Continuous plankton records stand the test of time: evaluation of flow rates, clogging and the continuity of the CPR time-series

The Continuous Plankton Recorder (CPR) survey is one of the most extensive biological time-series in existence and has been in operation over major regions of the North Atlantic since 1932. However, there is little information about the volume of water filtered through each sample, but rather a general assumption has persisted that each sample represents 3 m3. Data from electromagnetic flowmeters, deployed on CPRs between 1995 and 1998, was examined. The mean volume filtered through samples was 3.11 m3 and the effect of clogging on filtration efficiencies was not great. Consequently, even when the likely variations in flow due to clogging are taken into account, previously identified links between zooplankton abundance and climatic signals remain strong.

Measuring the filtration efficiency of the Continuous Plankton Recorder

The Continuous Plankton Recorder (CPR) survey has analysed over 160 000 plankton samples taken in the North East Atlantic over the last 60 years. However, flowmeters have not been routinely fitted to the recorders and there has been the implicit assumption that a CPR sample represents the organisms filtered from 3 m3 of sea water. We describe an electromagnetic flowmeter designed for the CPR and present results which show that on occasions the filtered volume per sample varied significantly from this value.