Zooplankton data was collected from 3 different devices: Ring net, Continuous automated Litter and Plankton Sampler (CALPS) and Plankton Image Identifier (PIA), during the PELTIC 2016 survey (Pelagic ecosystem in the western English Channel and eastern Celtic Sea, 3rd to the 19th October) on board the RV “Cefas Endeavour”. Zooplankton samples and images were collected at 40 stations during the night.
Depth-integrated vertical hauls using the Ring net were deployed with the aim of sampling from 3m above the seabed to the surface. Due to the absence of real time depth information, to attempt to achieve this the amount of cable used was 3m less than the water depth. During periods of slack water this was relatively successful, but due to the strong tides at certain times and locations, some samples did not sample the full water column. An 80-μm-mesh net was used, mounted on a 0.5-m-diameter ring frame equipped with a General Oceanics mechanical flowmeter (model 2030RC, which includes a mechanism to prevent the rotor from turning backwards) mounted in the centre of the aperture of the net. A mini-CTD (SAIV) was attached to the bridle recording pressure (depth), temperature and salinity. The net was hauled to the surface at a speed of 0.5 m/s. The net was washed down and the end bag thoroughly rinsed with sea water before preserving the sample in 4% formaldehyde. Position, date, time, seabed depth and sampled depth (from CTD attached to net) were recorded and the volume filtered was calculated from the flowmeter readings.
The CALPS consists of a pump system and additional elements fitted onto the research vessel. The additional elements include a water inlet of 20 cm diameter, a flowmeter, six cylinder traps and associated valves and level detectors to prevent overflowing (see full description and Fig 2 in Pitois et al., 2016). When activated, the system pumps sea water from a depth of 4m at rates of approximately 40 L/min and distributes the water into one or more of the six possible traps. Each trap consists of a PVC cylinder (height: 73.3 cm, diameter: 28.0 cm) containing a plankton net (length 66.0 cm and diameter 26.5 cm) of chosen mesh size. During the current survey, the samples were filtered through an 80-μm-mesh net, identical to that of the ring net. The volume of water filtered was measured with an electronic flowmeter, so that zooplankton abundance (m−3) could be determined for each sample. Sampling started while steaming at a fixed vessel speed of 10 knots, 20 min before arrival at the ring net station, continued during the deployment of the ring net at station (approximately 20 min), and was stopped 20 min after leaving the station at 10 knots vessel speed. The starting time and position, as well as end time, position and volume filtered were recorded for each station.
Samples collected with the ring net and CALPS system were analysed using the Zooscan Imaging system.
The PIA is a high-speed colour line scan-based imaging instrument. The flow cell is a 25mm brass tube that has two quartz optical windows halfway along its length. The flow cell at the windows is square with the same cross sectional area as the 25mm tube. A Basler 2048-70kc camera, sampling at 70k lines per second, images the water running through the flow cell. The flow rate is monitored by a Bell electro-magnetic flow meter and set to 40L/min. Colour images are captured using an EPIX E4 frame store. Raw images are stored to maximise dynamic range of the captured particles. For the purpose of consistency, the PIA was operated at the same time as the CALPS, and from the same pumped clean water supply, thus providing zooplankton information on the same body of water. Two thousand images, from a sample, were randomly selected as a subsample, analysed with the PIA for identification and validated by an expert taxonomist manually.