The effect of buoyant biofilms on the erodibility of sublittoral sediments of a temperate microtidal estuary

An in situ benthic flume (Sea Carousel) was deployed at eight stations along a transect (Upper South Cove, Nova Scotia) to examine the influence of biofilms on sediment erodibility. Subsamples of the material eroded within the Sea Carousel were collected by pumping and were analyzed for suspended particulate matter (SPM). Undisturbed syringe cores of the seabed were also collected and analyzed for major physical properties (bulk density, mineralogy, gain size) and organic character (chlorophyll, pheopigment, colloidal carbohydrate, organic content). Strong relationships between erosion thresholds and rates and sediment chlorophyll (r(2) = 0.948, r(2) = 0.875) and colloidal carbohydrate (r(2) = 0.854, r(2) = 0.774) content were observed, suggesting that both pigment and mucilage biofilm variables serve as good indicators of sediment stability. Erosion rates varied by a factor of 7 along the station transect, while erosion thresholds varied only by a factor of 2. Thus, erosion rate was a more sensitive indicator of trends in sediment stability than was erosion threshold. X-ray computed tomography revealed the presence of a buoyant chlorophyll-containing gel mud in the uppermost millimeters of the sediment column. The sectioning of undisturbed syringe cores revealed that the buoyant layer was associated with the surface chlorophyll layer. The reduction of sediment weight (geostatic load) in the surface biogenic layer may have a significant effect on sediment erodibility and explain the low erosion thresholds observed in this estuary.