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A New In Situ Chemical Analyzer for Mapping Coastal Nutrient Distributions in Real Time

Alfred K. Hanson

Abstract- A rapid-response, submersible chemical analyzer has been developed that is particularly suited for mapping nutrient distributions in 3-D space and time and for testing hydrodynamic models for chemical transport, within rivers, estuaries, coastal and off shore waters. The SubChemPak Analyzer™ is a novel 3-channel reagent delivery module that transforms underwater optical instruments into sensitive chemical analyzers for rapid measurements of nutrients and other environmentally important chemicals. A family of multi-nutrient analyzers is being developed for selected nutrients: nitrate, nitrite, ammonia, urea, phosphate, silicate, and iron. Continuous flow spectrophotometric and flourometric methodologies have been optimized for rapid in situ measurements of the nutrients. The operation, in situ calibration and data acquisition for the instrument are remotely controlled by a computer. The concentration readings for nutrients are instantaneously displayed on the computer monitor. The SubChemPak Analyzer may be co-deployed with standard oceanographic electronic sensor packages (CTD) for vertical and/or horizontal (to-yo) profiling.

A SubChemPak Analyzer was configured for the simultaneous determination of dissolved nitrite and iron (II) and co-deployed with a Sea Bird Electronics CTD system in the waters of Narragansett Bay during December 1999. The CTD system included modular sensors for the measurement of conductivity, temperature, pressure, dissolved oxygen, pH, chlorophyll fluorescence, and light transmission and irradiance. The results from laboratory and field tests in Narragansett Bay demonstrated that the SubChemPak Analyzer is capable of producing high-resolution vertical nutrient profiles in real time. The stable optical detection system and in situ calibration feature enabled accurate nutrient determinations at trace concentration levels (nanomolar to micromolar). The combination of this sensitive real-time chemical profiling technique with concurrent acoustic doppler current velocity and direction measurements will improve our ability to detect and track chemical plumes in coastal waters.

I. INTRODUCTION

     Characterization of the marine distributions of nutrients is critically important because, depending upon their concentrations, these biologically essential chemicals may greatly enhance or limit the growth of microscopic plants in aquatic waters. Natural and man-made environmental events can lead to dramatic changes in nutrient concentrations in aquatic waters, both in time and space. Such episodic changes in nutrient concentrations can drastically influence algal growth rates leading to troublesome eutrophication of lakes and estuaries, harmful algal blooms, anoxia and fishery problems [1,2]. There has been steady progress since the '70's in the development of analytical methods [3] and multi-nutrient autoanalyzer systems for both off-site and on-site determination of nutrients on collected water samples. However it is now apparent that there are significant practical limitations to the temporal and spatial resolution that can be obtained for nutrient measurements in aquatic waters using traditional water sampling and bench-top autoanalyzer technologies. One way to overcome these sampling limitations is in situ chemical analysis [4, 5, 6].

During 1995 a prototype dual-nutrient, submersible chemical analyzer was designed, developed and successfully tested in coastal waters [6]. The instrumentation was developed for the NAVY to demonstrate the existence micro- to fine-scale chemical gradients, associated with thin plankton layers, in stratified coastal waters. The prototype analyzer simultaneously determined two nutrients, dissolved nitrite and iron(II), at nanomolar concentration levels, in real-time and with submeter-scale depth resolution. Subsequent laboratory investigations demonstrated that additional nutrients of interest (nitrate, ammonium and phosphate) could also be determined accurately with the prototype analyzer, after kinetic optimization of the analytical methodologies [7]. In this paper we describe the features of a second-generation prototype, the SubChemPak Analyzer.  We also present some analytical results that were obtained while deploying the new instrument package in the estuarine waters of Narragansett Bay, RI.

II. METHODOLOGY

A.     Description of the Second Generation Prototype

The SubChemPak Analyzer (Fig. 1) is a 3-channel reagent delivery module that transforms underwater optical instruments into sensitive chemical analyzers for rapid measurements of nutrients and other environmentally important chemicals. Continuous flow spectrophotometric and flourometric methodologies have been optimized for rapid in situ determinations of several dissolved nutrients. 

