Transport Barriers and the Retention of Calanus finmarchicus on the Lofoten Shelf in Early Spring

Huizi Dong,Yisen Zhong,Sünnje L Basedow,Zhaoru Zhang,Walker O Smith,Meng Zhou,Ziyuan Hu

doi:10.1029/2021jc017408

Abstract

AbstractLarge aggregations of the copepod Calanus finmarchicus occur each spring in the shelf‐slope‐oceanic regions off the Lofoten‐Vesterålen Islands where productive fisheries have traditionally supported local economies. The retention and off‐shelf transport of populations of C. finmarchicus populations were studied by analyzing ocean color remote sensing, satellite altimetry data, and Lagrangian Coherent Structures (LCS) between 2010 and 2019. Our analysis revealed the existence of a transport barrier reoccurring at the shelf break that retains C. finmarchicus on the shelf for 30–70 days in the spring when C. finmarchicus were seasonally ascending to the surface layer. The analysis of baroclinic and barotropic energy conversions indicated that the topographically steered Norwegian Atlantic Current is the primary mechanism in the formation of the transport barrier, which restricts exchanges of C. finmarchicus populations between shelf and oceanic waters. In the mid‐ to late April, an increase in baroclinicity leads to an increase in mesoscale eddies generated on the shelf break near Lofoten‐Vesterålen Islands, breaking down transport barriers and causing off‐shelf transport of C. finmarchicus. The transport barrier predictably reoccurs in early spring which supports the entrapment of C. finmarchicus in the shelf region.

Highlights

Calanus finmarchicus is the dominant copepod species in the Norwegian Sea, and serves as a key link between primary producers and higher trophic levels (Melle et al, 2014; Planque, 2000)
Our analysis revealed the existence of a transport barrier reoccurring at the shelf break that retains C. finmarchicus on the shelf for 30–70 days in the spring when C. finmarchicus were seasonally ascending to the surface layer
The Finite-size Lyapunov-exponent (FSLE) fields computed from the satellite altimetry data and the RGB composite images from ocean color remote sensing data on April 5, 10, and 17, 2019 suggest the occurrence of a transport barrier near the shelf break (Figure 2)

Summary

Introduction

Calanus finmarchicus is the dominant copepod species in the Norwegian Sea, and serves as a key link between primary producers and higher trophic levels (Melle et al, 2014; Planque, 2000). Contribute more than 50% of total mesozooplankton biomass in North Atlantic and Subarctic Seas and are the main prey of several large commercial important fish such as the Northeast Arctic Cod (Gadus morhua) and the Norwegian herrings (Clupea harengus) (Carstensen et al, 2012; Melle et al, 2014; Planque, 2000). In the shelf-slope area off the coast of northern Norway, large aggregations of C. finmarchicus occur and are transported northward by both of the Norwegian Atlantic Slope Current (NwASC) and the Norwegian Coastal Current (NCC) during spring and summer

Results

Discussion

Conclusion