Initial results from the Espan Spring in Fürth, Germany

At present most knowledge on the impact of iron on O/O ratios (i.e. O) of dissolved oxygen (DO) 11 under circum-neutral conditions stems from experiments carried out under controlled laboratory conditions. These 12 showed that iron oxidation leads to an increase in ODO values. Here we present the first study on effects of 13 elevated Fe(II) concentrations on the ODO in a natural, iron-rich circum-neutral watercourse. Our results show 14 that iron oxidation was the major factor to cause rising oxygen isotopes in the first 85 meters of the system in the 15 cold season (February) and for the first 15 meters during the warm season (May). Further along the course of the 16 stream, the ODO decreased towards values known for atmospheric equilibration at 24.6 ‰ during both seasons. 17 Possible drivers for this decrease may be reduced iron oxidation, increased atmospheric exchange and DO 18 production by oxygenic phototrophic algae mats. In the cold season, the ODO values stabilized around 19 atmospheric equilibrium, whereas in the warm season stronger influences by oxygenic photosynthesis caused 20 values down to +21.8 ‰. In the warm season after 145 meters downstream of the spring, the ODO increased 21 again until it reached atmospheric equilibrium. This trend can be explained by a respiratory consumption of DO 22 combined with a relative decrease in photosynthetic activity and increasing atmospheric influences. Our study 23 shows that dissolved Fe(II) can exert strong effects on the ODO of a natural circum-neutral spring system even 24 under constant supply of atmospheric O2. However, in the presence of active photosynthesis, with active supply 25 of O2 to the system, direct effects of Fe oxidation on the ODO value becomes masked. Nonetheless, critical Fe(II) 26 concentrations may indirectly control DO budgets by enhancing photosynthesis, particularly if cyanobacteria are 27 involved. 28


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To the best of our knowledge, no study so far has systematically investigated the influences of elevated Fe(II) 73 concentrations on  18 ODO values in a natural and circum-neutral iron-rich system. In order to bridge this gap, we 74 investigated the aqueous chemistry and  18 ODO values in the iron-rich Espan Spring in Fürth, Germany (Fig. 1).

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This Fe(II)-rich artesian spring offers a complex biogeochemical natural field site to analyse effects of different

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The aims of this study were to establish an inventory of biology together with Fe and oxygen budgets in this natural 85 spring and stream system. We further aimed to investigate how increased Fe(II)-levels influence the oxygen budget 86 of the system and whether a combination of DO and  18 ODO measurements can help to assess this effect. This is 87 also timely because environmental impacts of Fe(II) become increasingly recognised for their negative effects on 88 ecosystems such as with the browning or brownification phenomenon (Kritzberg and  The Espan Spring is located in the city of Fürth, Germany (49°28'15.8''N 11°00'53.0''E, Fig. 1). It is an artesian 98 spring that originates from a confined aquifer that was tapped by a drilling project in 1935 from a depth of 448.5 99 m below ground. The water originates from the so called "lower mineral water horizon". This horizon is dominated 100 by artesian inflow from the lower Buntsandstein Formation. The Buntsandstein in Fürth consists of red sandstone 101 layers that are composed of light reddish to yellowish-white-grey sandstones of different grain sizes. The 102 sandstones are intercalated with various rubble, conglomerate, and clay layers as well as thin gypsum and salt 103 (Birzer, 1936   Sandstone and from the Eck conglomerate. This water, which is caught in the red sandstone and has a temperature 110 of about +22°C, was called the "Lower Mineral Water Horizon"; in 1936 its yield was about 10 L s -1 at a water 111 temperature of +23°C (Kühnau 1938). The water of this lower spring horizon is under artesian pressure and exits 112 the spring with a head of 13 m above ground level (Birzer, 1936). Nowadays the Espan Spring has a constant yield 113 of about 5 L s -1 .

