The next generation of Optical Plankton Counter: the Laser-OPC

Abstract

Optical Plankton Counters (OPCs) have been in operation for ∼12 years and while there have been considerable research studies by the user community, some operational issues have emerged over this time, such as 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 10 6 m -3 , nearly 100 times greater than its 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 sampling tunnel by making statistical estimates of the particle time-of-transit, (ii) a lower detection limit of 100 μm, (iii) high speed towing up to 8 m s -1 , and (iv) overall smaller physical size relative to the OPC. Data are presented from an LOPC mounted on the inside of a 0.5 m plankton net (75 μm mesh size) showing linear correlation between net samples (e.g. counts of copepod eggs, nauplii) and LOPC counts. The LOPC-measured shape profiles from the same LOPC/net tows clearly show copepods and appendages and, in some cases, euphausiids. By increasing the tunnel width relative to the original OPC, the volume sampled by the LOPC can be increased by 5×. Other towing platforms that have been tested to date are the Batfish vehicle towed at 8 knots and the Moving Vessel Profiler (MVP) towed at 12-14 knots.