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

Schütt, Julia LU (2017) In Student thesis series INES NGEM01 20171
Dept of Physical Geography and Ecosystem Science
Abstract
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... (More)
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. (Less)
Popular Abstract
The world’s oceans are taking up large quantities of CO2 and thus play an important role in regulating the global climate. The anthropogenically emitted CO2 is to a great extend taken up by the Southern Ocean, which makes it a very interesting region to study with regard to future climate change carbon dynamics. 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. To make reliable future climate predictions, it is necessary to understand the mechanisms controlling oceanic carbon budgets and to quantify biological uptake rates.
In my thesis, I used the data of two ocean gliders, autonomous... (More)
The world’s oceans are taking up large quantities of CO2 and thus play an important role in regulating the global climate. The anthropogenically emitted CO2 is to a great extend taken up by the Southern Ocean, which makes it a very interesting region to study with regard to future climate change carbon dynamics. 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. To make reliable future climate predictions, it is necessary to understand the mechanisms controlling oceanic carbon budgets and to quantify biological uptake rates.
In my thesis, I used the data of two ocean gliders, autonomous vehicles that can dive and sample surface waters and transmit the data via satellite, to model the biological net community production (NCP), which is a measure for the amount of carbon that is exported to the deep ocean. The model is based on Chlorophyll a data sampled by one of the gliders. I compared this time series to the seasonal development of surface water diurnal changes in dissolved inorganic carbon (DIC) concentration to see whether they behave in a similar way, which would support the assumption that carbon dynamics are mainly controlled by the biology. I also looked into the effect other physical forcing mechanisms could have, for example temperature, wind stress and the mixing depth.
The seasonal net community production was found to be of comparable magnitude and seasonal development as the C drawdown, which suggested that the carbon dynamics were mainly controlled by biological processes. The agreements could only be seen when smoothing the time series with a filter. There were no similarities visible when looking at the data without the filter, as the variability from day to day was very high. The agreements between both time series were found to be stronger in summer than in spring though. Looking into the other drivers for C dynamics, it showed that NCP and the DIC flux were not compared by the same processes, which was especially evident during spring, where wind and the mixed layer seemed to drive the flux of CO2 but not the biology. 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. During the latter half of the season, the days where the carbon dynamics appear not to be controlled by the biology coincide with the deepening of the mixed layer, which could add additional inorganic carbon to the surface waters.
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. This means that the dissolved inorganic carbon data could be used to derive information about biological community production and carbon uptake. (Less)
Please use this url to cite or link to this publication:
author
Schütt, Julia LU
supervisor
organization
course
NGEM01 20171
year
type
H2 - Master's Degree (Two Years)
subject
keywords
net community production, geography, physical geography, dissolved inorganic carbon, Southern Ocean
publication/series
Student thesis series INES
report number
408
language
English
additional info
External supervisor: Sandy Thomalla, CSIR/ SOCCO, South Africa
id
8905815
date added to LUP
2017-04-18 12:19:19
date last changed
2018-02-01 04:08:26
@misc{8905815,
  abstract     = {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.},
  author       = {Schütt, Julia},
  keyword      = {net community production,geography,physical geography,dissolved inorganic carbon,Southern Ocean},
  language     = {eng},
  note         = {Student Paper},
  series       = {Student thesis series INES},
  title        = {Assessment of forcing mechanisms on net community production and dissolved inorganic carbon dynamics in the Southern Ocean using glider data},
  year         = {2017},
}