Nitrate Flux Research Articles

Controls on Nutrient Cycling in Estuarine Mangrove Lake Sediments

We estimated the net exchange of nitrogen and phosphorus species using core incubations under light and dark conditions in estuarine lakes that are the aquatic interface between the freshwater Everglades and marine Florida Bay. These lakes and adjacent shallow water Florida Bay environments are sites where the restoration of hydrological flows will likely have the largest impact on salinity. Sediment respiration, measured by oxygen uptake, averaged (±S.D.) −2400 ± 1300, −300 ± 1000, and 1900 ± 1400 μmol m−2 h−1 for dark incubations, light incubations, and gross photosynthesis estimates, respectively, with dark incubations consistent with oxygen uptake measured by microelectrode profiles. Although most fluxes of soluble reactive phosphorus, nitrate, and N2–N were low under both light and dark incubation conditions, we observed a number of very high efflux events of NH4+ during dark incubations. A significant decrease in NH4+flux was observed in the light. The largest differences between light and dark effluxes of NH4+ occurred in lakes during periods of low coverage of the aquatic macrophyte Chara hornemannii Wallman, with NH4+ effluxes > 200 μmol m−2 h−1. Increasing freshwater flow from the Everglades is expected to expand lower salinity environments suitable for Chara, and therefore, diminish the sediment NH4+ effluxes that may fuel algal blooms.

Lateral advection supports nitrogen export in the oligotrophic open-ocean Gulf of Mexico

In contrast to its productive coastal margins, the open-ocean Gulf of Mexico (GoM) is notable for highly stratified surface waters with extremely low nutrient and chlorophyll concentrations. Field campaigns in 2017 and 2018 identified low rates of turbulent mixing, which combined with oligotrophic nutrient conditions, give very low estimates for diffusive flux of nitrate into the euphotic zone (< 1 µmol N m−2 d−1). Estimates of local N2-fixation are similarly low. In comparison, measured export rates of sinking particulate organic nitrogen (PON) from the euphotic zone are 2 – 3 orders of magnitude higher (i.e. 462 – 1144 µmol N m−2 d−1). We reconcile these disparate findings with regional scale dynamics inferred independently from remote-sensing products and a regional biogeochemical model and find that laterally-sourced organic matter is sufficient to support >90% of open-ocean nitrogen export in the GoM. Results show that lateral transport needs to be closely considered in studies of biogeochemical balances, particularly for basins enclosed by productive coasts.

Nitrate fluxes induced by turbulent mixing in dipole eddies in an oligotrophic ocean

AbstractThe vertical turbulent fluxes of nitrate to the base of the euphotic zone were quantified to assess the role of turbulent mixing in modulating new production inside cyclonic and anticyclonic eddies in an oligotrophic ocean. Although the turbulent nitrate flux (0.35 ± 0.11 mmol N m−2 d−1) at the base of the euphotic zone inside the cyclonic eddy was higher than that (0.07 ± 0.03 mmol N m−2 d−1) inside the anticyclonic eddy, this flux contributed to 19.7–76.3% of new production in the anticyclonic eddy and less than 4.94% in the cyclonic eddy, wherein new production was primarily driven by upwelling. Moreover, the turbulent nitrate flux in the cyclonic eddy center was higher than that at its edge because of differences in turbulent diffusivity. However, the opposite pattern appeared in the anticyclonic eddy, mainly due to discrepancies in the nitrate gradients and turbulent diffusivity. Elevated turbulent diffusivity at the edge of the anticyclonic eddy is the most likely attributed to submesoscale processes. These findings provide observatory evidence, confirming that a higher phytoplankton biomass exists in the anticyclonic eddy edge than in its center.

Nitrate Transport and Distribution in Soybean Plants With Dual-Root Systems.

