Phytoplankton Diversity Shapes Microbial Enzymatic Processes and Organic Matter Turnover in the Greenland Sea

Rapid warming, sea-ice decline, and Atlantification are shaping Arctic marine ecosystems and the pathways of carbon cycling and organic matter turnover. During the CASSANDRA cruise carried out across the Greenland Sea Gyre (75° N) in September 2021, we investigated the relationships between phytoplankton diversity, biomass, extracellular enzymatic activities (alkaline phosphatase, leucine aminopeptidase and β-glucosidase), chlorophyll a , phaeopigments, and prokaryotic biomass across contrasting thermohaline properties distribution and depth layers. By combining taxonomic and functional indicators, we assessed how phytoplankton community structure influences microbial metabolism and nutrient regeneration. Enzymatic activities were used as proxies of phosphorus stress and organic matter lability, allowing evaluation of the coupling between autotrophic biomass and heterotrophic processing. Marked spatial and vertical variability in phytoplankton assemblages was accompanied by changes in enzymatic patterns, indicating that microbial functioning was influenced not only by biomass but also by shifts in community composition and substrate quality. These observations are consistent with a role of phytoplankton diversity and size structure in influencing the balance between carbon export and microbial recycling. This study provides a process-based baseline for understanding ecosystem responses to ongoing Arctic change and supports the integration of biological indicators for future synoptic monitoring of Arctic marine ecosystems.