Abstract:
An apparatus and method for analyzing liquid samples employs first and second bidirectional ports that permit liquids to be introduced to first and second liquid waveguide capillary cells at the same time. The direction of liquid flow through the apparatus and roles of components may be reversed to prevent or overcome obstruction by particulates and other debris. The apparatus and method can detect and map colored dissolved organic matter and can detect, discriminate and map algal species or algal blooms including red tide in an automated, unattended manner within large aquatic systems.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to Provisional Application Ser. No. 60/638,789, filed Dec. 22, 2004 and entitled “Optical Phytoplankton Discriminator”, the disclosure of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates to the detection of algal species and colored dissolved organic matter in water, to the identification of such algal species, and to the detection and measurement of algal blooms.  
       BACKGROUND  
       [0003]     Blooms of the toxic dinoflagellate,  Karenia brevis , are a frequent occurrence off the coast of Florida in the Gulf of Mexico. Although such blooms generally develop and terminate offshore, wind and currents can concentrate assemblages in nearshore waters, creating a condition referred to locally and elsewhere as “red tide”. Harmful algal blooms pose a threat and efforts to reduce or eliminate their negative impacts and consequences are necessary. In the Gulf of Mexico, toxic blooms of K brevis regularly lead to untimely restrictions on commercial and recreational shellfish harvesting and deleterious effects on tourism and public health. Toxic blooms of  K. brevis  are generally detected by visual confirmation (e.g., water discoloration and fish kills), illness to shellfish consumers, or human respiratory irritation with actual toxicity verified through time-consuming chemical analyses for brevetoxins within shellfish samples and mouse bioassays. Currently, biological monitoring programs primarily rely on microscope-based cell enumeration and gross measurements of a biochemical parameter, such as chlorophyll-a concentration, for characterizing algal species and estimating biomass. Existing methods can be slow, labor intensive, intermittent, and do not always provide timely or accurate measurements. Developing optical detection methods for discriminating particular phytoplankton species in mixed phytoplankton assemblages has long been a goal of aquatic scientists. Phytoplankton pigment and light absorbance analyses could be incorporated into coastal monitoring programs and thus provide an effective and alternative means for monitoring  K. brevis . Mitigation of some of the harmful effects resulting from toxic blooms of  K. brevis  could be achieved by early detection of this species in mixed phytoplankton communities.  
         [0004]     The majority of organic carbon in the oceans is present as dissolved organic matter (“DOM”). A significant proportion of DOM has color (abbreviated “CDOM”), and is often present in concentrations sufficient to affect the color of lakes, estuaries, and near-shore coastal water. Due to the presence of CDOM in waters where phytoplankton assemblages occur, baseline CDOM measurements of water in the area are necessary. Additionally, mapping the distribution of CDOM including its time-space variability is central to understanding global carbon cycles. Characterizing CDOM variability is problematic because standard spectrometric methods for determining CDOM concentration are laborious and susceptible to methodological biases. For example, the collection and storage of discrete water samples for analysis in the laboratory often introduces artifacts such as contamination and photolysis. Also, the numerous steps required to conduct these analyses limit the number of samples that can be collected and effectively analyzed.  
         [0005]     The use of liquid waveguide capillary cell (“LWCC”) technology for CDOM absorption measurements has been described in E. J. D&#39;Sa et al., “Determining optical absorption of colored dissolved organic matter in seawater with a liquid capillary waveguide”, Limnol. Oceanogr. 44, 1142-1148 (1999). A near-continuous automated LWCC analytical technique for mapping CDOM distribution in an aquatic system is described in Kirkpatrick et al., “Continuous hyperspectral absorption measurements of colored dissolved organic material in aquatic systems”, Applied Optics, 42, 33, 6564-68 (20 Nov. 2003). Devices employing an LWCC are also described in U.S. Pat. Nos. 6,020,207 and 6,603,556 B2.  
       SUMMARY OF THE INVENTION  
       [0006]     During intense algal blooms, LWCC analytical devices and their associated components may require frequent cleaning. Instrument drift and local water changes may also necessitate sensor recalibration or more frequent reference spectrum collection. The above-mentioned Kirkpatrick et al. paper describes several measures to deal with these problems, including the use of a prefilter and a high water flow rate across the prefilter inlet with the majority of filtrate going to waste rather than through the instrument, and the use of two LWCCs operated on an alternating schedule to reduce gaps in the data record caused by cleaning and new reference spectrum collection.  