   

         

[a] SubChemPak Analyzer   [b] WET Labs detectors

Fig. 1. [a] The SubChemPak Analyzer, a remote controlled 3-channel reagent delivery module, with pressure housing, connecting cable and deck-box interface. [b] WET Labs, Inc. optical detectors; the A-Star single channel absorption meter and the FlashPak fluorometer.

The submersible components of the SubChemPak Analyzer system are the reagent delivery module and the electro-optical detectors. The reagent delivery module is comprised of a cylindrical black acetyl pressure housing (14 cm o.d. x 60 cm length) that contains the electro-fluidic, data acquisition, instrument control and power regulation systems. These submersible components are rated for depths up to 200 meters. A single underwater cable connects the SubChemPak Analyzer with a deck box that provides DC power to the submerged instrumentation and a data communications interface (serial cable or telemetry) to the computer. The SubChemPak can be readily configured for the determination of different nutrients by changing the reagents, standards and the modular electro-optical detectors.

1) Reagent delivery module: The primary components of the electro-fluidic flow system for the SubChemPak Analyzer are shown schematically in Fig 2. The components of the include a micro gear pump, flow rate and temperature sensors, a flexible tube heater, miniature solenoid pumps for reagents and calibration standards, reagent bags and a reaction manifold, reaction coil and debubbler. The three measurement channels are internally configured to be either analytical (reagents added) or reference (no reagents added).

The reagents and calibration standards are contained in small plastic bags located outside of the main pressure housing. They are usually placed in a separate cylindrical housing that is exposed to ambient underwater pressures. These bags are supplied pre-sterilized to help eliminate biofouling of reagents and nutrient standards. Two to four reagent bags are typically required for deployment; one to three 500 ml bags for the analytical reagents and one 250 ml bag for the calibration standard. The seawater and reagent flowrates are 20 ml/min/channel and 1 ml/min/channel, respectively. At these reagent volumes and flowrates the instrument can be operated continuously  for ~8 hours without replenishing the reagents. A five point calibration by the method of standard additions can be remotely initiated, while submerged, by incrementally changing the flowrate of the standard solution from 0 to 5 ml/min.

2) Electro-optical detectors: Two modular optical detection systems, spectrophotometric and fluorometric, have been developed by WET Labs, Inc. and tested with the SubChemPak Analyzer (Fig. 1b). One optical detector, the A-Star, is a submersible absorption meter with an optical flow cell (3 mm I.D. and 25 cm length). The second optical detector, the FlashPak, is a sensitive multi-purpose, submersible filter fluorometer. The SubChemPak supplies regulated power for, and acquires analog signals (0-5 vdc) from, up to three optical detectors. Optical detector configurations are listed for different analytical methodologies in Table I.

3) Remote Data acquisition and instrument control: The data acquisition and control (DAC) technology for the SubChemPak Analyzer involves a remote windows PC, and the DAC software and hardware. National Instruments software, Lab VIEW, and compatible data acquisition hardware have been adapted for this application. A stand-alone executable has been developed with a graphical user interface that provides user-friendly remote-control of all analyzer functions.  The DAC hardware includes state-of-the-art chips for analog-digital signal conversion and two-way serial communication via a multi-drop RS 485 network. The multi-channel data acquisition rate is one reading per second.

Fig. 2.  A schematic drawing of the electro-fluidic components of the SubChemPak Analyzer.


TABLE I. Analytical Methodologies for the SubChemPak Analyzer

 

Nutrient                  Detector      Wavelength          Reference

Nitrite                     A-Star            520 nm                [3,6]

Nitrate                    A-Star            520 nm                [3,6]

Urea                       A-Star            520 nm                [8]

Iron                        A-Star            560 nm                [6,9,10]

Phosphate              A-Star             800 nm                [3]

Silicate                   A-Star             800 nm                [3]

Ammonia               FlashPak          370/470 nm         [11]

