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  • View in gallery

    Basic arrangement of components within the SUV-6 instrument in plan and side views.

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    Surface ADCP current vectors for the two SeaSoar surveys: (a) the first survey and (b) the targeted second survey, with dynamic height at 10 m overlaid. Current vectors reveal a southeasterly flowing jet (indicated by red arrows) with two counterrotating eddies on either side; cyclonic (CYC) and anticyclonic (AC). The green stars in (a) show the positions of four CTD casts representing noneddy conditions, referred to in section 4. The green line in (b) represents the dogleg section also referred to in section 4.

  • View in gallery

    (a) Following individual calibrations to surface data, combined SUV-6 data from all tows fitted a 1:1 relationship with the underway data (dashed line). (b) Surface calibrated SUV-6 data plotted against CTD nitrate data down to 450 m. (c) Calibration curve of SUV-6 nitrate to CTD bottle nitrate based on mean profiles. (d) SUV-6 data calibrated to surface and to depth plotted against CTD nitrate data down to 450 m.

  • View in gallery

    (a) Laboratory nitrate concentration against absorbance ratio of 220:280 nm. Data points are indicated by filled circles. Solid line indicates a quadratic polynomial fit to the data. (b) Residuals after new calibration showing an accuracy better than 0.1 μM.

  • View in gallery

    Contoured sections of (a) temperature, (b) SUV-6 nitrate, and (c) dissolved oxygen for the dogleg section carried out during the second SeaSoar survey with isopycnals overlaid. The x axis represents distance from the beginning of the section. The anticyclonic, cyclonic, and jet components are indicated. Profiles from bottle samples collected from four full-depth CTD casts during the first SeaSoar survey for (d) nitrate and (e) dissolved oxygen. These casts were made near the boundaries of the eddy feature and can be considered to best represent noneddy conditions. The same four casts are shown in (d) and (e) although oxygen samples were taken in only two of the four casts. The profiles show the nitrate and oxygen signatures of the anomalous patch of water described in the results (20 and 238 μM, respectively) to be consistent with water found at ∼800 m, indicating localized vertical transport of over 400 m to the present location.

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A Novel Integration of an Ultraviolet Nitrate Sensor On Board a Towed Vehicle for Mapping Open-Ocean Submesoscale Nitrate Variability

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  • 1 National Oceanography Centre, Southampton, Southampton, United Kingdom
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Abstract

Initial results from a deployment of the SUV-6 ultraviolet spectrophotometer, integrated with the SeaSoar towed vehicle, are presented. The innovative, combined system measures nitrate concentration at high spatial resolution (4 m vertically, 5 km horizontally), high sensitivity (0.2 μM), and concomitantly with temperature, salinity, and dissolved oxygen. The authors demonstrate that this approach constitutes a powerful new tool for quantifying the role of mesoscale and submesoscale vertical nutrient fluxes to the euphotic zone, using measurements from a high-resolution survey of an eddy dipole in the Iceland Basin during the summer of 2007.

Corresponding author address: Rosalind Pidcock, National Oceanography Centre, Southampton (NOCS), University of Southampton Waterfront Campus, European Way, Southampton, SO14 3ZH, United Kingdom. Email: remp103@noc.soton.ac.uk

Abstract

Initial results from a deployment of the SUV-6 ultraviolet spectrophotometer, integrated with the SeaSoar towed vehicle, are presented. The innovative, combined system measures nitrate concentration at high spatial resolution (4 m vertically, 5 km horizontally), high sensitivity (0.2 μM), and concomitantly with temperature, salinity, and dissolved oxygen. The authors demonstrate that this approach constitutes a powerful new tool for quantifying the role of mesoscale and submesoscale vertical nutrient fluxes to the euphotic zone, using measurements from a high-resolution survey of an eddy dipole in the Iceland Basin during the summer of 2007.

