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

Nicolas Weidberg,Sünnje L Basedow

doi:10.1002/lno.11168

Abstract

AbstractCritical 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.

Highlights

Major spatiotemporal shifts in pelagic ecosystems have occurred recurrently over vast oceanic and coastal regions in the past and are ocurring increasingly today (IPCC Fifth Report 2014)
It has become crucial to fill these knowledge gaps to better understand how populations can cope with environmental changes due to natural and anthroprogenic climatic fluctuations (Ullah et al 2018) and to be able to predict how the marine ecosystem will respond to such changes
Herring migration patterns are variable and have changed after the collapse of the fishery in the 1960s, in the last years, nursery areas have been located in the Barents Sea from where young adults around 5 yr old migrate to southern Norway to spawn off Møre at the beginning of spring and move to the northeast toward the Lofoten Basin as the bloom progresses (Huse et al 2012; Huse 2016)

Summary

Introduction

Major spatiotemporal shifts in pelagic ecosystems have occurred recurrently over vast oceanic and coastal regions in the past and are ocurring increasingly today (IPCC Fifth Report 2014). In the Eastern North Atlantic and Arctic, the freshening and cooling of the Atlantic inflow since the late 1960s to the early 1990s, and the subsequent reversal, affected sea surface temperature (SST) and primary production (Jakobsson 1969; Curry et al 2003; Tiselius et al 2016) In all these cases, changes in the planktonic community scaled up through the trophic web eventually affecting key fisheries (yellowfin tuna, peruvian anchovy, herring, and cod, respectively) whose collapses caused severe economic and social. Problems in modern societies (Changnon 1999; Hamilton et al 2004) These trophic cascading effects and their impacts, usually exacerbated and sometimes triggered by overfishing, have been widely studied and described (e.g., Beaugrand and Kirby 2010; Pershing et al 2015a,b), but the mechanisms and species-specific underlying processes behind the long-term changes remain largely unknown. At its main population centers in the Nordic Seas C. finmarchicus has a 1-yr life cycle that consists of two contrasting phases: active feeding and reproduction in surface waters during few months in spring and summer and a longer dormancy phase, called diapause, at great depths in autumn and winter (Gaardsted et al 2011; Melle et al 2014)

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Discussion

Conclusion