Seasonal Variation in Transport of Zooplankton Into the Arctic Basin Through the Atlantic Gateway, Fram Strait

Abstract

The largest contribution of oceanic heat to the Arctic Ocean is the warm Atlantic Water (AW) inflow through the deep Fram Strait. The AW current also carries Atlantic plankton into the Arctic Basin and this inflow of zooplankton biomass through the Atlantic-Arctic gateway far exceeds the inflow through the shallow Pacific-Arctic gateway. However, because this transport has not yet been adequately quantified based on observational data, the present contribution is poorly defined, and future changes in Arctic zooplankton communities are difficult to project and observe. Our objective was to quantify the inflow of zooplankton biomass through the Fram Strait during different seasons, including winter. We collected data with high spatial resolution covering hydrography (CTD), currents (ADCP and LADCP) and zooplankton distributions (LOPC and MultiNet) from surface to 1,000 m depth along two transects crossing the AW inflow during three cruises in January, May and August 2014. Long-term variations (1997 in the AW inflow were analyzed based on moored current meters. Water transport across the inflow region was of the same order of magnitude during all months (January 2.2 Sv, May 1.9 Sv, August 1.7 Sv). We found a higher variability in zooplankton transport between the months (January 51 kg C s −1 , May 34 kg C s −1 , August 50 kg C s −1 ), related to seasonal changes in the vertical distribution of zooplankton. However, high abundances of carbon-rich copepods were observed in the AW inflow during all months. Surface patches with high abundances of C. finmarchicus, Microcalanus spp., Pseudocalanus spp., and Oithona similis clearly contributed to the advected biomass, also in winter. The data reveal that the phenology of species is important for the amount of advected biomass, and that the advective input of zooplankton carbon into the Arctic Basin is important during all seasons. The advective zooplankton input might be especially important for mesopelagic planktivorous predators that were recently observed in the region, particularly during winter. The inflow of C. finmarchicus with AW was estimated to be in the order of 500,000 metric tons C y −1 , which compares well to modeled estimates.

Microzooplankton Distribution and Dynamics in the Eastern Fram Strait and the Arctic Ocean in May and August 2014

“…These transects extended between 10 and 4 • E and crossed the warm core of WSC. During both cruises the dominating water mass in the upper 500 m of these transects was Atlantic (Basedow et al, 2018). Its inflow rate did not differ substantially between May and August compared to winter.…”

“…The dominant Atlantic expatriate Calanus finmarchicus and its Arctic congeners C. hyperboreus and C. glacilais are opportunistic omnivores that feed on both phytoplankton and microzooplankton (Ohman and Runge, 1994;Levinsen et al, 2000a;Campbell et al, 2009). The surface peak of the plastidic oligotrichs in May corresponded to low concentration of mesozooplankton based on the laser optical plankton counter high frequency vertical profiles (Basedow et al, 2018). In addition, the eastern end of transect D is the zone of fast northward AW flow over the shelf (Basedow et al, 2018) and the ciliates could have been advected there.…”

“…The eastern Fram Strait is dominated by the West Spitsbergen Current (WSC) -a northward continuation of the Norwegian Atlantic Current and the main conduit of Atlantic Water in the Arctic Ocean (Pnyushkov et al, 2015). In addition, WSC is transporting large quantities of Atlantic phyto- (Paulsen et al, 2016) and zooplankton into the Arctic Ocean (Kosobokova and Hirche, 2009;Basedow et al, 2018).…”

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

“…Both branches of the Norwegian Atlantic Current, the one continuing along the shelf edge toward the Fram Strait and the Arctic Ocean and the one turning into the Barents Sea (Fig. ), advect large amounts of C. finmarchicus into the respective ecosystems (Edvardsen et al ; Gluchowska et al ; Basedow et al ). For the Barents Sea, time scales of water mass advection and the proximity to overwintering habitats in the northern Norwegian Sea indicate that the temporal increase in overwintering stocks that was observed in our study might contribute directly to the increase in summer abundances in the Barents Sea (Aarflot et al ).…”

Food Web Functions and Interactions During Spring and Summer in the Arctic Water Inflow Region: Investigated Through Inverse Modeling

“…The vertically stratified samples (0-20, 20-50, 50-100, 100-200, 200-600, and 600-1,000 m) were collected with a MultiNet Midi (180 µm mesh size, 0.25 m 2 mouth opening, Hydro-Bios, Kiel, Germany) that was deployed vertically with a hauling speed of 0.5 m s −1 . Samples were preserved in a solution of 80% seawater and 20% fixation agent (75% formaldehyde buffered with hexamine, 25% anti-bactericide propandiol), resulting in a final formaldehyde concentration of 4% (for details see: Basedow et al, 2018).…”