Corresponding author address: Rosalind Pidcock, National Oceanography Centre, Southampton (NOCS), University of Southampton Waterfront Campus, European Way, Southampton, SO14 3ZH, United Kingdom. Email: remp103@noc.soton.ac.uk

1. Introduction

Observational evidence that mesoscale (1–100 km) phenomena, such as eddies and fronts, transport nutrient-rich water from the depths into the upper sunlit layer of the ocean is well established (Allen et al. 2005; Angel and Fasham 1983; Falkowski et al. 1991; Franks et al. 1986; Jenkins 1988; McGillicuddy et al. 2003; Oschlies and Garcon 1998; Siegel et al. 1999; Thomas and Lee 2005; Woods 1988). Recent modeling studies indicate that filamentary scales (1–10 km—the submesoscale) may contribute as much as eddy scales (10–100 km) to annual nutrient supply (Lapeyre and Klein 2006; Levy 2008; Levy et al. 2001; Mahadevan and Archer 2000; Spall and Richards 2000). Despite its potential importance, the submesoscale contribution to upper-ocean nutrient supply remains poorly quantified by in situ observations because of difficulties in collecting simultaneous hydrographic and nitrate data at the necessary temporal and spatial scales.

The SeaSoar towed, undulating vehicle (Allen et al. 2002; Pollard 1986) is well proven for surveying mesoscale fronts and eddies (Allen and Smeed 1996; Allen et al. 2005; Garabato et al. 2001; Legal et al. 2007; Martin et al. 1998; Pollard and Regier 1992; Rudnick 1996). Undulating between the surface and ∼500-m depth, carrying a variety of sensors, SeaSoar provides high-frequency (1 Hz) near-instantaneous mapping of upper-ocean properties: typically temperature, salinity, pressure, chlorophyll-a fluorescence, and photosynthetically active radiation (PAR). The conventional method for measuring vertical profiles of nitrate concentration, by colorimetric analysis (Mee 1986) of discrete bottle samples, is impractical for the high-frequency sampling required at the mesoscale, much less at the submesoscale. This has prompted the development of alternative approaches.

The potential for ultraviolet (UV) spectroscopy to provide high-frequency (1 Hz) nitrate concentration measurements has been recognized for over 40 years (Ogura and Hanya 1966, 1967). Ultraviolet sensors, such as the in situ ultraviolet spectrophotometer (ISUS; Johnson and Coletti 2002), can achieve the sampling frequency required to characterize submesoscale biophysical phenomena. In practice the instrumental accuracy of ISUS (±1.8 μM for a single measurement) restricts its utility to coastal applications where nitrate concentrations are high (up to 30 μM) and gradients are strong. A twofold improvement in accuracy for a single measurement (to ±0.65 μM) compared to discrete samples has been achieved with improved data processing (Sakamoto et al. 2009), with an operational response time of 1.7 s.

Developed at the National Oceanography Centre, Southampton (NOCS) in association with Valeport Ltd., Totnes, (United Kingdom), the SUV-6 achieves an accuracy of ±0.2 μM for a single measurement, with a response time of 1 s (Finch et al. 1998). We present data from an integrated deployment of the SUV-6 with a SeaSoar vehicle during a survey of an eddy dipole. Our approach is the first to adequately resolve filamentary-scale nitrate variability within an open-ocean mesoscale feature with simultaneous hydrographic and oxygen measurements.

2. Methods

a. SUV-6 instrument design and operation

The arrangement of components within the SUV-6 is illustrated in Fig. 1. A technical overview is given in Table 1. For a more detailed description of the components, including choice of materials, refer to Finch et al. (1998). The SUV-6 employs an internal reference path to monitor changes in the lamp spectrum over time. The detection limit of the SUV-6 is defined as the mean of the smallest difference between absorbance values that can be resolved by the sensor and is a function of the signal-to-noise ratio. Laboratory trials indicate a detection limit of 0.2 μM (Finch et al. 1998).

