Traits structure copepod niches in the North Atlantic and Southern Ocean

Realised niches describe the environmental and biotic conditions that a species occupies. Among marine zooplankton, species traits, including body size, dietary mode (herbivore, omnivore, or carnivore), and diapause strategy are expected to influence the realised niche of a species. To date, realised niches are known for only a small number of copepod species. Here we quantify the realised niches of 88 copepod species measured by the Continuous Plankton Recorder (CPR) in the North Atlantic and Southern Ocean using Maximum Entropy (MaxEnt) modelling. We estimate the univariate mean niche, niche breadth of copepods for several important environmental variables, and assess the relative effects of several key zooplankton traits on the mean niche. Sea surface temperature (SST) contributed the most information to the description of niches on average across all species, with the rank importance of the remaining variables varying between regions. In the North Atlantic SST, depth, salinity and chlorophyll niches separated omnivores and herbivores from carnivores while in the Southern Ocean niche differences across dietary modes were found for chlorophyll and wind stress only. Diapausing copepods were found to occur in colder temperatures compared with non-diapausing taxa, likely because of their capacity for accumulating lipids. A strong negative body size-niche breadth relationship was found only for diapausing copepods, suggesting that larger multi-year generation species are more reliant on a specific temperature range to successfully reach diapause. Our analysis demonstrates strong connections between copepod traits and their realised niches in natural populations.