Microalgal photosynthesis and spectral scalar irradiance in coastal marine sediments of Limfjorden, Denmark

Scalar irradiance and oxygenic photosynthesis were measured simultaneously at 100‐µm spatial resolution by a fiber‐optic scalar irradiance microsensor and an oxygen microelectrode spaced 120 µm apart. Marine microbial mats on sandy sediments along the coast of Limfjorden, 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 (phycocyanin), and 800 and 860 nm (bacteriochlorophyll a ). Infrared scalar irradiance reached 200% of incident light intensity at 0.0–0.3‐mm depth and IR penetration was independent of the development of a cyanobacterial surface film. At high incident light intensity, 740 µ Einst m −2 s −1 , the photosynthetic efficiency at 1.0‐mm depth was 10‐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 µ Einst m −2 s −1 .