     4) Remote power supply: The power requirements for the SubChemPak Analyzer are remotely supplied by an AC to DC switching power supply that is located in the deck box. The voltage and power outputs of the main power supply and the length and gauge of the conducting wires are designed to account for any voltage drop due to resistance losses over the long underwater cables. Miniature DC-DC converters convert the unregulated underwater DC supply into stabilized power (12 and 24 vdc) within the SubChemPak. The maximum total power requirements of the SubChemPak, are approximately 150 watts. The flexible tube heater is the major power consuming component. This device is used to heat the flowing seawater (typically to 30 C), in order to increase the rates of the color or fluorescence development reactions. In warmer waters, less power is used for this task than in cooler waters. The flexible tube heater is a requirement for profiled or towed applications that require rapid response times (seconds).

A.     Submerged Tests in Tanks and Narragansett Bay

     After bench top testing, evaluation and refinement, a SubChemPak Analyzer was installed on a winch-deployable, electronic profiling package. The profiling package also included a Sea Bird Electronics Model 25 Sea Logger CTD with modular sensors for the measurement of conductivity, temperature, pressure, dissolved oxygen, pH, chlorophyll fluorescence, and light transmission and irradiance. A 30-meter long umbilical was assembled that included two sea-cables and a plastic tube for water sample collection by a pump-to-surface technique. Two notebook computers were used to control the operation of the submerged instruments and acquire and display the real-time data stream from the CTD sensors and the SubChemPak Analyzer.

The integrated SubChemPak Analyzer and CTD system were then tested in a series of submerged experiments in out door test tanks filled with Narragansett Bay seawater (5 meters deep). On December 20, 1999 the CTD and chemical profiling system was deployed in Narragansett Bay (Fig. 3) aboard a URI Ocean Engineering research vessel, the CT-1. For these tank and field tests the SubChemPak Analyzer was configured for the simultaneous determination of the nutrients, dissolved nitrite and iron(II).

 

Fig. 3. Map showing station locations in Narragansett Bay for the SubChemPak Analyzer deployments.

Three A-Star detectors (520 nm) were used for the  chemical measurements. One A-Star detector was used to determine Nitrite by a spectrophotometric method that is based upon the formation of a colored azo dye. Nitrite reacts with sulfanilamide to form a diazonium ion that is subsequently coupled with N-(1-napthyl)-ethylenediamine dihydrochloride (molar absorbtivity = ~46,000 at 532 nm) [3]. A second A-Star detector (520 nm) was used to determine iron(II) using the classical Ferrozine methodology (molar absorbtivity = ~26,500 at 560 nm, somewhat less at 520 nm) [6,9]. The third A-Star detector was used as a reference detector (no reagents added or color formation) to correct for any background absorption due to the presence of colored dissolved organic matter (CDOM) in the seawater.

During the December cruise in Narragansett Bay, four vertical casts were completed using the SubChemPak Analyzer and CTD system near three station locations (Stations 6, 7, and 8 as shown on Fig. 3) while drifting with surface currents. The package descent rates were 2-3 m/min. Vertical profiles of current velocity and direction (RDI ADCP) were also recorded during the casts.

III. RESULTS

A.     Instrument Internal Calibration – In situ

One of the early goals of the submerged tank and field tests was to evaluate the accuracy and precision of the internal calibration while the instrument is submerged. The results of several calibration experiments are compiled and shown in Fig. 4. The calibration data (Fig. 4) includes results from a six point in situ calibration run, conducted at 9.5 m depth at station 7. The calibration coefficients obtained for nitrite and iron(II) at 9.5 m depth, in the cold (~6 C) bottom waters of the Bay, were statistically indistinguishable from those obtained in the lab and other submerged environments. The detection limits for nitrite and iron(II) were estimated to be 1.0 nM and 2.0 nM, respectively.

  Fig. 4. Summary comparison of SubChemPak Analyzer in situ calibration curves for dissolved nitrite and iron(II) from laboratory, test tanks and Narragansett Bay.

