Page 126 - (DK) Ocean - The Definitive Visual Guide

124 COASTS AND THE SEASHORE

Salt Marshes and Tidal Flats

A SALT MARSH IS A VEGETATED AREA OF COAST that is partly flooded
by the sea at high tide and completely flooded by the highest
spring tides. Many areas of salt marsh are bordered by tidal flats.
These are broad areas of mud or sand, mainly without
vegetation, that are uncovered at low tide and covered as the
tide rises. Salt marshes and tidal flats are depositories for large PA CIFIC ATLANTIC OCEAN
amounts of organic material, derived from decaying plants and OCEAN
INDIAN
animals. This provides the base for an extensive food chain.
OCEAN
S O U T H E R N O C E A N
Formation and Features

Tidal flats occur on low-energy sheltered coasts, such as estuaries and enclosed bays, where DISTRIBUTION
sediment held in the water settles out and builds up. The most extensive flats occur where Salt marshes and tidal flats occur only north
of the latitude of 32˚N and south of 38˚S.
there is a high tidal range. Tidal flats may consist either of sand (sandflats) or mud (mudflats),
In latitudes nearer the equator, they are
or a mixture of these. Mudflats contain a higher concentration of the decaying remains replaced by mangrove swamps.
of dead organisms than sandflats and are also the first stage in the
development of salt marshes. These develop on the landward side COASTAL SETTING
of mudflats. As various salt-tolerant plants grow, their roots trap sediment river delta Salt marshes commonly
and stabilize the mud. As the estuary develop in coastal
vegetated flat builds up, different lagoons or in estuarine
areas that are sheltered
types of plants become established. mainland mainland from the sea by spits
The result is a salt marsh, consisting or barrier islands.
of blocks of flat, low-growing channels The channels transport
vegetated areas of mud, broken up salt water, plankton,
by sinuous channels. nutrients, sediment, and
flood delta plant detritus into and
out of the marsh.
BAY OF FUNDY
In this small sub-estuary of Canada’s
Bay of Fundy, an area of salt marsh is dunes barrier island dunes KEY
visible in the background. In the salt marsh
foreground is a broad intertidal area ebb delta inlet lagoon
of mud and gravel. tidal flats

Zones and Evolution

Salt marshes have two main zones. The parts flooded by every high tide are called low
marsh, while the areas that are only occasionally flooded are termed high marsh. Each
zone is colonized by distinct species of salt-tolerant plants. Each species, of which SEA LAVENDER
there are many, has developed special mechanisms to deal with the high levels of salt Sea lavenders are
common high-marsh
they are exposed to: some possess salt-excreting glands, for instance, while others have colonizers. They
storage systems for collecting the salt until they can dilute it with water. Salt marshes bloom in summer,
and adjoining mudflats usually evolve over time. As sediment builds up, the mud producing purple or
surface in the marsh, the adjoining flats, and the bay or estuary as a whole lavender flowers.
tends to rise. As it does so, areas of low marsh become high marsh and
areas of mudflat are colonized by plants, turning into low marsh.

SALT-MARSH CORDGRASS
OCEAN ENVIRONMENTS upland upper high marsh high marsh lower high marsh low marsh highest spring tide
SALT-MARSH ZONES
Also called smooth cordgrass,
The low marsh is the part flooded once or
this species is the dominant
twice a day at high tide, while the high marsh
low-marsh plant throughout the
is the area above the mean high-tide level—it
Atlantic coast of North America.
is flooded only occasionally, by the highest
Stands of this grass grow to
spring tides. Each zone has distinctive
7 ft (2 m) high.
vegetation.
pool
mean high tide
mean sea level

mudflat