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.

Coccolithophores and the Continuous Plankton Recorder Survey

Samples historically collected and analysed by the Continuous Plankton Recorder (CPR) survey were used to describe the distribution of coccolithophores (class Prymnesiophyceae) in the north-east Atlantic and the North Sea. In the routine CPR analysis, members of this group are simply identified as ‘coccolithophores’ and not to any further taxonomic level. From this analysis, the 200-m depth contour marked a point of distinct transition between high coccolithophore occurrence (off the shelf) and low coccolithophore occurrence (on the shelf). Thirty-three CPR samples that had been collected between 1979–1992, were re-examined and the coccolithophores identified to a more detailed taxonomic level. Among the species identified was the bloom-forming coccolithophore, Emiliania huxleyi. Thus archived CPR samples could potentially be re-analysed to assess regional, seasonal and decadal changes in the occurrence of this species.

Spatio-temporal patterns in the Diel Vertical Migration of the Copepod Metridia lucens in the Northeast Atlantic derived from the Continuous Plankton Recorder Survey

The archived data set collected over a 45-yr period (1948-1992) by Continuous Plankton Recorders (CPRs) towed in near-surface waters was used to investigate the diel vertical migration of the copepod Metridia lucens in the northeast Atlantic (47-63?N and 10-30?W). Although the CPR sampling intensity was uniform during the day and the night, M. lucens was caught predominantly in samples collected at night, consistent with a normal diel vertical migration pattern involving movement from greater depth during the day to shallower depths at night. The length of time spent near the surface varied seasonally and was closely correlated (r2 = 0.80) with seasonal change in length of night. The residual variation in length of time spent at the surface was nonrandom, with more time being spent at the surface in spring before the onset of the spring bloom, and less time being spent at the surface in autumn, than that predicted from the length of night at these periods. The timing of this enhanced near-surface occupation in spring varied with latitude, occurring a mean of 3.4 d later per degree of latitude.

The volume of water filtered by a Continuous Plankton Recorder sample: the effect of ship speed

The Continuous Plankton Recorder (CPR) survey is one of the largest plankton-monitoring programmes in the world. Since 1931, the CPR has collected ∼380 000 samples, each representing 10 miles of tow. It has been assumed that the volume of seawater filtered by each sample remained constant and close to 3 m3. In this study, the volume filtered for each CPR sample was measured on two routes (SA and IN), monitored by the CPR survey, between 1995 and 2000. Although the filtered volume was near the theoretical value of 3 m3 on the SA route (3.2 m3), it was significantly higher on the IN route (3.8 m3). A significant negative relationship was found between the volume filtered and the speed of the ships. This relationship indicates that the faster the speed of the ship, the lower the volume filtered. This could have implications for the CPR survey as the speed of the ships has increased continuously since the end of the 1950s. However, no significant correlation was found between the long-term changes in the speed of the ships and two commonly used indicators of plankton variability: the Phytoplankton Colour and the Total Copepods indices. This absence of relationship may indicate that the effect found is small in comparison with the influence of hydroclimatic forcing, although a more extensive study is needed to confirm these findings.

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.

Long-term changes in abundance and distribution of microzooplankton in the NE Atlantic and North Sea

Long-term changes in mesozooplankton and phytoplankton populations have been well documented in the North Atlantic region, whereas data for microzooplankton are scarce. This neglected component of the plankton is a vital link in marine food-webs, grazing on smaller flagellates and cyanobacteria and in turn providing food for the larger mesozooplankton. We use the latest tintinnid (Ciliophora, Protista) data from the Continuous Plankton Recorder (CPR) survey in the NE Atlantic and North Sea to examine the phenology, distribution and abundance of this important group of ciliates. Presence/absence data came from 167 122 CPR samples collected between 1960 and 2009 and abundance data from 49 662 samples collected between 1996 and 2009. In the North Atlantic the genus Dictyocysta spp. dominated and Parafavella gigantea showed an increase in abundance around Iceland and Greenland. In the North Sea higher densities of Tintinnopsis spp., Favella serrata and Ptychocylis spp. were found. The presence of tintinnids in CPR samples collected in the North Atlantic has increased over the last 50 years and the seasonal window of high abundance has lengthened. Conversely in the North Sea there has been an overall reduction in abundance. We discuss possible drivers for these long-term changes and point the way forward to more holistic studies that examine how ecosystems, rather than just selected taxa, are responding to climate change.

Long-term changes in the diel vertical migration behaviour of zooplankton

Samples historically collected and analysed by the Continuous Plankton Recorder survey were used to examine long-term (1958 to 1994) patterns in the normal diel vertical migration (NDVM) behaviour of 7 copepod taxa in the North Sea: Calanus finmarchicus C5-C6; Calanus spp. C1-C4; Centropages typicus; Centropages hamatus; Temora longicornis; Acartia clausii and Para-Pseudocalanus (this last group included all Paracalanus and Pseudocalanus species). The ratio of night:day abundance near the surface was used as a measure of the extent of NDVM. For all 7 taxa, the extent of NDVM between 1958 and 1994 co-varied with the abundance of herring Clupea harengus in the North Sea. Fisheries data show that during this period the herring stock was a good indicator of the overall abundance of planktivorous fish in the North Sea. These results suggest that changes in the abundance of planktivorous fish in the North Sea over recent decades have resulted in modifications in the NDVM behaviour of many zooplankton taxa.

Ontogenetic and seasonal variation in the Diel Vertical Migration of the Copepods Metridia lucens and Metridia longa

Plankton samples collected and analyzed by the Continuous Plankton Recorder survey were used to examine the length of time spent near the surface in the North Atlantic and the North Sea by three closely related groups of zooplankton (copepodite stages 1-4 Metridia spp., copepodite stages 5-6 Metridia lucens, and copepodite stages 5-6 Metridia longa). For all three groups, the mean daily length of time spent near the surface in each month of the year covaried seasonally with day length. In addition, the amount of time spent near the surface varied significantly between the three groups, being longest for the copepods of smallest body size (C 1-C4 Metridia spp.) and shortest for the copepods of largest body size (C5-C6 M. longa). These results support the suggestion that diel vertical migration serves to reduce the risk of mortality from visually orienting predators