Manganese and iron oxidation during benthic oxygenic photosynthesis

The effect of benthic oxygenic photosynthesis on sediment-water fluxes of m anganese and iron was studied for an intertidal sediment. Undisturbed sedim ents were incubated at an incident surface irradiance of 250 mu E m(-2) s(- 1) 26 degrees C. Oxygenic photosynthesis was selectively inhibited by addin g [3-(3,4-dichloro)-1,1-dimethyl-urea] (DCMU). Benthic fluxes were determin ed experimentally from the change in manganese and iron concentrations in t he overlying water, and were predicted from the pore water concentration gr adients at the sediment-water interface assuming molecular diffusion as the transport mechanism. The experimental fluxes of manganese and iron in DCMU -treated cores amounted to - 0.84 and - 0.59 mmol m(-2) day(-1), respective ly, and were directed from the sediment towards the overlying water. Tn the control cores, showing high rates of benthic oxygenic photosynthesis, the fluxes of manganese and iron were directed towards the sediment, 0.06 and 0 .01 mmol m(-2) day(-1), respectively Mass balances for the 0.1-0.14 cm thic k oxic zone, calculated from the experimental fluxes and the predicted flux es, suggest a minimum areal reoxidation of 0.6 mmol m(-2) day(-1) for manga nese and of 0.48 mmol m(-2) day(-1) for iron in cores showing benthic photo synthesis. The estimated turnover times for dissolved Mn2+ and dissolved Fe 2+ in the oxic surface layer during benthic photosynthesis were 0.8 and 0.2 5 h, respectively. Sediment oxygen microprofiles and the sediment pH profil es suggest that chemical precipitation and reoxidation dominates the retent ion of manganese and iron during benthic oxygenic photosynthesis in shallow intertidal sediments. (C) 1998 Academic Press.