Master Thesis Defense by Clara Roskjær Møgeltoft Nielsen

Title: Seasonal Variability in Atlantic Water Inflow and its Influence on Microbial Eukaryote Community Composition at the Isfjorden-Adventfjorden Time Series Station

Abstract:
Arctic marine systems are crucial to the Earth’s ecological and climate balance. However, they are threatened by climate change. In particular, Atlantification has a growing influence on the Arctic Ocean by increasing the melting rate of sea ice and impacting the physical parameters that change the bottom-up factors shaping microbial life. Microbial eukaryotes carry out essential functions in the marine ecosystem, and perturbation of their communities can have cascading effects. Yet, it remains understudied how microbial eukaryotic communities respond to Arctic Water inflow.

This thesis addresses two aspects of Atlantification: whether the Atlantic Water inflow has changed in frequency, intensity, and characteristics, and whether Atlantic Water exposure explains the variation in community composition. These questions were investigated in Isfjorden, West Spitsbergen, using the Isfjorden South Mooring (I-SM), the Isfjorden South Entrance (I-SE), and Isfjorden-Adventfjorden (IsA) time series stations, established in December 2011.

Atlantic Water exposure was quantified from Conductivity, Temperature, and Depth (CTD) profiles at IsA and from temperature and salinity data from the two moorings. Trend analysis revealed mixed evidence for Atlantification at Isfjorden, with a significant decrease in the frequency and intensity of Atlantic Water inflow and shoaling of the Atlantic Water events measured at I-SM. Given the length of the time series and the non-linear progression of Atlantification through event-like pulses, it is not possible to fully distinguish trends from natural variability.

Within each season, redundancy analysis with variance partitioning was used to investigate how Atlantic Water exposure and the broader environmental conditions from the IsA time series explain the variance in eukaryotic community composition. This was determined by Illumina sequencing of the 18S rDNA V4 region. Since calendar seasons are not sufficient for explaining the seasonality in these data, new seasonal definitions were created based on chlorophyll-a measurements and sunrise information. Atlantic Water inflow shows strong seasonal spread in how much it explains variation in community composition, shared variance with other environmental variables (if any), and the relationship between them. The model design, including the conditions used for selecting predictors, might impact the explanatory power of AW exposure, the selection of other predictors, and the strength of their shared variance. Therefore, the model design should be considered when interpreting any results.    

Supervisor: Jens Hesselbjerg Christensen