Air-Sea Interaction, Coastal Circulation and Primary Production in the Eastern Arabian Sea: A Review

Air-sea interaction, coastal circulation and primary production exhibit an annual cycle in the eastern Arabian Sea (AS). During June to September, strong southwesterly winds (4∼9 m s⁻¹) promote sea surface cooling through surface heat loss and vertical mixing in the central AS and force the West India Coastal Current equatorward. Positive wind stress curl induced by the Findlater jet facilitates Ekman pumping in the northern AS, and equatorward-directed alongshore wind stress induces upwelling which lowers sea surface temperature by about 2.5°C (compared to the offshore value) along the southwestern shelf of India and enhances phytoplankton concentration by more than 70% as compared to that in the central AS. During winter monsoon, from November to March, dry and weak northeasterly winds (2–6 m s⁻¹) from the Indo-China continent enhance convective cooling of the upper ocean and deepen the mixed layer by more than 80 m, thereby increasing the vertical flux of nutrients in the photic layer which promotes wintertime phytoplankton blooms in the northern AS. The primary production rate integrated for photic layer and surface chlorophyll-a estimated from the Coastal Zone Color Scanner, both averaged for the entire western India shelf, increases from winter to summer monsoon from 24 to 70 g C m⁻²month and from 9 to 24 mg m⁻², respectively. Remotely-forced coastal Kelvin waves from the Bay of Bengal propagate into the coastal AS, which modulate circulation pattern along the western India shelf; these Kelvin waves in turn radiate Rossby waves which reverse the circulation in the Lakshadweep Sea semiannually. This review leads us to the conclusion that seasonal monsoon forcing and remotely forced waves modulate the circulation and primary production in the eastern AS. This revised version was published online in July 2006 with corrections to the Cover Date.     

Sea-ice retreat in the Sea of Okhotsk and the ice–ocean albedo feedback effect on it

Sea-ice retreat processes are examined in the Sea of Okhotsk. A heat budget analysis in the sea-ice zone shows that net heat flux from the atmosphere at the water surface is about 77 W m⁻² on average in the active ice melt season (April) due to large solar heating, while that at the ice surface is about 12 W m⁻² because of the difference in surface albedo. The temporal variation of the heat input into the upper ocean through the open water fraction corresponds well to that of the latent heat required for ice retreat. These results suggest that heat input into the ice–upper ocean system from the atmosphere mainly occurs at the open water fraction, and this heat input into the upper ocean is an important heat source for ice melting. The decrease in ice area in the active melt season (April) and the geostrophic wind just before the melt season (March) show a correlation: the decrease is large when the offshoreward wind is strong. This relationship can be explained by the following process. Once ice concentration is decreased (increased) by the offshoreward (onshoreward) wind just before the melt season, solar heating of the upper ocean through the increased (decreased) open water fraction is enhanced (reduced), leading to (suppressing) a further decrease in ice concentration. This positive feedback is regarded as the ice–ocean albedo feedback, and explains in part the large interannual variability of the ice cover in the ice melt season.     

Surface heat fluxes during hot events

We selected surface flux datasets to investigate the heat fluxes during “hot events”; (HEs), defined as short-term, large-scale phenomena involving very high sea surface temperature (SST). Validation of the heat fluxes against in-situ ones, which are estimated from in-situ observation in HE sampling conditions, shows the accuracies (bias ± RMS error) of net shortwave radiation, net long wave radiation, latent heat and sensible heat fluxes are 20 ± 45.0 W m⁻², −9 ± 12.3 W m⁻², −2.3 ± 31.5 W m⁻² and 1.5 ± 5.0 W m⁻², respectively. Statistical analyses of HEs show that, during these events, net solar radiation remains high and then decreases from 246 to 220 W m⁻², while latent heat is low and then increases from 100 W m⁻² to 124 W m⁻². Histogram peaks indicate net solar radiation of 270 W m⁻² and latent heat flux of 90 W m⁻² during HEs. Further, HEs are shown to evolve in three phases: formation, mature, and ending phases. Mean heat gain (HG) in the HE formation phase of 60 W m⁻² is larger than the reasonably estimated annual mean HG range of 0–25 W m⁻² in the Indo-Pacific Warm Pool. Such large daily HG in the HE formation phase can be expected to increase SSTs and produce large amplitudes of diurnal SST variations during HEs, which have been observed by both satellite and in-situ measurements in our previous studies.     

