Coastal wetlands are also important for keeping coastal waters clean. They filter sediment and chemicals, reducing the amount of pollution that washes into bays and the ocean. The mixing of nutrients from land and sea that occurs in salt marshes produces more organic matter per acre than the richest Midwestern farmland. A small percentage of salt marsh grasses are eaten directly by fish and invertebrates, but most is decomposed by bacteria, producing an organic soup that feeds organisms such as amphipods, shrimp, crabs, snails, shellfish and finfish. These organisms in turn support a broad food chain that includes a variety of shellfish and finfish populations.
Salt marshes are important to numerous species who are in the salt marsh only part-time.
Salt marsh plant stems create a very dense canopy with a diverse community of organisms growing on them. Rising and falling tides move both the organic soup and small feeding organisms between the salt marsh and adjacent coastal waters. The abundance of small habitats within a salt marsh allows for numerous organisms to hide from predators, feed without expending much energy, grow faster, and raise young. In addition to the transient organisms, salt marshes shelter species that may spend their entire life in the salt marsh. Although many species leave salt marshes with the ebbing tide, killifish take refuge in small salt marsh ponds, fiddler crabs hide in burrows, and mussels close their shells and await the next flood tide.