Effects of different freshwater sources on microplankton in the Western Arctic Ocean

While freshwater input into the western Arctic Ocean is increasing, studies evaluating the impacts of different freshwater sources—namely river water and sea-ice meltwater—on microplankton remain limited. In this study, we examined microplankton and hydrographic conditions during the late summers of 2021, 2023, and 2024 to evaluate the influences of different freshwater sources. Microplankton were identified and counted using an inverted microscope. In addition to water temperature, salinity, and nutrient concentrations, the fractions of river runoff (frro) and sea ice meltwater (fsimw) were estimated. High microplankton abundance (56,000 cells L−1) was observed in the group strongly influenced by river water (frro: 3%) but less affected by sea ice meltwater (fsimw: 7%), whereas the group strongly influenced by both freshwater sources (frro: 4%, fsimw: 15%) exhibited low abundance (530 cells L−1). In the former group, a lower overall freshwater proportion may have inhibited the full development of salinity stratification, allowing enhanced nutrient supply from deeper layers through upwelling. Therefore, nutrient-demanding taxa such as Chaetoceros dominated in the group. In contrast, strong salinity stratification in the latter group may have limited nutrient supply to surface waters. Consequently, heterotrophic taxa such as ciliophora tended to dominate. To our knowledge, this study provides one of the first evaluations of how different freshwater sources influence microplankton in the western Arctic Ocean. We compared the effects of freshwater accumulation on microplankton across different regions. Our results indicate that phytoplankton proliferation can be either promoted or suppressed depending on differences in freshwater sources and their complex interactions with environmental factors. This study provides a detailed assessment of how distinct freshwater sources, such as river water and sea ice meltwater, influence microplankton dynamics in the western Arctic Ocean. Furthermore, in 2021, sea ice melt was unusually delayed, and extensive sea ice remained even in the late summer. We also examined whether this unusually extensive sea ice influenced the microplankton community structure. Together, these results highlight the importance of considering both the magnitude and source of freshwater input, as well as sea-ice conditions, when evaluating microplankton dynamics in the western Arctic Ocean.