Microalgal Photosynthesis and Spectral Scalar Irradiance in Coastal Marine Sediments of Limfjorden, Denmark

Scalar irradiance and oxygenic photosynthesis were measured simultaneously at 100-μ m spatial resolution by a fiber-optic scalar irradiance microsensor and an oxygen microelectrode spaced 120 μ m 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 m2s-1.