Seasonal changes in sediment erodibility associated with biostabilization in a subarctic intertidal environment, St. Lawrence Estuary, Canada

Abstract

The sediment dynamics and the long-term evolution of intertidal environments depend on the erosion threshold, which is often increased by microbial biofilms. Intertidal biostabilization has been well studied in temperate and subtropical environments, but little is known for subarctic conditions with strong seasonal changes in vegetation cover and sea ice presence in winter because of the difficulty in studying the sediments under ice cover. The present project investigated the role of biofilms and their seasonal as well as spatial variations for sediment biostabilization in a subarctic intertidal area near Rimouski (Quebec, Canada). Grain size distribution reflected a seaward increasing trend in sand content and mean grain size. The poor sorting of sediments results from the contribution of sediment transport by ice rafting in addition to the hydrodynamic transport processes in this subarctic intertidal area. A Cohesive Strength Meter (CSM) was used to measure the erosion threshold (τcrit) across the entire intertidal area. Chlorophyll a (Chl a) and extra polymeric substances (EPS) were measured in the top 3 mm. The high marsh had a far higher τcrit than the low marsh, but there was much less variation between low marsh, mudflat and sandflat. τcrit was significantly correlated with Chl a, elevation, and organic matter content. EPS was not a good indicator for τcrit, except for the low marsh/high marsh difference. The results showed a seasonal cycle in intertidal bed erodibility. Lower intertidal areas had highest τcrit in autumn during the storm season, followed by lowest τcrit in winter under sea-ice cover, and no major difference between summer and spring. This study highlights the characteristics of cold climate intertidal areas, where low temperatures in autumn and land-fast ice cover during winter severely affect biofilms and intertidal vegetation, and ultimately their capacity to shield sediments from erosion.