Nitrate absorbed by soybean (Glycine max L. Merr.) roots from the soil can promote plant growth, while nitrate transported to nodules inhibits nodulation and nodule nitrogen fixation activity. The aim of this study was to provide new insights into the inhibition of nodule nitrogen (N) fixation by characterizing the transport and distribution of nitrate in soybean plants. In this research, pot culture experiments were conducted using a dual root system of soybeans. In the first experiment, the distribution of 15N derived from nitrate was observed. In the second experiment, nitrate was supplied–withdrawal–resupplied to one side of dual-root system for nine consecutive days, and the other side was supplied with N-free solution. Nitrate contents in leaves, stems, petioles, the basal root of pealed skin and woody part at the grafting site were measured. Nitrate transport and distribution in soybean were analyzed combining the results of two experiments. The results showed that nitrate supplied to the N-supply side of the dual-root system was transported to the shoots immediately through the basal root pealed skin (the main transport route was via the phloem) and woody part (transport was chiefly related to the xylem). There was a transient storage of nitrate in the stems. After the distribution of nitrate, a proportion of the nitrate absorbed by the roots on the N-supply side was translocated to the roots and nodules on the N-free side with a combination of the basal root pealed skin and woody part. In conclusion, the basal root pealed skin and woody part are the main transport routes for nitrate up and down in soybean plants. Nitrate absorbed by roots can be transported to the shoots and then retranslocated to the roots again. The transport flux of nitrate to the N-free side was regulated by transient storage of nitrate in the stems.

Evolution of N-balance with qualitative expert evaluation approach

Pollution of rivers by nitrate is a major issue. Many land use units are considered as net nitrate producers when the input dominates the uptake (e.g. agricultural areas), or in the opposite, net consumers (e.g. wetlands), but the role of their spatial organization and temporal dynamics together across the watershed is unclear. Here, we used a Nitrate-related Ecological Functions (NEF) concept, together with an expert-based analysis in a Geographical Information System, to investigate the role of two opposite landscape types in the nitrate regulation across the Garonne river watershed (France). At any point in a watershed, there is nitrate production (NP) and nitrate removal (NR). The nitrate net balance (NNB) between NP and NR functions can be neutral (NB, Neutral Balance) when nitrate fluxes balance over space and time. The first landscape type, called Actual, was obtained using a set of 7 actual environmental variables, as land cover types, soil organic matter content and wetlands presence. The second landscape type, called Natural, described a non-anthropized landscape, using the same layer types as the Actual landscape. Potentials in NP and NR for each class in each map layer were rated by a set of experts according to their scientific knowledge. NP, NR and by difference, NNB maps were obtained, overlaid and compared to provide an evaluation of the potential for each landscape. In both landscapes, NNB were largely balanced (Actual=48% and Natural=67%). In the Actual landscape, NNB were secondly dominated by an imbalance toward NP (43%) and in the Natural landscape secondly imbalanced toward NR (32%). We constructed 'maps of disagreement' between both landscapes to provide a spatially explicit assessment of NNB evolution caused by changing land cover. We found that 67% of the agricultural areas and 60% of the artificial areas of the watershed had been subjected to a loss of nitrate ecological functions from Natural to Actual landscapes. Some management practices able to modify these factors may improve ecological functions and diminish the NEF disagreement of the watershed.

Biogeochemical Responses to Nutrient Fluxes in the Open South China Sea: A 3‐D Modeling Study

AbstractRecent Bio‐Argo observations revealed a spatial difference of seasonal chlorophyll‐a pattern in the South China Sea (SCS) with permanent subsurface chlorophyll‐a maximum layer in the central SCS (CSCS) but intense winter chlorophyll‐a bloom in the northern SCS (NSCS). A three‐dimensional physical‐biogeochemical model was used to simulate the temporal change of upper ocean physics and biogeochemistry of the open SCS following the trajectories of the two Bio‐Argo floats to explore mechanisms driving the difference between the two very contrasting marine ecosystems. The model reasonably reproduced the Bio‐Argo vertical observations in both regions. In the model, high nitrate fluxes had supported a strong winter chlorophyll‐a and productivity in the NSCS, while low nitrate fluxes in the CSCS leading to a stable subsurface chlorophyll‐a layer throughout the year. We found that nitrate fluxes in the north were controlled by diffusion during the early winter but by advection during the late winter. Both buoyancy and wind forcing contributed to increased diffusive nutrient fluxes by mixing nutrients below the nutricline in the early winter of the NSCS. A strong upwelling during the late winter in the north was supported by eddy‐Ekman pumping, particularly the nonlinear effect induced by eddy vorticity gradient. The finding is consistent with the change of vertical advection in both directions and magnitudes in the north throughout the year. The eddy‐driven difference in advection between the two regions was further confirmed by substantially larger eddy kinetic energies and eddy activities in the north.

