A comparison of LHPR and OPC data from vertical distribution sampling of zooplankton in a Norwegian fjord

Sampling was carried out using a double Longhurst-Hardy Plankton Recorder (LHPR) system (200 µm and 53 µm mesh nets) in June 1993 in Storfjorden on the west coast of Norway to describe the vertical distribution of zooplankton. The results are compared with concurrent Optical Plankton Counter (OPC) data. Zooplankton was most abundant in a superficial (0–30 m depth) layer ofwarrn (8 to 12 °C), low salinity (23 to 34) water. Included in this shallow distribution were cladocerans, which comprised 62.7 % of all zooplankton identitied in the 200 µm LHPR samples. Most of the small zooplankton taken in the 53 µm LHPR samples was distributed in the upper 50 m of the water column|abundant amongst these small organisms were adult Microsetella norvegica together with nauplii and early copepodite stages of other copepods. Both the number and biovolume of particles recorded by the OPC were consistently about 4 times higher throughout the water column than for an equivalent size range of zooplankton identified in the 200 µm LHPR samples. A comparison of particulate size frequency distributions from the OPC and the 200 µm LHPR samples suggests that this difference could be due partly to the detection of flocculants and detritus by the OPC. However, the 53 µm LHPR results indicate that there was also significant under-representation of zooplankton < 0.35 mm Equivalent Spherical Diameter (ESD) in the 200 µm LHPR samples due to extrusion through the liltering mesh.