Assessment of forcing mechanisms on net community production and dissolved inorganic carbon dynamics in the Southern Ocean using glider data

Sammanfattning: In the Subantarctic Zone of the Southern Ocean, a combination of physical forcings, chemical solubility and biological fixation is controlling the carbon uptake and thus the role the Southern Ocean is playing in the remediation of global climate change. Therefore, it is necessary to understand the mechanisms controlling oceanic carbon budgets and to quantify biological uptake rates to make reliable future climate predictions. In this study, the data of two ocean gliders simultaneously sampling the ocean interior and the CO2 exchange processes at the ocean surface were used to model the biological net community production (NCP) based on Chlorophyll a. A comparison was made to the seasonal development of surface water diurnal changes in dissolved inorganic carbon (DIC) concentration, as well as to the physical forcing mechanisms controlling both processes. The cross-seasonal net community production was found to range between -90 and 242 mg m-2 d-1 with 118 mg m-2 d-1 on average and the seasonal average daily change in dissolved inorganic carbon concentration was -235 mg m-2 d-1, leaving the two processes at overlapping and comparable ranges. It was shown that both time series were following similar seasonal trends of daily carbon drawdown and release when comparing the time series smoothed with a running mean filter, leading to the conclusion that the here modeled daily dissolved inorganic carbon fluxes are largely controlled by the biology. Although, the dissolved inorganic carbon data is fluctuating with a higher amplitude and holds higher daily variability. The net community production was largely controlled by the mixed layer depth and by light, the dissolved inorganic carbon flux did not show any correlation with any of the physical drivers. It was reasoned that contrary to biological processes, the DIC dynamics are subject to chemical and thermodynamical forcings that are evident during short-lived events and might be most prominently occurring during spring. In the beginning of the productive season, variations in temperature and windstress could be held responsible in controlling an outgassing CO2 flux which reduces the daily dissolved inorganic carbon rate. During the latter half of the season the days of where net community production and dissolved inorganic carbon had the same sign coincide with periods of mixed layer depth entrainment. The comparison of the seasonal development of net community production and daily dissolved inorganic carbon fluxes and their physical drivers shows the analogy of both time series during summer and thus the possibility of using dissolved inorganic carbon data in the here presented way for deriving information about biological community production and carbon uptake.