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Rivers efficiently collect, process, and transport terrestrial‐derived carbon. River ecosystem metabolism is the primary mechanism for processing carbon. Diel cycles of dissolved oxygen (DO) have been used for decades to infer river ecosystem metabolic rates, which are routinely used to predict metabolism of carbon dioxide (CO2) with uncertainties of the assumed stoichiometry ranging by a factor of 4. Dissolved inorganic carbon (DIC) has been less used to directly infer metabolism because it is more difficult to quantify, involves the complexity of inorganic carbon speciation, and as shown in this study, likely requires a two‐station approach. Here, we developed DIC metabolism models using single‐ and two‐station approaches. We compared metabolism estimates based on simultaneous DO and DIC monitoring in the Upper Clark Fork River (USA), which also allowed us to estimate ecosystem‐level photosynthetic and respiratory quotients (PQEand RQE). We observed that metabolism estimates from DIC varied more between single‐ and two‐station approaches than estimates from DO. Due to carbonate buffering, CO2is slower to equilibrate with the atmosphere compared to DO, likely incorporating a longer distance of upstream heterogeneity. Reach‐averaged PQEranged from 1.5 to 2.0, while RQEranged from 0.8 to 1.5. Gross primary production from DO was larger than that from DIC, as was net ecosystem production by 100mmolm−2d−1. The river was autotrophic based on DO but heterotrophic based on DIC, complicating our understanding of how metabolism regulated CO2production. We suggest future studies simultaneously model metabolism from DO and DIC to understand carbon processing in rivers.

Ocean deoxygenation is predicted to threaten marine ecosystems globally. However, current and future oxygen concentrations and the occurrence of hypoxic events on coral reefs remain underexplored. Here, using autonomous sensor data to explore oxygen variability and hypoxia exposure at 32 representative reef sites, we reveal that hypoxia is already pervasive on many reefs. Eighty-four percent of reefs experienced weak to moderate (≤153 µmol O2kg−1to ≤92 µmol O2kg−1) hypoxia and 13% experienced severe (≤61 µmol O2kg−1) hypoxia. Under different climate change scenarios based on four Shared Socioeconomic Pathways (SSPs), we show that projected ocean warming and deoxygenation will increase the duration, intensity and severity of hypoxia, with more than 94% and 31% of reefs experiencing weak to moderate and severe hypoxia, respectively, by 2100 under SSP5-8.5. This projected oxygen loss could have negative consequences for coral reef taxa due to the key role of oxygen in organism functioning and fitness.

Rapid climate warming and sea-ice loss have induced major changes in the sea surface partial pressure of CO2(pCO2). However, the long-term trends in the western Arctic Ocean are unknown. Here we show that in 1994–2017, summer pCO2in the Canada Basin increased at twice the rate of atmospheric increase. Warming and ice loss in the basin have strengthened the pCO2seasonal amplitude, resulting in the rapid decadal increase. Consequently, the summer air–sea CO2gradient has reduced rapidly, and may become near zero within two decades. In contrast, there was no significant pCO2increase on the Chukchi Shelf, where strong and increasing biological uptake has held pCO2low, and thus the CO2sink has increased and may increase further due to the atmospheric CO2increase. Our findings elucidate the contrasting physical and biological drivers controlling sea surface pCO2variations and trends in response to climate change in the Arctic Ocean.

Changes may be occurring in the carbonate chemistry of fjords due to natural and anthropogenic disturbance of major freshwater sources. We present a high‐frequency time series study of seasonal pH and CO2partial pressure (pCO2) in a north Patagonian fjord with a focus on changes in freshwater inflows and biological processes. To do this, we monitored pH and pCO2in situ, along with river streamflow, salinity, temperature, and dissolved oxygen (DO) in the Reloncaví Fjord (41.5°S) for a full year (January to December 2015). Strong seasonal variability was observed in the pCO2, pH, and DO of the fjord's surface waters. During the summer, pCO2reached its annual minimum (range: 187–571 μatm) and pH its maximum (range: 7.98–8.24), coinciding with lower freshwater inflows (204–307 m3/s) and high DO (280–378 μmol/kg), as well as aragonite saturation states (ΩArag) higher than 1. In contrast, in winter, pCO2ranged from 461–1,008 μatm and pH from 7.57–8.03, coinciding with high freshwater inflows (1,049–1,402 m3/s), lower oxygen (216–348 μmol/kg), and constant undersaturation of ΩArag. Reloncaví Fjord had an annual air‐water CO2flux of 0.716 ± 2.54 mol·m−2·year−1during 2015 and thus acted as a low emission system. The annual cycle was mainly governed by seasonal changes in biological processes that enhanced the shift from a CO2sink in late spring and summer, caused by high primary production rates, to a CO2source during the rest of the year caused by high community respiration due to allochthonous organic carbon inputs. North Patagonian fjords are aquaculture‐heavy used environments, and there are not available data for understanding major biogeochemical process like air‐sea CO2fluxes and aragonite saturationA high‐frequency time series of the annual cycle of CO2, pH, and dissolved oxygen in a north Patagonian fjord is evaluatedThe fjord inorganic carbon cycle is very dynamic, primarily driven by varying contributions from primary production and respiration of allochthonous organic carbon

