Abstract
The impact of the rapidly changing Arctic on zooplankton community structure and seasonal behaviour is not yet understood. Here we examine 6 months of under-ice zooplankton observations from the N-ICE2015 expedition (January to June 2015) in the Nansen Basin and on the Yermak Plateau north of Svalbard. Stratified sampling in the water column was done with MultiNet during the entire expedition, and sampling in the upper 5 m below sea ice was performed during April-May by divers using a hand-held net. Hydrographic conditions were dominated by northward-flowing warm and saline Atlantic Water at intermediate depth, and southward-flowing cold Polar Surface Water in the upper 100 m. The mesozooplankton was dominated by copepods. Most numerous were the small ubiquitousOithona similisin the upper 200 m, withMicrocalanusspp. andTriconia borealisfurther down the water column.Calanus finmarchicusdominated among theCalanusspecies whileMetridia longawas also numerous. The most abundant deep-water copepods wereParaeuchaetaspp. andSpinocalanusspp. Arrow worms (Chaetognatha) and comb jellies (Ctenophora) were the most numerous non-copepods. The mesozooplankton community was more dependent on surrounding water mass characteristics, such as salinity and depth, than geographical location. Algal food availability, which was closely linked to seasonality, explained the community changes seen in surface waters in May and June due to seasonal ascent and recruitment. Seasonal changes from winter to spring mostly involved an increase in the herbivorousC. finmarchicusand its nauplii in the upper 200 m of the water column coinciding with the peak of the phytoplankton bloom in late May. The Yermak Plateau and adjacent Nansen Basin were characterised by oceanic North Atlantic and Arctic species, many of which are deep water specialists. Despite the late onset of the spring bloom due to consolidated sea ice, both North Atlantic and Arctic species successfully reproduced in the study area. This explains the species-rich mesozooplankton community in this region as opposed to the less productive central Arctic Ocean. Future prospects of less sea ice and earlier onset of the bloom will likely be positive for the overall secondary production by both Arctic and boreal zooplankton in this region.
Highlights
One of the main factors influencing the zooplankton community at high latitudes is strong seasonality
From samples collected by divers from right below the sea ice, the increase in Calanus nauplii had already started in early May (Figure 4)
The zooplankton is eventually carried into the Nansen Basin and the central Arctic Ocean (Kosobokova et al, 2011), with diminishing contribution by C. finmarchicus being subsequently replaced by C. glacialis and C. hyperboreus toward the central Arctic Ocean (Hirche and Mumm, 1992; Mumm et al, 1998; Kosobokova and Hirche, 2009; Ershova et al, 2021)
Summary
One of the main factors influencing the zooplankton community at high latitudes is strong seasonality. At 80◦–82◦N, the period of polar night without sunlight lasts for up to 4 months, marine organisms may experience darkness for longer time depending on sea ice and snow cover (Cohen et al, 2020). Most zooplankton studies in the Arctic Ocean are performed in late summer-autumn when sea ice is at its minimum (Conover and Huntley, 1991; Conover and Siferd, 1993; Auel and Hagen, 2002; Kosobokova and Pertsova, 2005; Daase et al, 2008; Søreide et al, 2010; Hirche and Kosobokova, 2011; Freese et al, 2016; Ershova et al, 2021). Knowledge about the mesozooplankton community structure and development during the winter-spring period at the main entrance to the Arctic Ocean proper is needed to investigate the expansion of boreal zooplankton into the Arctic Ocean (Wassmann et al, 2015)
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