Particulate Organic Nitrogen Fluxes Research Articles

High particulate organic carbon export during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean

Carbon fixation by phytoplankton plays a key role in the uptake of atmospheric CO2 in the Southern Ocean. Yet, it still remains unclear how efficiently the particulate organic carbon (POC) is exported and transferred from ocean surface waters to depth during phytoplankton blooms. In addition, little is known about the processes that control the flux attenuation within the upper twilight zone. Here, we present results of downward POC and particulate organic nitrogen fluxes during the decline of a vast diatom bloom in the Atlantic sector of the Southern Ocean in summer 2012. We used thorium-234 (234Th) as a particle tracer in combination with drifting sediment traps (ST). Their simultaneous use evidenced a sustained high export rate of 234Th at 100m depth in the weeks prior to and during the sampling period. The entire study area, of approximately 8000km2, showed similar vertical export fluxes in spite of the heterogeneity in phytoplankton standing stocks and productivity, indicating a decoupling between production and export. The POC fluxes at 100m were high, averaging 26±15mmol Cm−2d−1, although the strength of the biological pump was generally low. Only <20% of the daily primary production reached 100m, presumably due to an active recycling of carbon and nutrients. Pigment analyses indicated that direct sinking of diatoms likely caused the high POC transfer efficiencies (~60%) observed between 100 and 300m, although faecal pellets and transport of POC linked to zooplankton vertical migration might have also contributed to downward fluxes.

Wind-driven interannual variability of sea ice algal production in the western Arctic Chukchi Borderland

Abstract. Seasonal and interannual variability in the biogenic particle sinking flux was recorded using multi-year bottom-tethered sediment trap mooring systems in the Northwind Abyssal Plain (Station NAP: 75° N, 162° W, 1975 m water depth) of the western Arctic Chukchi Borderland. Trapped particle flux at a median depth of 184 m had an obvious peak and dominance of sea ice-related diatom assemblages in August 2011. The observed particle flux was considerably suppressed throughout summer 2012. In the present study, the response of ice algal production and biomass to wind-driven changes in the physical environment was addressed using a pan-Arctic sea ice–ocean modeling approach. A sea ice ecosystem with ice algae was newly incorporated into the lower-trophic marine ecosystem model, which was previously coupled with a high-resolution (i.e., 5 km horizontal grid size) sea ice–ocean general circulation model. Seasonal model experiments covering 2-year mooring periods indicated that primary productivity of ice algae around the Chukchi Borderland depended on basin-scale wind patterns via various processes. Easterly winds in the southern part of a distinct Beaufort High supplied nutrient-rich water for euphotic zones of the NAP region via both surface Ekman transport of Chukchi shelf water and vertical turbulent mixing with underlying nutricline water in 2011. In contrast, northwesterly winds flowing in the northern part of an extended Siberian High transported oligotrophic water within the Beaufort Gyre circulation toward the NAP region in 2012. The modeled ice algal biomass during summer reflected the differences in nutrient distribution. The modeled sinking flux of particulate organic nitrogen (PON) was comparable with the time series obtained from sediment trap data in summer 2011. In contrast, lateral advection of ice algal patches of shelf origin during a great cyclone event may have caused a modeled PON flux bias in 2012. Sensitivity experiments revealed several uncertainties of model configurations of ice algal productivity, particle sinking speed, and grazing activities. Extending the year-long measurements is expected to help illustrate the more general features of ice-related biological processes in the Arctic Ocean.

Heterotrophic bacterial production and extracellular enzymatic activity in sinking particulate matter in the western North Pacific Ocean

