Food Web Functions and Interactions During Spring and Summer in the Arctic Water Inflow Region: Investigated Through Inverse Modeling

English. We used inverse modeling to reconstruct major planktonic food web carbon flows in theAtlantic Water inflow, east and north of Svalbard during spring (18–25 May) and summer(9–13 August), 2014. The model was based on three intensively sampled stations duringboth periods, corresponding to early, peak, and decline phases of a Phaeocystis anddiatom dominated bloom (May), and flagellates dominated post bloom stages (August).The food web carbon flows were driven by primary production (290–2,850 mg Cm−2 d−1), which was channeled through a network of planktonic compartments, andultimately respired (180–1200 mg C m2 d−1), settled out of the euphotic zone asorganic particles (145–530 mg C m−2 d−1), or accumulated in the water columnin various organic pools. The accumulation of dissolved organic carbon was intense(1070 mg C m−2 d−1) during the early bloom stage, slowed down during the bloompeak (400 mg C m−2 d−1), and remained low during the rest of the season. Theheterotrophic bacteria responded swiftly to the massive release of new DOC by high butdecreasing carbon assimilation rates (from 534 to 330 mg C m−2 d−1) in May. The netbacterial production was low during the early and peak bloom (26–31 mg C m−2 d−1)but increased in the late and post bloom phases (>50 mg C m−2 d−1). Theheterotrophic nanoflagellates did not respond predictably to the different bloomphases, with relatively modest carbon uptake, 30–170 mg C m2 d−1. In contrast,microzooplankton increased food intake from 160 to 380 mg C m2 d−1 during thebuildup and decline phases, and highly variable carbon intake 46–624 mg C m2 d−1,during post bloom phases. Mesozooplankton had an initially high but decreasingcarbon uptake in May (220–48 mg C m−2 d−1), followed by highly variable carbon consumption during the post bloom stages (40–190 mg C m−2 d−1). Both, micro- andmesozooplankton shifted from almost pure herbivory (92–97% of total food intake)during the early bloom phase to an herbivorous, detritovorous and carnivorous mixeddiet as the season progressed. Our results indicate a temporal decoupling between themicrobial and zooplankton dominated heterotrophic carbon flows during the course ofthe bloom in a highly productive Atlantic gateway to the Arctic Ocean.

English. This work was funded by the Estonian Research Council (Grant1574P), and the Norwegian Research Council through the projectCarbonBridge (Project Number 226415).