Spatial variability in the primary productivity in the East China Sea and its adjacent waters

Primary productivity in the East China Sea and its adjacent area was measured by the¹³C tracer method during winter, summer and fall in 1993 and 1994. The depth-integrated primary productivity in the Kuroshio Current ranged from 220 to 350 mgC m⁻²d⁻¹, and showed little seasonal variability. High primary productivity (above 570 mgC m⁻²d⁻¹) was measured at the center of the continental shelf throughout the observation period. The productivity at the station nearest to the Changjiang estuary exhibited a distinctive seasonal change from 68 to 1,500 mgC m⁻²d⁻¹. Depth-integrated primary productivity was 2.7 times higher in the shelf area than the rates at the Kuroshio Current. High chlorophyll-a specific productivity (mgC mgChl.-a⁻²d⁻¹) throughout the euphotic zone was mainly found in the shelf area rather than off-shelf area, probably due to higher nutrient availability and higher activity of phytoplankton at the subsurface layer in the shelf area.     

Mixed layer variability in Northern Arabian Sea as detected by an Argo float

Seasonal evolution of surface mixed layer in the Northern Arabian Sea (NAS) between 17° N–20.5° N and 59° E-69° E was observed by using Argo float daily data for about 9 months, from April 2002 through December 2002. Results showed that during April - May mixed layer shoaled due to light winds, clear sky and intense solar insolation. Sea surface temperature (SST) rose by 2.3 °C and ocean gained an average of 99.8 Wm⁻². Mixed layer reached maximum depth of about 71 m during June - September owing to strong winds and cloudy skies. Ocean gained abnormally low ∼18 Wm⁻² and SST dropped by 3.4 °C. During the inter monsoon period, October, mixed layer shoaled and maintained a depth of 20 to 30 m. November - December was accompanied by moderate winds, dropping of SST by 1.5 °C and ocean lost an average of 52.5 Wm⁻². Mixed layer deepened gradually reaching a maximum of 62 m in December. Analysis of surface fluxes and winds suggested that winds and fluxes are the dominating factors causing deepening of mixed layer during summer and winter monsoon periods respectively. Relatively high correlation between MLD, net heat flux and wind speed revealed that short term variability of MLD coincided well with short term variability of surface forcing.     

Phytoplankton productivity in the western subarctic gyre of the North Pacific in early summer 2006

We deployed a profiling buoy system incorporating a fast repetition rate fluorometer in the western subarctic Pacific and carried out time-series observations of phytoplankton productivity from 9 June to 15 July 2006. The chlorophyll a (Chl a) biomass integrated over the euphotic layer was as high as 45–50 mg Chl a m⁻² in the middle of June and remained in the 30–40 mg Chl a m⁻² range during the rest of observation period; day-to-day variation in Chl a biomass was relatively small. The daily net primary productivity integrated over the euphotic layer ranged from 144 to 919 mg C m⁻² day⁻¹ and varied greatly, depending more on insolation rather than Chl a biomass. In addition, we found that part of primary production was exported to a 150-m depth within 2 days, indicating that the variations in primary productivity quickly influenced the organic carbon flux from the upper ocean. Our results suggest that the short-term variability in primary productivity is one of the key factors controlling the carbon cycle in the surface ocean in the western subarctic Pacific.     

