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

Abstract

The factors affecting the volume of water filtered by a Type II Mark III Continuous Plankton Recorder (CPR) were investigated in eastern Antarctica in February/March 2003. Three tows were conducted, one each using 270-, 224- and 125-µm nylon mesh. Volume filtered was measured at ∼3-s intervals with a Valeport electromagnetic flow meter, while ship speed, photosynthetically active radiation (PAR) and fluorescence were measured every minute. Substantial variation in measured volume filtered (MVF) was recorded on each transect. Ship speed was positively correlated with MVF and caused up to 30% reductions in MVF while clogging, predominantly by phytoplankton, resulted in up to 60% reductions in MVF. A maximum 78% reduction in MVF resulted from the combined effects of clogging and ship speed. The substantial impact of clogging on observed zooplankton densities highlights the need for flow meter measurements to quantify CPR data. However, observations from this study show that the CPR flow meter currently in use may itself have caused the positive correlation between MVF and ship speed, indicating the need for improved flow meter design. Continuing miniaturization and improved resolution of distance loggers for attachment to marine vertebrate predators holds promise in this area.