Spatial and temporal variation of sediment dynamics in a subarctic intertidal zone Atif Waqas, Urs Neumeier, André Rochon Institut des sciences de la mer de Rimouski, Université du Québec à Rimouski (
[email protected]) 1. Introduction
3. Objectives
Sediment erosion and transport is a continuous phenomenon on intertidal mudflats, which varies in time and in space. Microalgal biofilms consolidate the sediment surface and play a vital role for the erosive response to shear stress generated by waves and currents (Tolhurst et al., 2008). Sediment dynamics are controlled by the erosion threshold (crit), which can be measured in field with portable devices. Tidal flats in St. Lawrence Estuary are subjected to strong subarctic winter conditions with several months of sea ice, which reduces wave action on shorelines, whereas, summer coastal processes are relatively similar to temperate climates (Coulombier et al. 2012). The land fast ice constricts the space available for water flow and removes the marsh vegetation and hence, considerably changing hydrodynamics and sediment dynamics in the intertidal zones (Neumeier, 2011; Neumeier and Cheng, 2015).
The objectives of the Ph.D. project are: (1) To determine the role of biofilms and their seasonal variations for sediment biostabilization in a subarctic tidal flat. (2) To conduct the first winter measurements of sediment stability and sediment dynamics under land fast ice. (3) To construct a full-year sediment budget for a subarctic tidal flat taking into account the seasonal evolution of biofilm consolidation, marsh vegetation, storm frequency and sediment dynamics as well as sea ice action in winter. (A)
100
(B)90
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crit
Transmission (%)
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The Nazareth tidal flat is located west of Rimouski, Quebec on the southern shore of the St. Lawrence Estuary (Fig. 1). The tidal flat is partially protected by two islands, Canuel Island and Saint-Barnabé Island. This intertidal flat is ~1.3 km wide. The high marsh is separated from the low marsh by a 0.5 m marsh scarp. Seaward there is a mudflat followed by sand flat (Fig. 2).
Critical erosion threshold (crit ) measured by CSM shows a strong difference between high marsh and low marsh. Low marsh, mudflat and sand flat have relatively similar crit (Fig 5). There is a positive correlation between crit and organic matter content (measured by loss on ignition) as well as between crit and chl a especially for high marsh (Fig 5). There was an extensive vegetation cover of 20 to 30 cm height in transect B and C and up to 90 cm in Transect A on the upper marsh. Higher crit at C2 is probably caused by some pebbles and cobbles present in the sediments.
(C)
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2. Study Site
5. Preliminary Results
60 C
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PSI
Fig. 3: (A) CSM with test chamber and footprints of previous tests at site B3 in lower marsh with sparse Spartina alterniflora. (B) Erosion records by CSM at station B4. crit is defined where Transmission = 70%. The erosion records can be divided in three parts A, B and C. (C) Drilling through the land-fast ice to reach sediment surface. Fig. 4: Loss on ignition (LOI) across the tidal flat and salt marsh.
4. Methods and Materials 7
Erosion Threshold
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Transect C Transect B
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Transect A
N m–2
A first field campaign was conducted in August 2016 on 3 cross-shore transects (A-C) with 29 stations covering the entire intertidal area (Fig. 1). Additional field campaigns are planned in 2017. Erosion thresholds were quantified using a cohesive strength meter (CSM Mk IV). CSM is a specially designed portable device (Fig 2 and 3), which fires a series of water jets at the sediment surface within a small test chamber to observe the start of sediment resuspension (Tolhurst et al., 2003; Grabowski et al., 2012). The CSM was deployed to determine critical erosion threshold crit (n=5). Chlorophyll a (chl a) and colloidal carbohydrate concentrations were measured in the surface sediments (n=3). Small sediment cores were taken using modified 60 ml syringes (36 mm ø) and frozen in the field. The cores were sectioned on a cryo-microtome in 1 mm slices down to 3 mm. Chl a was analyzed with a Turner Design 10-AU Fluorometer (Riaux-Gobin et Klein, 1993).
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Fig. 1: Map showing location of the experiment sites in Nazareth tidal flat and marsh.
Transect A
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Fig 2: (A) High marsh site, (B) Low marsh site, (C) Mudflat site, (D) Sandflat site.
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Fig. 5: Erosion threshold crit and chl a measured on the three transects in August 2016.
References Coulombier, T., Neumeier, U., Bernatchez, P., 2012. Sediment transport in a cold climate salt marsh (St. Lawrence Estuary, Canada), the importance of vegetation and waves. Estuar. Coast. Shelf Sci. 101, 64-75. doi:10.1016/j.ecss.2012.02.014 Grabowski, R.C., Wharton, G., Davies, G.R., Droppo, I.G., 2012. Spatial and temporal variations in the erosion threshold of fine riverbed sediments. J. Soils Sediments 12, 1174-1188. doi:10.1007/s11368-012-0534-9 Neumeier, U., 2011. Boulder transport by ice on a St. Lawrence salt-marsh, pattern of pluriannual movements, Proceedings Coastal Sediments '11, World Scientific Publishing, 2533-2545. Neumeier, U., Cheng, C., 2015. Hydrodynamics and sediment dynamics in a ice-covered tidal flat. In : Coastal Sediments 2015.
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Acknowledgements World Scientific, 14 p. doi:10.1142/9789814689977-0083 Riaux-Gobin, C., Klein, B., 1993. Microphytobenthic Biomass Measurement Using HPLC and Conventional Pigment Analysis. In: Kemp, P., Sherr, B., Sherr, E., Cole, J. (eds) Handbook of methods in aquatic microbial ecology. Lewis Publishers, pp. 369-376. Tolhurst, T.J., Consalvey, M., Paterson, D.M., 2008. Changes in cohesive sediment properties associated with the growth of a diatom biofilm. Hydrobiologia. doi:10.1007/s10750-007-9099-9 Tolhurst, T.J., Jesus, B., Brotas, V., Paterson, D.M., 2003. Diatom migration and sediment armouring – an example from the Tagus Estuary, Portugal. Hydrobiologia 503, 183-193. doi:10.1023/B:HYDR.0000008474.33782.8d
We thank Bruno Cayouette and Alexandra Rao for their help in the field and Dominique Lavallée and Pascal Rioux for their help in laboratory.
