Biological export production controls upper ocean calcium carbonate dissolution and CO2 buffer capacity.

Marine biogenic calcium carbonate (CaCO3) cycles play a key role in ecosystems and in regulating the ocean's ability to absorb atmospheric carbon dioxide (CO2). However, the drivers and magnitude of CaCO3 cycling are not well understood, especially for the upper ocean. Here, we provide global-scale evidence that heterotrophic respiration in settling marine aggregates may produce localized undersaturated microenvironments in which CaCO3 particles rapidly dissolve, producing excess alkalinity in the upper ocean. In the deep ocean, dissolution of CaCO3 is primarily driven by conventional thermodynamics of CaCO3 solubility with reduced fluxes of CaCO3 burial to marine sediments beneath more corrosive North Pacific deep waters. Upper ocean dissolution, shown to be sensitive to ocean export production, can increase the neutralizing capacity for respired CO2 by up to 6% in low-latitude thermocline waters. Without upper ocean dissolution, the ocean might lose 20% more CO2 to the atmosphere through the low-latitude upwelling regions.

Significant CO2 emission in the shallow inshore waters of the southeastern Yellow Sea in 2020.

This study investigated the carbonate system and air-sea CO2 exchange in the inshore waters along South Korea's western coastline in 2020. Overlooking these waters might introduce significant errors in estimating air-sea CO2 fluxes of the southeastern Yellow Sea, given their interaction with land, offshore regions, and sediments. During periods other than summer, seasonal variations in seawater CO2 partial pressure (pCO2) could be generally explained by thermal effects. Tidal mixing and shallow depths resulted in weaker stratification-induced carbon export compared to offshore regions. However, during summer, inshore waters exhibited high spatial variability in pCO2, ranging from approximately 185 to 1000 µatm. In contrast to offshore waters that modestly absorbed CO2, inshore waters shallower than 20 m emitted ∼100 Gg C yr-1 to the atmosphere. However, considering the high heterogeneity of the study area, additional observations with high spatial and temporal resolution are required to refine estimates of air-sea CO2 exchange.

Identifying the external N and Hg inputs to the estuary ecosystem based on the triple isotopic information (δ15NNO3, Δ17ONO3 and δ18ONO3).

The supply and sources of N and Hg in the Geum estuary of the western coast of Korea were evaluated. Triple isotope proxies (δ15NNO3, Δ17ONO3 and δ18ONO3) of NO3- combined with conservative mixing between river and ocean waters were used to improve isotope finger-printing methods. The N pool in the Geum estuary was primarily influenced by the Yellow Sea water, followed by riverine discharge (821 × 106 mol yr-1) and atmospheric deposition (51 × 106 mol yr-1). The influence of the river was found to be greater for Hg than that of the atmosphere. The triple isotope proxies revealed that the riverine and atmospheric inputs of N have been affected by septic wastes and fossil fuel burning, respectively. From the inner estuary towards offshore region, the influence of the river diminishes, thus increasing the relative impact of the atmosphere. Moreover, the isotope proxies showed a significant influence of N assimilation in February and nitrification in May.

Food web structure for high carbon retention in marine plankton communities.

Total annual net primary productions in marine and terrestrial ecosystems are similar. However, a large portion of the newly produced marine phytoplankton biomass is converted to carbon dioxide because of predation. Which food web structure retains high carbon biomass in the plankton community in the global ocean? In 6954 individual samples or locations containing phytoplankton, unicellular protozooplankton, and multicellular metazooplankton in the global ocean, phytoplankton-dominated bottom-heavy pyramids held higher carbon biomass than protozooplankton-dominated middle-heavy diamonds or metazooplankton-dominated top-heavy inverted pyramids. Bottom-heavy pyramids predominated, but the high predation impact by protozooplankton on phytoplankton or the vertical migration of metazooplankton temporarily changed bottom-heavy pyramids to middle-heavy diamonds or top-heavy inverted pyramids but returned to bottom-heavy pyramids shortly. This finding has profound implications for carbon retention by plankton communities in the global ocean.

Isotopic investigation of sources and processes affecting gaseous and particulate bound mercury in the east coast, South Korea.

