Life strategies in an upwelling world: distribution patterns and niche partitioning of Calanidae copepods in the Benguela Current

Climate change is expected to alter coastal upwelling systems, making it essential to understand their current ecological structure. This study investigates the distribution and niche partitioning of Calanidae copepods in relation to upwelling intensities during austral summer in the northern (nBUS) and southern (sBUS) Benguela Current upwelling subsystems, which differ in upwelling seasonality, oxygen minimum zones and fisheries production. The six occurring Calanidae species were separated into three size categories each with potentially similar prey-size spectra. Similar-sized species differed in at least one of their horizontal, vertical or trophic niches. The dominant copepod Calanoides natalis was associated with cold, chlorophyll a-rich shelf waters and showed high levels of diatom fatty acid markers. Calanus agulhensis peaked offshore in the sBUS, with copepodids C5 also found deeper than 200 m and, unexpectedly, offshore in the nBUS, likely transported there by aged Agulhas retroflection rings drifting towards the nBUS and mixing with Subantarctic Mode Water, indicated by co-occurrences with Neocalanus tonsus. Copepodids C5 of C. agulhensis had elevated amounts of wax esters, suggesting special life-cycle adaptations of potential energy storage. Nannocalanus minor and Mesocalanus tenuicornis were associated with warm, offshore waters, with N. minor being more abundant in the nBUS and M. tenuicornis in the sBUS. They co-occurred to some extent but showed differences in their vertical and trophic niches. Fine-scale niche partitioning among closely related, similar-sized species likely supports persistence and diversity in highly dynamic upwelling systems. It raises questions about winners or losers in response to severe environmental changes related to future climate scenarios and how these changes may propagate to commercially important fish populations.