Indian River Lagoon
Species Inventory

Tidal Flats

In some areas of the Indian River Lagoon, when the tides recede, an ephemeral landscape reveals itself. At first glance, the lagoon’s silty-sandy bottom may seem like a barren mudscape – but just below the surface an abundance of life is burrowed in, waiting for the return of the lagoon’s waters.

Photo credit: D. Fischer

Covered at high tide and exposed at low tide, tidal flats are dominated by soft sediments and generally lack vegetation. Found worldwide, they are common elements of estuaries, and are the basic landform upon which coastal wetlands accumulate. In the Indian River Lagoon, tidal flats are most abundant near inlets, where tidal influence is strongest.

Tidal flats comprise only about 7 percent of total coastal shelf areas, but are highly productive ecosystems. Though overall biological diversity may be relatively low, tidal flats can contain astounding volumes of microorganisms and benthic infauna, or tiny animals that live in the top layer of sediment. In addition to recycling organic matter and nutrients from terrestrial and marine sources, benthic infauna are also prey for many fin and shellfish species, as well as resident and migratory wetland birds.

Credit: M. Donahue

Mud vs. Sand

Tidal flats are highly dynamic, with sediments continuously on the move. Depending on sediment grain size, tidal flats are generally categorized as either mud or sand flats.

Mudflats usually occur in the upper portion of the intertidal zone, and in areas with low-energy water movement. Here, sediments contain a high proportion of fine silt and clay particles. Mudflats have higher organic content, generally from microbial activity or from adjacent sources such as salt marshes, mangroves and seagrass beds.

Sandflats occur in areas with stronger currents and moderate wave action that can carry larger, heavier sediment particles. Sediments are mostly quartz (silica) derived from erosion. In southern Florida systems, mud-sand and combinations of calcium carbonate coral rock soft-bottom types are common.

Fort Pierce Inlet. Credit: D. Ramey Logan, Wikimedia Commons

Both types of tidal flat occur on Coon Island, along the north side of Fort Pierce Inlet in the Indian River Lagoon. The eastern edge of the island gives way to a relatively large tidal flat. On the southern end, where currents are relatively strong, sediments are sandy; on the northern and western areas, which are more protected from inlet currents, sediments are muddier.

Mud and sand flats also differ in their oxygen concentrations, which influence microbial activity. This activity stabilizes seasonal variation in organic material, ensuring a more consistent food supply for other organisms.

In mudflats, the fine sediments trap detritus and prevent water from easily percolating through. The higher surface area of the numerous fine grains allows for higher numbers of microbes, which leads to increased anaerobic decomposition of organic matter. This activity produces hydrogen sulfide, methane and ammonia in an oxygen-poor layer, roughly .4 inches (1 cm) below the surface. Often black in color, this layer is visually striking in its contrast to the thin, grayish oxygenated layer above it.

In sandflats, the large-grained particles allow water to percolate easily through sediments, which allows oxygen to penetrate as deep as 4 to 8 inches (10 to 20 cm) below the surface. Light can also filter deeply, allowing for prolonged activity by photosynthetic microorganisms.

Black-necked stilt with chick. Credit: U. Dubrick

Ecological Pros and Cons

For benthic organisms, life in the muddy sands of tidal flats affords many advantages. They can retreat into deeper sediments or burrows when threatened by predation. Able to move around, infaunal bivalves can survive partial predation as well as direct competition with burrowing neighbors. Desiccation is rarely an issue. Finally, organic materials accumulating on sediments provide a ready, constant food source.

But there are drawbacks: lack of a securing "anchor" in the sediment. In contrast to rocky intertidal habitats, where organisms are often securely attached to the rock via cement, byssal threads and muscular feet, tidal flat organisms are at the mercy of the sediments. During periods of severe storm erosion, larger infauna in soft bottom habitats may become easily dislodged and subsequently displaced.

Nine-armed seastar. Credit: M. Donahue

Threats to Tidal Flats

Clean water and sediments are critical for healthy lagoon habitats. Tidal flat areas face a number of human-made and natural threats, including sea level rise, loss of habitat, salinity fluctuations, pollution, erosion and invasive species. Threats to tidal flats directly mirror threats to the larger Indian River Lagoon.

For more information on the challenges facing the tidal flats and other areas of the lagoon, visit the Threats resource page.

Tidal Flat Species

Tidal flats host a diverse biotic assemblage, ranging from microscopic organisms to large crabs, fish and wading birds.

Man o’ war jellyfish during low tide on the IRL. Credit: M. Donahue

The majority of organisms in tidal flats are considered to be benthic, or living in or on the lagoon bottom. Though most are extremely tiny, such as bacteria and diatoms, some, including parchment worms and the nine-armed sea star, can grow to be quite large.

  • Microbenthos comprise primarily bacteria and diatoms.
  • Meiobenthos are usually less than a millimeter in length, which live in the void spaces between relatively large sand grains in sediments.
  • Hyperbenthos are slightly larger, a few millimeters in length, and live in the water just above the lagoon floor as well as in the very top layers of the sediment.
  • Macrobenthos are larger and can move freely through soft sediments, and include polychaete worms, bivalves and amphipods.
  • Epibenthos are large, predatory and grazing species including crabs, mollusks, fish, rays, wading birds and mammals.