Seasonal dynamics, spatial variability and shifts in plankton communities in Fram Strait: insights from 13 years of eDNA surveys

The HAUSGARTEN Long-Term Ecological Research (LTER) observatory serves as a key information source on the processes occurring in the Fram Strait —a pivotal water exchange zone between the North Atlantic and the Arctic Ocean. This article presents an analysis of the frequencies of amplicon sequence variants (ASVs), derived from 18S rRNA gene metabarcoding profiles of 775 samples, collected in the photic zone during annual expeditions conducted between late May and early October from 2009 to 2021. We investigated spatial and temporal changes in the composition of eukaryotic plankton communities which are major contributors to primary production and fuel the food webs in Arctic marine environments. Our study reveals significant annual and inter-annual patterns in the eukaryotic community composition. Aggregating data by month shows distinct seasonal and spatial differences in the eukaryotic community composition, with biodiversity generally increasing with depth (in both regions) and from June to September. This increase is primarily driven by a rise in Syndiniales and Dinophyceae diversity at greater depths and towards autumn. Inter-annually, biodiversity notably declined, and the June–July community structure shifted significantly in the eastern part, influenced by warmer Atlantic waters, with increased copepod ASVs and reduced frequencies of Syndiniales, Dinophyceae, and various primary producer ASVs, whereas the western part, influenced by polar waters, remained relatively stable. Our findings suggest that ongoing environmental change is altering planktonic communities, potentially affecting carbon circulation pathways and carbon sequestration rates in Fram strait, with its eastern part being stronger impacted, compared to the western part.