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After the water exits the basin in a pavilion with a temperature of ~ 20 °C, it discharges into a stream of about 300 115 m length that is known as the "Wetzendorfer Landgraben (WL)". This small stream drains into the Pegnitz River 116 without any further tributaries (Fig. 1b, c). The water can be classified as a Na-Ca-Cl-SO4 mineral water with 117 initially undersaturated DO values of 2.3 mg/L and Fe(II) contents of up to 6.6 mg/L (Table 1). Figure 1c shows 118 an aerial image of the spring and stream system that shows a distinct red coloring of the stream bed. The most 119 plausible explanation for this coloring are iron-oxide-precipitates ( Fig. 1 d, e). The WL has a water depth between

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Samples for water isotopes were collected in 15 mL Falcon tubes and treated in the same manner as the ones for 136 DO isotope measurements, except for preservation with HgCl2. All samples were stored in a mobile refrigerator 137 box at 4 °C directly after collection and carried to the laboratory where they were measured within 24 h.

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To obtain ratio changes in per mil (‰), the δ values were multiplied by factor of 1000.

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All samples were measured in triplicates and isotope values standard deviations (1σ) were less than 0.1 and 0.2 ‰ 183 for δ 18 OH2O and δ 18 ODO, respectively. The on-site parameters as displayed in Table 1

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Note that values before the forward slash are for cold season and after the slash for warm season.

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Most of the cyanobacteria and all eukaryotic algae were located in the topmost 1.2 mm of the biofilms (Fig. 2 246 O1). Close-up images show eukaryotic algae (Fig. 2.O2), thin filamentous cyanobacteria, possibly Persinema sp.

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In order to determine the identity of the predominant cyanobacterial species isolated from the E4.1 enrichment 251 cultures, a determination key was used to compare particular features of an isolate to those already in the literature 252 for specific cyanobacterial species (Komárek und Anagnostidis, 2005). Note that enrichment cultures for samples 253 E2 and E3 did not yield enough material for cyanobacterial determination after 5 weeks in culture.

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The Fe(II) content was highest at the faucet with 6.6 mg/L while its lowest content was below instrument precision.

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at sampling point E9 at 300 meters from the source (Fig. 4b). Fe(II) concentrations decreased constantly over the 289 stream course and were accompanied by increases in DO saturation (Fig. 4a). The decrease in Fe(II) could have 290 been caused by three major processes:

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(1) Oxidation of Fe(II) to form ferric iron minerals such as ferrihydrite, hematite and goethite

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(2) Precipitation of Fe(II) minerals such as the iron carbonate siderite (FeCO3) and/or an amorphous ferrous silicate

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The solubility of iron oxides in natural systems at a circum-neutral pH and under aerobic conditions is generally 311 very low (Cornell and Schwertmann, 2003) with values of the solubility product (Ksp) between 10 -37 and 10 -44 312 (Schwertmann, 1991). However, Fe(III) could still be detected in the water, thus showing that its dissolution was

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The curves are divided into two zones for the cold season and three zones for the warm season.

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The occurrence of  18 ODO values below + 24.6 ‰ in groundwater has been described in the literature (Wassenaar

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Increases in  18 ODO values in zone 1 were accompanied by increases in DO (Fig. 6a). In the cold season, a strong 358 positive correlation was evident between points E1a and E4. However, in the warm season, the same correlation 359 could only be observed between points E1a and E2 (Fig. 6b). Equilibration with the atmosphere would be 360 reasonable explanation for this trend until atmospheric equilibration was reached between point E2 and E3.

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However, the  18 ODO values, at least in the cold season, increased above this threshold to a value of + 25.7 ‰.

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This shows that another process in addition to atmospheric equilibration must have influenced the  18 ODO values 363 in zone 1. In the warm season, this was less evident, and the isotope atmospheric equilibrium value was only   with + 24.5 ‰ in E7 and + 24.8 ‰ in the Pegnitz River (Fig. 5a).

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In the warm season, zone 2 extended from sampling point E2 to point E5 at 145 meters distance from the spring.

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In this zone the values decreased from + 24.7 ‰ to a minimum value of + 21.8 ‰ in sampling point E5 (Fig. 5b).

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The fact that photosynthesising organisms seem to preferentially grow and impact the  18 ODO values between 410 sampling point E3 and E5 may be due to the availability of Fe(II). In addition, the growth could also be controlled 411 by changes in the pH or other environmental influences, with the site being located in a public park with the 412 associated perturbations. Cyanobacteria, especially aquatic strains prefer a neutral to alkaline pH (Brock, 1973)

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The temperature did not change significantly in this part of the watercourse and is therefore unlikely to have caused