Alex Herman
email: HermanA@mar.dfo-mpo.gc.ca
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Bedford Institute of Oceanography - Ocean Physics - DFO
1 Challenger Drive
Dartmouth, Nova Scotia, Canada B2Y 4A2
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Recent Papers, etc.
- The Next Generation of Optical Plankton Counter: the Laser-OPC
ABSRACT: Optical Plankton Counters (OPCs) have been in operation for approximately 12 years and while there has considerable research studies by the user community, some operational issues have emerged over this time, such as the its operational limitations in high densities, the lack of accompanying flow measurements and other measurement limitations. Reported here is the next generation of this device, the Laser-Optical Plankton Counter or LOPC which was designed at the Bedford Institute of Oceanography to address the future needs of optical plankton counting. Using a narrow laser beam and new sampling geometry, the LOPC is now capable of working in plankton densities of 106 /m3 nearly 100x greater than it predecessor, the OPC and is also capable of providing shape profiles of plankton >1.5 mm. Other new features include; i) the measurement of flow speeds through the tunnel by making a statistical estimate of particle time-of-transit, ii) a lower detection limit of 100 microns, iii) high speed towing up to 8 m/sec, and iv) overall smaller physical size. Data is presented from an LOPC mounted on the inside of 0.5 m plankton net (75 micron mesh size) showing good linear correlation between net samples (eg. copepod eggs, nauplii) and LOPC measurements. The LOPC-measured shape profiles from the same LOPC/Net tows clearly show copepods with antennae and in some case euphausiids. By increasing the tunnel width, the volume sampled by the LOPC can be increased by 5x. Other towing platforms that have been tested to-date are the Batfish vehicle towed at 8 kts and the Moving Vessel Profiler towed at 12-14 kts.
- Sampling Characteristics of Vertical Towed Plankton Nets and Intercomparisons with an Optical Plankton Counter
ABSTRACT: The problem of calibrating optical plankton counters (OPCs) against plankton net samples for smaller copepods in the upper layers was investigated, particularly in the size range of 200-500 µm where calibration problems occur for most OPC users. This was accomplished by conducting an intercomparison of plankton nets hauls to OPC measurements and developing a methodology for intercalibrations employing a variety of sampling nets, mostly vertical haul nets and nets attached to the OPC itself. The problem of extrusion of plankton through net mesh was examined for 3 mesh sizes, 80, 140 and 202 µm, and extrusion by different species of various shapes and sizes was quantified. It was found that the narrow diameter of the oblong-shaped copepod solely governed its extrusion through the net mesh and was totally independent of its length. The most dominant species in the spring were copepod nauplii which fell into the >equivalent= OPC size range of 200-300 µm and yet less than 1/30th of their numbers were retained in the 202 µm net, thus making it less than ideal for OPC intercomparisons. It was found that the 202 µm net extruded half the zooplankton species and 80-90% of the total counts as compared to the 80 µm net.
- Measurement of Euphausiid Length with a Lab Optical Plankton Counter
INTRO: We have implemented a modification in the towed OPC which enables the measurement of average flow through the tunnel. This is accomplished internally in the OPC by measuring the transit time of small particle passing through the light beam of a know width of approx. 4mm. Only moving particles that fall within the size range of (250-350 µ ) are selected by the OPC microprocessor and an average is estimated over 5-6 particles. The average flow is transmitted to surface at the repeating 1/2 second data stream of the OPC.
This same circuitry has been applied to the lab OPC for measuring the lengths of animals > 1cm which are in turn greater than the beam width. For a measured transit time and a known flow speed, an accurate estimate/calibration of the length can be made. The measurement is enabled by 2 factors; 1) laminar flow through the flow tube and , 2) forced alignment along the flow path by the narrow flow tube (2 x 2 cm cross-section). Measurements of lengths ranging from 1 - 4 cm were made on 2 species; Antarctic krill Euphausia superba supplied by University of California San Diego and Meganytiphanes norvegica from our local Scotian Shelf waters.
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