Understanding sources and processes affecting atmospheric mercury (Hg) are key to enabling targeted Hg managements under the Minamata Convention on Mercury. We employed stable isotopes (δ202Hg, Δ199Hg, Δ201Hg, Δ200Hg, Δ204Hg) and backward air trajectories to characterize sources and processes affecting total gaseous Hg (TGM) and particulate bound Hg (PBM) in a coastal city, South Korea, subjected to atmospheric Hg sources of a local steel manufacturing industry, coastal evasion from the East Sea, and long-distance transport from East Asian countries. Based on the simulated airmasses and the isotopic comparison with TGM characterized from other urban, remote, and coastal sites, TGM evaded from the coastal surface of the East Sea (warm seasons) and from the land surface in high latitude regions (cold seasons) act as important sources relative to local anthropogenic emissions at our study location. Conversely, a significant relationship between Δ199Hg and concentrations of PBM (r2 = 0.39, p < 0.05) and a seasonally uniform Δ199Hg/Δ201Hg slope (1.15), except for summer (0.26), suggest that PBM is generally sourced from local anthropogenic emissions and subjected to Hg2+ photo-reduction on particles. The striking isotopic similarity between our PBM (δ202Hg; -0.86 to 0.49 ‰, Δ199Hg; -0.15 to 1.10 ‰) and those previously characterized along the coastal and offshore regions of the Northwest Pacific (δ202Hg; -0.78 to 1.1 ‰, Δ199Hg; -0.22 to 0.47 ‰) infer that anthropogenically emitted PBM from East Asia and those processed in the coastal atmosphere serves as a regional isotopic end-member. The implementation of air pollution control devices can reduce local PBM, while regional and/or multilateral efforts are required to manage TGM evasion and transport. We also anticipate that the regional isotopic end-member can be used to quantify the relative influence of local anthropogenic Hg emissions and complex processes affecting PBM in East Asia and other coastal regions.

Observational evidence linking ocean sulfur compounds to atmospheric dimethyl sulfide during Icelandic Sea phytoplankton blooms.

In two Icelandic Sea spring blooms (May 2018 and 2019) in the North Atlantic Ocean (62.9-68.0°N, 9.0-28.0°W), chlorophyll-a and dimethylsulfoniopropionate (DMSP) concentrations and DMSP lyase activity (the DMSP-to-dimethyl sulfide (DMS) conversion efficiency) were measured at 67 stations, and the hourly atmospheric DMS mixing ratios were concurrently measured only in May 2019 at Storhofdi on Heimaey Island, located south of Iceland (63.4°N, 20.3°W). The ocean parameters for biology (i.e., chlorophyll-a, DMSP, and DMSP lyase activity) were broadly associated in distribution; however, the statistical significance of the association differed among four ocean domains and also between 2018 and 2019. Specifically, the widespread dominance of Phaeocystis, coccolithophores, and dinoflagellates (all rich in DMSP and high in DMSP lyase activity) across the study area is a compelling indication that variations in DMSP-rich phytoplankton were likely a main cause of the variations in statistical significance. For all the ocean domains defined here, we found that the DMS production capacity (calculated using the exposures of air masses to ocean biology prior to their arrivals at Heimaey and the atmospheric DMS mixing ratios of those air masses at Heimaey) was surprisingly consistent with in situ ocean S data (i.e., DMSP and DMSP lyase activity). Our study shows that the proposed computational approach enabled the detection of changes in DMS production and emission in association with changes in ocean primary producers.

Sedimentary processes dominate nitrous oxide production and emission in the hypoxic zone off the Changjiang River estuary.

Coastal oceans, known as the major nitrous oxide (N2O) source to the atmosphere, are increasingly subject to eutrophication and concurrent near-bottom hypoxia. The natural nitrogen cycle is likely to be altered markedly in hypoxic coastal oceans. However, the processes responsible for N2O production and emission remain elusive because of lacking field rate measurements simultaneously conducted in the water column and sediment. Here, we quantified N2O production rates using a 15N-labeled technique in the water-column and surface sediments off the Changjiang (Yangtze) River estuary, the largest hypoxic zone in the Pacific margins. Our results showed that the estuarine surface sediments were the major source for N2O production, accounting for approximately 90% of the total water-column accumulation and consequent efflux of N2O in the hypoxic zone, whereas the water-column nitrification and denitrification combined only contributed <10%. More importantly, the coupling of nitrification and denitrification at the presence of abundant supply and remineralization of labile organic matter was the main driver of the N2O release from the sediment-water interface in this region. This study highlights the dominant role of benthic processes occurring at the sediment-water interface controlling the coastal N2O budget, as the anthropogenic eutrophication and hypoxia are expanding in coastal oceans.