         [0007]     The present invention provides an improved apparatus and detection method for measuring algal blooms in liquids (e.g., in seawater or fresh water) and for discriminating harmful algal species in such blooms. In one aspect the invention provides an apparatus for analyzing liquid samples, the apparatus comprising: 
        (a) first and second bidirectional ports for providing liquid samples to the apparatus and discharging liquid samples from the apparatus,     (b) first and second liquid waveguide capillary cells in fluid communication with the first and second ports and arranged to analyze different liquid samples at the same time,     (c) at least one light source and at least one spectrometer in optical communication with the first and second cells, and     (d) a bidirectional pump or control arranged to flow samples in alternate directions through the first and second ports.        
 
         [0012]     In a preferred embodiment of the disclosed apparatus the first and second ports are each in fluid communication with one liquid waveguide capillary cell and in filtered fluid communication with another liquid waveguide capillary cell. In additional embodiments the cells are interchangeable, are arranged in series, or are arranged in parallel. In another preferred embodiment the disclosed apparatus can carry out continuous sample analysis.  
         [0013]     In another aspect the invention provides a method for analyzing liquid samples, the method comprising: 
        (a) obtaining one or more liquid samples through a first bidirectional port,     (b) flowing the sample(s) into two or more liquid waveguide capillary cells,     (c) spectroscopically analyzing the contents of the cells,     (d) discharging the sample(s) through a second bidirectional port,     (e) reversing the sample flow direction through the first and second ports, and     (f) repeating steps (b) and (c) using such reversed sample flow.        
 
         [0020]     In further embodiments of the disclosed method the first and second ports are each in fluid communication with one liquid waveguide capillary cell and in filtered fluid communication with another liquid waveguide capillary cell; or the cells are interchangeable, or are arranged in series, or are arranged in parallel; or the samples are analyzed continuously.  
         [0021]     The disclosed bidirectional ports facilitate automatic cleansing of debris that might otherwise block the flow of liquid through the disclosed apparatus. The disclosed apparatus and method may provide more accurate, more frequent or more reliable detection, mapping or tracking of algal blooms, algal species and CDOM characteristics, including time and space variability, in a real-time or unattended manner. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]     The present invention will be better understood by way of the specific embodiments shown in the following figures, the purpose of which is to illustrate the invention rather than to limit its scope:  
         [0023]      FIG. 1  is a schematic view of an Optical Phytoplankton Discriminator apparatus with parallel-connected LWCCs;  
         [0024]      FIG. 2  is a perspective view of a filter that may be used in the  FIG. 1  apparatus;  
         [0025]      FIG. 3  is a flow chart illustrating operation of the  FIG. 1  apparatus;  
         [0026]      FIG. 4  is a cross-sectional view of a bidirectional port;  
         [0027]      FIG. 5  is a plan sectional view of the  FIG. 4  port showing water flowing past the port;  
         [0028]      FIG. 6  is a schematic view of an Optical Phytoplankton Discriminator apparatus with series-connected LWCCs; and  
         [0029]      FIG. 7  is a schematic view of an Optical Phytoplankton Discriminator mounted on a mooring or buoy. 
     
    
       [0030]     Like reference symbols in the various figures of the drawing indicate like elements. The elements in the drawing are not to scale.  
       DETAILED DESCRIPTION  
       [0031]     Referring to  FIG. 1 , Optical Phytoplankton Discriminator apparatus  10  includes instrument housing  12  containing most of the components in the apparatus. Several components that may need more frequent cleaning or replacement are located outside housing  12 . Liquids enter and leave apparatus  10  via first bidirectional port  14  and second bidirectional port  15 . Port  14  is connected via suitable diameter tubing (e.g., 0.5 mm inside diameter tubing) and a suitable tubing wye to fittings  16   a  and  16   b  of dual channel bidirectional gear pump  18  and thence through fittings  22   a  and  22   b  to the remaining liquid circuits in apparatus  10 . Pump  18  contains three planetary mesh gears (not shown in  FIG. 1 ) arranged so that a central drive gear drives a first orbital gear that moves liquid in a first pump channel between fitting  16   a  and fitting  22   a , and drives a second orbital gear that moves liquid (flowing in the opposite direction through pump  18 ) in a second pump channel between fitting  16   b  and fitting  22   b . Pump  18  is magnetically coupled to drive motor  24  located inside housing  10 . Motor  24  can drive pump  18  in either direction so that ports  15  and  16  respectively serve as liquid inlet and outlet or as liquid outlet and inlet for apparatus  10 . When the liquid flow direction is such that port  14  is serving as an inlet, liquid exits pump  18  via fittings  22   a  and  22   b . Fitting  22   a  is connected via tubing to LWCC  40  located inside light-proof housing  44 . Fitting  22   b  is connected via tubing to fitting  24  of filter  26 .  