The SUV-6 measures absorption at six wavelengths: 205, 220, 235, 250, 265, and 280 nm. Currently, nitrate concentration is determined from the ratio of the absorption at 220 and 280 nm, following Finch et al. (1998; note that the original method used the 300-nm channel rather than 280 nm). At 280-nm absorption of UV light due to salt and nitrate is minimal. By normalizing the absorbance at 220 nm to the 280-nm output, changes in lamp intensity and scattering due to particles and bubbles in the pathway are corrected.

The manipulation of the output ratios from different pairs of channels (multiwavelength analysis) may allow interference from other anions in seawater absorbing at similar wavelengths as nitrate (principally Cl and Br plus others at lower concentrations) to be minimized (Thomas and Gallot 1990). In the SUV-6, the 205-nm channel is sensitive to salinity and a simple correction can be applied to measurements of nitrate for salt content. In the present study, the maximum range in salinity at any depth is 0.08 psu. This is equivalent to a change in nitrate concentration of just 0.04 μM. This is below the limit of detection of the instrument and hence can be neglected. Chromophoric dissolved organic matter (CDOM) in seawater exhibits a broadband absorption between 150 and 400 nm. Observations suggest that in the deep open ocean CDOM absorption at 220 nm is negligible relative to that of nitrate (Ogura and Hanya 1966). In the Iceland Basin Johnson et al. (2007) show that the variability of dissolved organic carbon in the surface ocean is less than 5% of the deep concentration (∼50 μM). Thus, we expect dissolved organic matter to have little impact on our patterns of UV absorption at 220 nm.

b. SeaSoar deployments and SUV-6 data collection

Data were collected as part of a research cruise carried out between 24 July and 23 August 2007 by NOCS (Allen 2008). Two spatial surveys were carried out using the SeaSoar vehicle within a 130 km × 130 km box centered near Ocean Weather Station India (OWSI) at 60°N, 20°W. The SeaSoar system carried a Chelsea Technologies Group (CTG) Minipack CTDF instrument, an Anderaa oxygen optode, a Seabird SBE43 dissolved oxygen sensor, a Turner Designs chlorophyll-a sensor, a Plymouth Marine Laboratory PAR sensor, and the NOCS/Valeport SUV-6 UV nitrate sensor. Average ship speed during the surveys was 8.5 kt, corresponding to an average SeaSoar vertical descent/ascent rate of ∼1.3 m s−1 and a vertical resolution of 1.3 m. This tow speed and surface-to-450-m profiling gave a horizontal resolution of ∼3.5 km at the surface and ∼2 km at middepth. Both surveys comprised several SeaSoar deployments, each lasting 6–12 h. During each deployment, the photocurrent outputs from the SUV-6 instrument’s six channels were integrated, digitized, and output in real time to the external Penguin Linux data acquisition system on SeaSoar (Allen et al. 2002).

Satellite images for the northeast North Atlantic together with current vector data from the vessel-mounted acoustic Doppler current profilers (VM-ADCPs) revealed an eddy dipole within the survey area, comprising a southeastward-flowing jet with counterrotating eddies on either side. Figure 2 shows 30-min-averaged ADCP current vector data for the two SeaSoar surveys. Dynamic height at 10 m is overlaid to indicate the positions of the eddy cores. The first survey comprised nine closely spaced parallel tracks ∼14 km apart. In the second survey, the tracks were targeted to bisect each eddy core and the central jet as the dipole propagated through the survey area. A “dogleg” section, carried out as part of the targeted second survey, is indicated in Fig. 2b. This section best illustrates the impact of the dipole on the water column and is discussed in section 4.

3. Postcruise processing and calibration

The integration of the SUV-6 onto the SeaSoar vehicle made calibration to seawater of known nitrate concentration physically impractical between deployments, as mounting and dismounting the instrument is a complex and delicate task. This necessitated the initial use of calibration values derived using a standard least squares fit to laboratory measurements prior to an earlier deployment (Quartly et al. 2006). SUV-6-determined nitrate concentrations were then compared to simultaneous underway surface samples from the ship’s pumped seawater supply and discrete samples taken from CTD rosette water bottles during the survey period. The discrete concentrations were determined using a Skalar Sanplus segmented continuous flow colorimetric autoanalyzer (Kirkwood 1996).