B.     Narragansett Bay Profiles

The vertical profiles obtained at Stations 6, 7 and 8 are shown in Figs. 5-8 for some of the parameters that were recorded during the SubChemPak Analyzer deployments. All parameters were measured simultaneously, and visualized in real-time during the deployment. All data was binned at 1 sample per second. The vertical cast at station 6 in the Providence River (Fig. 5) was taken at approximately 12:17 PM local time, during slack low tide. Water velocities at that station were appropriately low, below 10 cm/s, and spanned a range of directions.  The CTD data indicated a stratified water column with cooler, lower salinity water overlying denser waters that were warmer and saltier. An intense plume of phytoplankton (see chlorophyll data) was embedded in the water column from 1 to 4 meters depth.

These eutrophic waters also exhibited higher oxygen levels and a distinct pH gradient to higher levels with depth. Dissolved nitrite generally decreased with depth with a sharper gradient in deeper waters. Surprisingly high and variable levels of dissolved iron(II) (100-500 nM) were present in the water column at this station. We hypothesize that this anomalous plume of iron(II), is a thermodynamically unstable or transient signal, emanated from a sewage treatment plant outflow located near the sampling site. This chemical plume containing iron(II) was not apparent at Stations 7 or 8 (see below). Similar plumes of iron(II) were also observed during our earlier work in East Sound, WA [6].

   The vertical cast at station 7, near the mouth of the Providence River, (Fig. 6) was taken at approximately 1:13 PM local time, at the beginning of the incoming tide. The CTD data indicated a shallow layer of cool, lower salinity water (0-2 m) overlying denser waters (2-8 m) that were warmer and saltier. The shallow surface layer had elevated chlorophyll pigment levels, higher oxygen and lower pH levels, than the bottom layers. Dissolved nitrite decreased with depth and iron (II) increased with depth in the water column. Water velocities (not shown) were quite low in the top four meters, but increased to approximately 18 cm/s in the lower water column, at an average direction of 315 degrees.

Fig. 5. Vertical profiles obtained at Station 6 located in the Providence River.

Fig. 6. Vertical profiles obtained at Station 7 located in the Providence River

Fig. 7. Vertical profiles obtained for the first cast near Station 8 in the upper bay area (cast 8a). Note elevated nitrite and lower iron(II) near bottom.

Fig. 8. Vertical profiles obtained for the second cast near Station 8 in the upper bay area (cast 8b). Note scale change for nitrite and iron.

      The in situ trace chemical profiles for nitrite and iron(II) (Fig. 7b) exhibited a sharp discontinuity near 5.5 meters depth that is attributed to current shear associated with incoming tidal currents in deeper waters (northerly velocity component). Two consecutive vertical casts were taken near mid-bay station 8 (8a and 8b) while the ship was drifting into shallower water with the wind and surface currents (Figs. 7 and 8). Cast 8a was taken at approximately 2 PM local time, during the incoming tide, and cast 8b was about 10 minutes later. Northerly current velocities were quite high (15 to 30 cm/s) throughout the water column due to the flooding tide. The CTD data for cast 8a (Fig 7) indicated some variability in salinity, temperature and density, but within a narrower range of values than the earlier casts. The chlorophyll pigment levels and pH were fairly uniform at all depths. The elevated oxygen levels above four meters are indicative of photosynthetic activity. Dissolved nitrite and iron(II) concentrations were relatively uniform in the upper five meters of the water column for cast 8a. It is interesting to note that the in situ trace chemical profiles for nitrite and iron (II) exhibited a sharp discontinuity near 6 meters depth that is similar to that observed at 5.5 m for station 7 (Fig. 6). This near-bottom plume of water with elevated nitrite and lower iron(II) levels is tentatively attributed to the interaction of bottom currents (incoming tide) with surficial sediments. The property profiles for cast 8b (Fig. 8) were similar to those observed for the upper waters of cast 8a (Fig, 7). The dissolved nitrite and iron(II) profiles had considerable micro- to fine-scale structure and alternating maxima and minima.

