Since the start of the industrial revolution, human activities have caused a rapid increase in atmospheric carbon dioxide (CO₂) concentrations, which have, in turn, had an impact on climate leading to global warming and ocean acidification. Various approaches have been proposed to reduce atmospheric CO₂. The Martin (or iron) hypothesis suggests that ocean iron fertilization (OIF) could be an effective method for stimulating oceanic carbon sequestration through the biological pump in iron-limited, high-nutrient, low-chlorophyll (HNLC) regions. To test the Martin hypothesis, 13 artificial OIF (aOIF) experiments have been performed since 1990 in HNLC regions. These aOIF field experiments have demonstrated that primary production (PP) can be significantly enhanced by the artificial addition of iron. However, except in the Southern Ocean (SO) European Iron Fertilization Experiment (EIFEX), no significant change in the effectiveness of aOIF (i.e., the amount of iron-induced carbon export flux below the winter mixed layer depth, MLD) has been detected. These results, including possible side effects, have been debated amongst those who support and oppose aOIF experimentation, and many questions concerning the effectiveness of scientific aOIF, environmental side effects, and international aOIF law frameworks remain. In the context of increasing global and political concerns associated with climate change, it is valuable to examine the validity and usefulness of the aOIF experiments. Furthermore, it is logical to carry out such experiments because they allow one to study how plankton-based ecosystems work by providing insight into mechanisms operating in real time and under in situ conditions. To maximize the effectiveness of aOIF experiments under international aOIF regulations in the future, we therefore suggest a design that incorporates several components. (1) Experiments conducted in the center of an eddy structure when grazing pressure is low and silicate levels are high (e.g., in the SO south of the polar front during early summer). (2) Shipboard observations extending over a minimum of ∼40 days, with multiple iron injections (at least two or three iron infusions of ∼2000 kg with an interval of ∼10–15 days to fertilize a patch of 300 km² and obtain a ∼2 nM concentration). (3) Tracing of the iron-fertilized patch using both physical (e.g., a drifting buoy) and biogeochemical (e.g., sulfur hexafluoride, photosynthetic quantum efficiency, and partial pressure of CO₂) tracers. (4) Employment of neutrally buoyant sediment traps (NBST) and application of the water-column-derived thorium-234 (²³⁴Th) method at two depths (i.e., just below the in situ MLD and at the winter MLD), with autonomous profilers equipped with an underwater video profiler (UVP) and a transmissometer. (5) Monitoring of side effects on marine/ocean ecosystems, including production of climate-relevant gases (e.g., nitrous oxide, N₂O; dimethyl sulfide, DMS; and halogenated volatile organic compounds, HVOCs), decline in oxygen inventory, and development of toxic algae blooms, with optical-sensor-equipped autonomous moored profilers and/or autonomous benthic vehicles. Lastly, we introduce the scientific aOIF experimental design guidelines for a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES).

As the western North Pacific Ocean is located downwind of the source regions for spring Asian dust, it is an ideal location for determining the response of open waters to these events. Spatial analysis of spring Asian dust events from source regions to the western North Pacific, using long‐term daily aerosol index data, revealed three different transport pathways supported by the westerly wind system: one passing across the northern East/Japan Sea (40°N–50°N), a second moving over the entire East/Japan Sea (35°N–55°N), and a third flowing predominantly over the Siberian continent (>50°N). Our results indicate that strong spring Asian dust events can increase ocean primary productivity by more than 70% (>2‐fold increase in chlorophyll‐a concentrations) compared to weak/nondust conditions. Therefore, attention should be paid to the recent downturn in the number of spring Asian dust events and to the response of primary production in the western North Pacific to this change.

The input of aeolian mineral dust to the oceans is regarded as the major source in supplying bioavailable iron for phytoplankton growth. Severe dust events swept over East Asia during the 26 March to the 4 April 2018, decreasing air quality to hazardous levels, with maximum PM₁₀ mass concentrations above 3000 μg m⁻³ in northern China. Based on a comprehensive approach that combines multiple satellite measurements, ground observations, and model simulation, we revealed that two severe Asian dust events originating from the Taklimakan and Gobi deserts on 26 March and 1 April, were transported through northern China and the East/Japan Sea, to the North Pacific Ocean by westerly wind systems. Transportation pathways dominated by mineral dust aerosols were observed at altitudes of 2–7 km in the source regions, and then ascending to 3–10 km in the North Pacific Ocean, with relatively denser dust plumes within the second dust episode than there were during the first. Our results suggest that mineral dust emitted from the Taklimakan and Gobi deserts could increase ocean primary productivity in the North Pacific Ocean by up to ~50%, compared to average conditions. This emphasizes the potential importance of the deposition of Asian mineral dust over the North Pacific Ocean for enhancing the biological pump.