Heterotrophic activities on sinking particulate matter (SPM) play an important role in SPM fluxes in the ocean. To demonstrate regional differences in heterotrophic activities on SPM, we measured heterotrophic bacterial production (HBP) in seawater (HBPSW) and SPM (HBPSPM) as well as potential extracellular enzyme activity (EEA) in SPM on a transect along 155°E in the western North Pacific Ocean in the subarctic (44°N), the Kuroshio Extension area (35°N), and the subtropical gyre (20°N). Depth-integrated HBPSW from the surface to 500 m was comparable between the locations, whereas HBPSPM at 44°N was substantially lower than at the other sites. We found the highest particulate organic carbon (POC) export flux and export efficiency to bathypelagic depths, and the lowest water temperatures, at 44°N. We found significant correlations between leucine aminopeptidase (LAPase) activity, β-glucosidase (BGase) activity, POC flux and particulate organic nitrogen flux. LAPase activity was two orders of magnitude higher than BGase activity, with a BGase:LAPase activity ratio of 0.027. There were no significant correlations between HBP and EEA in SPM except for lipase, and lipase activity was significantly correlated with temperature. We propose that hydrographic conditions are an important factor controlling heterotrophic bacterial activity and export efficiency of organic carbon to the deep ocean, as are the sources and abundance of SPM produced in the euphotic zone via primary production.

The Intra-Annual Variability of Discharge, Sediment Load and Chemical Flux from the Monitoring: The Yukon River, Alaska

The covered-ice breakup in subarctic to arctic rivers in the early snowmelt season often gives any damage to instruments monitoring physical and chemical factors of water. The serious condition has brought few time series data during the snowmelt runoff except the river stage or discharge. In this study, the contribution of snowmelt runoff to the discharge and sediment load is quantified by monitoring water turbidity and temperature at the lowest gauging station of U. S. Geological Survey in the Yukon River, Alaska, for more than 3 years (June 2006 to September 2009). The turbidity was recorded by a self-recording turbidimeter with a sensor of infrared-ray back-scattering type, of which the window is cleaned by a wiper just before a measurement. The turbidity time series, coupled with frequent river water sampling at mid-channel, produce time series of suspended sediment (SS) concentration, particulate organic carbon (POC) concentration and particulate organic nitrogen (PON) concentration (mg?L–1) by using the high correlation (R2 = 0.747 to 0.790; P 11 to 2.01 × 1011 m3), 8.7% - 22.5% of the annual sediment load (3.94 × 107 to 5.08 × 107 ton), 11.6% - 23.7% of the annual POC flux (4.05 × 105 to 4.77 × 105 ton), and 10.3% - 24.5% of the annual PON flux (2.80 × 104 to 3.44 × 104 ton). In the snowmelt season, the peak suspended sediment concentration preceded the peak discharge by a few days. This probably results from the fluvial sediment erosion in the river channels.

Particulate nutrient fluxes over a fringing coral reef: relevant scales of phytoplankton production and mechanisms of supply

Seasonal observations of phytoplankton uptake at Ningaloo Reef, Western Australia, reinforce the importance of particulate organic nitrogen (PON) and carbon (POC) in reef nutrient budgets and identify wave action and the dynamics of regional currents (over a range of temporal and spatial scales) as important factors determining plankton supply to the reef. Phytoplankton uptake rates, calculated from declining chlorophyll a concentrations as water moved over the reef, appeared to be near the physical limits of mass transfer. Phytoplankton-derived PON flux of 2 to 5 mmol N m -2 d -1 was on the order of that typical for dissolved N uptake — confirming that particle feeding may supply the N missing in reef N budgets — while POC flux of 14 to 27 mmol C m -2 d -1 was on the order of net community metabolism. Phytoplankton supply was highly variable at daily-to- seasonal time scales in response to the dynamics of a regional current system dominated by the downwelling-favourable Leeuwin Current (LC). Acceleration of the LC in the austral autumn may supply as much phytoplankton to the reef as sporadic upwelling associated with the Ningaloo Cur- rent (NC) in summer. The ocean catchment concept is introduced as a basis for examining the spatial scale of pelagic processes influencing benthic systems: every day, Ningaloo may completely consume the phytoplankton over 87 km 2 of LC water, compared to only 20 km 2 of NC water. Production within this catchment appears insufficient to maintain offshore phytoplankton concentrations, and advec- tion of remotely sourced production into the catchment is required to balance reef uptake. A func- tional dependence by reef organisms on externally sourced ocean productivity increases the poten- tial scale at which human- or climatically induced changes may affect reef communities and suggests that processes such as changes in offshore currents and plankton communities require further con- sideration in reef-level biogeochemistry.

