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

[摘要] At present most knowledge on the impact of iron on 18 O  /   16 O ratios (i.e. δ 18 O) 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 δ 18 O DO values. Here we present the first study on effects of elevated Fe(II) concentrations on the δ 18 O 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 δ 18 O DO decreased towards values known for atmospheric equilibration around + 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 δ 18 O DO values stabilized around atmospheric equilibrium, whereas in the warm season stronger influences by oxygenic photosynthesis caused values down to + 21.8 ‰. In the warm season from 145 m downstream of the spring, the δ 18 O 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 δ 18 O DO of a natural circum-neutral spring system even under constant supply of atmospheric O 2 . However, in the presence of active photosynthesis, with supply of O 2 to the system, direct effects of Fe oxidation on the δ 18 O DO value become masked. Nonetheless, critical Fe(II) concentrations may indirectly control DO budgets by enhancing photosynthesis, particularly if cyanobacteria are involved.

[学科分类] 大气科学