Since the start of the industrial revolution, human activities have caused a rapid increase in atmospheric CO₂ concentrations, which have in turn been cited as the cause of a variety climate changes such as global warming and ocean acidification. Various approaches have been proposed to reduce atmospheric CO₂ concentrations. The ''Martin (or Iron) Hypothesis'' suggests that ocean iron fertilization (OIF) should be an efficient method for stimulating the biological pump in iron-limited high nutrient-low chlorophyll regions. To test the Martin hypothesis, a total 13 OIF experiments have been performed since 1990 in the Southern Ocean (7 times), in the subarctic Pacific (3 times), in the equatorial Pacific (twice), and in the subtropical Atlantic (once). These OIF field experiments demonstrated that primary production could be significantly increased after artificial iron addition. However, export production efficiency revealed by the OIF experiments was unexpectedly low compared to production from natural processes in all, except one of the experiments (i.e., the Southern Ocean European Iron Fertilization Experiment, EIFEX). These results, including side effects such as N₂O production and hypoxia development, have been scientifically debated amongst those who support and oppose OIF experimentation. In the context of increasing global and political concerns associated with climate change, it is valuable to examine the validity and usefulness of the OIF. We provide a general overview of the OIF experiments conducted over the last 25 years (past), a discussion of OIF considerations including possible side effects (present), and an introduction to the OIF experiment plan currently being designed by Korean oceanographers (future).

The northern East China Sea (a.k.a., “The South Sea”) is a dynamic zone that exerts a variety of effects on the marine ecosystem due to Three-Gorges Dam construction. As the northern East China Sea region is vulnerable to climate forcing and anthropogenic impacts, it is important to investigate how the remineralization rate in the northern East China Sea has changed in response to such external forcing. We used an historical hydrographic dataset from August 1997 to obtain a baseline for future comparison. We estimate the amount of remineralized phosphate by decomposing the physical mixing and biogeochemical process effect using water column measurements (temperature, salinity, and phosphate). The estimated remineralized phosphate column inventory ranged from 0.8 to 42.4 mmol P m-2 (mean value of 15.2 ± 12.0 mmol P m-2). Our results suggest that the Tsushima Warm Current was a strong contributor to primary production during the summer of 1997 in the study area. The estimated summer (June - August) remineralization rate in the region before Three-Gorges Dam construction was 18 ± 14 mmol C m-2 d-1

Natural climate variability influences phytoplankton community both directly and indirectly by altering ocean stratification and availabilities of nutrient and light, or grazing pressure. The world’s largest ecosystem, North Pacific Subtropical Gyre (NPSG), is largely controlled by basin-scale decadal climate variability, such as the North Pacific Gyre Oscillation and the Pacific Decadal Oscillation. These indices have two phases, known as warm phase and cool phase, respectively. Previous studies reported that warm phase was related to the dominance of pico-phytoplankton induced by warm temperature anomaly (i.e., strong stratification) while cool phase was related to the dominance of nano-phytoplankton induced by cold temperature anomaly (i.e., weak stratification). Besides the impact of natural climate variability, anthropogenic global warming has accelerated in recent years and it might have abnormal impact on marine ecosystems. However, there is little information about the responses of phytoplankton community to recent climate change in the NPSG. Here, we present the temporal variations of deseasonalized and normalized NPSG phytoplankton community using phytoplankton pigment concentrations and cell densities, obtained on monthly intervals over the period 1988−2018 at Station ALOHA (22°45’N, 158°W). These variations were compared with the variations of climate indices, physical, and biogeochemical parameters from Station ALOHA. The NPSG climate indices showed five phase transitions; warm (~1997) – cool (1998−2002) – warm (2003−2006) – cool (2007−2013) – warm (2014~). Before 2006 year, the phase transitions of phytoplankton community (pico→nano→pico) were coincident with physical factors (e.g., stratification; strong→weak→strong) and biogeochemical factors (e.g., particle export; low→high→low), coupling with phases of climate indices. However, interestingly, following the recent rapid rise in greenhouse gas emission (since 2007), phytoplankton community, even under continued coupling of climate indices and physical factors, showed only dominance of pico-phytoplankton, decoupling with the phases of climate indices. These findings suggest that the contribution of pico-sized plankton to NPSG phytoplankton community will increase gradually in response to the acceleration of the global warming.