Seasonal dynamics of autotrophic and heterotrophic plankton metabolism and P_CO2 in a subarctic Greenland fjord

We measured net planktonic community production (NCP), community respiration (CR), and gross primary production (GPP) in September, February, and May in a subarctic Greenland fjord influenced by glacial meltwater and terrestrial runoff. Potential controls of pelagic carbon cycling, including the role of terrestrial carbon, were investigated by relating surface‐water partial pressure of CO 2 (P CO2 ), NCP, GPP, and CR to physicochemical conditions, chlorophyll a (Chl a) concentration, phytoplankton production, inventories of particulate (POC) and dissolved organic carbon (DOC) and vertical flux of POC. The planktonic community was net heterotrophic in the photic zone in September (NCP 5 ‒21 ± 45 mmol O 2 m ‒2 d ‒1 ) and February (NCP 5 ‒17 mmol O 2 m ‒2 d ‒1 ) but net autotrophic during a developing spring bloom in May (NCP 5 129 ± 102 mmol O 2 m ‒2 d ‒1 ). In September, higher temperatures, shorter day lengths, and lower Chl a concentrations compared with May caused increased rates of CR, lower GPP rates, and net heterotrophy in the photic zone. The GPP required to exceed CR and where NCP becomes positive was low (in May: 1.58 ± 0.48 µ mol O 2 L ‒1 d ‒1 and September: 3.06 ± 0.82 µ mol O 2 L ‒1 d ‒1 ) and in the range of open ocean values, indicating that allochtonous carbon did not stimulate CR. The P CO2 in the surface water was below atmospheric levels (September average 25.0 ± 0.71 Pa, February 35.4 ± 0.40 Pa, and May 19.8 ± 1.21 Pa), rendering the ecosystem a sink of atmospheric CO 2 . NCP was identified as an important driver of surface P CO2 , with high rates of autotrophy and vertical export of POC reducing surface P CO2 during summer. In winter, net heterotrophy added CO 2 to the water column, but this postive effect on P CO2 was balanced by simultaneous cooling of the water column, which decreased P CO2 because of increased solubility of CO 2 . High autochthonous production implies a relatively limited influence of allochthonous carbon on pelagic carbon balance and CO 2 dynamics in the fjord.