Solute exclusion during sea ice formation is a potentially important contributor to the Arctic Ocean inorganic carbon cycle that could increase as ice cover diminishes. When ice forms, solutes are excluded from the ice matrix, creating a brine that includes dissolved inorganic carbon (DIC) and total alkalinity (AT). The brine sinks, potentially exporting DIC and ATto deeper water. This phenomenon has rarely been observed, however. In this manuscript, we examine a ~1 year pCO2mooring time series where a ~35‐μatm increase in pCO2was observed in the mixed layer during the ice formation period, corresponding to a simultaneous increase in salinity from 27.2 to 28.5. Using salinity and ice based mass balances, we show that most of the observed increases can be attributed to solute exclusion during ice formation. The resulting pCO2is sensitive to the ratio of ATand DIC retained in the ice and the mixed layer depth, which controls dilution of the ice‐derived ATand DIC. In the Canada Basin, of the ~92 μmol/kg increase in DIC, 17 μmol/kg was taken up by biological production and the remainder was trapped between the halocline and the summer stratified surface layer. Although not observed before the mooring was recovered, this inorganic carbon was likely later entrained with surface water, increasing the pCO2at the surface. It is probable that inorganic carbon exclusion during ice formation will have an increasingly important influence on DIC and pCO2in the surface of the Arctic Ocean as seasonal ice production and wind‐driven mixing increase with diminishing ice cover. When sea ice forms, the dissolved solutes are squeezed out of the ice matrix, forming a dense brine that can sink into deeper water. This process has a potentially important role in the inorganic carbon cycle because dissolved inorganic carbon can sink with the brine, exporting it from the surface ocean and increasing the sea surface CO2partial pressure (pCO2). We deployed a pCO2sensor at 30‐m depth in the Canada Basin, the largest of the Arctic Ocean subbasins, that documented an increase in pCO2during the fall to winter ice formation period. This increase correlated with an increase in salinity over the same time period. A simple model calculation supports that the increase in pCO2and inorganic carbon is primarily due to ice formation. This finding has important implications for the future inorganic carbon cycle as seasonal ice formation increases due to loss of perennial ice cover in the Arctic. We deployed a pCO2sensor in the northwest Canada Basin of the Arctic Ocean; the time series shows a steady increase from fall to winterThe pCO2increase is attributed to sea ice formation that excludes inorganic carbon; the inorganic carbon concentrated in the mixed layerIce exclusion of inorganic carbon could become an increasingly important part of the Arctic Ocean carbon cycle with diminishing ice cover

Understanding the physical and biogeochemical processes that control CO2and dissolved oxygen (DO) dynamics in the Arctic Ocean (AO) is crucial for predicting future air‐sea CO2fluxes and ocean acidification. Past studies have primarily been conducted on the AO continental shelves during low‐ice periods and we lack information on gas dynamics in the deep AO basins where ice typically inhibits contact with the atmosphere. To study these gas dynamics, in situ time‐series data have been collected in the Canada Basin during late summer to autumn of 2012. Partial pressure of CO2(pCO2), DO concentration, temperature, salinity, and chlorophyll‐a fluorescence (Chl‐a) were measured in the upper ocean in a range of sea ice states by two drifting instrument systems. Although the two systems were on average only 222 km apart, they experienced considerably different ice cover and external forcings during the 40–50 day periods when data were collected. The pCO2levels at both locations were well below atmospheric saturation whereas DO was almost always slightly supersaturated. Modeling results suggest that air‐sea gas exchange, net community production (NCP), and horizontal gradients were the main sources of pCO2and DO variability in the sparsely ice‐covered AO. In areas more densely covered by sea ice, horizontal gradients were the dominant source of variability, with no significant NCP in the surface mixed layer. If the AO reaches equilibrium with atmospheric CO2as ice cover continues to decrease, aragonite saturation will drop from a present mean of 1.00 ± 0.02 to 0.86 ± 0.01. Arctic Ocean sea surface pCO2and dissolved O2dynamics are examined using in situ sensorsVariability in low ice regions was dominated by air‐sea gas exchange, net community production, and advection/mixingVariability in high ice regions was dominated by horizontal advection with negligible contribution from net community production The Arctic Ocean (AO) is changing rapidly. Decreasing sea ice thickness and extent is increasing exposure of the sea surface to heat and gas exchange. It is possible that decreasing sea ice extent has increased air‐sea exchange of CO2 leading to higher surface ocean CO2 levels and accelerating ocean acidification. This study has used autonomous carbon dioxide and oxygen sensors deployed on ice tethered drifters to study the carbon cycle in the Arctic Ocean. Two different sensor‐equipped drifters were deployed, one in open water and one where there was extensive ice coverage. The open water system found higher rates of biological production and uptake of atmospheric CO2 compared to the sensors in the ice covered region. This studies finds that with continuous exposure to the atmosphere the AO sea surface will increase towards atmospheric CO2 reducing the Arctic Ocean sink of anthropogenic CO2 while rapidly decreasing sea surface pH (accelerating ocean acidification).