The significance of phytoplankton photo-adaptation and benthic–pelagic coupling to primary production in the South China Sea: Observations and numerical investigations

The primary production in the South China Sea (SCS) has been assessed by a coupled physical–biogeochemical model with a simple NPZD ecosystem [Liu et al., 2002. Monsoon-forced chlorophyll distribution and primary production in the SCS: observations and a numerical study. Deep-Sea Research I 49(8), 1387–1412]. In recent years there have been an increasing number of observations in the SCS that may be used to check the validity of the previous approach. The coupled model of the SCS mentioned above employs a photo-adaptation scheme for the phytoplankton growth and uses the simplest bottom boundary condition of an inert benthic layer. These adopted schemes are checked against observations at the South-East Asian Time-series Study (SEATS) Station in the northern SCS and in the Gulf of Thailand. Numerical experiments with or without photo-adaptation or active benthic processes are carried out in this study. Additional experiments are performed with different parameters used for these processes. The observations at the SEATS Station provide direct evidence for the variable chlorophyll-to-nitrogen ratio in phytoplankton as required by photo-adaptation. It is concluded that a photo-adaptation scheme is critical to the phytoplankton growth, especially for the development of the subsurface chlorophyll maximum (SCM). Without photo-adaptation, the average value of the vertically integrated primary production (IPP) over the whole SCS domain would be 35% lower. It is noted that, the modeled SCM occurs at depths shallower than observations due to physical as well as biological processes employed by the model. Increasing the upper limit of the chlorophyll-to-nitrogen ratio, as suggested by observations, enhances chlorophyll level in the lower part of the euphotic zone and raises primary productivity in areas with rich nutrient supply. The observed values of the IPP in the Gulf of Thailand clearly demonstrate the importance of the benthic–pelagic coupling to the nutrient cycle. Without benthic nutrient regeneration the model grossly underestimates primary production due to failure to build up the nutrient reserve in the Gulf. On the other hand, a fully regenerated flux of particulate organic nitrogen at the sea floor without denitrification produces too strong a primary production. The improved model uses a higher upper limit for the chlorophyll-to-phytoplankton ratio of 3.5 g Chl/mol N and adopts benthic processes of a coupled nitrification–denitrification scheme with denitrification consuming 14% of the detritus flux at the bottom. The model predicts a mean annual IPP value of 406 mg C m −2 d −1 for the SCS, which is 44% higher than that predicted by the original model. The increase can be broken down to 39% attributed to the benthic nutrient regeneration and 5% to the enhanced photo-adaptation. The average IPP is 390 mg C m −2 d −1 for the basin region (>200 m) and 429 mg C m −2 d −1 for the shelf region (<200 m), both of which compare favorably with observed mean values. The model also predicts a mean nitrogen removal flux of 0.16 mmol N m −2 d −1 during denitrification for the shelf region.

A comparison of major and minor elemental fluxes collected in neutrally buoyant and surface-tethered sediment traps

We compare material collected in Neutrally Buoyant Sediment Traps (NBSTs) to material collected in standard Particle Interceptor Traps (PITs) deployed during June and September 2001 at the Bermuda Atlantic Time-series Study site in order to compare sediment traps with various hydrodynamic properties. Current meters located on the PITs array indicated that both experiments were conducted during conditions of relatively low horizontal flow, which are optimal conditions for the unbiased collection of sinking particles by PITs. The NBSTs and PITs both recorded a range of overall flux conditions, as supported by the fluxes of mass, particulate organic carbon, and other elements (S, Mg, Sr, Fe, Mn, Ni, Ti, P, V), which do not significantly differ between the two types of traps. However, the fluxes of Si, Ca, Al, and Ba collected by the PITs are significantly larger than the NBST fluxes of those same elements. Additionally, the fluxes of particulate organic nitrogen (PON) collected by the PITs are significantly smaller than the NBST PON fluxes. How these results compare under conditions of higher flow or differing particle characteristics cannot be assessed from this study.

