Species classification
Species classification was performed by a random forest (RF) model using the R package randomForest (Liaw & Wiener, 2002) using 2000 trees and the square root of peak number as randomly sampled variables at each split. To avoid overrepresentation of most abundant species, the number of specimens per species in each tree was limited to the abundance of the least abundant species, respectively. The species classification RF model was applied to all specimens from these 27 species: Acartia (Acanthacartia) bifilosa (N=5), A. (Acanthacartia) tonsa (N=34), A. (Acartiura) clausi (N=48), A. (Acartiura) longiremis ( N=81), A. (Acartia) danae (N=18), A. (Acartia) negligens(N=6), Anomalocera patersonii (N=9), Calanus finmarchicus(N=77), C. helgolandicus (N=29), C. glacialis(N=10), C. hyperboreus (N=29), Centropages bradyi(N=4), C. typicus (N=47), C. hamatus (N=53), C. chierchiae (N=6), Ditrichocorycaeus anglicus(N=10), Eurytemora affinis affinis (N=12), Limnocalanus macrurus macrurus (N=12), Metridia longa (N=13), M. lucens (N=24), Microcalanus sp. (N=12), Nannocalanus minor (N=24), Paraeuchaeta norvegica (N=10), Pseudocalanus elongatus (N=16), P. moultoni (N=15), Temora longicornis(N=91) and T. sytlifera (N=16). Please note that these are the taxa names accepted as valid by the World Register of Marine Species, but for simplification we will use genus and species name only from here on.