Continuous Plankton Recorder flow rates revisited: clogging, ship speed and flow meter design

The factors affecting the volume of water filtered by a TypeII Mark III Continuous Plankton Recorder (CPR) were investigatedin eastern Antarctica in February/March 2003. Three tows wereconducted, one each using 270-, 224- and 125-µm nylonmesh. Volume filtered was measured at 3-s intervals with a Valeportelectromagnetic flow meter, while ship speed, photosyntheticallyactive radiation (PAR) and fluorescence were measured everyminute. Substantial variation in measured volume filtered (MVF)was recorded on each transect. Ship speed was positively correlatedwith MVF and caused up to 30% reductions in MVF while clogging,predominantly by phytoplankton, resulted in up to 60% reductionsin MVF. A maximum 78% reduction in MVF resulted from the combinedeffects of clogging and ship speed. The substantial impact ofclogging on observed zooplankton densities highlights the needfor flow meter measurements to quantify CPR data. However, observationsfrom this study show that the CPR flow meter currently in usemay itself have caused the positive correlation between MVFand ship speed, indicating the need for improved flow meterdesign. Continuing miniaturization and improved resolution ofdistance loggers for attachment to marine vertebrate predatorsholds promise in this area.