Annual and Seasonal Variations of the Sea Surface Heat Fluxes in the East Asian Marginal Seas

Based on the twice-daily marine atmospheric variables which were derived mostly from the weather maps for 18 years period from 1978 to 1995, the surface heat flux over the East Asian marginal seas was calculated at 0.5°×0.5° grid points twice a day. The annual mean distribution of the net heat flux shows that the maximum heat loss occurs in the central part of the Yellow Sea, along the Kuroshio axis and along the west coast of the northern Japanese islands. The area off Vladivostok turned out to be a heat-losing region, however, on the average, the amount of heat loss is minimum over the study area and the estuary of the Yangtze River also appears as a region of the minimum heat loss. The seasonal variations of heat flux show that the period of heat gain is longest in the Yellow Sea, and the maximum heat gain occurs in June. The maximum heat loss occurs in January over the study area, except the Yellow Sea where the heat loss is maximum in December. The annual mean value of the net heat flux in the East/Japan Sea is −108 W/m² which is about twice the value of Hirose et al. (1996) or about 30% higher than Kato and Asai (1983). For the Yellow Sea, it is about −89 W/m² and it becomes −75 W/m² in the East China Sea. This increase in values of the net heat flux comes mostly from the turbulent fluxes which are strongly dependent on the wind speed, which fluctuates largely during the winter season. This revised version was published online in August 2006 with corrections to the Cover Date.     

Station Papa Time Series: Insights into Ecosystem Dynamics

The Northeast Pacific has one of the longest time series of any open ocean station, primarily as a result of the weathership station at Station P from the 1950s to 1981. This review summarizes our understanding of the plankton ecosystem for this station and examines interannual variability for the primary producers. The weathership era characterized a period of high temporal sampling resolution with a limited number of parameters being measured. In contrast, the post-weathership period focussed on seasonal sampling (usually three times per year), but a wider range of parameters were measured and sediment traps were deployed to estimate carbon and opal flux rates. The mixed layer depth is shallow compared to the Atlantic Ocean, ranging from 40 to 120 m in late summer and winter respectively. Nitrate, silicate and phosphate are saturating year round with only a few exceptions in the 1970s. Winter and summer Si:N ratios are the same (1.5:1). Ammonium and urea are 0.5 uM in winter and near detection limits (∼0.1 uM) in late summer. Iron is limiting (∼0.05 nM) in late spring and summer for the growth of large diatoms, but iron is co-limiting with irradiance in winter. Chlorophyll and primary productivity are low and show little seasonal variation (about 2 times). Summer chl is about 20 mg m⁻² while primary productivity ranges from 400–850 mg C m⁻²d⁻¹. The f-ratio of 0.25 does not vary with seasons and indicates that primary productivity is fueled by regenerated nitrogen (e.g. NH₄ and urea). Small cells (<5 um) are normally abundant and they utilize regenerated nitrogen produced by the micrograzers; they do not appear to be Fe-limited, but rather controlled by the micrograzers. Shipboard carboy experiments indicate that large diatoms become dominant when iron is added. Therefore top down control is exerted by the micrograzers on the small cells, while there is bottom up control of the large phytoplankton due to low Fe concentrations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nutrient variability in the mixed layer of the subarctic Pacific Ocean, 1987–2010

Data collected primarily from commercial ships between 1987 and 2010 are used to provide details of seasonal, interannual and bidecadal variability in nutrient supply and removal in the surface ocean mixed layer across the subarctic Pacific. Linear trend analyses are used to look for impacts of climate change in oceanic domains (geographic regions) representing the entire subarctic ocean. Trends are mixed and weak (generally not significant) in both winter and summer despite evidence that the upper ocean is becoming more stratified. Overall, these data suggest little change in the winter resupply of the mixed layer with nutrients over the past 23 years. The few significant trends indicate a winter increase in nitrate (~0.16 μM year⁻¹) in the Bering Sea and in waters off the British Columbia coast, and a decline in summer phosphate (0.018 μM year⁻¹) in the Bering. An oscillation in Bering winter nutrient maxima matches the lunar nodal cycle (18.6 years) suggesting variability in tidal mixing intensity in the Aleutian Islands affects nutrient transport. Nitrate removal from the seasonal mixed layer varies between 6 μM along the subarctic–subtropical boundary and 18 μM off the north coast of Japan, with April to September new production rates in the subarctic Pacific being estimated at 2 and 6 moles C m⁻². Changes in nutrient removal in the Bering and western subarctic Pacific (WSP) suggest either the summer mixed layer is thinning with little change in new production or new production is increasing which would require an increase in iron transport to these high-nutrient low-chlorophyll (HNLC) waters. Si/N and N/P removal ratios of 2.1 and 19.7 are sufficient to push waters into Si then N limitation with sufficient iron supply. Because ~3 μM winter nitrate is transferred to reduced N pools in summer, new production calculated from seasonal nutrient drawdown should not be directly equated to export production.     

