Impacts of temperature and CO2 on the coupling between phytoplankton growth and microzooplankton grazing in the western Arctic Ocean

The Arctic Ocean is experiencing about 2 to 3 times faster warming than the global average. In addition, carbon dioxide (CO₂) input from the atmosphere has increased due to the expanding in sea ice free area during summer. However, the effects of those environmental perturbations on marine lower trophic level organisms and those interactions are poorly investigated in the western Arctic Ocean. This study demonstrated on-board temperature (in situ and in situ plus 4°C) and CO₂ (in situ and in situ plus 300–500 µatm pCO₂) manipulation experiments using natural plankton communities in the western Arctic Ocean in September 2017 (5 experiments) and July 2018 (4 experiments). In this study, a modified two-point dilution technique was applied to investigate the effects of environmental perturbations on both phytoplankton growth and microzooplankton grazing and their interactions. Overall, the growth of phytoplankton was enhanced by temperature rise at a pace of 0.1 day⁻¹ °C⁻¹ in both years. In 2017, higher CO₂ levels enhanced the growth of smaller phytoplankton traits. In 2018, higher CO₂ levels enhanced the growth of larger phytoplankton traits only under low in situ CO2 conditions. However, the fertilization effects of CO₂ diminished and the CO₂ have appeared negative impact on larger phytoplankton traits under the increased temperature. Microzooplankton grazing rates were often linearly related to the specific growth rates of phytoplankton but the grazing did not overwhelm phytoplankton growth. Higher CO₂ levels did not affect microzooplankton grazing. The slope between phytoplankton growth rate and microzooplankton grazing rate tended to closer with increasing in situ temperature. Those results demonstrated that higher temperature and CO₂ levels are beneficial to smaller phytoplankton traits and the trophic link tended to be tight with the warming of the western Arctic Ocean.