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Photosynthetic biofilms in streams can influence element cycles through changes in physicochemical conditions over diel time scales. Through diffusion and advection, the products of algal metabolism may modify chemical speciation and ecosystem function in sediments. We quantified the magnitude of change in physicochemical gradients over diel periods in sediments underlying photosynthetic biofilms. Hourly micron-scale physicochemical depth profiles from the water column into sediment were collected using oxygen microelectrodes. The sediment oxic layer was shallowest (~3 mm) at nighttime just before dawn and deepest (>10 mm) between 13:00–19:00. Deepest oxic layer was concurrent with the time that sediment oxygen concentration was highest (~140% saturation). Maximum sediment oxygen concentration was stable for an additional 4 hours following maximum water column concentration. Water column oxygen concentrations indicated that the stream was slightly autotrophic (NEP = 0.28 g O2/d/m2). Increased depth in the sediment oxic layer over diel periods suggests that algal biofilms can drive sediment physicochemistry and coupled elemental cycles at fine temporal scales. Time lag in oxygen concentrations within sediments suggests that biogeochemical reactions favored under oxic conditions would be sustained at depth beyond the period of peak primary production.

Andrea Fitzgibbon (Primary Presenter/Author), Kent State University ,;

David Costello ( Co-Presenter/Co-Author), Kent State University,;