         [0032]     Filter  26  (which may for example be a MICROKROS™ cross-flow filter module from Spectrum Laboratories, Inc.) is shown in perspective view in  FIG. 2 . Liquid can flow between fittings  24  and  26  through hollow fibers  27  disposed in housing  29 . Fibers  27  have porous sidewalls through which filtered liquid (filtrate) passes when exiting filter  26 . Thus when unfiltered liquid flows into fitting  24  and suitable back pressure exists at fitting  28 , filtrate will exit through fittings  30  and  32 .  
         [0033]     Referring again to  FIG. 1 , check valve  34  can provide such back pressure and prevent liquid from flowing out of filter  26  via fitting  28  when the liquid flow direction is such that port  14  is serving as an inlet. At the same time, filtrate exits filter  26  through fittings  30  and  32 , then passes through a suitable tubing wye, check valve  36  and thence into LWCC  42 . Meanwhile, unfiltered (or, if any liquid screening or other prefiltration has taken place in port  14 , less finely filtered) liquid travels from pump fitting  22   a  to LWCC  40 . Liquid from LWCC  40  can exit apparatus  10  by passing through check valve  52  into fitting  58  and thence through filter  54 , fitting  56 , a suitable tubing wye, and port  15 . Filter  54  may for example be a hollow fiber filter like filter  26 . Filter  54  may if desired be different from filter  26 . For example, filter  54  may be sized to pass larger or smaller particles than filter  26 , thereby permitting analysis of differently sized particles, or different organisms, in each liquid flow direction through apparatus  10 . Meanwhile, liquid from LWCC  42  can exit apparatus  10  by passing through suitable tubing and the above-mentioned tubing wye and thence through port  15 .  
         [0034]     Accordingly, when the liquid flow direction is such that port  14  is serving as an inlet, the  FIG. 2  embodiment supplies an unfiltered (or less finely filtered) liquid sample to LWCC  40 . This liquid sample may for example contain CDOM, organisms and other particulate matter, and may be referred to as the “discriminate” sample. Simultaneously, a filtered (or more finely filtered) liquid sample is supplied to LWCC  42 . This latter liquid sample may for example contain CDOM but no or few algal species and may be referred to as a locally-obtained “reference” CDOM sample. Reference CDOM measurements will vary depending on CDOM concentration at each sampling site. Desirably, a further reference spectrum will have earlier been obtained using pure seawater collected far offshore away from terrestrial sources of CDOM, or using filtered seawater collected from an open ocean site where CDOM content may be at a minimum. This earlier reference spectrum may be referred to as the CDOM baseline. The CDOM baseline will be constant whereas the CDOM reference will vary based on the site where the local sample was taken. The locally measured reference CDOM sample is compared to the CDOM baseline spectrum to determine a CDOM absorption spectrum. The CDOM absorption spectrum may in turn be used as a reference spectrum for determining a particle absorbance spectra and discriminating algal groups in an unfiltered or less finely filtered sample. The particle absorbance spectrum may for example be used to calculate the logarithm of the ratio of the CDOM reference sample transmitted spectrum to the particulate light transmitted spectrum using an algorithm such as those described below. This effectively eliminates CDOM from the resultant spectrum and leaves only light absorbance due to undissolved particles that passed through the filter.  
