Rank abundance distributions—RAD

The characterisation of abundance and distribution of most Oithona species calls for the use of sampling nets with a mesh size smaller than 200 μm^12,40. Parallel comparison of efficiency of different sampling devices in catching Oithona species has been only seldom faced and investigated¹⁵ and no conclusive guidelines have been issued to scale the catches or merge the data collected with different approaches in single tows.

Nevertheless, for the scope of the present study, it was necessary to detail the presence/absence and relative abundances of to the two species (O. davisae and O. nana) within the picture of the overall copepod community in each station and possibly in each seasonal/environmental condition.

Given the quite even distribution of the samplings events during the year (monthly with the 80 μm net, and almost seasonally during 2014–2017 period with the 200 μm net), we approached the issue by assuming as “typical” the yearly trend of Oithona species coming from the samplings with 80 μm net (Supplementary Fig. ^S1). The linear interpolation of the expected abundances in specific calendar days made possible their merging with the specimen counts of the 200 μm samplings.

In this way we were able to construct the synthetic Rank Abundance Distributions (RAD) of the overall copepod community in each station by adding together all the catches collected with both nets (80 μm and 200 μm) and then calculating the relative abundances and ranks of every species. In particular we obtained, for each station, a pool of copepod communities that (1) integrates the species abundances of standard 200 μm net and the 80 μm net counts for O. nana and O. davisae, and (2) corresponds to several seasonal conditions during the year. These pools were also used to investigate the mechanisms of niche apportionment.