The great majority of the shelf’s soft-bottom communities lack significantamounts of seaweeds or seagrasses. They are therefore known as unvegetated communities. The absence of large sea- weeds and plants is the defining featureof these communities. Sea lettuce (Ulva) and other green algae such as Entero-morpha may grow in the shallows, butonly on hard surfaces such as rocks andshell fragments. The main primary pro-ducers are diatoms and a few other mi-croscopic algae and bacteria that grow on sand or mud particles in shallow water. This almost complete absence of seaweeds and plants means that, as onsandy beaches, the primary productionby benthicprimary producersis practi-cally nil. Nearly all of the primary pro-duction is by phytoplankton that are notpart of the bottom communities. Because there is little primary pro-duction by benthic organisms, detritus is a very important food source for many in-habitants. Detritus is brought in by cur-rents from estuaries, rocky shores, andother more productive coastal commu-nities. It is also present as feces, dead individuals, and other debris from theplankton and nekton in the water column.