A polymer-clad fiber optic is used as an evanescent field absorbance sensor for solvents that penetrate into the polymer cladding. The sensor is coupled to an FT-NIR spectrometer for spectral measurements from 1.0 to 2.2 μm. A 400-μm core fiber optic, 1.5 m in length, was coiled with a 1.5-cm radius on a Teflon® support. The coiled sensor was then immersed in various nonpolar organic liquids which partitioned into the hydrophobic polysiloxane cladding. The evanescent-field near-infrared spectra of pure hexane, chloroform, and carbon tetrachloride are shown along with ethanol in chloroform. Various amounts of chloroform in carbon tetrachloride and toluene in cyclohexane were used to test the quantitative response. This sensor data are compared with data from a conventional NIR spectrometer using principal component regression.

Coral reefs are predicted to be one of the ecosystems most sensitive to ocean acidification. To improve predictions of coral reef response to acidification, we need to better characterize the natural range of variability of pH, partial pressure of carbon dioxide (pCO2) and calcium carbonate saturation states (Ω). In this study, autonomous sensors for pH and pCO2were deployed on Media Luna reef, Puerto Rico over three seasons from 2007 to 2008. High temporal resolution CaCO3saturation states were calculated from the in situ data, giving a much more detailed characterization of reef saturation states than previously possible. Reef pH, pCO2and aragonite saturation (ΩAr) ranged from 7.89 to 8.17 pH units, 176–613 μatm and 2.7–4.7, respectively, in the range characteristic of most other previously studied reef ecosystems. The diel pH, pCO2and Ω cycles were also large, encompassing about half of the seasonal range of variability. Warming explained about 50% of the seasonal supersaturation in mean pCO2, with the remaining supersaturation primarily due to net heterotrophy and net CaCO3production. Net heterotrophy was likely driven by remineralization of mangrove derived organic carbon which continued into the fall, sustaining high pCO2levels until early winter when the pCO2returned to offshore values. As a consequence, the reef was a source of CO2to the atmosphere during the summer and fall and a sink during winter, resulting in a net annual source of 0.73 ± 1.7 mol m−2year−1. These results show that reefs are exposed to a wide range of saturation states in their natural environment. Mean ΩArlevels will drop to 3.0 when atmospheric CO2increases to 500 μatm and ΩArwill be less than 3.0 for greater than 70% of the time in the summer. Long duration exposure to these low ΩArlevels are expected to significantly decrease calcification rates on the reef. Aragonite saturation states vary over a large rangeReef inorganic carbon is controlled by local and open ocean processesCaCO3 saturation approaches 3.0 when atmospheric CO2 is 500 uatm

A unique time series of moored bio‐optical measurements documented the 2004 spring‐summer bloom in the southern Labrador Sea. In situ and satellite chlorophyll data show that chlorophyll levels in the 2004 bloom were at the upper end of those typically observed in this region. Satellite chlorophyll and profiling float temperature/salinity data show that the main bloom, which typically peaks in June/July, is often preceded by ephemeral mixed layer shoaling and a lesser, short‐lived bloom in May; this was the case in 2004. The particulate backscatter to beam attenuation ratio (bbp[470 nm]/Cp[660 nm]) showed peaks in the relative abundance of small particles at bloom initiation and during the decline of the bloom, while larger particles dominated during the bloom. Chlorophyll/Cpand bbp/chlorophyll were correlated with carbon export and dominated by changes in the pigment per cell associated with lower light levels due to enhanced attenuation of solar radiation during the bloom. An NPZ (nutrients, phytoplankton, zooplankton) model captured the phytoplankton bloom and an early July peak in zooplankton. Moored acoustic Doppler current profiler (ADCP) data showed an additional mid‐June peak in zooplankton biomass which was attributed to egg‐laying copepods. The data reported here represent one of the few moored time series of Cp, bbpand chlorophyll extending over several months in an open ocean region. Interpretation of data sets such as this will become increasingly important as these deployments become more commonplace via ocean observing systems. Moreover, these data contribute to the understanding of biological‐physical coupling in a biogeochemically important, yet poorly studied region. Satellites underestimate chlorophyll, but not carbon, at high latitudesRatios of backscatter, chl and beam attenuation are correlated with C exportRatios of backscatter and beam attenuation reflect changes in phytoplankton