A comparison of major and minor elemental fluxes collected in neutrally buoyant and surface-tethered sediment traps

We compare material collected in Neutrally Buoyant Sediment Traps (NBSTs) to material collected in standard Particle Interceptor Traps (PITs) deployed during June and September 2001 at the Bermuda Atlantic Time-series Study site in order to compare sediment traps with various hydrodynamic properties. Current meters located on the PITs array indicated that both experiments were conducted during conditions of relatively low horizontal flow, which are optimal conditions for the unbiased collection of sinking particles by PITs. The NBSTs and PITs both recorded a range of overall flux conditions, as supported by the fluxes of mass, particulate organic carbon, and other elements (S, Mg, Sr, Fe, Mn, Ni, Ti, P, V), which do not significantly differ between the two types of traps. However, the fluxes of Si, Ca, Al, and Ba collected by the PITs are significantly larger than the NBST fluxes of those same elements. Additionally, the fluxes of particulate organic nitrogen (PON) collected by the PITs are significantly smaller than the NBST PON fluxes. How these results compare under conditions of higher flow or differing particle characteristics cannot be assessed from this study.

Pulsing versus constant supply of nutrients (N, P and Si): effect on phytoplankton, mesozooplankton and vertical flux of biogenic matter

An experiment with eight vertically stratified seawater enclosures of 27 m 3 (depth 9.3 m, diameter 2 m, 90% penetration of PAR) was run in order to test whether pulsed addition of nutrients may cause: 1, higher primary production; 2, higher build-up of phytoplankton biomass; 3, larger temporal mismatch between herbivores and phytoplankton biomass; and 4, higher sedimentation rates, distinguishing in each case between silicate and non-silicate fertilised systems. Nitrate and phosphate were added to all enclosures (NP), while silicate was added to four of the enclosures (NPS). Each enclosure received the same total amount of nutrients, but the nutrients were supplied at four different intervals ranging from one single load to continuous additions. Spring bloom-like systems developed where nutrients were added in one or two pulses as they were characterised by high primary production, high suspended biomass of chlorophyll a (Chl a) and particulate organic carbon (POC) and high sedimentation rates. In contrast, the seawater enclosures receiving nutrients about every third day or in a continuous supply resembled regenerated systems with low concentrations of suspended Chl a and POC and with low and stable loss rates. Due to a typical autumn inoculum with dominance of dinoflagellates and flagellates, diatoms did not dominate the NPS enclosures. The only significant effect of the silicate addition was higher vertical flux of particulate organic nitrogen in the NPS enclosures, and higher microzooplankton biomass. The mesozooplankton did not show responses to the different frequencies of nutrient additions. However, accumulation of mesozooplankton biomass was higher in the NP-mesocosms, probably reflecting better feeding conditions. We conclude that the frequency of nutrient additions had a stronger influence on the development of the phytoplankton and vertical flux of carbon than the +/- silicate treatment in this experiment.

Is Southern Ocean organic carbon and biogenic silica export enhanced by iron-stimulated increases in biological production? Sediment trap results from SOIREE

During the first in situ, mesoscale iron fertilisation experiment conducted in the Southern Ocean (SOIREE, 61°S 140°E, austral summer, February 10–22, 1999), export processes beneath an iron-stimulated phytoplankton bloom were monitored using free-drifting sediment traps. Duplicated arrays were deployed during days T7–9 and T11–13 inside the iron-fertilised patch with single “control” arrays deployed outside the patch on days T0–2 and T11–13. Average total mass and chlorophyll a flux at 110 m approximately doubled between T0–2 and T7–9 (395–735 mg dry weight m−2 d−1 and 50–110 μg chl a m−2 d−1, respectively), while particulate organic carbon (POC), particulate organic nitrogen (PON) and phaeopigment fluxes increased only slightly (15–30%) and biogenic silica (BSi) fluxes were essentially constant (90–100 mg m−2 d−1). Inside the patch between T7–9 and T11–13, mass flux remained at about 735–745 mg m−2 d−1 while other flux components increased by 30–40%. Mean POC and silica fluxes inside the patch at T11–13 were about 185 and 150 mg m−2 d−1, respectively, compared with 80 and 90 mg m−2 d−1 measured outside. However, flux variations of >50% between the two “control” deployments at T0–2 and T11–13 confounded the trapping experiment. POC and PON fluxes at deep traps were typically 30–50% of those at shallow traps, whereas total mass and silica fluxes at 310 m were generally 70–>90% of the flux at 110 m, indicating substantial remineralisation of POC, relative to silica, over the top 300 m of the water column. As also suggested by other biogeochemical proxies (234Th, δ13C), POC and biogenic silica export from the SOIREE patch did not increase measurably in response to iron-stimulated increases in primary production and a floristic shift to large, heavily silicified, chain-forming diatoms. The temporal decoupling between new and export production was accompanied by considerable algal accumulation in the mixed layer, reductions in phytoplankton sinking rates inside the patch, and low levels of mesozooplankton grazing during SOIREE. Since elevated satellite-derived algal concentrations persisted near the experimental site for 30–45 d after SOIREE, a conservative estimate of potential total flux from the bloom is calculated (6–9 g C m−2). Considerable logistical constraints must be overcome, however, in order to quantify the magnitude of carbon export from iron-mediated blooms at high southern latitudes before we can establish links between Southern Ocean productivity and global climate change, as encapsulated in the “Iron Hypothesis”.

