Laser Optical Particle Counter (LOPC)


Laser Optical Plankton Counter (LOPC) deployed in the CalBOBL (CalCOFI bongo) net is used to assess distribution, abundance, and size of zooplankton and other particles.


1. Principle

The Laser Optical Plankton Counter (LOPC, Herman et al. 2004) senses zooplankton and other particles optically during the standard deployment of the CalCOFI bongo net. The LOPC is mounted with an external battery pack and CTD in one side of the CalCOFI bongo net. The LOPC senses particles > ~ 100 microns equivalent spherical diameter (ESD) as they pass through a 1-mm thick sheet of red light in a 7cm x 7cm tunnel. The ESD of particles that occlude one or two 1mm x 1mm optical elements (‘single element particles’, SEPs) are accumulated by size bins in histograms every 0.5sec. Size and transparency of particles that occlude three or more optical elements (‘multiple element particles’, MEPs) are recorded as they are encountered and used to estimate their ESD. Combined ESD for all particles provides a depth-resolved size spectrum of zooplankton and other particles in the sampled water column for comparison with the zooplankton collected simultaneously in the CalCOFI bongo net. From the combined LOPC and net-derived data, inference is made about the distribution, abundance, size, and type of zooplankton and other particles. It is noted that inference about particle type is limited, especially for small particles for which only ESD is known; size and transparency data for larger particles allow inference about relatively transparent aggregates and relatively opaque zooplankters.

For details on CalBOBL net sampling protocols at sea, see: CalBOBL (Bongo) Deployment and

2. Pre-Cruise


Past data are checked to ensure data in LOPC memory have been downloaded and archived.


LOPC memory is reformatted.


LOPC batteries are replaced with new batteries.


LOPC response to passage, in air, of calibration beads is measured.

3. Cruise


The LOPC, with battery pack and CTD, is installed in the CalCOFI bongo net frame.


The LOPC deck unit and computer are installed in the ship’s interior.


Deck tests are performed with the LOPC and deck unit.


At each station, the LOPC is turned on and begins recording on deck; the CalCOFI bongo, with LOPC, is deployed in according to CalCOFI protocol; the bongo-LOPC is retrieved; the LOPC is turned off.


Periodically, the LOPC is attached to the deck unit, the stored data are transferred to a shipboard computer, and the LOPC battery voltage checked.


Midway during a cruise, the LOPC batteries are replaced.


After the final station, the LOPC is attached to the deck unit and all data are transferred to a shipboard computer.


The LOPC, with battery pack and CTD, is removed from the CalCOFI bongo frame.

4. Post-Cruise


The LOPC, battery pack, and CTD are thoroughly rinsed in freshwater, dried, and stored until next use.


Data are archived on a CD and on a server.

5. Data-Processing


Raw data from the LOPC, in binary format, are converted into text (ASCII) files using LOPC software provided by the manufacturer (Brooke Ocean Technology, Dartmouth, Nova Scotia, Canada). CTD data are integrated in the LOPC data during acquisition and hereafter considered part of the LOPC data.


Text files are processed in a variety of ways. No single processing protocol has been adopted. The principles of processing are described in Checkley et al. 2008 and are listed below.

  1. ESD for each MEP is estimated from its size and transparency using an empirical relationship.
  2. An ESD spectrum is created by combining ESD data from SEPs and MEPs. Such a spectrum can be displayed in different ways (e.g., as a normalized biomass size spectrum [NBSS, Yurista et al. 2005] or normalized volume spectrum [nVd, Jackson et al. 1997, Checkley et al. 2008).
  3. Estimates can be made from MEPs of the abundance of aggregates and zooplankton based on size and transparency (e.g., Checkley et al. 2008).
  4. For each station, spectra and abundance estimates (i.e., 5.2.2. and 5.2.3.) can be computed for an entire deployment, an entire upcast, and depth bins. Similarly, data can be accumulated within and among cruises, seasons, and years.
  5. These data are used to investigate patterns and processes involving the zooplankton and other particles. They are also compared with data from microscopic analysis of net-collected zooplankton.

6. Equipment/Supplies

  • Clipboard, stopwatch, stainless steel ratchet strap
  • Calibration Beads (BOT Brooke Ocean Technology Ltd., Cat. No. GL-0191 and GL-0271)
  • USB driver and utility applications
  • Shipboard Computer with (Windows 2000/XP)
  • Procell C 1.5V Batteries (DURACELL)
  • Flowmeter, Digital, Mechanical, w/standard rotor (General Oceanic Cat. No. 2030R)
  • Laser Optical Plankton Counter Software (BOT Brooke Ocean Technology Ltd)
  • Laser Optical Plankton Counter LOPC-660 (BOT Brooke Ocean Technology Ltd)
  • Data Logger Deck Box with Interface and Programming Cable (BOT Brooke Ocean Technology Ltd., Model No. LOPC-DL-DU-1)
  • Laser Optical Plankton Counter Data Logger with internal battery pack (BOT Brooke Ocean Technology Ltd., Model No. LOPC-DL-2)

7. References

  • Checkley, D.M. Jr., R.E. Davis, A.W. Herman, and G.A. Jackson. 2008. Assessing plankton and other particles in situ with the SOLOPC. Limnology and Oceanography 53: 2123-2136.
  • General Oceanics Digital Flowmeter Mechanical and Electronic Operators Manual 
  • Herman, A.W., B. Beanlands, and E.F. Phillips. 2004. The next generation of Optical Plankton Counter: the Laser-OPC. Journal of Plankton Research 26: 1135-1145 [doi:10.1093/plankt/fbh095].
  • Jackson, G.A., R. Maffione, D.K. Costello, A.L. Alldredge, B.E. Logan, and H.G. Dam. 1997. Particle size spectra between 1 micron and 1 cm at Monterey Bay determined using multiple instruments. Deep-Sea Research Part I 44: 1739-1767.
  • Yurista, P.M., J.R. Kelly, and S.E. Miller. 2005. Evaluation of optically acquired zooplankton size-spectrum data as a potential tool for assessment of condition in the Great Lakes. Environmental Management 35: 34-44.