Seasonal dynamics of autotrophic and heterotrophic plankton metabolism and PCO2 in a subarctic Greenland fjord

Abstract

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 CO2 (PCO2), 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 O2 m‒2 d‒1) and February (NCP 5 ‒17 mmol O2 m‒2 d‒1) but net autotrophic during a developing spring bloom in May (NCP 5 129 ± 102 mmol O2 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 µmol O2 L‒1 d‒1 and September: 3.06 ± 0.82 µmol O2 L‒1 d‒1) and in the range of open ocean values, indicating that allochtonous carbon did not stimulate CR. The PCO2 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 CO2. NCP was identified as an important driver of surface PCO2 , with high rates of autotrophy and vertical export of POC reducing surface PCO2 during summer. In winter, net heterotrophy added CO2 to the water column, but this postive effect on PCO2 was balanced by simultaneous cooling of the water column, which decreased PCO2 because of increased solubility of CO2. High autochthonous production implies a relatively limited influence of allochthonous carbon on pelagic carbon balance and CO2 dynamics in the fjord.