Abstract
Three plankton collection methods were used to gather plankton samples in the Celtic Sea in October 2016. The Plankton Image Analysis (PIA) system is a high-speed colour line scan-based imaging instrument, which continuously pumps water, takes images of the passing particles, and identifies the zooplankton organisms present. We compared and evaluated the performance of the PIA against the Continuous Automatic Litter and Plankton Sampler (CALPS) and the traditional ring net vertical haul. The PIA underestimated species abundance compared to the CALPS and ring net and gave an image of the zooplankton community structure that was different from the other two devices. There was, however, good agreement in the spatial distribution of abundances across the three systems. Our study suggests that the image capture and analysis step rather than the sampling method was responsible for the discrepancies noted between the PIA and the other two datasets. The two most important issues appeared to be differences in sub-sampling between the PIA system and the other two devices, and blurring of specimen features due to limited PIA optical depth of field. A particular advantage of the CALPS over more traditional vertical sampling methods is that it can be integrated within existing multidisciplinary surveys at little extra cost without requiring additional survey time. The biggest advantage of using the PIA, in addition to those associated with using the CALPS, is that it removes the need to collect physical preserved samples for subsequent analysis in the laboratory. Provided a taxonomist is present on-board, the system in its current form can also integrate the sampling and analysis steps, thus increasing the speed, and cutting down substantially on the cost, of obtaining zooplankton information. Another clear advantage is that the images are available in near real-time. Once the noted issues have been dealt with, a revised PIA system will have the potential to become an important element of an integrated monitoring program, for the measurement of zooplankton.
Highlights
In pelagic ecosystems, zooplankton occupy a central position in the food web, often controlling smaller organisms by grazing and providing food for many important larval and adult fish and seabirds (Pitois et al, 2012; Lauria et al, 2013)
To test whether water column depth affected the sample size and the abundance of the organisms collected by the Plankton Image Analyser (PIA) and Continuous Automatic Litter and Plankton Sampler (CALPS) compared to the ring net, we looked at the relationships between depth and volume filtered as well as with species-specific RingNet:CALPS and RingNet:PIA ratios (Figure 5)
Despite some differences resulting from the changes in community composition from 2014 to 2016, we have confirmed that the CALPS is suited to describe broad geographic patterns in zooplankton community structure and diversity
Summary
Zooplankton occupy a central position in the food web, often controlling smaller organisms by grazing and providing food for many important larval and adult fish and seabirds (Pitois et al, 2012; Lauria et al, 2013) Their short life cycle render zooplankton sensitive to environmental changes (Edwards and Richardson, 2004; Beaugrand et al, 2010; Harris and Edwards, 2014; Serranito et al, 2016), and their position in the food chain between primary producer (bottom-up control) and fish (top-down control), make them a prerequisite for an understanding of ecosystem approach to management. For GES to be achieved, zooplankton must be present and “occur at levels that are within acceptable ranges that will secure their long-term viability and functioning” and the “distribution and abundance of species are in line with prevailing physiographic, geographic, and climatic conditions” (Borja et al, 2013)
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