First-year sea ice leads to an increase in dimethyl sulfide-induced particle formation in the Antarctic Peninsula.

Dimethyl sulfide (DMS) produced by marine algae represents the largest natural emission of sulfur to the atmosphere. The oxidation of DMS is a key process affecting new particle formation that contributes to the radiative forcing of the Earth. In this study, atmospheric DMS and its major oxidation products (methanesulfonic acid, MSA; non-sea-salt sulfate, nss-SO42-) and particle size distributions were measured at King Sejong station located in the Antarctic Peninsula during the austral spring-summer period in 2018-2020. The observatory was surrounded by open ocean and first-year and multi-year sea ice. Importantly, oceanic emissions and atmospheric oxidation of DMS showed distinct differences depending on source regions. A high mixing ratio of atmospheric DMS was observed when air masses were influenced by the open ocean and first-year sea ice due to the abundance of DMS producers such as pelagic phaeocystis and ice algae. However, the concentrations of MSA and nss-SO42- were distinctively increased for air masses originating from first-year sea ice as compared to those originating from the open ocean and multi-year sea ice, suggesting additional influences from the source regions of atmospheric oxidants. Heterogeneous chemical processes that actively occur over first-year sea ice tend to accelerate the release of bromine monoxide (BrO), which is the most efficient DMS oxidant in Antarctica. Model-estimates for surface BrO confirmed that high BrO mixing ratios were closely associated with first-year sea ice, thus enhancing DMS oxidation. Consequently, the concentration of newly formed particles originated from first-year sea ice, which was a strong source area for both DMS and BrO was greater than from open ocean (high DMS but low BrO). These results indicate that first-year sea ice plays an important yet overlooked role in DMS-induced new particle formation in polar environments, where warming-induced sea ice changes are pronounced.

Atmospheric deposition of inorganic nutrients to the Western North Pacific Ocean.

We evaluated the potential impacts of atmospheric deposition on marine productivity and inorganic carbon chemistry in the northwestern Pacific Ocean (8-39°N, 125-157°E). The nutrient concentration in atmospheric total suspended particles decreased exponentially with increasing distance from the closest land-mass (Asia), clearly revealing anthropogenic and terrestrial contributions. The predicted mean depositional fluxes of inorganic nitrogen were approximately 34 and 15 µmol m-2 d-1 to the west and east of 140°E, respectively, which were at least two orders of magnitude greater than the inorganic phosphorus flux. On average, atmospheric particulate deposition would support 3-4% of the net primary production along the surveyed tracks, which is equivalent to ~2% of the dissolved carbon increment caused by the penetration of anthropogenic CO2. Our observations generally fell within the ranges observed over the past 18 years, despite an increasing trend of atmospheric pollution in the source regions during the same period, which implies high temporal and spatial variabilities of atmospheric nutrient concentration in the study area. Continued atmospheric anthropogenic nitrogen deposition may alter the relative abundances of nitrogen and phosphorus.

Feeding diverse prey as an excellent strategy of mixotrophic dinoflagellates for global dominance.

Microalgae fuel food webs and biogeochemical cycles of key elements in the ocean. What determines microalgal dominance in the ocean is a long-standing question. Red tide distribution data (spanning 1990 to 2019) show that mixotrophic dinoflagellates, capable of photosynthesis and predation together, were responsible for ~40% of the species forming red tides globally. Counterintuitively, the species with low or moderate growth rates but diverse prey including diatoms caused red tides globally. The ability of these dinoflagellates to trade off growth for prey diversity is another genetic factor critical to formation of red tides across diverse ocean conditions. This finding has profound implications for explaining the global dominance of particular microalgae, their key eco-evolutionary strategy, and prediction of harmful red tide outbreaks.

The impacts of ocean acidification on marine trace gases and the implications for atmospheric chemistry and climate.