While the initial calibration gave nitrate values in a reasonable range for the open ocean (0–20 μM) (Pidcock and Srokosz 2008), they needed adjustment to match discrete concentrations taken from deep CTD samples. Some stability issues regarding changes in instrument calibration between deployments were also highlighted in postprocessing. Both issues were rectified by calibration to the discrete samples, detailed below.

Scatterplots of surface (4–5 m) SUV-6 data against discrete underway bottle samples exhibited an upward step in calibration with each deployment (not shown). Therefore, an average offset between the SUV-6 measurements and the simultaneous discrete bottle samples was calculated and applied to the SUV-6 data for each individual deployment. Subsequently, all the SUV-6 data fitted a 1:1 relationship with the associated underway data (Fig. 3a).

Calibration of the SUV-6 data with surface samples involved only a small range of nitrate values (∼2–6 μM). It was therefore necessary to further calibrate against CTD bottle samples from full-depth casts. A comparison revealed SUV-6 nitrate concentrations 10%–30% lower than obtained with the CTD over the full-depth range (Fig. 3b). A considerable range in nitrate concentration at all depths (up to 5 μM) due to real variability in the data is demonstrated by both the SUV-6 and the CTD scatter. Although both datasets cover the same spatial region, they were not collected simultaneously and so cannot be compared on a point-by-point basis. Mean SUV-6 and CTD sample profiles of nitrate were therefore obtained using all the data available for each. The number of data points was increased by including the calibrated underway data from Fig. 3a. There are relatively few points in the midrange concentrations (5–9 μM) as these are within the sharp gradient of the nitracline. A cubic polynomial provided the optimal fit for the SUV-6 nitrate profile to the CTD sample data, as assessed by an f test. The surface data had already been calibrated, so the cubic function was forced through zero. The SUV-6 data were calibrated according to the following function (see Fig. 3c), where x is the surface-calibrated SUV-6 nitrate data:
i1520-0426-27-8-1410-eq1

Following calibration of the SUV-6 data, the fit to the CTD samples was much improved (Fig. 3d). Comparison of the mean CTD and calibrated SUV-6 nitrate profiles revealed a mean postcalibration accuracy of the SUV-6 of ±0.2 μM over the full-depth range. The full, calibrated SeaSoar dataset, including SUV-6 nitrate, was then interpolated onto a regular 5 km (horizontal) × 4 m (vertical) grid.

Following the successful deployment of the SUV-6 instrument described here, a detailed laboratory calibration has been conducted using sodium nitrate spiked seawater samples from near 0- to ∼50-μM nitrate concentration. Compared against the discrete colorimetric autoanalyzer-derived nitrate concentrations, the SUV-6 now demonstrates an accuracy of ±0.1 μM (Fig. 4). This calibration will be applied to future SUV-6 deployments but the data presented here has not been further corrected.

4. Results and discussion

Figure 5 shows temperature, SUV-6 nitrate and dissolved oxygen for the dogleg section highlighted in Fig. 2, with isopycnals overlaid and with the cyclonic, anticyclonic, and jet components of the dipole indicated. The section was composed of one single SeaSoar tow of ∼12-h duration. The x axis represents the distance from the beginning of the section. Raised isopycnals are seen in the top 200 m of both eddy cores (Fig. 5a). A section of deeper CTD casts (to 800 m) within the anticyclonic eddy (not shown) showed upward doming of isopycnals at the base of the seasonal thermocline and depression of isopycnals at the permanent thermocline, revealing it as a winter mode-water eddy.