Temporal variations of surface oceanic and atmospheric CO2 fugacity and total dissolved inorganic carbon in the northwestern North Pacific

In order to examine temporal variations of the surface oceanic and atmospheric fCO₂ and the DIC concentration, we analyzed air and seawater samples collected during the period May 1992–June 1996 in the northwestern North Pacific, about 30 km off the coast of the main island of Japan. The atmospheric CO₂ concentration has increased secularly at a rate of 1.9 ppmv yr⁻¹, and it showed a clear seasonal cycle with a maximum in spring and a minimum late in summer, produced mainly by seasonally-dependent terrestrial biospheric activities. DIC also showed a prominent seasonal cycle in the surface ocean; the minimum and maximum values of the cycle appeared in early fall and in early spring, respectively, due primarily to the seasonally-dependent activities of marine biota and partly to the vertical mixing of seawater and the coastal upwelling. The oceanic fCO₂ values were almost always lower than those of the atmospheric fCO₂, suggesting that this area of the ocean acts as a sink for atmospheric CO₂. Values varied seasonally, mainly reflecting seasonal changes of SST and DIC, with a secular increase at a rate of 3.7 μatm yr⁻¹. The average values of the annual net CO₂ flux between the ocean and the atmosphere calculated by using the different bulk equations ranged between −0.8 and −1.7 mol m⁻²yr⁻¹, and its magnitude was enhanced and reduced late in spring and mid-summer, respectively, due mainly to the seasonally varying oceanic fCO₂.     

Export Production in the Equatorial and North Pacific Derived from Dissolved Oxygen, Nutrient and Carbon Data

A global ocean inverse model that includes the 3D ocean circulation as well as the production, sinking and remineralization of biogenic particulate matter is used to estimate the carbon export flux in the Pacific, north of 10°S. The model exploits the existing large datasets for hydrographic parameters, dissolved oxygen, nutrients and carbon, and determines optimal export production rates by fitting the model to the observed water column distributions by means of the “adjoint method”. In the model, the observations can be explained satisfactorily with an integrated carbon export production of about 3 Gt C yr⁻¹ (equivalent to 3⋅10¹⁵ gC yr⁻¹) for the considered zone of the Pacific Ocean. This amounts to about a third of the global ocean carbon export of 9.6 Gt C yr⁻¹ in the model. The highest export fluxes occur in the coastal upwelling region off northwestern America and in the tropical eastern Pacific. Due to the large surface area, the open-ocean, oligotrophic region in the central North Pacific also contributes significantly to the total North Pacific export flux (0.45 Gt C yr⁻¹), despite the rather small average flux densities in this region (13 gC m⁻²yr⁻¹). Model e-ratios (calculated here as ratios of model export production to primary production, as inferred from satellite observations) range from as high a value as 0.4 in the tropical Pacific to 0.17 in the oligotrophic central north Pacific. Model e-ratios in the northeastern Pacific upwelling regions amount to about 0.3 and are lower than previous estimates. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of cold eddy on phytoplankton production and assemblages in Luzon strait bordering the South China Sea

