The variability of total alkalinity (TA) and its relationship with salinity in the tropical and subtropical surface ocean were examined using data collected in various marine environments from a ship of opportunity. In the open ocean regions of the Atlantic, Pacific, and Indian Oceans, sea surface TA variability was observed to be mainly controlled by the simple dilution or concentration (SDC) effect of precipitation and evaporation, and the measured concentrations of TA agreed well with those predicted from salinity and temperature. Non‐SDC changes in alkalinity in ocean margins and inland seas were examined by comparing the salinity‐normalized alkalinity with that of the open ocean end‐member. Non‐SDC alkalinity additions to the western North Atlantic margin, eastern North Pacific margin, and Mediterranean Sea were identified, which mainly resulted from river inputs and shelf currents. In contrast, removal of TA through formation and sedimentation of calcium carbonate was observed to be an important control in the Red Sea. The concentration of the river end‐member can only be reliably derived from the y intercept of TA‐S regression (TA_S0) in river‐dominated systems such as estuaries and river plumes. In coastal regions where other processes (evaporation, shelf currents, upwelling, calcification, etc.) are more influential, TA_S0 can significantly deviate from the river water concentration and hence be an unreliable indicator of it. Negative values of TA_S0 can result from non‐SDC TA removal at the low salinity end (relative to the salinity of the oceanic end‐member) and/or non‐SDC TA addition at high salinities (as occurs in the Mediterranean Sea).

The capacity of CO₂ uptake in the Chukchi Sea is particularly sensitive to rapid physical and biological changes. However, scarce field observations pose a challenge in understanding the long‐term trend of CO₂ uptake capacity on this continental shelf. We adopted a machine‐learning‐based approach to construct a 17‐years (2003–2019) long‐term time series of summer sea surface partial pressure of CO₂ (pCO₂) from remote sensing products. We show that the long‐term increase in CO₂ uptake capacity can be attributed to strong and enhanced biological uptake. In addition, the intraseasonal variability of surface pCO₂ in early summer confirms the crucial role of sea ice melt and the subsequently enhanced photosynthesis as soon as the surface ocean converts into an open system. Our results thus highlight the use of remote sensing data in interpolating/extrapolating the highly dynamic carbonate system in the continental shelf sea and shed light into future studies involving machine learning or algorithms.

The driving mechanisms of submarine groundwater discharge (SGD) in Sanya Bay in the northern South China Sea were investigated using a combination of time‐series observation and modeling, as well as the influences of SGD on the carbonate system of a coastal coral reef. SGD flux, characterized by high variability on flood‐ebb and spring‐neap tidal cycles, was found to be mainly driven by tidal pumping. SGD posed more significant impacts on coastal water at the ebb phase during the spring tide (higher SGD flux and extended offshore reach of SGD impact), with nearshore water to be more heavily affected. Under the influence of SGD, the diurnal ranges of the carbonate variables observed in the coral reef system were 124 – 313 μmol kg⁻¹ for dissolved inorganic carbon, 29 – 101 μmol kg⁻¹ for total alkalinity, 179 – 717 μatm for partial pressure of CO₂, and 0.20 – 0.45 for pH. The variations of the CO₂ system were dominated by the enhanced SGD input during the spring tide, while biological metabolism of coral reef played a predominant role during the neap tide. The intensified SGD input resulted in higher diurnal variations of the carbonate variables, enhanced acidification, and oceanic CO₂ emission during the spring tide. The SGD‐associated inorganic carbon flux is an additional stressor influencing coastal acidification in the context of rising atmospheric carbon dioxide.

Net community production (NCP) in the surface water of the northern Gulf of Mexico (nGOM) and its coupling with the CO₂ system were examined during the productive spring season. NCP was estimated using multiple approaches: (1) underway O₂ and Ar ratio, (2) oxygen changes during light/dark bottle oxygen incubations, and (3) non-conservative changes in dissolved inorganic carbon or nutrients. These methods all showed high spatial variability of NCP and displayed similar patterns along the river–ocean mixing gradient, showing high production rates in plume regions. NCP${}_{{\mathrm{O}}_{\mathrm{2}}\mathrm{Ar}}$ estimated from high-resolution O₂ and Ar underway measurement indicated heterotrophic conditions at the high-nutrient and high-turbidity Mississippi River end ($-\mathrm{51.3}\pm \mathrm{11.9}$ mmol C m⁻² d⁻¹ when salinity < 2) resulting from the influence of terrestrial carbon input and light limitation on photosynthesis. High NCP${}_{{\mathrm{O}}_{\mathrm{2}}\mathrm{Ar}}$ rates (105.0±59.2 mmol C m⁻² d⁻¹, up to 235.4 mmol C m⁻² d⁻¹) were observed in the Mississippi and Atchafalaya plumes at intermediate salinities between 15 and 30 where light and nutrients were both favorable for phytoplankton production. NCP${}_{{\mathrm{O}}_{\mathrm{2}}\mathrm{Ar}}$ rates observed in the high-salinity, oligotrophic offshore waters (salinity > 35.5) were close to zero due to nutrient limitation. Air–sea CO₂ fluxes generally showed corresponding changes, from being a strong CO₂ source in the river channel (55.5±7.6 mmol C m⁻² d⁻¹), to a CO₂ sink in the plume ($-\mathrm{13.4}\pm \mathrm{5.5}$ mmol C m⁻² d⁻¹), and to being nearly in equilibrium with the atmosphere in offshore waters. Overall, the surface water of the nGOM was net autotrophic during spring 2017, with an area-weighted mean NCP${}_{{\mathrm{O}}_{\mathrm{2}}\mathrm{Ar}}$ of 21.2 mmol C m⁻² d⁻¹, and was a CO₂ sink of −6.7 mmol C m⁻² d⁻¹. A temporal mismatch between in situ biological production and gas exchange of O₂ and CO₂ was shown through a box model to result in decoupling between NCP${}_{{\mathrm{O}}_{\mathrm{2}}\mathrm{Ar}}$ and CO₂ flux (e.g., autotrophic water as a CO₂ source outside the Mississippi River mouth and heterotopic water as a CO₂ sink in the Atchafalaya coastal water). This decoupling was a result of in situ biological production superimposed on the lingering background pCO₂ from the source water because of the slow air–sea CO₂ exchange rate and the buffering effect of the carbonate system.