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Variability and regulation of the planktonic respiratory quotient in a eutrophic lake (Lake Vombsjön) in summer 2016

Münzner, Karla LU (2017) In Student thesis series INES NGEM01 20171
Dept of Physical Geography and Ecosystem Science
Abstract
Bacterial respiration and biomass growth are important processes for carbon cycling in freshwater ecosystems and bacterial respiration is often quantified from measured O2 concentrations by using a respiratory quotient (RQ=O2 consumed to produce CO2, in moles) of 1. Recent studies have shown, however, that RQs can vary a lot in different aquatic ecosystems (0.5-5), which may lead to an under- or overestimation of respiration. In this study, we conducted in-situ measurements of O2 and CO2 fluxes in a eutrophic lake (Lake Vombsjön) to assess the magnitude and variability of the RQ and bacterial growth efficiency (BGE) during summer 2016. The RQ was mostly <1 and increased with increasing water depth while the BGE was high (0.2-0.5) and... (More)
Bacterial respiration and biomass growth are important processes for carbon cycling in freshwater ecosystems and bacterial respiration is often quantified from measured O2 concentrations by using a respiratory quotient (RQ=O2 consumed to produce CO2, in moles) of 1. Recent studies have shown, however, that RQs can vary a lot in different aquatic ecosystems (0.5-5), which may lead to an under- or overestimation of respiration. In this study, we conducted in-situ measurements of O2 and CO2 fluxes in a eutrophic lake (Lake Vombsjön) to assess the magnitude and variability of the RQ and bacterial growth efficiency (BGE) during summer 2016. The RQ was mostly <1 and increased with increasing water depth while the BGE was high (0.2-0.5) and decreased with increasing water depth. In both cases, this could be attributed to the preferential use of autochthonous organic matter by bacterioplankton. Still, no coupling between the RQ and BGE was observed. All in all, our observed RQs were much lower than any reported values (between 0.5 and 0.2). No single explaining factor could be found; instead, a combination of high primary productivity, increasing nitrification and denitrification rates over the summer and possibly the occurrence of methane oxidation may be responsible. This indicates that using a theoretical RQ of 1 may lead to an overestimation of bacterial respiration in eutrophic ecosystems and that other metabolic processes (nitrification, denitrification, methane oxidation) should be considered when studying respiration processes in those ecosystems. (Less)
Popular Abstract
Freshwater ecosystems are an important part of the global carbon cycle, releasing as much carbon dioxide to the atmosphere as is absorbed by the oceans during the same time. Carbon in lakes is cycled via primary production, i.e. the fixation of inorganic carbon by autotrophs, and respiration, i.e. the remineralisation of organic carbon by the lake community. Bacterial respiration is often determined by measuring O2 concentrations and converting them into CO2 concentrations by using a respiratory quotient (RQ). The RQ is described as the ratio of O2 consumed to CO2 produced and is usually assumed to be 1. Recent studies have shown, however, that RQs can vary a lot in different aquatic ecosystems (0.5-5), which may lead to an under- or... (More)
Freshwater ecosystems are an important part of the global carbon cycle, releasing as much carbon dioxide to the atmosphere as is absorbed by the oceans during the same time. Carbon in lakes is cycled via primary production, i.e. the fixation of inorganic carbon by autotrophs, and respiration, i.e. the remineralisation of organic carbon by the lake community. Bacterial respiration is often determined by measuring O2 concentrations and converting them into CO2 concentrations by using a respiratory quotient (RQ). The RQ is described as the ratio of O2 consumed to CO2 produced and is usually assumed to be 1. Recent studies have shown, however, that RQs can vary a lot in different aquatic ecosystems (0.5-5), which may lead to an under- or overestimation of respiration.
In this study, we conducted in-situ measurements of O2 and CO2 fluxes in a eutrophic lake (Lake Vombsjön) in Southern Sweden. We used those measurements to assess the magnitude and variability of the RQ as well as the bacterial growth efficiency in summer 2016. The RQ was mostly below 1 and increased with increasing water depth. The BGE was high (0.2 – 0.5) and showed no pattern similar to the RQ. Both trends could partly be attributed to the preferential use of autochthonous organic matter by bacterioplankton. However, the majority of our observed RQs was lower than any reported values (0.2 – 0.5) and we found no single explaining factor for this. Instead, a combination of different processes may be responsible for the low RQs: bacterioplankton use abundant algal organic matter from primary production, which yields a theoretical RQ of 0.7 – 0.8. Nitrification, the stepwise oxidation of NH4+ to NO3-, removes O2 from the water column without producing CO2, thus lowering the RQ we measured. Methane oxidation, having a theoretical RQ of 0.5, may also occur and lower the overall RQ.
Our findings indicate that using a theoretical RQ of 1 may lead to an overestimation of bacterial respiration in eutrophic ecosystems. Since many temperate lakes are eutrophic, their role in the global carbon cycle as sources of atmospheric carbon may be wrongly represented on a global scale. It is thus necessary to gain deeper knowledge about the size and variability of RQs in those lakes and take other metabolic processes (nitrification, denitrification, methane oxidation) into consideration when studying respiration on an ecosystem level. (Less)
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author
Münzner, Karla LU
supervisor
organization
course
NGEM01 20171
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Physical geography and ecosystem analysis, biogeochemistry, bacterioplankton respiration, eutrophic lake, carbon cycling, Lake Vombsjön
publication/series
Student thesis series INES
report number
441
language
English
id
8933782
date added to LUP
2018-01-24 14:45:41
date last changed
2019-01-24 03:45:24
@misc{8933782,
  abstract     = {Bacterial respiration and biomass growth are important processes for carbon cycling in freshwater ecosystems and bacterial respiration is often quantified from measured O2 concentrations by using a respiratory quotient (RQ=O2 consumed to produce CO2, in moles) of 1. Recent studies have shown, however, that RQs can vary a lot in different aquatic ecosystems (0.5-5), which may lead to an under- or overestimation of respiration. In this study, we conducted in-situ measurements of O2 and CO2 fluxes in a eutrophic lake (Lake Vombsjön) to assess the magnitude and variability of the RQ and bacterial growth efficiency (BGE) during summer 2016. The RQ was mostly <1 and increased with increasing water depth while the BGE was high (0.2-0.5) and decreased with increasing water depth. In both cases, this could be attributed to the preferential use of autochthonous organic matter by bacterioplankton. Still, no coupling between the RQ and BGE was observed. All in all, our observed RQs were much lower than any reported values (between 0.5 and 0.2). No single explaining factor could be found; instead, a combination of high primary productivity, increasing nitrification and denitrification rates over the summer and possibly the occurrence of methane oxidation may be responsible. This indicates that using a theoretical RQ of 1 may lead to an overestimation of bacterial respiration in eutrophic ecosystems and that other metabolic processes (nitrification, denitrification, methane oxidation) should be considered when studying respiration processes in those ecosystems.},
  author       = {Münzner, Karla},
  keyword      = {Physical geography and ecosystem analysis,biogeochemistry,bacterioplankton respiration,eutrophic lake,carbon cycling,Lake Vombsjön},
  language     = {eng},
  note         = {Student Paper},
  series       = {Student thesis series INES},
  title        = {Variability and regulation of the planktonic respiratory quotient in a eutrophic lake (Lake Vombsjön) in summer 2016},
  year         = {2017},
}