Grain sizes retained by diatom biofilms during erosion on tidal flats linked to bed sediment texture

• We eroded fine intertidal sediment with natural and bleached surface biofilms.
• Mud biofilms were found to preferentially retain clays and very fine silts.
• Findings indicate a positive feedback between sizes retained and sediment texture.

Size-specific sediment retention by diatom biofilms was measured by eroding intertidal muds at increasing shear stresses (0.01–0.60 Pa) using a Gust microcosm. The grain sizes eroded from biofilm-covered sediment were compared to those from control cores from which the biofilms were destroyed using bleach. Biofilms were quantified using carbohydrate measurements. Cores from an intertidal mud flat in the Minas Basin of the Bay of Fundy (Canada) showed biofilms preferentially retained clays and very fine silts relative to fine and medium silts. In contrast, prior field observations on an intertidal sand flat indicated that fine and medium silts were preferentially retained by biofilms relative to clays and very fine silts. These contrasting results suggest a link between size-specific sediment retention and sediment texture, where sand biofilms retain coarser, non-cohesive sediment grains, while mud biofilms retain finer, cohesive sediment grains. This relationship implies that biofilms could contribute to a positive feedback that would preserve existing sediment texture.