Surface ocean biogeochemistry and photochemistry regulate ocean-atmosphere fluxes of trace gases critical for Earth's atmospheric chemistry and climate. The oceanic processes governing these fluxes are often sensitive to the changes in ocean pH (or pCO2) accompanying ocean acidification (OA), with potential for future climate feedbacks. Here, we review current understanding (from observational, experimental and model studies) on the impact of OA on marine sources of key climate-active trace gases, including dimethyl sulfide (DMS), nitrous oxide (N2O), ammonia and halocarbons. We focus on DMS, for which available information is considerably greater than for other trace gases. We highlight OA-sensitive regions such as polar oceans and upwelling systems, and discuss the combined effect of multiple climate stressors (ocean warming and deoxygenation) on trace gas fluxes. To unravel the biological mechanisms responsible for trace gas production, and to detect adaptation, we propose combining process rate measurements of trace gases with longer term experiments using both model organisms in the laboratory and natural planktonic communities in the field. Future ocean observations of trace gases should be routinely accompanied by measurements of two components of the carbonate system to improve our understanding of how in situ carbonate chemistry influences trace gas production. Together, this will lead to improvements in current process model capabilities and more reliable predictions of future global marine trace gas fluxes.

Precipitation of hydrogen peroxide during winter storms and summer typhoons.

Hydrogen peroxide (H2O2) affects the activity of microbes, including archaea, and thereby influences the biogeochemical cycles of critical elements in marine and terrestrial environments. In this study, we measured the levels of H2O2 associated with three classes of extreme wet precipitation events: winter storms, tropical storms, and typhoons. In conjunction with precipitation data, the measured H2O2 concentration in a seawater reservoir receiving precipitation was used to estimate rainwater H2O2 concentration and flux. The rainwater H2O2 concentration during winter storms and coexisting storms (storms having combined maritime and continental origins) was a factor of 2-3 higher than the levels observed during the typhoons. Fluxes of H2O2 in rainwater of 6 µM min-1 or greater resulted in H2O2 concentrations ~1 µM in the seawater reservoir. During all precipitation events, the H2O2 concentration in the seawater reservoir was dominated by wet precipitation and reached levels greater than would be produced in situ by photochemical processes. During winter and coexisting storms, the rainwater H2O2 concentrations were likely to have been enhanced by atmospheric photochemical reactions probably involving pollutants. An increase in the H2O2 concentration in surface aqueous environments during extreme precipitation events will directly affect the microbial cycling of nitrogen and organic carbon.

Quantification of the paralytic shellfish poisoning dinoflagellate Alexandrium species using a digital PCR.

A ubiquitous dinoflagellate, Alexandrium, produces paralytic shellfish toxin (PST), and its outbreaks have negative impacts on aquaculture, fisheries, human health, and the marine ecosystem. To minimize such damages, a routine monitoring program of toxic species must be implemented with a suitable analytical technique for their identification and quantification. However, the taxonomic identification and cell quantification of Alexandrium species based on their external morphology under a light microscope, or by using conventional molecular approaches have limited sensitivity and reproducibility. To address these challenges, we have developed an advanced protocol using droplet-digital PCR (ddPCR) for the discrimination and enumeration of three co-occurring Alexandrium species (A. affine, A. catenella, and A. pacificum) in environmental samples. Copies of species-specific internal transcribed spacer (ITS) per cell, which were calculated from environmental samples spiked with various numbers of culture cells, were used to estimate the abundance of species in the field samples. There were no significant differences in ITS copies estimated by the digital PCR assay between environmental samples from different localities, spiked artificially with a consistent number of cells from Alexandrium cultures. This sensitive assay was applied to determine the abundance and vertical distribution of those populations in the southern coastal waters of Korea. In spring, A. catenella was the dominant species, followed by the non-toxic A. affine in summers. A novel digital PCR assay can also be used to monitor other harmful marine protists that require high sample throughput and low detection limit with high accuracy and precision.

Input of terrestrial organic matter linked to deglaciation increased mercury transport to the Svalbard fjords.