Sedimentation of Particulate Matter in the Dona Paula Bay, West Coast of India during November to May 1995-1997

Data on hydrography, nutrients, suspended particles, and sedimented particles were collected at weekly intervals from November to May during 1995 to 1997 at a station in the coastal waters of Dona Paula Bay, India. Suspended and sedimented particles were analyzed for total suspended matter (SPM), total sedimented particulate matter (TPM), particulate organic carbon (POC), particulate organic nitrogen (PON), chlorophylla (chla), and diatom abundance. Variations in hydrography and nutrients influenced the quantity and composition of sedimented particles. The TPM, POC, PON, and chla fluxes showed small-scale seasonal variations and were higher in the summer (February to May) than in the winter (November to January). Resuspension of carbon accounted for approximately 25% of the gross POC and was highest in April 1997 (45%). The mean net POC flux was 197±90 mg C m−2 d−1 and accounts for 4.6% of the TPM. The average C∶N (w∶w) ratio of the sedimented material was 13.2±6.6. The POC:chla ratio was relatively higher in the sedimented material as compared to the suspended material. The particulate carbon reaching the bottom sediment was 39% of the primary production. The low organic carbon concentration (approximately 0.1% of dry sediment) in the sediments implies that about 98% of the sedimented carbon was either consumed at the sedimentwater interface or resuspended/advected before it was finally buried into the sediments.

Particulate matter distribution and [formula omitted] disequilibrium along the Northern Iberian Margin: implications for particulate organic carbon export

Light beam attenuation and scattering profiles were obtained from a total of 94 stations from the shelf edge and slope of the Iberian Margin in the early summer of 1997 and winter of 1998. Hydrographic and satellite data suggest that the summer cruise took place just prior to the seasonal upwelling period present along this margin. Our results from the slope show well-developed, high-concentration, intermediate nepheloid layers (INL) present in North Atlantic Central Water, underlain by relatively clear Mediterranean Water, which, at its salinity maximum, shows an off-shelf decrease in particle load to clear water at the outer edge of the water mass. Only low-concentration INL and weak bottom nepheloid layer (BNL) activity is observed in North Atlantic Deep Water. The presence of low BNL activity suggests weak off-shelf transport of material and weak bottom current resuspension. Good agreement between particulate matter concentration estimated from the empirical calibration of beam attenuation and scattering is found in clear water, while differences in instrumental response within the BNL and surface layer are most likely related to the differing nature of the particles present. In the upper ocean particle load calculated from beam attenuation shows a considerable excess over that calculated by scattering. Offshore spatial and temporal differences in the relationship between both measurements are consistent with a response to large particle sized biological material present in the surface layer. The upper ocean Th 234 activity was measured at a total of 23 stations and particulate organic carbon (POC) and particulate organic nitrogen (PON) at each station occupied during the summer cruise. The broadly uniform distribution of total Th 234 , and more variable particulate Th 234 , suggests that during pre-upwelling conditions on the Iberian Margin, Th 234 distribution is strongly influenced by particle type. As a consequence, particle residence times derived from Th 234 / U 238 disequilibria show a considerable range between 4 and 193 d. Estimates of export flux of POC and PON from the upper 50 m were made using the Th 234 , POC and PON results. POC and PON export ranged from 1–56 mmol C m −2 d −1 and 0.15–7.7 mmol N m −2 d −1 respectively, showing a broad offshore decrease. Comparison of the POC export with estimates of carbon fixation measured during the same period suggests that an average of 19% of the primary production is lost as a sinking particulate flux. Overall, the pre-upwelling situation observed during CD-105 was characterised by low phytoplankton biomass, primary production and low POC export fluxes.