How a Small Reef in the Kuroshio Cultivates the Ocean

AbstractVertical nitrate fluxes associated with turbulent mixing and upwelling around a small reef in the Kuroshio are quantified by continuously deploying a turbulence microstructure profiler with an attached submersible ultraviolet nitrate analyzer while drifting from the upstream to the downstream of the reef. Flow separations and trains of Kelvin‐Helmholtz billows (thickness = 60 m) are identified using a shipboard ADCP and an echo‐sounder. The turbulence diffusivity associated with the vigorous turbulent mixing reaches up to O(10−1 m2 s−1), resulting in strong nitrate fluxes of O(1–103 mmol m−2 day−1). In addition, large differences between the upstream and downstream density profiles suggest a strong upwelling velocity of O(10−3 m s−1), as well as an upwelling nitrate flux of O(102 mmol m−2 day−1) in the entire subsurface layer.

Vertical Changes in the Flux of Atmospheric Nitrate From a Forest Canopy to the Surface Soil Based on Δ17O Values

AbstractTo better understand the factors that control N retention and N export in forested watersheds, it is necessary to understand the relationships among atmospheric nitrogen (N) deposition, internal N cycling within plant‐soil systems, and N leaching. The relative contributions of atmospheric nitrate (NO3−atm) and remineralized nitrate produced by microbial nitrification to total nitrate (NO3−) in stream water have been investigated in many studies. However, the dynamics of these two types of NO3− from the forest canopy to the soil are not well understood. Therefore, we determined the changes in the NO3− flux and the 17O excess (Δ17O) of NO3−, a robust tracer of NO3−atm, from bulk deposition to the soil water beneath oak and spruce trees as well as dwarf bamboo‐dominated canopy gaps in a natural coniferous‐broadleaved mixed forest in northern Japan. The Δ17O values in NO3− dramatically decreased after passing through the forest floor, indicating that the dominant source of NO3− leaching is nitrification in the forest floor. In contrast, a large decrease in NO3−atm flux was observed between bulk deposition and throughfall, especially for oak and spruce, suggesting that the forest canopy is an important sink for deposited NO3−atm. The retention of NO3−atm by the canopy was higher for oak (86.3 ± 10.1%) and spruce (87.7 ± 8.8%) than for Sasa in the canopy gap (49.9 ± 26.6%). Our study demonstrated that the Δ17O value of NO3− is a promising tool for quantifying the atmospheric nitrate dynamics in complex forest N cycling.

Simulation of nitrogen fertigation schedule for drip irrigated paddy

A robust soil-water system is essential for achieving most of the Sustainable Development Goals of United Nations. Systems approach is essential to keep the soil-water system to derive sustainable benefits. Prevention is better than cure; hence accurate prediction of the likely results of our action on soil-water system is needed. Hydrus is one of the handy tools for simulating root zone under drip irrigation to derive best management practices for water and nutrient application. Hydrus is parameter intensive software. Hence, mostly, there is a tendency to use typical values of parameters available in the literature for modelling. In this research work, we generated data for Hydrus by intensive field experimentation to improve the reliability of modelling. The first order rate constants of urea-ammonium-nitrate reactions were obtained by field experimentation. HYDRUS was used numerically to simulate nitrogen transport for paddy crop under drip irrigation. From the simulation, various fluxes of urea, ammonium and nitrate were quantified, to develop a N fertigation schedule for entire crop duration. The fertigation schedule was implemented by raising a paddy crop and evaluation was done by estimating N assimilation of crop by plant sample analysis. The simulated values of nitrogen uptake were in good agreement with the observed values of nitrogen uptake.

Modeling NO3–N leaching during establishment of turfgrasses irrigated with tailored reclaimed water