The biochemical effects of a cold-core eddy that was shed from the Kuroshio Current at the Luzon Strait bordering the South China Sea (SCS) were studied in late spring, a relatively unproductive season in the SCS. The extent of the eddy was determined by time-series images of SeaWiFS ocean color, AVHRR sea surface temperature, and TOPEX/Jason-1 sea surface height anomaly. Nutrient budgets, nitrate-based new production, primary production, and phytoplankton assemblages were compared between the eddy and its surrounding Kuroshio and SCS waters. The enhanced productivity in the eddy was comparable to wintertime productivity in the SCS basin, which is supported by upwelled subsurface nitrate under the prevailing Northeastern Monsoon. There were more Synechococcus, pico-eucaryotes, and diatoms, but less Trichodesmium in the surface water inside the eddy than outside. Prochlorococcus and Richelia intracellularis showed no spatial differences. Water column-integrated primary production (IPP) inside the eddy was 2–3 times that outside the eddy in the SCS (1.09 vs. 0.59 g C m⁻²d⁻¹), as was nitrate-based new production (INP) (0.67 vs. 0.25 g C m⁻²d⁻¹). INP in the eddy was 6 times that in the Kuroshio (0.12 g C m⁻²d⁻¹). IPP and INP in the eddy were higher than the maximum production values ever measured in the SCS basin. Surface chlorophyll a concentration (0.40 mg m⁻³) in the eddy equaled the maximum concentration registered for the SCS basin and was higher than the wintertime average (0.29 ± 0.04 mg m⁻³). INP was 3.5 times as great and IPP was doubled in the eddy compared to the wintertime SCS basin. As cold core eddies form intermittently all year round as the Kuroshio invades the SCS, their effects on phytoplankton productivity and assemblages are likely to have important influences on the biogeochemical cycle of the region.     

Cold deep water in the South China Sea

Two deep channels that cut through the Luzon Strait facilitate deep (>2000 m) water exchange between the western Pacific Ocean and the South China Sea. Our observations rule out the northern channel as a major exchange conduit. Rather, the southern channel funnels deep water from the western Pacific to the South China Sea at the rate of 1.06 ± 0.44 Sv (1 Sv = 10⁶ m³s⁻¹). The residence time estimated from the observed inflow from the southern channel, about 30 to 71 years, is comparable to previous estimates. The observation-based estimate of upwelling velocity at 2000 m depth is (1.10 ± 0.33) × 10⁻⁶ ms⁻¹, which is of the same order as Ekman pumping plus upwelling induced by the geostrophic current. Historical hydrographic observations suggest that the deep inflow is primarily a mixture of the Circumpolar Deep Water and Pacific Subarctic Intermediate Water. The cold inflow through the southern channel offsets about 40% of the net surface heat gain over the South China Sea. Balancing vertical advection with vertical diffusion, the estimated mean vertical eddy diffusivity of heat is about 1.21 × 10⁻³ m²s⁻¹. The cold water inflow from the southern channel maintains the shallow thermocline, which in turn could breed internal wave activities in the South China Sea.     

East-west distribution of POC fluxes estimated from <Superscript>234</Superscript>Th in the northern North Pacific in autumn

Observations of primary productivity, ²³⁴Th, and particulate organic carbon (POC) were made from west to east across the northern North Pacific Ocean (from station K2 to Ocean Station Papa) during September–October 2005. Primary productivities in this region varied longitudinally from approximately 236 to 444 mgC m⁻²d⁻¹ and clearly indicate the West High East Low (WHEL) trend. We estimated east-west variations in the POC flux from the surface layer (0–100 m) by using ²³⁴Th as a tracer. POC fluxes in the western region (44–53 mgC m⁻²d⁻¹) were higher than those in the eastern region (21–34 mgC m⁻²d⁻¹). However, the export ratios (e-ratios) ranged from approximately 8% to 16% and did not show the WHEL trend. Contrary to our expectation, no relation between POC flux (or e-ratio) and diatom biomass (or dominance) was apparent in autumn in the northern North Pacific.     

Spatiotemporal Decreases of Nutrients and Chlorophyll-a in the Surface Mixed Layer of the Western North Pacific from 1971 to 2000

Using time series of hydrographic data in the wintertime and summertime obtained along 137°E from 1971 to 2000, we found that the average contents of nutrients in the surface mixed layer showed linear decreasing trends of 0.001∼0.004 μmol-PO₄ l⁻¹ yr⁻¹ and 0.01∼0.04 μmol-NO₃ l⁻¹ yr⁻¹ with the decrease of density. The water column Chl-a (CHL) and the net community production (NCP) had also declined by 0.27∼0.48 mg-Chl m⁻² yr⁻¹ and 0.08∼0.47 g-C-NCP m⁻² yr⁻¹ with a clear oscillation of 20.8±0.8 years. These changes showed a strong negative correlation with the Pacific Decadal Oscillation Index (PDO) with a time lag of 2 years (R = 0.89 ± 0.02). Considering the recent significant decrease of O₂ over the North Pacific subsurface water, these findings suggest that the long-term decreasing trend of surface-deep water mixing has caused the decrease of marine biological activity in the surface mixed layer with a bidecadal oscillation over the western North Pacific.     

