Scalar irradiance and oxygenic photosynthesis were measured simultaneously at 100~pm spatial resolution by a fiber-optic scalar irradiance microsensor and an oxygen microelectrode spaced 120 pm apart. Marine microbial mats on sandy sediments along the coast of Limtjorden, Denmark, were dominated by cyanobacteria with a surface layer populated by pennate diatoms. In dim light Oscillatoria sp. migrated upward and a dense surface film of cyanobacteria developed. The spectral distribution of scalar irradiance showed absorption peaks at 430 and 675 nm (Chl a), 630 (phy- cocyanin), and 800 and 860 nm (bacteriochlorophyll cc). Infrared scalar irradiance reached 200% of incident light intensity at 0.0-0.3-mm depth and IR penetration was independent of the de- velopment of a cyanobacterial surface film. At high incident light intensity, 740 WEinst rnZ s-l, the photosynthetic efficiency at 1 .O-mm depth was lo-fold higher than in the uppermost 0.0-0.6 mm of the sediment. The lower boundary of the euphotic zone (detectable gross photosynthesis) was at a mean light level of ~7.5 FEinst rn+ s-l.