Abstract The capacity of CO 2 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 CO 2 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 CO 2 ( p CO 2 ) from remote sensing products. We show that the long‐term increase in CO 2 uptake capacity can be attributed to strong and enhanced biological uptake. In addition, the intraseasonal variability of surface p CO 2 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.
Abstract Nutrient transfer into the sunlit surface ocean by cyclonic eddies is potentially crucial for sustaining primary productivity in the stratified subtropical gyres. However, the nature of productivity enhancements, including the flow of matter to higher trophic levels and its impact on carbon fluxes, remain poorly resolved. Here, we report a detailed assessment of the biogeochemical response to a cyclonic eddy in the subtropical Northwest Pacific via a combination of ship‐based and autonomous platforms. Primary production was enhanced twofold within the eddy core relative to reference sites outside, whereas phytoplankton biomass even decreased. Pico‐phytoplankton (< 2 μ m) dominated (> 80%) both phytoplankton biomass and primary production inside and outside the eddy. The stimulated primary production in the eddy core was accompanied by an approximately twofold increase in mesozooplankton abundance, an approximately threefold increase in particle formation in the deep chlorophyll maximum layer, as well as significantly enhanced surface oceanic CO 2 uptake and net community production. We suggest these observations carry important implications for understanding carbon export in the subtropical ocean and highlight the need to include such subtropical eddy features in ocean carbon budget analyses.
This study evaluates deployment strategies for artificial oxygenation devices to mitigate coastal hypoxia, particularly in mariculture regions. Focusing on a typical mariculture region in the coastal waters of China, we examined the combined effects of topography, hydrodynamics, and biogeochemical processes. A high-resolution three-dimensional physical-biogeochemical coupled model, validated against observational data from three summer cruises in 2020, accurately captured key drivers of hypoxia. Results reveal that hypoxic zones exhibit an uneven distribution, driven by persistent offshore jets at specific locations. Nearshore deployment of oxygenation devices upstream of hypoxic zones significantly improves oxygen delivery and is more cost-efficient due to reduced construction and maintenance requirements. Uncertainty analysis explored the impacts of varying water mass properties, oxygen concentration, injection flow rates, and biogeochemical content. The influence varies depending on the deployment site. Particularly, buoyant plumes can notably reduce the effectiveness of hypoxia mitigation. Artificial oxygenation may lead to unintended ecological impacts, including increased nutrient release and enhanced primary production, which can prolong the duration of hypoxia. Furthermore, simulations indicate that natural downwelling currents are insufficient to transport oxygen-enriched surface water to the bottom hypoxic zones. These findings underscore the importance of comprehensive predeployment assessments and the advancement of oxygenation technologies to ensure both immediate effectiveness and long-term ecological sustainability.
Nitrite, an intermediate product of the oxidation of ammonia to nitrate (nitrification), accumulates in upper oceans, forming the primary nitrite maximum (PNM). Nitrite concentrations in the PNM are relatively low in the western North Pacific subtropical gyre (wNPSG), where eddies are frequent and intense. To explain these low nitrite concentrations, we investigated nitrification in cyclonic eddies in the wNPSG. We detected relatively low half-saturation constants (i.e., high substrate affinities) for ammonia and nitrite oxidation at 150 to 200 meter water depth. Eddy-induced displacement of high-affinity nitrifiers and increased substrate supply enhanced ammonia and nitrite oxidation, depleting ambient substrate concentrations in the euphotic zone. Nitrite oxidation is more strongly enhanced by the cyclonic eddies than ammonia oxidation, reducing concentrations and accelerating the turnover of nitrite in the PNM. These findings demonstrate a spatial decoupling of the two steps of nitrification in response to mesoscale processes and provide insights into physical-ecological controls on the PNM.
Abstract 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 CO 2 , and 0.20 – 0.45 for pH. The variations of the CO 2 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 CO 2 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.
Abstract 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 environmental conditions in estuaries display distinct variability along the river-ocean mixing continuum from turbid, eutrophic freshwater to clear, oligotrophic offshore oceanic water. In order to understand the effects of suspended particulate matter (SPM), nutrient, and salinity on phytoplankton growth, this study investigated the response of a harmful dinoflagellate ( Amphidinium carterae Hulburt) to the ecological gradients in estuary environments. Rapid nutrient uptake and growth of A. carterae were detected in the nutrient-rich clear water, while nutrient concentration had little impact on the cellular chlorophyll a (Chl- a ) content at the stationary phase. Light attenuation caused by SPM not only inhibited the specific growth rate of A. carterae but also prolonged its adaption period in turbid water, resulting in a delayed and weakened growth response. The elevated cellular Chl- a content under high SPM conditions resulting from photo-acclimation led to the decoupling of cell density and Chl- a concentration, indicating that Chl- a is not a reliable indicator for phytoplankton abundance in turbid environments. The combined effect of SPM and nutrient on specific growth rate of A. carterae can be explained by the comparative effect model, while the multiplicative effect model better predicted their interactive effect on the growth inhibitory rate (GIR). There is a transit of dominant limiting factor for phytoplankton growth along the salinity gradient in estuary environments. Salinity (for marine phytoplankton cannot survive under low salinity condition) and SPM are the dominant limiting factors at low salinities in nearshore turbid environments, while nutrient depletion exerts the dominant inhibitory effect in high salinity offshore water. Depending on the balance between enhancing nutrient limitation and reducing light limitation with increasing salinity, blooms most likely occur in the “optimal growth region” at intermediate salinities where light and nutrient are both suitable for phytoplankton growth.
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