Journal cover Journal topic
Biogeosciences An interactive open-access journal of the European Geosciences Union
doi:10.5194/bg-2017-81
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
13 Mar 2017
Review status
This discussion paper is under review for the journal Biogeosciences (BG).
Tune in on 11.57 µHz and listen to primary production
Tom J. S. Cox1,2, Justus E. E. van Beusekom3, and Karline Soetaert2 1University of Antwerp, Department of Biology, Ecosystem Management research group, Universiteitsplein 1, B-2060 Anwerpen, B
2Royal Netherlands Institute of Sea Research (NIOZ) and University of Utrecht, Korringaweg 7, P.O. Box 140, 4400 AC Yerseke, Nl
3Helmholtz-Zentrum Geesthacht. Institute for Coastal Research, Max-Planck-Strasse 1, 21502 Geesthacht, Germany
Abstract. In this manuscript we present an an elegant approach to reconstruct slowly varying GPP as a function of time, based on O2 time series. The approach, called complex demodulation, is based on on a direct analogy with amplitude modulated (AM) radio signals. The O2 concentrations oscillating at the diel frequency (or 11.57 μHz) can be seen as a carrier wave, while the time variation in the amplitude of this carrier wave is related to the time varying GPP. The relation follows from an analysis in the frequency domain of the governing equation of O2 dynamics. After the theoretical derivation, we assess the performance of the approach by applying it to 3 artificial O2 time series, generated with models representative for a well mixed vertical water column, a river and an estuary. These models are forced with hourly observed incident irradiance, resulting in a variblity of GPP on scales from hours to months. The dynamic build-up of algal biomass further increases the seasonality. Complex demodulation allows to reconstruct with great precision time varying GPP of the vertical water column and the river model. Surprisingly, it is possible to derive daily averaged GPP – complex demodulation thus reconstructs the amplitude of every single diel cycle. Also in estuaries time varying GPP can be reconstructed to a great extent. But there, the influence of the tides prevent achieving the same temporal resolution. In particular, the combination of horizontal O2 gradients with the O1 and Q1 harmonics in the tides, interferes with the complex demodulation procedure, and introduces spurious amplitude variation that can not be attributed to GPP. But also other tidal harmonics, in casu K1 and P1, introduce diel fluctuations that can not be distinguished from GPP. We demonstrate that these spurious effects also occur in real-world time series (Hörnum Tief, De). The spurious fluctuations introduced by O1 and Q1 can be removed to a large extent by increasing the averaging time to 15 days. As such, we demonstrate that a good estimate of the running 15 day average of GPP can be obtained in tidal systems. Apart from the direct merits to estimating GPP from O2 time series, the analysis in the frequency domain enhances our insights in O2 dynamics in tidal systems in general, and in the performance of O2 methods to estimate GPP in particular.

Citation: Cox, T. J. S., van Beusekom, J. E. E., and Soetaert, K.: Tune in on 11.57 µHz and listen to primary production, Biogeosciences Discuss., doi:10.5194/bg-2017-81, in review, 2017.
Tom J. S. Cox et al.
Tom J. S. Cox et al.
Tom J. S. Cox et al.

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Short summary
Oxygen concentration (O2) reveal many things about water quality. With mathematical techniques we demonstrate how we can calculate the photosynthesis done by microscopic plants that drift in ponds, rivers, coasts or the open ocean. The techniques have also allowed us to better understand how the tides in coasts and estuaries influence O2. The results are important for correctly interpreting the data that is gathered by a growing set of continuous O2 sensors that are deployed around the world.
Oxygen concentration (O2) reveal many things about water quality. With mathematical techniques...
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