The possible copepod link between kelp forests, the pelagic ecosystem and deep-sea carbon sequestration

Kelp forests are dynamic coastal habitats that generate large amounts of carbon-rich detritus. The fate of this detritus is largely unknown and considered a missing link in global carbon budgets. Kelp detritus can serve as food for benthic invertebrates and pelagic invertebrate larvae, but we know close to nothing about the role of kelp detritus as food source for other zooplankton. Lipid-rich pelagic copepods constitute a key link from primary producers to higher trophic levels in marine boreal and arctic ecosystems, and they transport vast amounts of carbon into the deep sea. We conducted feeding experiments to test if the copepod Calanus finmarchicus can feed on fragments of two dominant kelp species, Saccharina latissima and Laminaria hyperborea. Such feeding would constitute an undescribed pathway from blue forests to pelagic consumers and deep-sea carbon sequestration. The experiment indicated that C. finmarchicus can ingest kelp particles, but the digestion is limited compared to a regular phytoplankton diet. Moreover, the results provide initial evidence that L. hyperborea contains substances that are toxic to copepods, an observation that warrants further research. Using specific qPCR assays to trace the consumption of kelp, we found that kelp DNA amplification signals were stronger for copepods fed with S. latissima than L. hyperborea, but we were not able to conclusively separate consumed kelp from DNA attached to the outside of the copepods. ### Competing Interest Statement The authors have declared no competing interest.