Longhurst-hardy Plankton recorders: their design and use to minimize bias

Field studies and tow basin experiments of Longhurst-Hardy Plankton Recorder (LHPR) biases were conducted by injecting various types of particles into an LHPR having a variety of net and recorder box configurations. Tow speed and clogging were also considered. The following factors contributed to biases: (1) low recorder gauze aperture area to throat area ratios, causing backing up of flow into the net cod end, (2) tunnel gauze slot design, allowing significant losses of particles; (3) steep side angles of the net at the recorder entrance due to bagging or radial stretching; (4) clogging of the neta head of the LHPR; (5) short nets; and (6) tow speeds greater than about 125 cm s⁻¹. Experiments with an LHPR modified to minimize biases showed that resolution to 15 m in oligotrophic waters is possible. In eutrophic waters, we doubt it is feasible to use a net with the LHPR.