Surface nitrate concentrations (2–6 μM) were nonlimiting (Fig. 5b). Considerable spatial variability in nitrate is observed at scales down to 5 km, associated with submicromolar changes in concentration. The maximum nitrate concentration (20 μM) is very localized, found 150 km from the start of the section, on the periphery of the cyclonic eddy at a depth of approximately 375 m (Fig. 5b). The oxygen signature of the patch (238 μM) is anomalously low compared to its surroundings (Fig. 5c). Bottle samples taken from four full-depth CTD casts on the edge of the survey area, most representative of noneddy conditions (positions marked on Fig. 2), show that nitrate and oxygen signatures of 20 and 238 μM, respectively, are consistent with water found at ∼800 m (Figs. 5d,e). This indicates localized upward transport of nitrate and oxygen of over 400 m to the present location. The position of the high-nitrate/low-oxygen patch on the periphery of an eddy is suggestive of filament related upwelling, associated with frontogenetic processes, as modeled by Lapeyre and Klein (2006). Those authors report a near doubling of annual eddy-driven nutrient supply (from 0.14 to 0.26 mol N m−2 yr−1) when these small-scale features are resolved (Lapeyre and Klein 2006).

5. Conclusions and future work

The resolution of such finescale spatial structure in nitrate concentrations illustrates the potential of our integrated SUV-6–SeaSoar approach. Vertical velocities and nitrate gradients can be combined at the necessary scales to estimate local nitrate fluxes. Previously, mean or “representative” nitrate profiles have been used. This improvement will allow more precise estimates of the contribution of eddy-driven processes to new production, the amount of primary production directly fueled by newly available nitrate in the euphotic zone. Current estimates vary by an order of magnitude (Falkowski et al. 1991; Lapeyre and Klein 2006; Levy et al. 2001; Mahadevan and Archer 2000; McGillicuddy and Robinson 1997; McGillicuddy et al. 2003; Oschlies 2002). Our technique provides a powerful tool for quantifying the role of the submesoscale in reducing this uncertainty and therefore in improving our understanding of global marine biogeochemistry.

Systematic laboratory testing of the SUV-6 has been ongoing since the work presented here in order to resolve the stability issues and to improve the calibration. Future work centers on miniaturizing the SUV-6 and developing the multichannel capability to ensure versatility for a broad range of uses and aquatic environments. This is part of a continuing collaborative effort within NOCS research and engineering departments.

Acknowledgments

This work was carried out by the National Oceanography Centre, Southampton (NOCS) as part of the Natural Environment Research Council (NERC) Oceans 2025 Strategic Marine Science Programme. R. Pidcock acknowledges NERC for her studentship NE/F00639X/1. The authors thank their colleagues and the RRS Discovery’s captain, officers, and crew for valuable discussions.

REFERENCES

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    • Search Google Scholar
    • Export Citation
  • Allen, J. T., , Cornell V. , , Moore C. M. , , Crisp N. , , and Dunning J. , 2002: Operational oceaography using the new SeaSoar undulator. Sea Technol., 43 (4) 3540.

    • Search Google Scholar
    • Export Citation
  • Allen, J. T., and Coauthors, 2005: Diatom carbon export enhanced by silicate upwelling in the Northeast Atlantic. Nature, 437 , 728732.

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    • Search Google Scholar
    • Export Citation
  • Falkowski, P. G., , Zieman D. , , Knap A. H. , , and Bienfang P. K. , 1991: Role of eddy pumping in enhancing primary production in the ocean. Nature, 352 (6330) 5558.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Finch, M. S., , Hydes D. J. , , Clayson C. H. , , Weigl B. , , Dakin J. P. , , and Gwilliam T. J. P. , 1998: A low power ultra violet nitrate sensor for use in seawater: Introduction, calibration and initial sea trials. Analytica Chim. Acta, 377 , 167177.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Franks, P. J. S., , Wroblewski J. S. , , and Flierl G. R. , 1986: Prediction of phytoplankton growth in response to the frictional decay of a warm-core ring. J. Geophys. Res., 91 , (C6). 76037610.

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  • Garabato, A. C. N., , Leach H. , , Allen J. T. , , Pollard R. T. , , and Strass V. H. , 2001: Mesoscale subduction at the Antarctic Polar Front driven by baroclinic instability. J. Phys. Oceanogr., 31 , 20872107.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Jenkins, W. J., 1988: Nitrate flux into the euphotic zone near Bermuda. Nature, 331 (6156) 521523.