         [0035]     After apparatus  10  has been in operation for a time with port  14  serving as an inlet, large particles and other debris may obstruct one or more parts of apparatus  10 . For example, debris may obstruct port  14 , may be carried into filter  26  and clog or otherwise obstruct the hollow ends of fibers  27  near fitting  24 , or may be carried into LWCC  40 . The onset of such obstruction may be detected using a variety of measures such as by monitoring a suitable pressure drop transducer (not shown in  FIG. 2 ) located in an appropriate part of the liquid flow circuit. The onset of such obstruction may also be forecasted by measuring elapsed operating time, integrating a detected species count, or through any other suitable scheduling measure. Meanwhile, while port  14  has served as an inlet, the flow of liquid from LWCC  40  through filter  54  and port  15 , and the flow of liquid from LWCC  42  through port  15 , will have helped flush away particles that might otherwise have obstructed port  15  or filter  54 . Once obstruction has taken place (or is forecasted to take place) in apparatus  10 , the flow of liquid may be reversed by reversing motor  20 . When such reversal takes place, the roles of ports  14  and  15 , filters  26  and  54 , and LWCCs  40  and  42  will also be reversed. Liquid will flow into apparatus  10  through port  15 . Check valve  52  will provide back pressure at fitting  58 . Filtrate will flow from filter  54  through fittings  60  and  62  and check valve  50  to LWCC  40 , then exit apparatus  10  by passing through fitting  22   a , pump  18 , fitting  16   a  and port  14 . Meanwhile, unfiltered (or less finely filtered) liquid will flow from port  15  to LWCC  42 , and then will exit apparatus  10  by passing through check valve  34  into fitting  28  and thence through filter  26 , fittings  24  and  22   b , pump  18 , fitting  16   b  and port  14 . The flow of liquid from LWCC  42  through filter  26 , and the flow of liquid from LWCC  40  through port  14 , will help flush away particles that might otherwise obstruct port  14  or filter  26 . The above-described flow reversal may be repeated again and again as needed so that apparatus  10  will remain unobstructed by debris, or will recover normal operation if it becomes obstructed.  
         [0036]     Accordingly, by periodically reversing the direction of liquid flow through ports  14  and  15 , filters  26  and  54 , and LWCCs  40  and  42 , large particles and other debris can be flushed away and discouraged or prevented from clogging or otherwise obstructing apparatus  10 . Meanwhile, the flow of liquid through LWCCs  40  and  42  can take place on an essentially continuous or semi-continuous basis while spectral measurements are obtained.  
         [0037]     If desired pump  18  and motor  20  may be eliminated and the flow of liquid through apparatus  10  may be controllably reversed using other measures. For example, when installed in a vessel, ports  14  and  15  may be oriented so that one port is always exposed to higher water pressure and the connections to the two ports may be reversed using a suitable valve, by exchanging the tubing connections at each port, or through any other convenient changeover measure. Ports  14  and  15  may also or instead be mounted in movable fashion on a suitable through-hull fitting capable of alternately exposing one or the other of port  14  and port  15  to higher water pressure relative to the other port (for example, by alternately turning port  14  or port  15  to face towards or more towards the bow).  
         [0038]     The  FIG. 1  apparatus thus simultaneously supplies different liquid samples to two LWCCs using bidirectional ports and a filter at each such port. Liquid samples are drawn into the apparatus through a first port and are directed either through a first filter and a first LWCC and out of the apparatus through a second port, or are directed into a second LWCC and then through a second filter and out of the apparatus through the second port. The roles of these first and second components are reversed when the direction of liquid flow is reversed. For example, the resulting role and flow reversal can exchange an LWCC measuring a CDOM reference with an LWCC measuring a discriminate sample, while changing the flow direction in each LWCC. Such role and flow reversal can automatically clean intake ports, tubing, and LWCCs, and can prevent such components from becoming clogged or otherwise impaired by particulate matter or other debris.  