Sedimentation rates and subsidence in the Southern Tyrrhenian Basin

The petrophysical properties of sediment drill core samples recovered from the Sardinian margin and the abyssal plain of the Southern Tyrrhenian Basin were used to estimate the downhole change in porosity and rates of deposition and mass accumulation. We calculated how the deposited material has changed its thickness as a function of depth, and corrected the thickness for the compaction. The corresponding porosity variation with depth for terrigenous and pelagic sediments and evaporites was modelled according to an exponential law. The mass accumulation rate for the Plio-Quaternary is on average 4.8×10⁴ kg m⁻² my⁻¹ on the Sardinian margin and for the Pliocene in the abyssal plain. In the latter area, the Quaternary attains its greatest thickness and a mass accumulation rate of 11–40×10⁴ kg m⁻² my⁻¹. The basement response to sediment loading was calculated with Airy-type backstripping. On the lower part of the Sardinian margin, the basement subsidence rate due to sediment loading has decreased from a value of 300 m my⁻¹ in the Tortonian and during the Messinian salinity crisis (7.0–5.33 Ma) to about 5 m my⁻¹ in the Plio-Quaternary. In contrast, on the abyssal plain this rate has changed from 8–50 m my⁻¹ during the period 3.6–0.46 Ma, to 95–130 m my⁻¹ since 0.46 Ma, with the largest values in the Marsili Basin. The correlation between age and the depth to the basement corrected for the loading of the sediment in the ocean domain of the Tyrrhenian Basin argues for a young age of basin formation.     

Nitrogen circulation in the water column in the Oyashio region with reference to its high productivity

In order to determine quantitatively the reason for the high productivity in the Oyashio Region, which is the southwest part of the Pacific Subarctic Region, the annual-mean vertical circulation of nitrogen in the region was estimated from the vertical profiles of nitrate, dissolved oxygen and salinity, and sediment-trap data by adapting them to the balance equations. Estimates of the upwelling velocity (1.7×10⁻⁵cm sec⁻¹) and the vertical diffusivity (2.1 cm² sec⁻¹) in the abyssal zone and the primary and secondary productivities (44 and 4 mgN m⁻²day⁻¹, respectively) in the euphotic zone were close to those of previous works. The estimated vertical circulation of nitrogen strongly suggested that, since the divergence (5 mgN m⁻²day⁻¹) is caused by the abyssal convergence (6 mgN m⁻²day⁻¹) and the positive precipitation, the local new production (22 mgN m⁻²day⁻¹) necessarily exceeds not only the sinking flux (10 mgN m⁻²day⁻¹) itself but also the sum of the sinking flux and the downward diffusion of dissolved and particulate organic matter (7 mgN m⁻²day⁻¹) produced probably in the euphotic zone. The important roles of the abyssal circulation, the winter convection, and the metabolic activity in the bathyal zone to support the high productivity in the euphotic zone were clarified quantitatively.     

Roles of Continental Shelves and Marginal Seas in the Biogeochemical Cycles of the North Pacific Ocean

