MEPS 545:35-47 (2016)  -  DOI: https://doi.org/10.3354/meps11630

ABSTRACT: The close coupling of photosynthesis and light was studied in 2 contrasting diatom-dominated coastal sediments (sand and mud flats) using O₂ microelectrodes and fiber-optic microprobes for scalar irradiance. The diatom community of the muddy sediment was composed almost exclusively of motile epipelic species, whereas in the sandy sediment similar contributions of epipsammic and epipelic diatoms were observed. The attenuation coefficient of scalar irradiance (K₀) was significantly higher in the mud, where light was attenuated exponentially with depth from the sediment surface. In the sand, scalar irradiance levels increased in the first 0.1–0.2 mm due to high scattering and low absorption. Attenuation of scalar irradiance was highest for wavelengths of absorption by major diatom photopigments (chlorophylls and carotenoids). Higher areal and volumetric rates of O₂ respiration were found in illuminated sediments than those in the dark, resulting from an increase in both O₂ concentration and penetration depth and a direct stimulation of heterotrophic processes. A lower light acclimation index (E_k) was observed for the muddy sediment community, indicating lower light acclimation compared to the sandy sediment community. Areal and volumetric rates of photosynthesis were ~3 times higher in the muddy sediment. We conclude that higher photosynthetic rates in the finer sediment were determined by (1) a thinner and more densely populated photic zone, where the contribution of active photopigments to total light absorption relative to that of photosynthetically inactive components was higher, and (2) differences in diatom species composition and dominant life-strategies, specifically the capacity of cells to actively search for optimal light microenvironments in the fine-grained sediment.

KEY WORDS: Microsensors · Diatoms · Migration · Photosynthesis · Respiration · Scalar irradiance