         [0039]     Referring again to  FIG. 1 , liquid discriminate sample analysis may be carried out by illuminating LWCCs  40  and  42  with a suitable energy source and analyzing light transmission through each LWCC. The light source  68  may be any suitable source of ultraviolet, visible or infrared light. For example, light source  68  may incorporate a deuterium lamp and a tungsten lamp modified with a color correction filter (not shown in  FIG. 1 ) to balance the light emitted. For example, the color correction filter may employ stacked FG3 and BG26 filters (both from CVI Laser Corporation) so that the lamps transmit wavelengths from 320 nm to 840 nm. The resulting light may be supplied in optical communication with LWCCs  40  and  42  in any convenient fashion, such as through optical fibers  70  and  72 . Electromechanical shutters  74  and  76  may be positioned between light source  68  and optical fibers  70  and  72  so that light will enter LWCCs  40  and  42  in intermittent (e.g., alternating) fashion. Light transmitted through LWCCs  40  and  42  can be collected in any convenient fashion for analysis using one or more spectrometers such as spectrometer  82 . For example, such light may be collected using optical fibers  78  and  80 , filtered if need be through suitable optical filters (not shown in  FIG. 1 ) and optically communicated to spectrometer  82 . Spectrometer  82  may for example be a USB2000fiber optic spectrometer from Ocean Optics, Inc. calibrated to detect transmitted light over a wavelength range from ultraviolet to near infrared (e.g., from 250 nanometers to 850 nanometers). Electrical control lines  84   a ,  84   b ,  84   c  and  84   d  can connect pump motor  20 , light source  68 , spectrometer  82  and microcomputer  86  with power control circuitry  88 . Additional electrical control lines (not shown in  FIG. 1 ) may be used to operate shutters  74  and  76 , or appropriate signal detection circuitry may for example be used to determine when shutters  74  and  76  are open. Microcomputer  86  may for example record data; control pump  18 , light source  68  and spectrometer  82 ; or perform calculations. After sampling is complete, the data or calculations may be stored in onboard computer memory or downloaded from apparatus  10  through a suitable communications scheme such as a wireless or wired link and a serial communications protocol.  
         [0040]      FIG. 3  is a flow chart showing several steps in the operation of apparatus  10 . The liquid samples may be aspirated through apparatus  10  using pump  18 . The aspiration volume may be controlled by operating motor  20  for a defined time period, and may be very small, e.g., about 250 microliters to 2 milliliters, or may be considerably larger. The ability to adjust the aspiration volume makes the disclosed apparatus more versatile and better suited for use in a variety of sampling situations. A first circuit in pump  18  supplies liquid to filter  26  and filtrate to LWCC  42 , and a second circuit in pump  18  supplies unfiltered or less finely filtered liquid to LWCC  40 . When motor  20  is reversed, the second pump  18  circuit supplies liquid to filter  54  and filtrate to LWCC  40 , and the first pump  18  circuit supplies unfiltered or less finely filtered liquid to LWCC  42 . Meanwhile, fiber optic spectrometer  88  collects transmitted spectral data from LWCCs  40  and  42 . The collected data may be used for example to identify or map phytoplankton species or to characterize or map CDOM characteristics in liquid water samples or in aquatic systems. Several species of algae can be measured simultaneously, e.g., by programming several target algal species absorbance spectra into the apparatus. The disclosed apparatus and method are especially well suited for detecting and if need be providing early warning of the presence of  K. brevis  in natural, mixed phytoplankton communities or in laboratory cultures. The disclosed apparatus and method may rely in part on the high sensitivity of derivative analysis to uncover subtle characteristics in the curvature of the absorbance spectrum. The disclosed apparatus and method are especially well-suited for making continuous or a semi-continuous, automated, unattended measurements in fresh or salt water environments. Such measurement may be made by moving through a bed of varying analyte or via stationary analysis amidst moving water currents.  
         [0041]     Fourth derivative analysis and similarity index algorithms may be used to determine the presence of various materials in the liquid samples. The similarity index is a ratio comparing absorbance spectra from unknown organisms in a sample to absorbance spectra of known target algal species. For example, a processing algorithm for phytoplankton absorbance spectra to detect the red tide dinoflagellate,  K. brevis , has been described in Millie et al., “Detection of harmful algal blooms using photopigments and absorption signatures: A case study of the Florida red tide dinoflagellate, Gymnodinium breve”, Limnol. Oceanogr. 42, 1240-1251 (1997). This algorithm evaluates the fourth derivative of the particulate absorbance spectrum of an unknown sample and compares it to a standard fourth derivative spectrum for  K. brevis . Generally, the closeness of the curves is calculated using a ratio of several peaks in the spectra. Similarity indices greater than 0.7 are generally considered positive for the target organism. This algorithm may be used in the disclosed apparatus and method. Previous optical approaches to phytoplankton discrimination typically provide only bulk composite signals for a given water mass. Signatures for distinct phytoplankton species are difficult to discriminate. The disclosed apparatus and method can provide a useful field survey system capable of making spectral absorbance measurements and providing early warning to the presence of organisms including  K. brevis  in a variety of water and weather conditions including oligotrophic waters. The disclosed apparatus and method can also provide real-time automated CDOM measurement with greatly increased spatial and temporal sampling coverage compared to standard, discrete sampling methods. A high degree of variability in CDOM concentration exists in coastal waters and is difficult or impossible to resolve via traditional discrete CDOM sampling techniques. The disclosed apparatus can be used to obtain comprehensive three-dimensional views over time and space of CDOM absorption characteristics in large aquatic systems at both high and low CDOM or algal species levels.  
