Microbial degradation of dissolved organic carbon (DOC) in aquatic environments can cause oxygen depletion, water acidification, and CO₂ emissions. These problems are caused by labile DOC (LDOC) and not refractory DOC (RDOC) that resists degradation and is thus a carbon sink. For nearly a century, chemical oxygen demand (COD) has been widely used for assessment of organic pollution in aquatic systems. Here, we show through a multicountry survey and experimental studies that COD is not an appropriate proxy of microbial degradability of organic matter because it oxidizes both LDOC and RDOC, and the latter contributes up to 90% of DOC in high-latitude forested areas. Hence, COD measurements do not provide appropriate scientific information on organic pollution in natural waters and can mislead environmental policies. We propose the replacement of the COD method with an optode-based biological oxygen demand method to accurately and efficiently assess organic pollution in natural aquatic environments.

A series of 8 methyl iodide (CH₃I) production experiments were conducted during Meteor cruise 55 to the tropical Atlantic. Natural seawater samples were incubated in quartz flasks, on‐deck, for 24 hours both in the dark and under the natural light‐dark cycle. A subset of incubations was conducted with samples that had been either filtered or poisoned to reduce or eliminate within‐flask biological activity. The accumulation of CH₃I in the dark incubations was <20% of the ‘light’ production. The experimental results supported prior inferences that CH₃I is produced in the open ocean by a light‐dependent production pathway that is not directly dependent on biological activity. The average observed net production rate in full sunlight incubations was 2–5 nmol m⁻³ day⁻¹. A mass balance for the tropical Atlantic surface layer implies that this rate of production can support at least 50% of the average sea‐to‐air flux of 23 nmol m⁻² day⁻¹.

Nitrification is a central process of the aquatic nitrogen cycle that controls the supply of nitrate used in other key processes, such as phytoplankton growth and denitrification. Through time series observation and modeling of a seasonally stratified, eutrophic coastal basin, we demonstrate that physical dilution of nitrifying microorganisms by water column mixing can delay and decouple nitrification. The findings are based on a 4-y, weekly time series in the subsurface water of Bedford Basin, Nova Scotia, Canada, that included measurement of functional (amoA) and phylogenetic (16S rRNA) marker genes. In years with colder winters, more intense winter mixing resulted in strong dilution of resident nitrifiers in subsurface water, delaying nitrification for weeks to months despite availability of ammonium and oxygen. Delayed regrowth of nitrifiers also led to transient accumulation of nitrite (3 to 8 μmol · kg_sw⁻¹) due to decoupling of ammonia and nitrite oxidation. Nitrite accumulation was enhanced by ammonia-oxidizing bacteria (Nitrosomonadaceae) with fast enzyme kinetics, which temporarily outcompeted the ammonia-oxidizing archaea (Nitrosopumilus) that dominated under more stable conditions. The study reveals how physical mixing can drive seasonal and interannual variations in nitrification through control of microbial biomass and diversity. Variable, mixing-induced effects on functionally specialized microbial communities are likely relevant to biogeochemical transformation rates in other seasonally stratified water columns. The detailed study reveals a complex mechanism through which weather and climate variability impacts nitrogen speciation, with implications for coastal ecosystem productivity. It also emphasizes the value of high-frequency, multiparameter time series for identifying complex controls of biogeochemical processes in aquatic systems. Significance Changes in both quantity and speciation of nitrogen in coastal waters impact phytoplankton communities, contributing to eutrophication and harmful algal blooms. Multidisciplinary oceanographic time series of high resolution are rare but crucial for identifying complex mechanisms that underlie such anthropogenic impacts. Analysis and modeling of such a time series from a seasonally stratified fjord showed that dilution of nitrifier biomass by variable winter mixing altered the timing and rates of nitrification, which converts ammonia to nitrite and nitrate. This reveals a link among climate-sensitive physical dynamics, nitrifier abundance, and diversity, with controls on phytoplankton ecology. The findings imply that explicit measurement and modeling of microbial communities will be required to project impacts of climate change on coastal ecosystems.

We use a 30‐year time series (1986–2016) of dichlorodifluoromethane (CFC‐12) concentrations with a refined transit time distribution (TTD) method, to estimate the temporal variation of anthropogenic carbon (C_ant) in the Central Labrador Sea. We determined that the saturation of CFC‐12 and sulfur hexafluroide (SF₆) in newly‐formed Labrador Sea Water had departed significantly from 100% and varied systematically with time. Multiple linear regression of the time‐varying saturation, with the tracer's atmospheric growth rate and the wintertime mixed layer depth as independent variables, allowed reconstruction of the saturation history of CFC‐12 and SF₆ in wintertime surface waters, which was implemented in the TTD method. Use of the time‐varying saturation for CFC‐12 gave C_ant concentrations ∼7 μmol kg⁻¹ larger than estimates obtained assuming a constant saturation of 100%. The resulting C_ant column inventories were ∼20% larger and displayed lower interannual variability compared to conventional TTD‐based estimates. The column inventory of C_ant increased at an average rate of 1.8 mol m⁻² y⁻¹ over the 30‐year period. However, the accumulation rate of C_ant was higher than this average in the early 1990s and since 2013, whereas inventories remained almost unchanged between 2003 and 2012. The variation in the C_ant accumulation rate is shown to be linked to temporal variability in the relative layer thickness of the annually ventilated Labrador Sea Water and the underlying Deep Intermediate Water. The non‐steady C_ant accumulation highlights the importance of sampling frequency, especially in regions of variable deep mixing and high carbon inventories, and potential misinterpretation of C_ant dynamics.

CDIAC's ocean carbon data collection includes discrete and underway measurements from a variety of platforms, including research ships, commercial ships, and buoys. The measurements come from deep and shallow waters from all oceans. All ocean carbon data CDIAC receives come from individual investigators and groups following initial data review.

Water column data of carbon and carbon-relevant hydrographic and hydrochemical parameters from 188 previously non-publicly available cruise data sets in the Arctic Mediterranean Seas, Atlantic and Southern Ocean have been retrieved and merged into a new database: CARINA (CARbon dioxide IN the Atlantic Ocean). The data have gone through rigorous quality control procedures to assure the highest possible quality and consistency. The data for the pertinent parameters in the CARINA database were objectively examined in order to quantify systematic differences in the reported values, i.e. secondary quality control. Systematic biases found in the data have been corrected in the three data products: merged data files with measured, calculated and interpolated data for each of the three CARINA regions, i.e. the Arctic Mediterranean Seas, the Atlantic and the Southern Ocean. These products have been corrected to be internally consistent. Ninety-eight of the cruises in the CARINA database were conducted in the Atlantic Ocean, defined here as the region south of the Greenland-Iceland-Scotland Ridge and north of about 30° S. Here we present an overview of the Atlantic Ocean synthesis of the CARINA data and the adjustments that were applied to the data product. We also report the details of the secondary QC (Quality Control) for salinity for this data set. Procedures of quality control – including crossover analysis between stations and inversion analysis of all crossover data – are briefly described. Adjustments to salinity measurements were applied to the data from 10 cruises in the Atlantic Ocean region. Based on our analysis we estimate the internal consistency of the CARINA-ATL salinity data to be 4.1 ppm. With these adjustments the CARINA data products are consistent both internally as well as with GLODAP data, an oceanographic data set based on the World Hydrographic Program in the 1990s, and is now suitable for accurate assessments of, for example, oceanic carbon inventories and uptake rates and for model validation.