Open-channel measurement of denitrification in a large lowland river

submitted to Aquatic Sciences

authored by Stephanie Ritz1*, Kirstin Dähnke2, Helmut Fischer1

1 Federal Institute of Hydrology – BfG, Am Mainzer Tor 1, 56068 Koblenz

2 Helmholtz-ZentrumGeesthacht - Institute for Coastal Research, Max-Planck-Straße 1
21502 Geesthacht

* Correspondig author:

Online Resource 2: MIMS results for summer 2012 and 2013

This supplementary material contains information about thefailed MIMS measurementsof the campaigns insummer 2012 and summer 2013.

Table S2.1: Summary of measured N2 super saturations. sd=standard deviations, CV = coefficient of variation.

Campaign Reach / N2 super saturation / Mean analytical error of field samples
sd / CV
(µM) / (%) / Mean analytical error of standards1
sd / CV
(µM) / (%)
Average
(µM) / min – max
(µM)
Summer 2011 / A / 2.6 / 1.3 – 4.3 / 0.6 / 0.1 / 0.7 / 0.2
B / 4.3 / 3.2 – 5.1 / 1.0 / 0.2
C / 1.9 / 1.1 – 2.4 / 0.7 / 0.1
Spring 2012 / A / 4.5 / 2.1 – 6.8 / 1.1 / 0.2 / 0.5 / 0.1
B / 4.3 / 2.5 – 7.4 / 1.0 / 0.2
C / 2.0 / -0.7 – 3.8 / 1.2 / 0.3
Summer 2012 / A / -2.0 / -35.0 – 5.1 / 9.1 / 1.7 / 0.5 / 0.1
B / -1.2 / - 8.6 – 4.3 / 9.6 / 2.2
C / -3.9 / -12.4 – 3.6 / 9.1 / 1.5
Summer 2013 / A / 8.7 / 4.2 – 14.2 / 5.0 / 0.9 / 0.4 / 0.1
B / 7.8 / 5.1 – 12.2 / 5.2 / 1.0
C / 7.2 / 5.4 – 10.3 / 3.9 / 0.7

1: Salt solutions with a defined gas concentration that were interspersed among samples during analysis (one standard after every second sample)

In summer 2012 and 2013 the CV (coefficent of variation) of the N2 measurements among the field replicates ranged between 0.7 and 2.2 % (table S2.1). This was much higher compared to summer 2011 and spring 2012. In summer 2011 and spring 2012 the CV among the field replicates was around 0.2 µM. However, N2 in the standard solutions was measured in high precision (CV ≈ 0.1%) for all campaigns. In August 2012 under-saturations were measured in some samples, which were occasionally high (down to -35 µM in reach A). This lead to average under-saturations for all reaches in this campaign. For August 2013 no under-saturations were measured. Instead a comparably high average super saturation of 7.9 µM was measured. The single gas measurements of Argon (m/z 40) showed a constant CV for all campaigns (CV ≈ 1.5%) for both, field replicates and standard solutions (table S2.2).

Table S2.2: Variability of the Argon concentration measured in the field samples. The values correspond to average standard deviations (sd) and average coefficients of variation (CV) among all sets of field replicates taken at the different sampling points along the 580 km long river stretch.

August 2011 / May 2012 / August 2012 / August 2013
sd (µM) / 0.2 / 0.3 / 0.2 / 0.2
CV (%) / 1.6 / 1.7 / 1.1 / 1.6

These findings imply that:

  1. An instrument defect was not the reason for the increased standard deviations of the field samples. Otherwise, measuremets of the standard solution would also show a decreased precision analogously to the field samples.
  2. Leakage during storage did not occur. Otherwise, the Argon concentration in the field samples must have been affected as well and should show an incerased vraiability analogous to N2.

In August 2013 it was additionally tested if the river water itself affected the analytical accuracy of MIMS at that time (e.g. due to higher seston concentrations that can clog the capillaries and disturb the uniform water flow over the MIMS membrane). For this test in brief, river water was taken from the impounded section of the Elbe (at Elbe-km 581) and fixed with ZnCl2 (final concentration 0.5%, v/v). In the laboratory, the sample water was mixed with defined amounts of saline solution and equilibrated with the atmosphere at room temperature (25.3°C). Thereby a series of defined gas concentrations (with final salinities of 0‰, 6‰, 12‰) was accomplished. These Elbe water test solutions were left open and stored at room temperature. Like the field samples, the test solutions were stored upright for 12 h to allow for sedimentation before the measuremets, which were done in quadruplicate.The CV for the Elbe water test solutions averaged 0.1% and the measured concetrations correspondet to the expexted concentrations according to temperature and salinity (figure S2.1). Thus no negative effect of the river water on measurement precision could be detected.

Figure S2.1: MIMS test with air equilibrated Elbe water of different salinities. Dashed lines represent the expected N2 concentrations according toHamme and Emerson (2004).

Since the sampling procedure remained the same for all campaigns and artefacts based on incorrect storage or defects during the MIMS measurements could be excluded, the reason for the increased variability among the field replicates in summer 2012 and 2013 could not be clarified.Therefore, these campaigns were excluded from the denitrification calculations in the main text.

However for summer 2013 acalculation of denitrification rates is principally possible, due to the positive super sauturations that have been measured. Considering the entire range of possible scenarios (including uncertainties in gas exchange, N2 concentration, channel morphology & discharge), themedian denitrification rates accounted for 47, 25 and 24 mg N m-2 h-1 for reach A, B and C, respectively.The uncertainty in these denitrification rates was high because the incerased varibility of the measured N2 concentrations (figure S2.2).

Figure S2.2: Denitrification rates calculated from the N2 concentrations measured in the river Elbe in August 2013.