Long-term variability in overwintering copepod populations in the Lofoten Basin: The role of the North Atlantic oscillation and trophic effects

Critical gaps in knowledge hinder our ability to infer spatiotemporal dynamics in pelagic ecosystems. In particular, environmental changes affecting key copepod species while overwintering in deep waters are still not well understood. Here, we analyzed an 11 yr time series (2000-2010) of winter (January/February) samplings in the Lofoten Basin to characterize the spatial distribution of Calanus finmarchicus overwintering abundances and to infer their long-term temporal trends. The spatial structure of populations at depths between 700 and 900 m corresponded to mesoscale aggregations consistent with eddies in the region. Over time, increased abundances of copepods and of one of its main predators, the herring (Clupea harengus), matched a negative trend in the 7 yr lagged winter NAO index. However, this progressive climatic shift did not affect surface conditions in the region or southward but corresponded to an increase in salinity and a deepening of the vertical extension of the Atlantic Water layer. We hypothesized that this change in salinity structure across the water column increased the density contrast between copepods and ambient water masses and facilitates the ascent rates during seasonal vertical migration. We suggest a step-wise mechanism from NAO large-scale forcing to copepod and herring populations mediated by hydrographical changes in intermediate water masses to explain the observed trends in abundances. Thus, large-scale, lagged climatic patterns affecting overwintering copepods might scale up to succesive trophic levels in the pelagic ecosystem.