ANALYSIS OF SPATIAL AND TEMPORAL TRENDS IN INORGANIC SEDIMENT CHARACTERISTICS AND DISTRIBUTION ACROSS A NEW ENGLAND SALT MARSH

Salt marshes will be among the first environments to respond to changes in sea level and seasonality, via changes in bioproductivity and the accretion of tidally deposited inorganic sediment. Shifting patterns of precipitation and evaporation, and duration of summer drought, winter ice-over, and the growing season all directly influence organic and inorganic sediment retention on the marsh surface. The tidally-deposited sediment load varies seasonally and spatially across the study site on the Essex River marsh in Ipswich, Massachusetts, a 18.8 km2 lower-mesotidal New England meadow marsh. Sediment pads deployed in a grid and transect pattern across the low, transitional, and high marsh platform were monitored for quantity and quality of tidally deposited sediment. GIS was used to assess the spatial and temporal variability in deposition. Grain size data was obtained using a Laser Diffraction Particle Size Analyzer. Percent organics was determined by loss-on-ignition. Surface peat samples provide multi-year trends, which are used to determine seasonal control on long-term salt marsh deposition and grain size. The highest rates of deposition occurred in summer, followed by winter and fall seasons. Ice rafted sediment deposits were excluded from analysis, resulting in an average coarser mean grain size in the summer months. The average grain size distribution curve in fall is similar to the distribution in winter and is significantly skewed towards fines. Spatially, grain size distributions were more negatively skewed at higher elevations within the marsh, farther from creeks and ditches. These trends controvert expectations that summer's higher vegetation density would preclude larger grains from deposition farther from tidal channels and creeks. The influence of elevation, vegetation, temperature patterns and storms are examined in reference to these trends.