A first approach to build and test the Copepod Mean Size and Total Abundance (CMSTA) ecological indicator using in-situ size measurements from the Plankton Imager (PI)

•Zooplankton mean size can reflect the ecosystem health status in the Celtic Sea.•Zooplankton size is an important trait to be monitored.•The Plankton Imager offers a unique opportunity to collect routine zooplankton size information.•Geometric mean is more representative of the zooplankton community than average size. Being located between primary producers and fish, zooplankton are a key element of marine food webs, the trophic structure of which is dependent upon the size distribution of species. Changes in zooplankton community size structure have the potential to alter the food web structure and ultimately the quality of food for planktivorous fish. Zooplankton therefore play a key role in overall ecosystem health and are ideal indicators of environmental variability due to their short life cycle and sensitivity to environmental changes. In this study, we used a first approach to build and test the Copepod Mean Size and Total Abundance (CMSTA) ecological indicator in the Celtic Sea, using a similar methodology to that of the HELCOM Mean Size and Total Stock (MSTS) indicator. We explored relationships between zooplankton mean size, total abundance and biomass with hydrographic and biological variables representative of both lower and higher trophic levels (i.e. Chlorophyll-a levels and the biomass distribution of herring, sardine, anchovy, sprat and horse mackerel). Herring, the species with the strongest preference for larger prey, was the only fish that displayed a statistically significant positive correlation with copepod mean size. Displaying the stations spatially within the 2-dimentional (copepod mean size versus abundance) indicator plots, resulted in a strong pattern of herring distribution related to areas where copepods were the largest rather than the most abundant. Our preliminary results are in line with those obtained from previous studies, confirming that zooplankton mean size is able to reflect the state of the food web, and thus reinforce the importance as zooplankton size as a key trait to be routinely monitored. Our in-situ zooplankton size measurements were collected with the Plankton Imager (PI). The ability of this automated system to take measurements for every single organism in a sample meant that the mean size was calculated based on the actual size distribution of the sampled population. Additionally, because of the non-normal distribution of copepod size, the geometric mean was found to be a much more representative descrip