AbstractTurfgrass systems have been identified as potential sources of nitrate leaching and subsequent groundwater contamination. The HYDRUS (2D/3D) model was used to quantify nitrate leaching from bermudagrass [Cynodon dactylon (L.) Pers.] and buffalograss [Buchloe dactyloides (Nutt.) Engelm] established by either seed or sod and irrigated with either tailored (defined as reclaimed water with an N concentration of 15 mg L−1) or potable water and fertilized with calcium nitrate. A parameter sensitivity analysis conducted prior to model calibration revealed that soil texture, denitrification rate, and plant uptake all affected nitrate leaching. Simulated nitrate flux matched the experimental data more accurately when denitrification rate varied by soil depth. Moreover, nitrate leaching also differed between turfgrass species and between establishment methods. Leaching was higher from grasses propagated by seed than from sod at the beginning of the establishment period. Increasing the concentration of nitrate in tailored water from 0 to 200 mg L–1 increased concentrations at 50‐cm depths for both species but the increase was significantly higher under buffalograss than bermudagrass and was attributed to lower nutrient uptake and denitrification rates under buffalograss. Nitrate concentrations at 50‐cm depth were significantly higher for coarse sand compared with loamy sand, which was attributed to differences in retention times of the two soil types. Soil texture was even more important than nitrate application rate in predicting nitrate leaching losses and the results of the sensitivity analysis demonstrated that nitrate leaching was affected more by denitrification than by plant uptake.

Biogeochemical responses over 37 years to manipulation of phosphorus concentrations in an Arctic river: The Upper Kuparuk River Experiment

AbstractThe climate of the Arctic region is changing rapidly, with important implications for permafrost, vegetation communities, and transport of solutes by streams and rivers to the Arctic Ocean. While research on Arctic streams and rivers has accelerated in recent years, long‐term records are relatively rare compared to temperate and tropical regions. We began monitoring the upper Kuparuk River in 1983 as part of a long‐term, low‐level, whole‐season phosphorus enrichment of a 4–6 km experimental reach, which was subsequently incorporated into the Arctic Long‐Term Ecological Research (Arctic LTER) programme. The phosphorus enrichment phase of the Upper Kuparuk River Experiment (UKRE) ran continuously for 34 seasons, fundamentally altering the community structure and function of the Fertilized reach. The objectives of this paper are to (a) update observations of the environmental conditions in the Kuparuk River region as revealed by long‐term, catchment‐level monitoring, (b) compare long‐term trends in biogeochemical characteristics of phosphorus‐enriched and reference reaches of the Kuparuk River, and (c) report results from a new ‘ReFertilization’ experiment. During the UKRE, temperature and discharge did not change significantly, though precipitation increased slightly. However, the UKRE revealed unexpected community state changes attributable to phosphorus enrichment (e.g., appearance of colonizing bryophytes) and long‐term legacy effects of these state changes after cessation of the phosphorus enrichment. The UKRE also revealed important biogeochemical trends (e.g., increased nitrate flux and benthic C:N, decreased DOP flux). The decrease in DOP is particularly notable in that this may be a pan‐Arctic trend related to permafrost thaw and exposure to new sources of iron that reduce phosphorus mobility to streams and rivers. The trends revealed by the UKRE would have been difficult or impossible to identify without long‐term, catchment level research and may have important influences on connections between Arctic headwater catchments and downstream receiving waters, including the Arctic Ocean.

Modeling the seasonal and interannual variations in nitrate flux through Bering Strait

The nutrient flux through Bering Strait has significant impacts on the biomass and community composition of phytoplankton in the western Arctic Ocean. Based on a three-dimensional ocean-sea ice-biogeochemical model, the hindcast simulations from 1998 to 2015 suggest that the nitrate flux through Bering Strait reaches the peak during February–May (~12 kmol N/s) and then decreases to a relatively lower level (~8 kmol N/s) in July and maintains this level until January. The interannual nitrate fluxes in winter and summer were analyzed with winter defined as February–March and summer defined as July–August. The main findings are summarized as follows. (1) Shelf-slope exchange near Cape Navarin plays a key role in poleward nutrient supply since the main nutrient transport belt over the Bering shelf corresponds to the pathway of Anadyr Current from Cape Navarin to Bering Strait. (2) In the winter season, the interannual nitrate flux through Bering Strait has a significant correlation (r = 0.80, p < 0.01) with that through the section adjacent to Cape Navarin (CN section) in the fall season. This remote interannual signal from upstream appears to regulate the Bering Strait nitrate flux via horizontal advection so that it can be considered as a predictor factor. (3) In the summer season, the nitrate budget in the water column from Cape Navarin to Bering Strait indicates that biological activity plays a limited role in the interannual variation in poleward nitrate flux. Instead, this interannual variability is determined by the volume transport through Bering Strait and upstream advection.

