Consistency of Towing Speed and Sampling Depth for the Continuous Plankton Recorder

The mean annual towing speed of the Continuous Plankton Recorder (CPR) variedsystematically between 1946 an 1991d B.y mounting a pressure transducer on the CPR torecord towing depth, it was shown, however, that changes in towing speed did not causea significant change in towing depth, although the mean towing depth (6-7 m, SD=l-7 m,N=77) was shallower than the previously assumed towing depth of 10 m. Thus theobserved changes in towing speed are unlikely to have caused discontinuities in the CPRtime-series by affecting sampling depth.Long-term data sets play an important role in attempts to understand the causes of fluctua-tions in plankton abundance. The Continuous Plankton Recorder (CPR) survey provides multi-decadal information on plankton abundance in the North Sea and North Atlantic (McGowan,1990), and is one of the longest standing marine plankton abundance time-series. However,while the CPR time-series has great potential, as with all other data sets spanning many years,questions may be asked regarding the consistency with which the data have been collected andhence the true continuity of the time-series.While the internal design of the CPR has remained unchanged (Colebrook, 1986), there havebeen radical changes in the external design (Figure 1). The older version of the CPR incorporateda forward diving plane. However, between 1975 and 1986 recorders were modified to improvetheir stability at high speed with the diving plane being removed and a box-tail replacing thesingle horizontal tail fin.Since CPRs are towed from ships of opportunity, rather than dedicated research ships, there isno active control of towing speed, but rather the recorders are towed at the operational speed ofeach particular ship. The effect of towing speed on the plankton catch therefore needs to beB