When blooms are extensive, they can blanket large areas. Macroalgal blooms can alter types and numbers of animals living in sediments - benthic animals. Animals in sediments represent important food that may be affected through a variety of pathways. Initially, the algal mats may cause an increase in numbers of individuals and species. They can form localised deposits of fine particles that provide food or the algae can be a direct source of food for herbivores.
Seaweeds can, however, also provide a refuge for or from predators.
Then, numbers of individuals and species are reduced and the whole assemblage is impoverished. Seaweeds eventually get buried by sediments and successively decompose. When the effects are only small-scale, diversity of animals in an assemblage may increase, especially by increases in animals which move slowly. When there are blankets of algal mats, the system as a whole may change dramatically. Most studies of effects of deposition of seaweeds have been done in fine, organically rich, clay sediments.