Active DNRA and denitrification in oxic hypereutrophic waters

Since the start of synthetic fertilizer production more than a hundred years ago, the coastal ocean has been exposed to increasing nutrient loading, which has led to eutrophication and extensive algal blooms. Such hypereutrophic waters might harbor anaerobic nitrogen (N) cycling processes due to low-oxygen microniches associated with abundant organic particles, but studies on nitrate reduction in coastal pelagic environments are scarce. Here, we report on 15N isotope-labeling experiments, metagenome, and RT-qPCR data from a large hypereutrophic lagoon indicating that dissimilatory nitrate reduction to ammonium (DNRA) and denitrification were active processes, even though the bulk water was fully oxygenated (> 224 µM O2). DNRA in the bottom water corresponded to 83% of whole-ecosystem DNRA (water + sediment), while denitrification was predominant in the sediment. Microbial taxa important for DNRA according to the metagenomic data were dominated by Bacteroidetes (genus Parabacteroides) and Proteobacteria (genus Wolinella), while denitrification was mainly associated with proteobacterial genera Pseudomonas, Achromobacter, and Brucella. The metagenomic and microscopy data suggest that these anaerobic processes were likely occurring in low-oxygen microniches related to extensive growth of filamentous cyanobacteria, including diazotrophic Dolichospermum and non-diazotrophic Planktothrix. By summing the total nitrate fluxes through DNRA and denitrification, it results that DNRA retains approximately one fifth (19%) of the fixed N that goes through the nitrate pool. This is noteworthy as DNRA represents thus a very important recycling mechanism for fixed N, which sustains algal proliferation and leads to further enhancement of eutrophication in these endangered ecosystems.

Long term response of water and nitrogen fluxes to Good Agricultural Practices at field and catchment scales

Facing the nitrate pollution problem, the European Union has encouraged a code of “Good Agricultural Practices” (GAP) in order to recover a good chemical and ecological status of waterbodies. In this study, we hypothesized that the systematic application of GAP in time and space would allow to meet the EU standard of nitrate concentration (50 mg NO3 L−1) under arable cropping systems. Water and nitrogen fluxes were determined in an agricultural catchment (187 ha) during 22 years after GAP implementation, at field and catchment scales. The aquifer outlet is a set of springs which were monitored for water flow and nitrate concentration. GAP management mainly consisted in adjusting N fertilization rates and establishing catch crops. Crop N uptake, soil water and mineral N were measured respectively two and three times per year on 36 sites representing soil variability. These data were used to initialize the STICS model which simulated the fluxes of infiltrated water and nitrate leached below the rooting zone at field scale. The elementary fluxes (calculated for each site-year) were then used as independent inputs of the hydrological model MODCOU which made the integration at catchment scale. Simulations of agricultural scenarios allowed to calculate the cost/efficiency ratio of GAP implementation per soil type. The mean amounts of infiltrated water and N leached calculated below rooting depth in agricultural fields were 179 mm yr−1 and 19 kg N ha−1 yr−1, respectively, yielding a mean weighted nitrate concentration of 41 mg NO3 L−1 during 22 years over the whole catchment. The mean residence time of water in the catchment was estimated at 17–22 years using tritium and CFC tracers. The observed nitrate concentration in the main spring declined 11 years after GAP implementation and levelled off to 49 mg NO3 L−1. The agro-hydrological model satisfactorily predicted water flow and nitrate concentration in springs but overestimated their response time. It predicted a positive impact of GAP application on water quality. GAP management appeared to be efficient on the long term with a low cost/efficiency ratio. A more flexible and motivating management could consist in including GAP as a first step in a loop progress towards agro-ecological systems.

Low‐temperature phase formation in the SrF2–LaF3 system

AbstractPhase formation in the SrF2–LaF3 system was studied at temperatures ranging from 300°C to 450°C using nitrate flux. The solubility of LaF3 in SrF2 decreases with decreasing temperature. The equilibrium width of the solid solution region Sr1−xLaxF2+x at 400°C, it is 44.6 ± 0.4 mol% LaF3 (x = 0.446), at 350°C — 38.3 ± 0.7 mol% LaF3 (x = 0.383), and decreases almost to zero at 300°C.

