Contrasting seasonal patterns in particle aggregation and DOM transformation in a sub-Arctic fjord

Abstract. Particulate (POM) and dissolved (DOM) organic matter in the ocean are important components of the Earth’s biogeochemical cycle and in constant dynamic change through physical and biochemical processes. However, they are mostly treated as two distinct entities, separated operationally by a filter. We studied the transition between the DOM and POM pools and its drivers in different seasons in a sub-Arctic fjord by monthly environmental sampling and performing aggregation-dissolution experiments. For the experiments, surface water (5 m) was either pre-filtered through a GF/F filter (0.7 µm), or left unfiltered, followed by 36 h incubations. Before and after the incubation, samples were collected for dissolved and particulate organic carbon concentrations (DOC, POC), microbial community (flow cytometry) and in-depth analysis of the molecular composition of DOM (HPLC-HRMS). During the biologically productive period, when environmental POC concentrations were high (April, June, September), the filtered water showed a rapid increase of POC concentrations by up to 88 % within 36 h, indicating net-aggregation processes. During this process in September, DOM lability decreased based on changes in average hydrogen saturation and aromaticity of DOM molecules. In contrast, during the winter period (December, February), when environmental POC concentrations were low, the experiments indicated a dissolution of POC with a net-loss up to 58 %. Simultaneously, the DOM pool became more labile during the incubation period indicated by changes in average hydrogen saturation, aromaticity, and oxygen saturation. In both periods, bacterial activity increased throughout the incubation, showing that bacterial degradation likely plays a role in the transformation of POM and DOM. Our data highlights the importance of both physically driven DOM aggregation and biologically driven POM dissolution during different periods of the year, together determining the fate of the OM pool in high latitude marine ecosystems.