The Italian contribution to the Synoptic Arctic Survey programme: the 2021 CASSANDRA cruise (LB21) through the Greenland Sea Gyre along the 75°N transect

Abstract. In September 2021, as part of the Italian Arctic research programme, a multidisciplinary cruise along the 75th parallel north through the Greenland Sea Gyre was conducted aboard the Italian icebreaker Laura Bassi as part of the CASSANDRA project, which also contributed to the Synoptic Arctic Survey (SAS) 2020/22. The cruise took place during the period of the lowest summer sea ice extent ever measured. The data show strong horizontal gradients with temperatures between 1.5 °C and 9.0 °C and salinity between 30 and 35. Warm and salty Atlantic Water (AW, θ > 3.0 °C, S around 35) dominates on the eastern side of the transect in the upper 500 m with surface temperatures of 4.5–9.0 °C, while Polar Water (PW, θ < 0 °C, S < 33) occupies the surface layer (50–80 m) in the west. The intermediate layer (100–500 m) consists of mixed water, and below 500 m the deep water of the Greenland Sea and the Norwegian Sea predominates. The oxygen enrichment is higher in the intermediate layers, while the values in deep layers and western regions are lower (< 300 µmol kg^-1). A stratified upper layer (30–50 m deep) with low surface nutrients, especially nitrate, is observed, while an accumulation of silicate occurs in deep water masses. The surface water in the eastern part of the transect has high pH_T and total alkalinity values due to photosynthesis and the presence of salty AW, while the fresh PW in the west has a lower alkalinity. Respiratory activity and organic matter concentrations (particulate/dissolved organic carbon) vary horizontally at the surface, decrease with depth, and increase slightly near the seafloor. A west-east gradient is also observed for δ¹⁸O and δD, with the ratios indicating the influence of freshwater at the surface near the Greenland coast. The abundance of prokaryotes decreases from the photic zone (< 100 m depth) to the sea floor. Carbohydrates and carboxylic acids are identified as well-utilised polymers at every station and in every layer. Overall, the microbial enzyme patterns show a decrease from the surface to deeper layers, with some hotspots of metabolic activity at 20–40 m and in the aphotic layer. The enzyme patterns vary spatially, with activity peaks at the ends and in the middle of the transect. Phytoplankton biomass, measured as chlorophyll-a, varies across the transect, with higher values at its extremities. Micro-phytoplankton fraction dominates in PW, replacing the nano-phytoplankton fraction, which is prevalent in AW, even at the interface between the two water masses. Data of phytoplankton communities show low abundances and dominance of nano-sized organisms, with diatoms being more abundant in the western part. Microzooplankton represents an important fraction of the planktonic community in this area, with tintinnids being the most important groups along the transect. Micrometazoans and aloricate ciliates are more abundant in the AW, resulting in higher biomass values at the eastern stations. Copepods are the most abundant mesozooplanktonic taxon both at the surface and in the upper 100 m water layer (97 % and 94 % of total mesozooplankton abundance, respectively), mainly represented by the genus Calanus.

The data are publicly available at the Italian Arctic Data Centre (IADC), see section Data availability.