Deglaciation has accelerated the transport of minerals as well as modern and ancient organic matter from land to fjord sediments in Spitsbergen, Svalbard, in the European Arctic Ocean. Consequently, such sediments may contain significant levels of total mercury (THg) bound to terrestrial organic matter. The present study compared THg contents in surface sediments from three fjord settings in Spitsbergen: Hornsund in the southern Spitsbergen, which has high annual volume of loss glacier and receives sediment from multiple tidewater glaciers, Dicksonfjorden in the central Spitsbergen, which receives sediment from glacifluvial rivers, and Wijdefjorden in the northern Spitsbergen, which receive sediments from a mixture of tidewater glaciers and glacifluvial rivers. Our results showed that the THg (52 ± 15 ng g-1) bound to organic matter (OM) was the highest in the Hornsund surface sediments, where the glacier loss (0.44 km3 yr-1) and organic carbon accumulation rates (9.3 ~ 49.4 g m-2 yr-1) were elevated compared to other fjords. Furthermore, the δ13C (-27 ~ -24‰) and δ34S values (-10 ~ 15‰) of OM indicated that most of OM were originated from terrestrial sources. Thus, the temperature-driven glacial melting could release more OM originating from the meltwater or terrestrial materials, which are available for THg binding in the European Arctic fjord ecosystems.

Atmospheric deposition of anthropogenic inorganic nitrogen in airborne particles and precipitation in the East Sea in the northwestern Pacific Ocean.

The atmospheric deposition of anthropogenic nitrogen is an increasingly important new source of nitrogen to the ocean. Coastal areas east of the Korean Peninsula are suitable for the investigation of the effects of atmospheric anthropogenic nitrogen on the ocean nutrient system because of the low riverine discharge rates and the prevailing influence of the East Asian outflow. Thus, we measured the concentrations of nitrate (NO3-) and ammonium (NH4+) in airborne particles and in precipitation from March 2014 to February 2016 at a coastal site (37.08°N, 129.41°E) on the east coast of Korea. The dry deposition of NO3- (27-30 mmol N m-2 yr-1) was far greater than that of NH4+ (6-8 mmol N m-2 yr-1). The greater rate of dry NO3- deposition was associated with air masses traveling over northeastern China and central Korea. In contrast, the rates of wet deposition of NO3- (17-24 mmol N m-2 yr-1) and NH4+ (14-27 mmol N m-2 yr-1) were comparable and were probably associated with in-cloud scavenging of these ions. The results indicate that the total deposition of NO3- and NH4+ combined could contribute to ~2.4% and ~1.9% of the primary production in the coastal areas east of the Korean Peninsula and in the East Asian marginal seas, respectively, which would be a lower bound because the dry deposition of reactive nitrogen gas was not included. Our study shows that the atmospheric input of anthropogenic NO3- and NH4+ may substantially increase phytoplankton biomass in the coastal waters of the East Sea near the Korean Peninsula.

Effects of warming and eutrophication on coastal phytoplankton production.

Phytoplankton production in coastal waters influences seafood production and human health and can lead to harmful algal blooms. Water temperature and eutrophication are critical factors affecting phytoplankton production, although the combined effects of warming and nutrient changes on phytoplankton production in coastal waters are not well understood. To address this, phytoplankton production changes in natural waters were investigated using samples collected over eight months, and under 64 different initial conditions, established by combining four different water temperatures (i.e., ambient T, +2, +4, and + 6 °C), and two different nutrient conditions (i.e., non-enriched and enriched). Under the non-enriched conditions, the effect of warming on phytoplankton production was significantly positive in some months, significantly negative in others, or had no effect. However, under enriched conditions, warming affected phytoplankton production positively in all months except one, when the salinity was as low as 6.5. These results suggest that nutrient conditions can alter the effects of warming on phytoplankton production. Of several parameters, the ratio of initial nitrate concentration to chlorophyll a concentration [NCCA, µM (µg L-1)-1] was one of the most critical factors determining the directionality of the warming effects. In laboratory experiments, when NCCA in the ambient or nutrient-enriched waters was ≥1.2, warming increased or did not change phytoplankton production with one exception; however, when NCCA was <1.2, warming did not change or decreased production. In the time series data obtained from the coastal waters of four target countries, when NCCA was 1.5 or more, warming increased phytoplankton production, whereas when NCCA was lower than 1.5, warming lowered phytoplankton production, Thus, it is suggested that NCCA could be used as an index for predicting future phytoplankton production changes in coastal waters.

