Winter-Spring Development of the Zooplankton Community Below Sea Ice in the Arctic Ocean

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 ubiquitous Oithona similis in the upper 200 m, with Microcalanus spp. and Triconia borealis further down the water column. Calanus finmarchicus dominated among the Calanus species while Metridia longa was also numerous. The most abundant deep-water copepods were Paraeuchaeta spp. and Spinocalanus spp. 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 herbivorous C. finmarchicus and 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.

Seasonal Enzyme Activities of Sympatric Calanus glacialis and C. finmarchicus in the High-Arctic

“…However, sea ice, snow cover and open leads can strongly modulate the underwater light regime, which impacts the timing of the blooms. The arrival of Calanus in surface waters under the ice well before the bloom (Hop et al, 2021), the connection between enzyme activity and Chl a as shown in our study, and the increase in catabolism of stored lipids observed in C. finmarchicus prior to the spring bloom, indicate that external triggers such as food are important for the final steps of terminating diapause. Thus, while endogenous mechanisms, like a circannual clock, may be important for initiating termination of diapause, there appears to be mechanisms for tuning the timing between the producers and consumers in the planktonic food web to ensure a good match, even with interannual and spatial variability in bloom timing.…”

“…However, we note that the ascent of Calanus spp. was recorded in early May both off-shelf in 2015 (Hop et al, 2021) and in Isfjorden in 2016 (Hatlebakk et al, 2022), and whilst this is not sufficient to state synchronized ascent across the two sampling sites, it is in line with the hypothesis of Häfker et al (2018). Even if the ascent to the surface occurred at the same time of the year, the increases in metabolic enzymes as well as digestive enzyme activities were first observed in connection with the increase in Chl a concentration.…”

“…Based on gene expression patterns and synchronized ascent despite lack of light in their study, Häfker et al (2018) suggest that an endogenous circannual clock is involved in the termination of diapause. Gene expression was not involved in our study, and unfortunately the community data are not overlapping for the two sites at the time of ascent (Hop et al, 2021, Hatlebakk et al, 2022. However, we note that the ascent of Calanus spp.…”

“…Sampling off-shelf was focused on the period from March to June 2015, when the N-ICE 2015 campaign allowed for frequent sampling in the area. Then, the community was strongly dominated by C. finmarchicus (Hop et al, 2021), and thus, there are too few data for C. glacialis to compare with Isfjorden. The MDH and CS activities of C. finmarchicus showed similarly low activity levels prior to the bloom and similarly high levels in the bloom as in Isfjorden.…”

Editorial: Effects of Ice Loss on Marine Biodiversity

“…Reduction of sea-ice in the Arctic resulted in a massive loss of the previous typical multiyear ice, with a reduction of biodiversity of the sea-ice associated protists (Hop et al, 2021) and an increase of primary production (Arrigo and Van Dijken, 2015). North of Svalbard Ehrlich et al show loss in biodiversity for the sympagic meiofauna, with nematodes totally absent and only a low number of flatworms.…”