To study O. davisae and O. nana, quantitative samples were collected monthly, from August 2016 to July 2017, by horizontal hauls with the HydroBios Apstein net (mouth diameter 0.17 m, mesh size 80 μm). The 80 μm Apstein net was operated by hand from the boat by towing 15 m rope length at 0.3 m s−1 approximately, resulting a filtered volume of 0.34 m31. This particularly small mesh size was adopted, as it is known that both O. davisae and O. nana, being relatively small species, may pass through the classically used nets with a 200 μm mesh40. The characterization of the rest of the mesozooplankton community was based instead on seasonal samplings, from May 2014 to February 2017 (Supplementary Table S4), performed by the ongoing long-term zooplankton monitoring activities. Those zooplankton community samples were collected by horizontal hauls of HydroBios Apstein net (mouth diameter 0.4 m, mesh size 200 μm) and of WP2 standard net (mouth diameter 0,57 m, mesh size 200 µm) equipped with a flowmeter HydroBios to calculate the filtered water volumes (7 to 50 m3).

A multiparametric CTD probe (SBE 19plus) was used to measure environmental parameters like temperature, salinity, oxygen and chlorophyll a at the sampling sites, while the water transparency was measured with the Secchi disk. The zooplankton samples were fixed in a solution of 4% sodium tetraborate buffered formalin and ambient water. In the laboratory, copepods were identified and counted, according to the International Council for the Exploration of the Sea (ICES) protocol41 to species level, where possible, or to higher taxonomic levels. As males of the genus Oithona could not be easily identified at species level, only copepodites and females were considered regarding the two species O. davisae and O. nana. While adult males and earlier stages (nauplii) were not counted.

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