration of DOC also shows an inter-annual variability with higher DOC in 2005 (higher stream flow) than 2004 (lower stream flow). Average inorganic N/P molar ratio was 110 ± 124, with up to 442 under the ice and 38-70 during the ice-open season. While dissolved organic matter had a higher C/N ratio under the ice (45-62), the particulate C/N ratio was lower during winter (21-26) and spring freshet (19). Apparent fractionation factors of C, N, P, Si and dD and d 18 O between ice and river water varied considerably, with high values for inorganic species such as DIC and Si(OH) 4 (45 and 9550, respectively) but lower values for DOC (4.7). River ice formation may result in fractionation of inorganic and organic solutes and the repartitioning of seasonal flux of carbon and nutrient species. estimation without spring freshet sampling results in considerable underestimation for organic species but significant overestimation for inorganic species regardless of the flux estimation methods used. Without time-series sampling that includes frozen season, an over-or under-estimation in carbon and nutrient fluxes will occur depending on chemical species. Large differences in carbon export fluxes between studies and sampling years indicate that intensive sampling together with long-term observations are needed to determine the response of the Yukon River to a changing climate.

Water samples were collected biweekly from the Chena River, Alaska, during [2005][2006] for analysis of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total dissolved carbohydrate (TCHO), including monosaccharide (MCHO) and polysaccharide (PCHO), particulate organic carbon (POC) and its isotopic compositions, and Si(OH) 4 . Carbon species exhibit strong temporal variations with elevated DOC, POC, and TCHO but depleted DIC and Si(OH) 4 during the spring freshet and decreased DOC, POC, and TCHO but elevated DIC and Si(OH) 4 concentrations under winter ice. Organic matter is mostly derived from surface soil leaching, whereas DIC and Si(OH) 4 are associated with groundwater and mineral layer leaching. On average, DIC was the predominant carbon species, accounting for 77% 6 13% of the total carbon pool, whereas DOC and POC comprised 19% 6 10% and 4% 6 4%, respectively. However, DOC became the dominant carbon species during the spring freshet. TCHO comprised 15% 6 4% of DOC with higher CHO : DOC ratios during spring runoff and summer. Within the TCHO pool, MCHO was the predominant CHO component (89% 6 10%), leaving 11% 6 10% as PCHO. The particulate organic matter source during the summer drought season was mostly autochthonous, with low POC

The abundance of colloidal organic phosphorus (COP) and colloidal inorganic phosphorus (CIP) was quantitatively determined using an ultrafiltration permeation model for riverine, estuarine, and coastal waters in the northern Gulf of Mexico. Dissolved inorganic phosphorus (DIP) was the dominant species in Mississippi and Pearl River waters, while dissolved organic phosphorus was dominant in marine environments. The abundance of COP was highest in the Pearl River (up to 88%), followed by the Mississippi Bight (,67%), and the Bay of St. Louis (,59%), but was lowest in the Mississippi River (41-50%). These variations highlight the roles of terrestrial inputs, autochthonous production, and anthropogenic activities in controlling the COP abundance in different aquatic environments. In the Gulf of Mexico, COP abundance generally decreased with increasing depth and coincided with chlorophyll a, reflecting the production of COP by phytoplankton and remineralization during downward transportation in the water column. The colloidal organic C : P molar ratios were substantially higher than the Redfield ratio but consistently lower than those of the bulk dissolved organic matter except for the Mississippi River, indicating a diagenetically fresher or younger COP pool. The percentage of CIP in the DIP pool was generally negligible or very low (#3%) except in the Pearl River, where CIP abundance was as high as 27-47%, likely contributed from colloidal soils, minerals, and iron oxyhydroxides. We hypothesize that high COP abundance and seasonal P-limitation play an important role in regulating the biogeochemical cycling of P and the development of hypoxia in the northern Gulf of Mexico.

Partially eroded ice wedges and lenticularly shaped bodies of massive thermokarst‐cave ice in ice‐rich syngenetic permafrost (yedoma) are exposed in the CRREL tunnel near Fairbanks, Alaska. The ice wedges, which formed 25 000 − 40 000 years ago, were subsequently affected by localised thermal erosion, resulting in underground cavities that filled with surface water infiltrating through a network of conduits. This water froze inward from the walls of the cavity. We report the biogeochemical characteristics of one of these thermokarst‐cave ice features and four nearby ice wedges. The thermokarst‐cave ice has 30 times the dissolved organic carbon concentration, 20 times the total dissolved nitrogen concentration and five to 20 times the inorganic solute concentrations of the surrounding (original) ice wedge material. Based on these results we present a schematic model to describe how the thermokarst‐cave ice was formed and preserved and what processes led to its current biogeochemical characteristics. Current estimates of soluble solutes stored in permafrost may underestimate the total carbon and nutrient load where wedge material has been extensively replaced by surface water rich in organic carbon, nutrients or inorganic solutes. Published in 2011 by John Wiley & Sons, Ltd.