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

Abstract

Large 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.

Large scale patches of Calanus finmarchicus and associated hydrographic conditions off the Lofoten archipelago

“…m − 3 that we found have been previously recorded within the usual distribution range of C. finmarchicus in spring (Wishner et al, 1995;Planque et al, 1997;Nash and Geffen, 2004) and are also consistent with the size of overwintering populations off Northern Norway (Halvorsen et al, 2003;Weidberg and Basedow, 2019). These aggregations matched the spatial patterns observed in the satellite imagery obtained from a concurrent study, which was able to identify large reddish surface patches over the shelf (Basedow et al, 2019;Dong et al, 2021). Moreover, those images point to copepod swarms in areas typically influenced by coastal, cold, fresh waters from the Norwegian Coastal Current, as reddish surface areas included the shelf but also zones well into Vestfjorden (Basedow et al, 2019).…”

“…In spring typical estuarine circulation develops in these fjords causing surface offshore export of recently ascended copepods and euphausids (Zhou et al, 2005;Espinasse et al, 2016;Espinasse et al, 2018). Once over the shelf, the front between the NCC and Atlantic Waters could be an effective barrier for offshelf transport, thus enhancing aggregations within the NCC (Dong et al, 2021). It is unclear if overwintering populations in the deep fjords would be large enough to substantially contribute to a large swarm over the shelf, although in Vestfjorden high wintertime copepod concentrations in the same order as those found in the Lofoten Basin have been observed (Espinasse et al, 2016;Weidberg and Basedow, 2019).…”

Dynamical Controls of the Eastward Transport of Overwintering Calanus finmarchicus From the Lofoten Basin to the Continental Slope

“…To trace the origin of the particles that reached the continental slope in the deep layer, four groups of particles were released at 643, 763, 902, and 1,062 m between 2008 and 2019 in the backward LCSs. Since the diapause of the overwintering C. finmarchicus population ends by February and C. finmarchicus first appears in upper layers on the Lofoten shelf from late February to early March (Dong et al., 2021; Edvardsen et al., 2006), 221 particles were released every 10 days between December and February in the 60‐day LCSs simulation. The location of the particle release was chosen based on the highest EKE and the most baroclinically unstable part off the continental slope region (Isachsen, 2015; Trodahl & Isachsen, 2018).…”

“…Eddies that are shed from the NwASC propagate westward, sustaining the LBE and carrying warm and salty AW into the LB (Ghaffari et al, 2018;Søiland & Rossby, 2013). This process plays an important role in driving the cross-slope transport of C. finmarchicus and their large-scale surface distribution in spring (Dong et al, 2021).…”

The Role of Fronts in Horizontal Transports of the Changjiang River Plume in Summer and the Implications for Phytoplankton Blooms

“…LCSs take the form of material lines, which are smooth and continuous curves of fluid particles advected by the flow, to reflect the fluid‐flow transport (Peacock & Haller, 2013). The LCSs analysis has been applied to many oceanic research areas, including open ocean (Duran et al., 2018; Harrison & Glatzmaier, 2012), continental shelves (Dong et al., 2021; Hu & Zhou, 2019), and coastal and estuarine waters (Ghosh et al., 2018; Ghosh, Suara, Mccue, & Brown, 2021; Giudici et al., 2021; Suara et al., 2020; Wei et al., 2013), which provides a useful tool to investigate the accumulation and transport of material. The LCSs method was also used to study the interactions between physical processes and biogeochemical processes, such as the persistency of debris accumulation in estuaries (Ghosh, Suara, Mccue, Yu, & Brown, 2021), the development and dispersion of algal blooms (Olascoaga et al., 2008; Son et al., 2016), and the track of marine predators (Kai et al., 2009).…”