         [0042]      FIG. 4  shows a cross-sectional view of bidirectional port  400  mounted on a hull or other underwater housing  402 . The outer cover  404  of port  400  may be made, for example, from clear or opaque plastic, with a machined or molded opening  406  that permits liquid to pass into port  400  and two machined or molded openings  408  that permit liquid to pass out of port  400 . Desirably the liquid volume passing from opening  406  to openings  408  is much larger (e.g., several hundreds or thousands of times larger) than the liquid volume actually entering or exiting the disclosed apparatus, thereby helping to sweep port  400  clean and to assist in collecting accurate liquid samples. A perforated copper gasket  410  is situated directly over fitting  412  to discourage biofouling. Gasket  410  has a small opening  411  to permit liquid samples to pass into or out of the disclosed apparatus. When entering the disclosed apparatus, the sample passes through replaceable stainless steel mesh or size fractionation prefilter  416  situated in fitting  412 , and thence through tubing  414  to the remainder of the disclosed apparatus inside housing  402 . Prefilter  416  may have openings sized to permit algal cells of a target species of interest to pass through prefilter  416  while preventing larger cells or other large particles from being aspirated into the disclosed apparatus and obstructing its components. The openings in prefilter  416  may for example be sized at about 15 μm to 100 μm and desirably for detection of  K. brevis  may be sized at about 40 μm. Port  400  may be attached to housing  402  using seawater-resistant fasteners  418 .  
         [0043]      FIG. 5  shows bidirectional port  400  in a plan sectional view taken at line  5 - 5 ′ in  FIG. 4 . The housing on which port  400  is mounted is moving in direction A. Due to the Bernoulli Effect, liquid (e.g., seawater) passes into port  400  via opening  406  as indicated generally by flow arrows B and passes out of port  400  via openings  408  as indicated generally by flow arrows C. So long as the motion indicated by arrow A continues at a sufficient rate, liquid will flow through port  400  as shown by arrows B and C whether port  400  is serving as an intake or an outlet for the disclosed apparatus. The liquid sample flowing into or out of the disclosed apparatus will however be reversible and when flowing outward will help keep port  400  free of debris.  
         [0044]      FIG. 6  is a schematic view of an Optical Phytoplankton Discriminator apparatus  600  with series-connected LWCCs  640  and  642  mounted inside housing  612 . Liquids enter and leave apparatus  600  via first bidirectional port  614  and second bidirectional port  615 . Port  614  is connected via suitable tubing to single channel bidirectional gear pump  618 . When the liquid flow direction is such that port  614  is serving as an inlet, liquid exits pump  618  through suitable tubing and enters one end of LWCC  640 . Liquid can exit the other end of LWCC  640  by traveling through removable inline stop-flow filter  660  and check valve  662  to one end of LWCC  642 . Liquid can exit the other end of LWCC  642  and apparatus  600  through port  615 . In this fashion different liquid samples are supplied to LWCCs  640  and  642 , with LWCC  640  receiving an unfiltered or less finely filtered sample and LWCC  642  receiving a more finely filtered sample representing a filtering version of the sample exiting LWCC  640 . The remaining components in  FIG. 6  numbered like those in  FIG. 1  have the same description and function as was given for  FIG. 1 . When pump  618  is reversed, the flow of liquid through apparatus  600  reverses, and liquid flows from LWCC  642  through filter  664  and check valve  666 . The resulting liquid flow reversal can remove debris from ports  614  and  615  and from LWCCs  640  and  642 . The apparatus shown in  FIG. 6  may require more frequent filter replacement (and thus might be less well suited for unattended field use) than the apparatus shown in  FIG. 1 . However, the  FIG. 6  apparatus may have particular utility for laboratory benchtop applications. The  FIG. 6  apparatus should for example be less expensive to build and operate (particularly since it could employ relatively inexpensive filters) and may provide a more compact apparatus. It would also be useful when it is desired to save material collected on the filter for other applications such as quantitative measurement of the amount of collected material, pigment analyses, carrying out absorbance measurements directly in the spectrometer for calibration or other purposes, identification of species using molecular probes, and other applications that will be familiar to those skilled in the art.  