Increase in anthropogenic mercury in marginal sea sediments of the Northwest Pacific Ocean.

Over the past century, the addition of anthropogenic mercury (HgANTH) to vast areas of North Pacific marginal seas adjacent to the northeast Asian continent has tripled. Analysis of sediment cores showed that the rate of HgANTH addition (HgANTH flux) was greatest in the East China and Yellow Seas (9.1 µg m-2 yr-1) in the vicinity of China (the source continent), but was small in the Bering and western Arctic Ocean (Chukchi Sea) (0.9 µg m-2 yr-1; the regions furthest from China). Our results show that HgANTH has reached open ocean sedimentary environments over extended areas of the northwestern Pacific Ocean, via the formation of organic-mercury complexes and deposition. The implication of these findings is that the addition of HgANTH (via atmospheric deposition and riverine input) to the ocean environment is responsible for elevated Hg flux into sedimentary environments in the northwest Pacific Ocean.

Widespread Anthropogenic Nitrogen in Northwestern Pacific Ocean Sediment.

Sediment samples from the East China and Yellow seas collected adjacent to continental China were found to have lower δ15N values (expressed as δ15N = [15N:14Nsample/15N:14Nair - 1] × 1000‰; the sediment 15N:14N ratio relative to the air nitrogen 15N:14N ratio). In contrast, the Arctic sediments from the Chukchi Sea, the sampling region furthest from China, showed higher δ15N values (2-3‰ higher than those representing the East China and the Yellow sea sediments). Across the sites sampled, the levels of sediment δ15N increased with increasing distance from China, which is broadly consistent with the decreasing influence of anthropogenic nitrogen (NANTH) resulting from fossil fuel combustion and fertilizer use. We concluded that, of several processes, the input of NANTH appears to be emerging as a new driver of change in the sediment δ15N value in marginal seas adjacent to China. The present results indicate that the effect of NANTH has extended beyond the ocean water column into the deep sedimentary environment, presumably via biological assimilation of NANTH followed by deposition. Further, the findings indicate that NANTH is taking over from the conventional paradigm of nitrate flux from nitrate-rich deep water as the primary driver of biological export production in this region of the Pacific Ocean.

Pyrosequencing revealed SAR116 clade as dominant dddP-containing bacteria in oligotrophic NW Pacific Ocean.

Dimethyl sulfide (DMS) is a climatically active gas released into the atmosphere from oceans. It is produced mainly by bacterial enzymatic cleavage of dimethylsulfoniopropionate (DMSP), and six DMSP lyases have been identified to date. To determine the biogeographical distribution of bacteria relevant to DMS production, we investigated the diversity of dddP­the most abundant DMS-producing gene­in the northwestern Pacific Ocean using newly developed primers and the pyrosequencing method. Consistent with previous studies, the major dddP-containing bacteria in coastal areas were those belonging to the Roseobacter clade. However, genotypes closely related to the SAR116 group were found to represent a large portion of dddP-containing bacteria in the surface waters of the oligotrophic ocean. The addition of DMSP to a culture of the SAR116 strain Candidatus Puniceispirillum marinum IMCC1322 resulted in the production of DMS and upregulated expression of the dddP gene. Considering the large area of oligotrophic water and the wide distribution of the SAR116 group in oceans worldwide, we propose that these bacteria may play an important role in oceanic DMS production and biogeochemical sulfur cycles, especially via bacteria-mediated DMSP degradation.

Chemical oceanography. Increasing anthropogenic nitrogen in the North Pacific Ocean.

The recent increase in anthropogenic emissions of reactive nitrogen from northeastern Asia and the subsequent enhanced deposition over the extensive regions of the North Pacific Ocean (NPO) have led to a detectable increase in the nitrate (N) concentration of the upper ocean. The rate of increase of excess N relative to phosphate (P) was found to be highest (~0.24 micromoles per kilogram per year) in the vicinity of the Asian source continent, with rates decreasing eastward across the NPO, consistent with the magnitude and distribution of atmospheric nitrogen deposition. This anthropogenically driven increase in the N content of the upper NPO may enhance primary production in this N-limited region, potentially leading to a long-term change of the NPO from being N-limited to P-limited.