The role of local and regional environmental factors for Calanus finmarchicus and C. hyperboreus abundances in the Nordic Seas

In the advective realm of the seas, it is challenging to disentangle the role of regional and local processes on zooplankton populations. However, comparative studies of spatially separated zooplankton populations can provide valuable insights into this issue. We studied interannual abundance variation of the key zooplankton species Calanus finmarchicus and C. hyperboreus in three near-shore locations of the Nordic Seas: off northern Norway, Svalbard, and northern Iceland. Average abundances of both species were similar among locations, while in each location the abundance of C. finmarchicus was about an order of magnitude higher than the abundance of C. hyperboreus. The abundance of both species decreased in northern Norway, while C. finmarchicus abundance increased in northern Iceland. C. finmarchicus abundance in northern Norway covaried with regional climate, while the Svalbard Calanus populations were related to local environment (hydrography, phytoplankton). In northern Iceland, C. finmarchicus abundance covaried with local environmental factors, while C. hyperboreus abundance covaried with climate variability. Top-down forcing could not be investigated. The results indicate that the mechanisms relating regional climate variability (North Atlantic and Arctic oscillations) to Calanus abundance are mediated through advection of water masses, while more local environmental variability involved bottom-up processes or advection.