         [0045]     The LWCCs in the disclosed apparatus may desirably be sufficiently like one another so that they will be “interchangeable”, that is, so that these components may be used for reference sample measurements or discriminate sample measurements without materially adversely affecting the accuracy of the disclosed apparatus. More than two LWCCs may also be employed if desired in the disclosed apparatus. For example, the  FIG. 1  parallel-connected LWCC circuit and  FIG. 6  series-connected LWCC circuit could be combined in a single apparatus and the two circuits used together (e.g., with four LWCCs), or may be selected in the alternative using two LWCCs and suitable valves or other controls. Three or more LWCCs could be used in a single apparatus, and need not interchangeably analyze the same species. For example, samples could be supplied to two or more LWCCS and compared to yet another reference LWCC operating at the same time.  
         [0046]     The liquid samples may be collected semi-continuously on a pre-determined schedule, e.g., every 15 minutes or another suitable interval, or may be collected continuously without any gaps in liquid intake through one or the other of the bidirectional ports. The frequency of CDOM reference and discriminate sample measurement may be adjusted if more frequent measurement of the reference or discriminate sample is desired. Also, more frequent sampling of one analyte compared to other analytes may also be carried out.  
         [0047]     The disclosed apparatus and method can greatly reduce or can essentially eliminate data recording gaps caused by cleaning and new reference spectrum collection. Additionally, by operating two or more LWCCs simultaneously, the reference sample and discriminate sample will come from the same water, as opposed to coming from similar but at best nearby water samples. The disclosed apparatus may be adapted for use in an autonomous underwater vehicle, such as a buoyancy-driven underwater glider, vertical profiler, or a propeller-driven surveyor, for carrying out roving underwater analysis. Such analysis may for example be performed in or through a bed of varying analyte, thereby permitting continuous or semi-continuous, automated unattended sampling under conditions that might normally clog the apparatus with debris. The disclosed apparatus may also be moored or otherwise installed at a fixed maritime location such as a buoy, piling, marker, or other marine platform, and used to collect data over time (e.g., for several weeks) to allow continuous monitoring at a specific location. Suitably-constructed field models may be used under high pressure conditions such as sampling in deep water (e.g., for depths down to 50 or 60 meters or more) or under ambient pressure on a platform near the surface. Position information for such maritime devices may be obtained directly from a dedicated global positioning system interfaced to the onboard microcomputer and may be updated continuously or at surfacing intervals. The disclosed apparatus may also be adapted for use in a fixed onshore location, such as the above-mentioned benchtop model for analysis of liquid samples in a laboratory. Liquid samples can be introduced into such a benchtop model in a number of ways that will be apparent to those skilled in the art, including discrete injection and continuous sample flow.  
         [0048]      FIG. 7  shows an Optical Phytoplankton Discriminator  700  encased in watertight underwater instrument housing  702  attached to channel marker  704 . Housing  702  desirably is positioned a suitable distance below the mean low water mark  706  (e.g., at least 30 cm below mean low water mark  706 ) so that normal wave action  710  will not expose the bidirectional ports (not shown in  FIG. 7 ) in housing  702 . A wireless modem and battery box  712  connected to housing  702  desirably is positioned a suitable distance above mean high water mark  708  so that it normally remains unimmersed. An antenna  714  placed atop marker  704  enables data to be transmitted from (or if need be, to) Optical Phytoplankton Discriminator  700 .  
         [0049]     Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from this invention. This invention should not be restricted to that which has been set forth herein only for illustrative purposes.