Parts of the ocean bed are covered with sand and mud.  These materials are evidence of the work of erosion.  Natural forces break down rocks into billions of grains of sand, and weak carbonic acid in the rains helps to loosen the little flakes of clay.  Rivers and streams carry the sand and clay downhill toward the sea..  These mineral particles form the beaches, sand dunes, and tidal flats that stretch out along the ocean shores in many places in the world.

     When we look at expanses of sand and mud, they often look lifeless, because there may be nothing growing on them.  Pounding waves make anchorage impossible in sand, and so algae do not grow on sand in the intertidal zone.  

Sand

    The sea rolls over the sand, and when the waves pull back, the surface of the sand is wet and smooth.  In a few seconds bubbles appear on the  surface and pop, leaving behind small holes.   What causes the holes?

    Even though there is nothing stable here for seaweeds to cling to, there are living creatures that burrow down into the wet sand to hide from predators.  When the tides are out, these animals wait in safety.  When the water covers them they can come to the surface of the sand to trap tiny particles of food in the incoming waves.  Below the low tide line, burrowers can feed when they wish.  Other life forms in the sand are tiny, tiny snails and worms and little creatures with segmented bodies like tiny shrimp.

     There are sandy patches in many places in the sea near the shores.  They provide habitats for organisms that could not live on the rocks, and so contribute to the diversity of the undersea community.

Mud
     Mud flats often underlie productive waters.  Mud is carried to the sea by mature rivers that are slowly flowing over their flood plains.  When they get to the sea, or even in their twisting and turning, they drop the fine mineral flakes that make up the mud.  This mud, called silt, is rich in organic materials, and so can support many micro- organisms, little worms, and small crustaceans.

     Many of our wetlands are mudflats.  As you can see in this picture, there is some low-growing vegetation above the high water line.  The sea water comes in further with the tides.  Mud flats  really are flat, so a small difference in  the height of the sea changes the water level.

     Tidal mud flats are visited by migrating water birds, and mud flats covered by deeper water support the vigorous growth of phytoplankton.  Newly hatched fishes and invertebrates, such as oysters and sea anemonies, eat the phytoplankton and grow. 

     However it is difficult for algae to anchor themselves in mud or sand, and they face the possibilities of being buried or washed away. 

  Interestingly enough, these areas can be colonized by sea grasses.  Sea grasses are true plants that developed on land and then returned to the sea.  These land plants have roots and underground runners, and so can anchor themselves in these difficult environments.  These plants have tough blades, but support a rich assortment of life forms by providing hiding places and many leaves.  When the leaves die, the  detritovores can break down to liberate the nutrients for the use of phytoplankton.  Areas below the low tide line, where the bottom is soft, are very often underwater meadows of Eelgrass.