Sporadic silicate limitation of phytoplankton productivity in the subarctic NE Pacific

A time series (1970–1980) of silicate concentrations in the surface mixed layer at Ocean Station P (OSP, 50°N, 145°W) in the subarctic NE Pacific Ocean in high-nutrient and low-chlorophyll (HNLC) waters shows nearly total depletion of silicate (<1 μmol kg −1) in the summers of 1972, 1976 and 1979. From a mixed-layer model for the spring–summer period, we calculated silicate and nitrate utilization. The silicate utilization (ΔSiO 4) during the growing season displays large interannual variations, suggesting that diatom production would experience similar fluctuations. The years 1972 and 1979 had both high-silicate utilization (ΔSiO 4) and high-nitrate utilization (ΔNO 3). During these two high-production years, the lack of available silicate appeared to limit diatom production. For 1972 and 1979, the ratio of ΔSiO 4 to ΔNO 3 was 1.4 and 2.5, respectively. The 1979 ratio supports the conclusion that high-nutrient utilization in the mixed layer is dominated by diatoms. The 1972 ratio is consistent with the average value calculated from the time-series data and suggests that the high-nutrient utilization resulted from a combination of diatom and non-siliceous production. A time series of particle fluxes (1980–1994) collected in deep-moored sediment traps at OSP showed that the averaged monthly flux ratio of opal to particulate organic nitrogen (PON) remained constant except during two high-PON flux periods. These two periods occurred during the late summers of 1983 and 1993 when the PON flux was more than double the average. During these two high-PON flux periods a significant decrease in the opal to PON flux ratio occurred. If these high-PON fluxes were caused by increased diatom production, the composition and preservation of nitrogen in the sinking organic matter also must increase during these diatom blooms. In 1983, the high-PON flux period was associated with a high-opal flux but not in 1993. It appears that the two high-PON periods were driven by two different processes. In 1983, the high-PON and opal fluxes are consistent with increased diatom production, while in 1993, the high-PON flux and average opal flux suggest increased non-diatom production. The switch between high production of diatoms and high production by non-diatoms is consistent with the silicate and nitrate utilization, which also had years with both high silicate and nitrate utilization and years with only high-nitrate utilization.

Enhancement of new production in the northeast subarctic Pacific Ocean during negative North Pacific index events

We examined interannual variability in the 1965–1990 winter‐summer differences in surface nitrate (ΔNO3) in the northeast subarctic Pacific Ocean (Station P, 50°N, 145°W). Increases in ΔNO3, of 30–70% above the mean value occurred during four events (1969, 1971‐1972, 1979, 1983). The ΔNO3 time series was highly correlated (r = 0.86) with the spring‐summer nitrate utilization rate from 1970 to 1980. The ΔNO3 time series was negatively correlated with the North Pacific index (NPI) (r = ‐0.72) and positively correlated with solar radiation in the spring (r = 0.48) and summer (r = 0.46). A simple ecosystem model forced with observed incident solar radiation and mixed‐layer depth during the spring‐summer period predicted increases in export production that were 50% of the observed increase for 1970–1972 and 1977–1979. Comparison between the ΔNO3 and sedimenting particulate organic nitrogen (PON) showed that the high ΔNO3 event of 1983 was associated with a dramatic increase in PON fluxes. A simple ecosystem model underpredicts observed interannual variations in export production, and this result combined with the observed PON fluxes suggests that a combination of increased phytoplankton production and increased f ratio produced the observed increase in ΔNO3. The increase in f‐ratio implies a change in the community structure of the phytoplankton during the elevated ΔNO3 events that appears connected to changes in the NPI.

