The impact of the ice edge on zooplankton in the Fram Strait: A comparison of species abundance and vertical distribution at two stations of the AWI HAUSGARTEN

The Fram Strait between Greenland and Spitsbergen is the only deep-water connection between the Arctic Ocean and its surrounding water bodies. Here, the WSC (West Spitsbergen Current) transports warm Atlantic Water into the Arctic, which accounts for a major part of the warming trend measured there in the past. The northern part of the Fram Strait is strongly influenced by the MIZ (Marginal Ice Zone), which causes a stratification of the water column that retains nutrients at the surface. These sustain intense algal blooms, leading to high abundances of zooplankton, linking the primary production and higher trophic levels. Changes in the position of the MIZ and the timing of the algae blooms might affect the zooplankton community and thus the entire Arctic food chain. The aim of this study was to investigate the effect of the ice edge on the zooplankton community in the Fram Strait. Zooplankton samples were taken at the northern (ice influenced) stations N4/5 and the southern (ice-free) station S3 of the Long-Term Ecological Research Observatory HAUSGARTEN established by the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI) in the Fram Strait in 1999. MultiNet samples of both stations were collected in 6 years. For 2015, 2017 and 2018, samples covered a depth of 1,000 m with 4 depth intervals (0-50-200-500-1,000 m). Additionally, surface samples (0-50 m) from 2011, 2012 and 2016 were added to the analysis. CTD data and ice parameters were included to test for correlations between species occurrences and the shifting ice edge. It was hypothesized 1) that the plankton community differs between the northern stations N4/5 and the southern station S3, 2) that these differences are caused by the influence of the ice edge and 3) that interannual differences are stronger at N4/5 due to the shifting ice edge. Furthermore, the zooplankton camera LOKI (Lightframe On-sight Key species Investigation) was deployed at S3 in 2017. This approach offers continuous abundance data combined with simultaneously measured environmental parameters and thus allows a closer insight into small-scale zooplankton distributions. Here, it is hypothesized 4) that the results of the LOKI and the MultiNet will differ regarding zooplankton abundance and community composition. An ANOSIM revealed sampling depth to be the only significant factor for differences between samples. Year and location both tested insignificant. While the northern station showed a higher interannual variability of environmental parameters than the southern station, this was not reflected in the zooplankton communities. It is likely that the WSC which transports Atlantic species northwards is the major factor determining the community composition while the influence of the ice edge is minor. Abundances ranged from 4,500 ind.m-3 at the surface level (0-50 m) at N5 in 2015 to almost 14,000 ind.m-3 at the surface at N4 in 2018, with copepods being the most abundant taxon in all samples. While overall abundances were higher at the surface, the number of taxa increased with depth. In general, there was no trend with time or location. However, in 2017 and 2018 large blooms of the appendicularian Fritillaria sp. were found, especially at the northern station. These might be an indicator of warmer temperatures and an increasing borealization of the Arctic. The LOKI and MultiNet hauls compared at station S3 in 2017 showed differences regarding species abundances, with the LOKI often recording only a fraction of what was found in the MultiNet data. This disparity was mostly accounted for by copepods of the genus Oithona, whose translucent bodies were not well captured by the LOKI. On the other hand, the high vertical resolution of the LOKI allowed to detect a close correlation of Metridia longa to Atlantic Water and a niche separation of developmental stages of M. longa with only females and CV performing a diel vertical migration. These results, while not finding a statistically significant difference between the northern stations N4/5 and the southern station S3, illustrate the importance of time series stations in indicator regions such as the Fram Strait in order to monitor climate change processes