Most marginal seas in the North Pacific are fed by nutrients supported mainly by upwelling and many are undersaturated with respect to atmospheric CO₂ in the surface water mainly as a result of the biological pump and winter cooling. These seas absorb CO₂ at an average rate of 1.1 ± 0.3 mol C m⁻²yr⁻¹ but release N₂/N₂O at an average rate of 0.07 ± 0.03 mol N m⁻²yr⁻¹. Most of primary production, however, is regenerated on the shelves, and only less than 15% is transported to the open oceans as dissolved and particulate organic carbon (POC) with a small amount of POC deposited in the sediments. It is estimated that seawater in the marginal seas in the North Pacific alone may have taken up 1.6 ± 0.3 Gt (10¹⁵ g) of excess carbon, including 0.21 ± 0.05 Gt for the Bering Sea, 0.18 ± 0.08 Gt for the Okhotsk Sea; 0.31 ± 0.05 Gt for the Japan/East Sea; 0.07 ± 0.02 Gt for the East China and Yellow Seas; 0.80 ± 0.15 Gt for the South China Sea; and 0.015 ± 0.005 Gt for the Gulf of California. More importantly, high latitude marginal seas such as the Bering and Okhotsk Seas may act as conveyer belts in exporting 0.1 ± 0.08 Gt C anthropogenic, excess CO₂ into the North Pacific Intermediate Water per year. The upward migration of calcite and aragonite saturation horizons due to the penetration of excess CO₂ may also make the shelf deposits on the Bering and Okhotsk Seas more susceptible to dissolution, which would then neutralize excess CO₂ in the near future. Further, because most nutrients come from upwelling, increased water consumption on land and damming of major rivers may reduce freshwater output and the buoyancy effect on the shelves. As a result, upwelling, nutrient input and biological productivity may all be reduced in the future. As a final note, the Japan/East Sea has started to show responses to global warming. Warmer surface layer has reduced upwelling of nutrient-rich subsurface water, resulting in a decline of spring phytoplankton biomass. Less bottom water formation because of less winter cooling may lead to the disappearance of the bottom water as early as 2040. Or else, an anoxic condition may form as early as 2200 AD. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nutrient regeneration at bottom after a massive spring bloom in a subarctic coastal environment,Funka Bay,Japan

Nutrient regeneration and oxygen consumption after a spring bloom in Funka Bay were studied on monthly survey cruises from February to November 1998 and from March to December 1999. A high concentration of ammonium (more than 4 μmol l⁻¹) was observed near the bottom (80–90 m) after April. Phosphate and silicate gradually accumulated and dissolved oxygen decreased in the same layer. Salinity near the bottom did not change until summer, leading to the presumption that the system in this layer is semi-closed, so regenerated nutrients were preserved until September. Nitrification due to the oxidation of ammonium to nitrate was observed after June. Nitrite, an intermediate product, was detected at 4–7 μmol L⁻¹ in June and July 1999. Assuming that decomposition is a first order reaction, the rate constant for decomposition of organic nitrogen was determined to be 0.014 and 0.008 d⁻¹ in 1998 and 1999, respectively. The ammonium oxidation rate increased rapidly when the ambient ammonium concentration exceeded 5 μmol L⁻¹. We also performed a budget calculation for the regeneration process. The total amount of N regenerated in the whole water column was 287.4 mmol N m⁻² in 4 months, which is equal to 22.8 gC m⁻², assuming the Redfield C to N ratio. This is 34% of the primary production during the spring bloom and is comparable to the export production of 25 gC m⁻² measured by a sediment trap at 60 m (Miyake et al., 1998).     

Turbulence investigation in a tidal coastal region

A high-frequency (1.2 MHz) four-beam Acoustic Doppler Current Profiler (ADCP) moored on the sea bottom was used for the direct measurements of the turbulence parameters in the shallow (20 m) coastal zone of the eastern English Channel. The measurements were as long as four tidal cycles during the period of the spring tide development. The measurements in the ocean and estimates showed that the Reynolds stress variability coincided with the semidiurnal tide. Their maximum values during the flood phase were approximately 1.5 Pa, while, during the ebb phase, they reached −1.2 Pa. The variations of the turbulence’s kinetic energy (TKE) and the rate of its production (P) coincided with the period of the tidal harmonic M4. Their maximum values were found during the flood phase near the bottom, and they were approximately equal to 0.03 m²/s² and 0.8 W/m³, respectively. These values decreased rapidly with the distance from the bottom. During the periods of low stagnant water, the values of TKE and P in the water column decreased to the minimum values (2 × 10⁻³ m²/s² and 3 × 10⁻⁵ W/m³, respectively), which coincided with the moment of the current’s reversal flow. The results demonstrated the dominating role of the tidal motion, which controls the structure and intensity of the turbulence in the bottom layer, and revealed the characteristic asymmetry of its distribution related to the nonlinear character of the tidal cycle.