Seasonal variability in primary production and particle flux in the northwestern Sargasso Sea: U.S. JGOFS Bermuda Atlantic time-series study

The relationship between primary production and sediment trap-derived downward flux of particulate organic matter was characterized over a 2 year period at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site to evaluate the importance of temporal variations in upper ocean biogeochemical processes. Water column-integrated primary production (∫ PP), determined once each cruise using 14C incubations ( in situ dawn-to-dusk), peaked in late winter/early spring of both 1989 and 1990. Smaller increases in ∫ PP also occured in July 1989 and October–December 1990. Annual ∫ PP was 9.2 mol C m −2 y −1 in 1989 and 12 mol C m −2 y −1 in 1990. This was higher than the 1959–1963 annual average (6.8 mol C m −2 y −1) determined at Station “S” located approximately 50 km northwest of the BATS site. Fluxes associated with sinking of total particulate mass, particulate organic carbon (POC) and particulate organic nitrogen (PON) were measured at 150, 200, 300 and 400 m using a free-floating sediment trap array generally deployed once each cruise for 72 h. Fluxes varied seasonally, and within our ability to resolve differences (i.e. monthly sampling), there was no distinguishable time offset between peaks in ∫ PP and corresponding peaks in elemental flux. Fluxes generally decreased with increasing depth, and fluxes of POC and PON were positively correlated with particulate mass flux at all depths. POC/PON (C/N) ratios at 150 m during periods of high ∫ PP were generally characteristics of live planktonic biomass. Higher C/N ratios in material collected by the deeper traps were consistent with more rapid losses of PON than POC from sinking particles. POC and PON fluxes at 150 m, nominally the base of the euphotic zone, were positively correlated with ∫ PP. The fraction of ∫ PP leaving the euphotic zone in the form of sinking particles (i.e. collected in traps) varied seasonally and was inversely proportionato ∫ PP. Surface export of organic matter estimated by sediment traps at 150m was 0.78 mol C m −2y −1 (0.10 mol N m −2y −1) in 1989 and 0.77 mol C m −2y −1 (0.11 mol N m −2y −1) in 1990.

Vertical nitrogen flux from the oceanic photic zone by diel migrant zooplankton and nekton

Where the photic zone is a biological steady-state, the downward flux of organic material across the pycnocline to the interior of the ocean is thought to be balanced by upward turbulent flux of inorganic nitrogen across the nutricline. This model ignores a significant downward dissolved nitrogen flux caused by the diel vertical migration of interzonal zooplankton and nekton that feed in the photic zone at night and excrete nitrogenous compounds at depth by day. In the oligotrophic ocean this flux can be equivalent to the flux of particulate organic nitrogen from the photic zone in the form of faecal pellets and organic flocculates. Where nitrogen is the limiting plant nutrient, and the flux by diel migration of interzonal plankton is significant compared to other nitrogen exports from the photic zone, there must be an upward revision of previous estimates for the ratio of new to total primary production in the photic zone if a nutrient balance is to be maintained. This upward revision is of the order 5–100% depending on the oceanographic regime.

Primary production, new production and vertical flux in the eastern Pacific Ocean

The sinking of participate organic matter in the ocean links food webs beneath the euphotic zone to surface primary production and is an important pathway for the downward transport of many elements1–3. The flux of particulate organic carbon (POC) is also an important parameter in the global carbon cycle and may be related to long-term changes in atmospheric CO2 4,5. In 1980, Suess6 synthesized existing measurements from sediment trap studies into a model to predict the vertical flux of POC from depth (z) and primary production (PP)6. The Suess model has become the standard for evaluating vertical flux data7, for estimating the annual flux of POC in the ocean8 and for parameterizing ocean carbon cycle models4,5. We present here a new model of the vertical flux of POC and particulate organic nitrogen (PON) from a set of contemporaneous measurements of PP and fluxes made during the VERTEX (Vertical Transport and Exchange) programme in the north-east Pacific. The VERTEX model indicates that PP and vertical fluxes of POC and PON, in the oligotrophic ocean are greater than previously suggested. In addition, the vertical flux of PON from the photic zone represents a measure of the PP that is supported by new nitrogen (new production)9,10. In the north-east Pacific, new production ranged from 13 to 25% of primary production and was positively related to total PP.