  • Johnson, K. S., , and Coletti L. J. , 2002: In situ ultraviolet spectrophotometry for high resolution and long-term monitoring of nitrate, bromide and bisulfide in the ocean. Deep-Sea Res. I, 49 , 12911305.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Johnson, M., and Coauthors, 2007: Ammonium accumulation during a silicate-limited diatom bloom indicates the potential for ammonia emission events. Mar. Chem., 106 , 6375.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • Kirkwood, D., 1996: Nutrients: Practical notes on their determinations in seawater. ICES Techniques Mar. Environ. Sci., 17 , 2549.

  • Lapeyre, G., , and Klein P. , 2006: Impact of the small-scale elongated filaments on the oceanic vertical pump. J. Mar. Res., 64 , 835851.

  • Legal, C., , Klein P. , , Treguier A. , , and Paillet J. , 2007: Diagnosis of the vertical motions in a mesoscale stirring region. J. Phys. Oceanogr., 37 , 14131424.

    • Crossref
    • Search Google Scholar
    • Export Citation
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Fig. 1.
Fig. 1.

Basic arrangement of components within the SUV-6 instrument in plan and side views.

Citation: Journal of Atmospheric and Oceanic Technology 27, 8; 10.1175/2010JTECHO780.1

Fig. 2.
Fig. 2.

Surface ADCP current vectors for the two SeaSoar surveys: (a) the first survey and (b) the targeted second survey, with dynamic height at 10 m overlaid. Current vectors reveal a southeasterly flowing jet (indicated by red arrows) with two counterrotating eddies on either side; cyclonic (CYC) and anticyclonic (AC). The green stars in (a) show the positions of four CTD casts representing noneddy conditions, referred to in section 4. The green line in (b) represents the dogleg section also referred to in section 4.

Citation: Journal of Atmospheric and Oceanic Technology 27, 8; 10.1175/2010JTECHO780.1

Fig. 3.
Fig. 3.

(a) Following individual calibrations to surface data, combined SUV-6 data from all tows fitted a 1:1 relationship with the underway data (dashed line). (b) Surface calibrated SUV-6 data plotted against CTD nitrate data down to 450 m. (c) Calibration curve of SUV-6 nitrate to CTD bottle nitrate based on mean profiles. (d) SUV-6 data calibrated to surface and to depth plotted against CTD nitrate data down to 450 m.

Citation: Journal of Atmospheric and Oceanic Technology 27, 8; 10.1175/2010JTECHO780.1

Fig. 4.
Fig. 4.

(a) Laboratory nitrate concentration against absorbance ratio of 220:280 nm. Data points are indicated by filled circles. Solid line indicates a quadratic polynomial fit to the data. (b) Residuals after new calibration showing an accuracy better than 0.1 μM.

Citation: Journal of Atmospheric and Oceanic Technology 27, 8; 10.1175/2010JTECHO780.1

Fig. 5.
Fig. 5.

Contoured sections of (a) temperature, (b) SUV-6 nitrate, and (c) dissolved oxygen for the dogleg section carried out during the second SeaSoar survey with isopycnals overlaid. The x axis represents distance from the beginning of the section. The anticyclonic, cyclonic, and jet components are indicated. Profiles from bottle samples collected from four full-depth CTD casts during the first SeaSoar survey for (d) nitrate and (e) dissolved oxygen. These casts were made near the boundaries of the eddy feature and can be considered to best represent noneddy conditions. The same four casts are shown in (d) and (e) although oxygen samples were taken in only two of the four casts. The profiles show the nitrate and oxygen signatures of the anomalous patch of water described in the results (20 and 238 μM, respectively) to be consistent with water found at ∼800 m, indicating localized vertical transport of over 400 m to the present location.

Citation: Journal of Atmospheric and Oceanic Technology 27, 8; 10.1175/2010JTECHO780.1

Table 1.

Technical summary of the SUV-6.

Table 1.
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