DISSOLVED METALS IN SURFACE SEDIMENT AND A MICROBIAL MAT AT 100-MU-M RESOLUTION

Sensors such as electrodes and optical fibre devices, optrodes, can be used to determine steep concentration gradients of chemical species i n aquatic microenvironments, such as in the pore waters of surface sed iments' and microbial mats(2-4), but are limited to a restricted range of determinands. The highest-resolution measurements of trace-metal c oncentrations in pore waters, at about 1.25 mm, have been provided by a recently developed thin-film gel technique(5,6), but the resultant m etal distributions suggest that sub-millimetre-scale gradients need to be determined if the fluxes and cycling of the metals are to be fully quantified and understood. Here we report the development of this thi n-film gel technique to measure Zn, Mn, Fe and As fluxes and concentra tions at a resolution of 100 mu m, and demonstrate the utility of the method in situ within the surface sediments and overlying microbial ma t of a stream. Vertical profiles through the mat and sediments, and ho rizontal two-dimensional mapping just below the sediment-water interfa ce, reveal the contrasting gradients, fluxes and remobilization niches of the four metal species at a submillimetre scale. The microbial mat appears to be an important regulator of the cycling of these metals. This technique has the potential to be extended to other chemical spec ies and applied to other microenvironments with steep concentration gr adients, such as redox boundaries, plant roots, animal burrows and sit es of precipitation/dissolution in soils and sediments.