DESIGN AND CALIBRATION OF A NEW OPTICAL PLANKTON COUNTER CAPABLE OF SIZING SMALL ZOOPLANKTON

A new design of optical plankton counter (OPC) capable of sizing zooplankton in the size range of 250-mu-m to 2 cm is presented. The detection sensitivity is the result of new optical design of the previous system (HERMAN, 1988, Continental Shelf Research, 8, 205-221) that originally had a lower detection limit of 550-mu-m. Both theoretical and experimental calibrations for the OPC are derived and compared. Preserved copepods and eggs introduced in a flow tank demonstrate the response of the OPC and its capability for detecting plankton to a limit of 250-mu-m. Copepod profiles measured by the OPC mounted on plankton net samplers provided intercomparisons that showed good agreement in identifying copepod layers and identifying some species, for example, copepods as small as Calanus finmarchicus II and up to the adult stages. Profiles containing marine snow and Ceratium illustrate some of the limitations of the OPC in directly measuring and identifying copepods. An additional modification to the OPC may result in a potential lower limit detection of 120-mu-m, and the practicality of such applications are discussed.