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 influ ence of biofilms on sediment erodibility. Subsamples of the material e roded within the Sea Carousel were collected by pumping and were analy zed for suspended particulate matter (SPM). Undisturbed syringe cores of the seabed were also collected and analyzed for major physical prop erties (bulk density, mineralogy, gain size) and organic character (ch lorophyll, pheopigment, colloidal carbohydrate, organic content). Stro ng relationships between erosion thresholds and rates and sediment chl orophyll (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 pi gment and mucilage biofilm variables serve as good indicators of sedim ent 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 st ability than was erosion threshold. X-ray computed tomography revealed the presence of a buoyant chlorophyll-containing gel mud in the upper most 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 (geostati c load) in the surface biogenic layer may have a significant effect on sediment erodibility and explain the low erosion thresholds observed in this estuary.