How are oxygen budgets influenced by dissolved iron and growth of oxygenic phototrophs in an iron-rich spring system? Initial results from the Espan Spring in Fürth, Germany

At present most knowledge on the impact of iron on inline-formula¹⁸O inline-formula $M2inlinescrollmathml/$ 8pt14ptsvg-formulamathimg527256ea34e0af356380afd605ccefc0 bg-18-4535-2021-ie00001.svg8pt14ptbg-18-4535-2021-ie00001.png  inline-formula¹⁶O ratios (i.e. inline-formulaδ¹⁸O) of dissolved oxygen (DO) under circum-neutral conditions stems from experiments carried out under controlled laboratory conditions. These showed that iron oxidation leads to an increase in inline-formulaδ¹⁸Oinline-formula_DO values. Here we present the first study on effects of elevated Fe(II) concentrations on the inline-formulaδ¹⁸Oinline-formula_DO in a natural, iron-rich, circum-neutral watercourse. Our results show that iron oxidation was the major factor for rising dissolved oxygen isotope compositions in the first 85 m of the system in the cold season (February) and for the first 15 m during the warm season (May). Further along the course of the stream, the inline-formulaδ¹⁸Oinline-formula_DO decreased towards values known for atmospheric equilibration around inline-formula+24.6 ‰ during both seasons. Possible drivers for these changes may be reduced iron oxidation, increased atmospheric exchange and DO production by oxygenic phototrophic algae mats. In the cold season, the inline-formulaδ¹⁸Oinline-formula_DO values stabilized around atmospheric equilibrium, whereas in the warm season stronger influences by oxygenic photosynthesis caused values down to inline-formula+21.8 ‰. In the warm season from 145 m downstream of the spring, the inline-formulaδ¹⁸Oinline-formula_DO increased again until it reached atmospheric equilibrium. This trend can be explained by respiratory consumption of DO combined with a relative decrease in photosynthetic activity and increasing atmospheric influences. Our study shows that dissolved Fe(II) can exert strong effects on the inline-formulaδ¹⁸Oinline-formula_DO of a natural circum-neutral spring system even under constant supply of atmospheric Oinline-formula₂. However, in the presence of active photosynthesis, with supply of Oinline-formula₂ to the system, direct effects of Fe oxidation on the inline-formulaδ¹⁸Oinline-formula_DO value become masked. Nonetheless, critical Fe(II) concentrations may indirectly control DO budgets by enhancing photosynthesis, particularly if cyanobacteria are involved.