IV. DISCUSSION

 A. Nutrient Determinations in Marine Waters

There has been steady progress in the development of water sample collection and preservation techniques, and bench-top analytical methods and instrumentation, for the determination of nutrients in aquatic waters. Water samples are generally collected by tripping water samplers at discrete depths during hydro or CTD-rosette casts or by sampling seawater pumped from depth. Collected water samples are often filtered and frozen to preserve them for future nutrient determinations. Well characterized spectrophotometric and fluorometric analytical methodologies have been established for the determination of nutrients in water samples [3]. Optical instrumental techniques have also been developed for important micronutrients like iron [10,12]. Several multi-nutrient autoanalyzer systems, that apply segmented (air-bubble) continuous flow analysis, are commercially available and routinely used for off-site determinations of nutrients on collected water samples. Autoanalyzer systems have also been used on-site, either in portable field laboratories or on-board coastal and off-shore research vessels, in an effort to obtain faster and more reliable nutrient data. On-site nutrient measurements are often conducted continuously, or semi-continuously, by connecting the autoanalyzer to a flowing sample stream. Such "on-line" systems allow for continuous vertical profiling [13, 14] or horizontal surface mapping [15, 16] of chemical distributions. It is well known that on-site field measurements can offer cost and time advantages over sample analyses by off-site laboratories.

In spite of this notable progress, there are significant practical limitations to the temporal and spatial resolution that can be obtained for nutrient measurements in aquatic waters using these established sampling and autoanalyzer technologies. Although off-site measurements by university, state, federal and private laboratories persist as the primary method for many environmental chemical analyses, now there is a need for enhanced on-site measurement capabilities and, particularly for chemical sensors and analyzers that operate in situ [4, 5, 6,17].

B. In Situ Nutrient Analyzers for Environmental Monitoring

 In situ sensors for physical (salinity, temperature), bio-optical (radiometers, chlorophyll fluorometers, light transmissometers) and some chemical parameters (oxygen, pH) have been available for many years to the marine research and environmental monitoring community. These sensors have become so reliable that it is now a rare occurrence to have water samples collected for salinity or oxygen measurements. Fortunately, our evolving environmental monitoring and oceanographic research needs for in situ chemical measurements has coincided with considerable technological advancement in our ability to construct submersible chemical analyzers and sensors that may operate in remote environments. Advances in flow-injection and continuous flow analysis techniques, osmotic and electro-osmotic pumps, fiber-optics technology, electrochemical sensors and biosensors offer exciting opportunities for the development of submersible instrumentation to monitor most chemical constituents of interest. There has been considerable progress during the past ten years in the development of submersible chemical analyzers for either stationary monitoring or profiling for selected chemical constituents [4, 5, 6, 16, 18,19, 20, 21].

Two profiling chemical analyzers for nutrients have been  developed and described in the scientific literature. Johnson et al. [18] were the first to successfully develop a submersible chemical analyzer (the dual channel "scanner") by applying continuous flow analysis, without air-bubble segmentation. The "scanner" system was successfully applied to obtain continuous profiles for nitrate, sulfide, iron, manganese and hydrogen peroxide in marine waters [16,18,19]. A similar approach has also used [21] to construct a 3 channel instrument, the "in situ-CFA", with a demonstrated applicability for nitrate profiling. 

 The commercial  development of submersible chemical sensors and analyzers has been economically hindered within the United States [17] limiting their availability for use by our environmental and oceanographic research communities. Two in situ nutrient analyzers for nitrate and phosphate are commercially available in Europe for ‘longer-term’ stationary monitoring of nutrient levels in marine waters: 1) W.S Ocean Systems “Nutrient Monitor” and 2) Chelsea Instruments “AquaSensor”.  Since several minutes (or more) are required per analysis, these instruments are presently not designed or suited for determining  nutrient concentrations at the high data rates required for vertical profiling and 3-D mapping of nutrient distributions.

A.     The SubChemPak Analyzer in Narragansett Bay

 The field deployments in the Providence River and Narragansett Bay demonstrated the unique capability of the SubChemPak Analyzer to profile trace chemical concentrations, in real time, and continuously (1 reading per second) while submerged. The vertical depth resolution was estimated to be 20 cm at a 3 m/min descent rate. The instrument operated in a consistent manner at room temperatures (20 C), submerged in the cold waters (6-9 C) of Narragansett Bay, and when subjected to natural gradients of salinity, temperature, biology and chemicals. The experimental results demonstrate that the in situ calibration feature works very well, and that the SubChemPak Analyzer is capable of producing accurate and reproducible analytical results while submerged.

