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 Furth, Germany

At present most knowledge on the impact of iron on O-18 / O-16 ratios (i.e. delta O-18) 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 delta O-18(DO) values. Here we present the first study on effects of elevated Fe(II) concentrations on the delta O-18(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 85m of the system in the cold season (February) and for the first 15m during the warm season (May). Further along the course of the stream, the delta O-18(DO) decreased towards values known for atmospheric equilibration around +24.6 parts per thousand 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 delta O-18(DO) values stabilized around atmospheric equilibrium, whereas in the warm season stronger influences by oxygenic photosynthesis caused values down to +21.8 parts per thousand. In the warm season from 145m downstream of the spring, the delta O-18(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 delta O-18(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 delta O-18(DO) value become masked. Nonetheless, critical Fe(II) concentrations may indirectly control DO budgets by enhancing photosynthesis, particularly if cyanobacteria are involved.