Dynamics of Soil Nitrogen Availability Following Sunn Hemp Residue Incorporation in Organic Strawberry Production Systems

Sunn hemp (Crotalaria juncea L.), as a summer leguminous cover crop, is often grown before fall planting of strawberries (Fragaria ×ananassa Duch.) in Florida. Although sunn hemp has been suggested as a green manure for supplying nitrogen (N) to subsequent crops, limited information is available regarding the contribution of sunn hemp biomass to soil N availability in Florida sandy soils with low levels of organic matter. This is especially true for organic strawberry production where nutrient management remains one of the major yield-limiting factors. This study was conducted in Citra, FL, and assessed the dynamics of N availability after soil incorporation of sunn hemp in organic strawberry production systems established on sandy soils in a subtropical environment. Sunn hemp was planted at a seeding rate of 44.9 kg·ha−1 on 19 July 2017 and 24 July 2018 and terminated 65 days after seeding; a summer weedy fallow was used as the control. Containerized strawberry seedlings of Sweet Sensation® ‘Florida127’ were transplanted on 13 Oct. 2017 (22 days after sunn hemp incorporation) and 4 Oct. 2018 (8 days after sunn hemp incorporation). Immediately after sunn hemp incorporation, anion exchange membranes (AEMs) were buried in the soil to monitor soil NO3-N fluxes, together with traditional soil testing to measure extractable soil NO3-N concentrations. In the 2018 season, soils incorporated with sunn hemp residues were also incubated in the laboratory at 24 °C over 8 weeks to determine the N release pattern by quantifying soil NO3-N and NH4-N. Overall, nitrate fluxes monitored by AEMs in the first 3 weeks after sunn hemp incorporation were significantly higher in the sunn hemp treatment than in the weedy fallow control (by 66% to 185%) in both years. Sunn hemp incorporation also led to a considerable increase in extractable soil NO3-N concentration (by 20% to 94%). The early and fast release of plant available N (PAN) from sunn hemp residues was confirmed by the 8-week laboratory incubation study, which demonstrated that the net N mineralization rate of sunn hemp remained highest over the first 2 weeks of the incubation period. Sunn hemp showed a positive impact on organic strawberry early-season fruit yield in both years, with significant increases in marketable (by 59%) and total (by 52%) fruit weight yields and marketable fruit number (by 46%) in 2017 and total fruit number (by 15%) and weight yield (by 14%) and marketable fruit number (by 13%) in 2018. Given the typical waiting period between sunn hemp residue soil incorporation and strawberry planting as well as the lag in nutrient uptake shortly after transplanting, a large fraction of N released from sunn hemp residues is likely not taken up by strawberry plants. Our findings highlight the challenges of using sunn hemp residues to improve N availability for meeting crop demand and enhance fruit yield in organic strawberry production while minimizing environmental N losses in Florida sandy soils.

Enhanced vertical turbulent nitrate flux in the intermediate layer of the Kuroshio in the Tokara Strait

Vertical mixing in the ocean is an important physical process that brings deep nutrients to the upper layer. The Tokara Strait, where the Kuroshio flows out to the North Pacific from the East China Sea, is known for a mixing hotspot, but the associated vertical nutrient transport has not been well quantified yet, especially in the intermediate layer below 200 m depth. This study involved flow, turbulence, and nitrate observations to quantify the vertical turbulent nitrate flux in the Tokara Strait and discuss its impact on the Kuroshio nutrient transport. Temporal variation of current velocity, derived from a 3.5-day mooring observation, captured semi-diurnal internal waves propagating vertically at depths between 200 and 400 m. Vertical shear associated with the semi-diurnal flow was elevated at this depth range. Vertical shear due to the mean and the diurnal flow was also elevated at about 220 m and 400 m depths, respectively. Mean vertical eddy diffusivity was elevated to O(10–3–10–2 m2 s−1) at the density layers of 25.2–26.4 σθ (200–450 m depth). Associated vertical turbulent nitrate flux reached O(10 mmol m−2 day−1), especially at around 25.6 σθ (250 m depth) and 26.2–26.4 σθ (400–450 m depth). The enhanced vertical turbulent nitrate flux suggests a large amount of nitrate is injected vertically to the nutrient stream from the layer just above the North Pacific Intermediate Water and is transported throughout the core of the Kuroshio nutrient stream situated at 25.0–26.2 σθ.

