Soft-Bottom and Other Mobile Substrates

More than 90% of the estuary’s bottom is composed of particles small enough to be moved by tidal currents. Soft-bottom habitat includes the soft substrate, organisms living on or within the substrate, and the overlying water column.

Soft-bottom habitat includes sediments that range in size from clay (0.001 - 0.0039 mm) to silt (0.0039 - 0.0625 mm), and sand (0.0625 - 2 mm). “Mud” refers to clay and silt together. All of these particles can readily be moved by tidal currents. Larger particles such as gravel (2 - 64 mm) and cobble (64 - 256 mm), are somewhat mobile and are also included in this category. Deposits of bivalve shells can be mobile and are also considered in this section.
This habitat is threatened by construction activities, dredging and sand mining activities and deposition of dredged material, wakes from ships and ferries, and a variety of contaminants, including some toxic “hot spots.” Soft-bottom habitat may also be threatened by an overall decrease in sediment supply from upstream, and by sea-level rise. The report therefore recommends that the quality of this habitat be improved and that it be managed properly.

To read more, download the soft-substrate section of our report.




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