Arctic mesozooplankton dynamics over the last decade in the Fram Strait.
Supervisors: Barbara Niehoff, Ellen Oldenburg, Ovidiu Popa (AWI)
The Fram Strait, located between Greenland and the Svalbard Archipelago, is considered the only deep-water connection between the Arctic and North Atlantic Ocean. Here, the West Spitsbergen Current (WSC) is the main transporter of warm and saline Atlantic Water (AW), which is the biggest oceanic heat and boreal species contributor to the Arctic basin. Since the Arctic is subject to severe climate-related changes, the aim of this study is to evaluate the Arctic epipelagic mesozooplankton dynamics over the last decade in the Fram Strait, and to elucidate whether the zooplankton community has undergone changes, and if yes, how these changes could be explained by environmental conditions.

To determine zooplankton taxonomic composition abundance and horizontal distribution, vertical MultiNet hauls from 1500 m to the surface were taken at 9 stations of the LTER HAUSGARTEN observatory in the Fram Strait during late spring/summer between 2011 to 2022. This thesis focuses on data resulting from the analyses of samples from the 0-50 m depth interval assuming that the zooplankton community in this layer is most affected by changes in sea-ice conditions. The dataset for this thesis was constructed using data from both microscopy (during the early years from 2011-2016) and ZooScan analysis (late years from 2017-2022) to complete 10 years of study, as these two methods yielded comparable results. Mesozooplankton data were evaluated in relation to hydrographic data observations to assess any influence of sea ice melt (stratification), AW input from year to year.

Abundances from a maximum of approximately 20,000 ind.m-3 at stations in the HG (Hausgarten) while the lowest were in the stations at East Greenland Current (EGC) with approximately 1,000 ind.m-3. Copepods were the most abundant taxon at all stations with a contribution of 95.9% of the total abundance, dominated by boreal species Oithona spp and C. finmarchicus. While mesozooplankton dominance of Simpson index was relatively high, the number of taxa, Shannon diversity and Pielous’ evenness were low in the upper 50 meters. Our results indicate that looking at each station was not possible to decipher a trend. However, when stations were grouped according to their large-scale hydrography, a possible trend in the EGC realm was observed between 2017 to 2019. Lack of samples in 2018 did not allow to clarify this pattern and studies in the EGC realm are rare. Temperature and salinity were the most correlated environmental parameters driving the mesozooplankton community. Annual events such as melting ice, and Atlantic water inflow in the Fram Strait could influence the dynamic of the community, promoting zooplankton peaks of boreal species when more saline and warm water was observed in the WSC realm.