The high-resolution vertical profiles obtained for nitrite and iron(II) in the stratified waters of Narragansett Bay exhibited micro- to fine-scale chemical gradients. These observations provide further supporting evidence for the fine-scale chemical gradients and thin layers that were detected in  East Sound, WA during an earlier investigation [6]. Continuous vertical profiling with an in situ chemical analyzer is the only way to document such dynamic chemical variability. These chemical gradients can not be detected by the placement of in situ chemical analyzers (stationary monitors) at selected fixed depths within the estuary.

V. CONCLUSIONS

The results from laboratory and field tests in Narragansett Bay demonstrate that the SubChemPak Analyzer is capable of producing high-resolution vertical nutrient profiles in real time. The stable optical detection system, fast data acquisition rates and in situ calibration feature enabled accurate nutrient determinations at trace concentration levels (nanomolar to micromolar). The instrument can be readily integrated with a winch-deployable oceanographic profiling package that includes a CTD system components and auxiliary sensors. The combination of this sensitive, rapid, real-time chemical profiling technique with concurrent acoustic doppler current velocity and direction measurements will improve our ability to detect and track chemical plumes in coastal waters.

ACKNOWLEDGMENTS

Casey Moore, Eugene Morin and Percy Donaghay provided invaluable assistance with engineering and instrumentation. The assistance of John King,, Chris Kincaid, Beth Lacey-Laliberte, William Deleo and Captain Fred Pease with the field deployments is also greatly appreciated. This work was supported by funds provided by the RI-Economic Policy Commission and the URI-Ocean Technology Center, the NOAA/UNH  Cooperative Institute for Coastal and Estuarine Environmental Technology, the Office of Naval Research and the National Ocean Partnership Program.

REFERENCES

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[9]     L.L. Stookey, ‘Ferrozine - A new spectrophotometric reagent for iron”, Analytical Chemistry 42(7) 779-781, 1970.

 

[10]  D.W. O'Sullivan, A.K. Hanson, Jr. and D.R. Kester, “The distribution and redox chemistry of iron in the Pettaquamscutt Estuary”. Estuarine , Coastal and Shelf Science, 45:769-788, 1998.

 

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[12]  D.W. O’Sullivan, A.K. Hanson, Jr. and D.R. Kester, “Stopped flow luminol chemiluminescence determination of Fe(II) and reducible iron in seawater at subnanomolar levels” Marine Chemistry, 49:65-77, 1995.

 

[13]  J.J. Anderson and A. Okubo, “Resolution of chemical properties with a vertical profiling pump”, Deep-Sea Research, 29(8A): 1013-1019, 1982.

 

[14]  L.A. Codispotti, G.E. Friederich, J.W. Murray and C.M. Sakamoto, “Chemical variability in the Black Sea: implications of continuous vertical profiles that penetrated the oxic/anoxic interface”, Deep-Sea Research, 38:2, S691-S710, 1991.

 

[15]  F.P. Chavez, H.W. Jannasch, K.S. Johnson, C.M. Sakamoto, G.E. Friederich, G.D. Thurmond, R.A. Herlien and L.A. Codispotti, “The MBARI Program for obtaining real-time measurements in Monterey Bay”. IEEE, 327-333, 1992.

 

[16]  K.H. Coale, C.S. Chin, G.J. Massouth, K.S. Johnson and E.T. Baker. “In situ chemical mapping of dissolved iron and manganese in hydrothermal plumes”, Nature 352:325-328, 1991.

 

[17]  MarChem, MarChem 93: “The Proceedings of a Workshop on Marine Chemistry Instrumentation”, S. J. Martin, Ed., Martin Laboratories, 1993.

 

[18]  K.S. Johnson, C.L. Beehler, and C.M. Sakamoto-Arnold. 1986a. “A submersible flow analysis system”, Analytica Chimica Acta. 179:245-257, 1986.

 

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