Elevated turbulent and double-diffusive nutrient flux in the Kuroshio over the Izu Ridge and in the Kuroshio Extension

While the Kuroshio is known to be a nutrient stream, as these nutrients are in dark subsurface layers, they are not immediately available for photosynthesis unless they are supplied to the sunlit surface layers. Recent microstructure observations have revealed that strong diapycnal mixing caused by the Kuroshio flowing over topographic features and double diffusion in the subsurface layers of the Kuroshio. However, it is still unclear how much nutrient flux can be provided by these microscale mixing processes. In this study, using an autonomous microstructure float and nutrient samplings, nutrient flux caused by the Kuroshio over the Izu Ridge, and that caused by double diffusion in the Kuroshio Extension are quantified. The nitrate diffusive flux is estimated to be >1 ,hbox {mmol} ,hbox {N},hbox {m}^{-2}hbox {day}^{-1} over a distance, 20–30 km near the Izu Ridge and >0.1 ,hbox {mmol} ,hbox {N}, hbox {m}^{-2}hbox {day}^{-1}, which persists further downstream direction over 100 km along the Kuroshio, increasing the subsurface chlorophyll-a concentration in the region 200 km downstream. The double-diffusion-induced nitrate flux is estimated to be 1-10 ,hbox {mmol} ,hbox {N} ,hbox {m}^{-2}hbox {day}^{-1} in the pycnostad 26–26.5,hbox {kgm}^{-3} of the Kuroshio Extension, suggesting that whether this double-diffusion-induced nutrient flux in the subsurface layers can ultimately contribute to surface primary production depends on additional eddy up- and northward fluxes.

Internal hydraulic jump in the Tsugaru Strait

Sea straits are known for important sites to modify water-mass properties and transport nutrients through vigorous vertical mixing. The first microstructure measurements ever taken in the Tsugaru Strait, between the Sea of Japan and the Northwest Pacific Ocean, showed elevated turbulent mixing. The turbulent energy dissipation rate was especially enhanced to O(10−6 W kg−1) via an internal hydraulic jump when the throughflow passed over an abrupt sill. Beyond the sill, the associated vertical eddy diffusivity reached O(10−2 m2 s−1) and the vertical turbulent nitrate flux below the subsurface chlorophyll-a maximum layer was estimated to reach 1 mmolN m−2 day−1, which could elevate the new production of phytoplankton within the layer. Large vertical velocities exceeding 0.1 m s−1 were also observed near the seafloor of the sill where the flow was hydraulically controlled. A simple advection–diffusion model suggested that the energy dissipation rate of O(10−5 W kg−1) was necessary to reproduce the observed modification of subsurface salinity maximum across the strait. Upwelling and/or vertical straining associated with the internal hydraulic jump is also believed to play an important role in the water-mass modification in the strait and could enhance new primary production for a wide area downstream.

Vertical turbulent nitrate flux from direct measurements in the western subarctic and subtropical gyres of the North Pacific

Vertical turbulent nitrate fluxes were estimated in the western North Pacific from direct measurements of vertical turbulent mixing and vertically continuous nitrate profiles during the summer of 2008. We made three north–south transects that covered the area from the subarctic to the subtropics including a section along the Emperor Sea Mounts. Subsurface fluxes generally showed an increasing trend with increasing vertical gradient of nitrate from oligotrophic subtropical to non-oligotrophic subarctic waters. Enhanced fluxes [O(10−6) mmol m−2 s−1] due to elevated mixing [vertical diffusivity: O(10−5) m2 s−1] were observed, especially over the Emperor Sea Mounts. It is suggested that the internal tide generated by the topography enhanced the vertical mixing. In other subarctic areas, the fluxes were estimated as O(10−7) mmol m−2 s−1. The same order of fluxes was also found in the frontal area between the subarctic and subtropical gyres, the Kuroshio–Oyashio Transition Area. Enhancement of fluxes in the frontal area, including the Kuroshio Extension, was also observed at mid-depth regions, and their vertical divergence suggested nitrate transport from North Pacific Intermediate Water to lighter densities. In the frontal areas, the enhancement of turbulence is caused by the surface wind rather than the internal tide. In contrast, in the subtropical regions, subsurface fluxes were estimated as O(10−8) mmol m−2 s−1 owing to the small nitrate gradient even where diffusivity was enhanced. In these regions, enhancement of diffusivity, including that at mid-depths, corresponded to the elevation of the internal-tide dissipation, in addition to that of surface turbulence.