Under conditions suitable for deposition, particles of various sizes accumulate to form a loose substrate of pebbles, sands, silts, or clays, as opposed to the solid rock of rocky coasts. Actual particle size is a function of the velocity of longshore currents, strength of wave action, and the types of particles available for transport to the site.
Four main kinds of soft-sediment habitats are recognized:
- Shingle or pebble beaches with steep slopes and strong wave action
- Open sandy beaches semi-exposed to the open sea and experiencing significant wave action. The sands and profile of the beach are often reworked during storms. Wind-blown dunes form along the landward margin.
- Protected sandy beaches sheltered from strong wave action. Their gentle slopes are formed in finer sands.
- Protected mudflats at the heads of inlets and on the landward sides of barrier islands. Wave action is minimal, allowing silts and the finest particles of organic detritus to settle out. Salt marshes characterize such habitats in temperate climes; mangroves in tropical regions.
Soft-sediment environments differ from rocky coasts in two key ways:
1. They are three dimensional: not only are horizontal zones formed according to elevation and tidal range, but vertical layers are formed within the substrate. Organisms live on the beach as well as in the beach.
2. Instability is a given since small particles move in the swash and backwash of waves on the beach. Bioturbation—disturbance of the sediments by the action of the beach’s inhabitant also commonly occurs. Inhabitants of the beach, themselves, are highly mobile and their distribution patterns change frequently in response to the physical changes in the habitat that occur daily and seasonally. Indeed, the entire beach may disappear and reappear after storms and from summer to winter.
A major control of the distribution of life on a beach is the rate at which water infiltrates the sediments. In coarser deposits, upper levels are dry at low tide and the ebb and flow of seawater removes wastes and re places nutrients. Fine particle beaches and tidal flats, on the other hand, remain saturated even at low tide and become stagnant.
Vertical stratification develops in sandy beaches at low tide. The surface zone becomes dry due to evaporation and the downward draining of water due to gravity. A zone of retention lies below the surface where water is lost by gravity but replaced by water drawn upward by capillary action. This zone has the best combination of water, oxygen, food, and substrate stability to support life. Below it is a zone of resurgence which receives the water pulled down from above by gravity. Deepest is the zone of saturation; it becomes stagnant and depleted of oxygen.