Tidal Flats

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.

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.

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.

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.

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.

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.

Further Reading

• Ambrose, W. G. 1984. Role of predatory infauna in structuring marine soft-bottom communities. Mar. Ecol. Prog. Ser. 17(2): 109-115.
• Amos, C. L. 1995. Siliciclastic tidal flats. In: Perillo, G. M. (Ed.), Geomorphology and Sedimentology of Estuaries. Elsevier, Amsterdam. pp. 273-306.
• Bell, S. and B. Coull 1978. Field evidence that shrimp predation regulates meiofauna. Oecologia 35: 141-148.
• Beyer, F. 1958. A new, bottom-living trachymedusa from the Oslo fjord. Nytt Mag. Zool. 6: 121-143.
• Bertness, M. D. 1999. The Ecology of Atlantic Shorelines. Sinauer Associates, Inc., Sunderland. 417 pp.
• Black, K. S., T. J. Tolhurst, S. E. Hagerthey and D. M. Paterson. 2002. Working with natural cohesive sediments. J. Hydraulic Eng. Forum 128: 1-7.
• Bottom, M. L. 1984. The importance of predation by horseshoe crabs, Limulus polyphemus, to an intertidal sand flat community. J. Mar. Res. 42: 139-161.
• Coelho, V. D., R. A. Cooper and S. Rodrigues. 2000. Burrow morphology and behavior of the mud shrimp Upogebia omissa (Decapoda: Thalassinidea: Upogebiidae). Mar. Ecol. Prog. Ser. 200: 229-240.
• Commito, J. A. and W. G. Ambrose. 1985. Multiple trophic levels in soft-bottom communities. Mar. Ecol. Prog. Ser. 26: 289-293.
• Coull, B. C. 2009. Role of meiofauna in estuarine soft-bottom habitats. Austral Ecol. 24(4): 327-343.
• de Brouwer, J. F. and L. J. Stal. 2001. Short-term dynamics in microphytobenthos distribution and associated extracellular carbohydrates in surface sediments of an intertidal mudflat. Mar. Ecol. Prog. Ser. 218: 33-44.
• Dyer, K. R. (Ed.), 1979. Estuarine Hydrography and Sedimentation. Estuarine and Brackish Water Sciences Association. Cambridge University Press, Cambridge. 230 pp.
• Dyer, K .R., M.C. Christe and E. W. Wright. 2000. The classification of mudflats. Cont. Shelf Res. 20: 1061-1078.
• Felder, D. L. and R. B. Manning. 1986. A new genus and two new species of Alpheid shrimps (Decapoda: Caridea) from south Florida. J. Crust. Biol. 6(3): 497-508.
• Giere, O. 2009. Meiobenthology. The microscopic motile fauna of aquatic sediments. Springer-Verlag, Berlin. 527 pp.
• Hendler G., J. E. Miller, D. L. Pawson , and P. M. Kier. 1995. Sea Stars, Sea Urchins, and Allies. Smithsonian Institution Press, Washington, D. C. 390 pp.
• Higgins, R. P. and H. Thiel. 1988. Introduction to the study of meiofauna. Smithsonian Institution Press, Washington, D. C. 488 pp.
• Holligan, P. M. and W. A. Reiners. 1992. Predicting the responses of the coastal zone to global change. Adv. Ecol. Res. 22: 211-215.
• Koulouri, P. Preliminary study of hyperbenthos in Heraklion Bay (Cretan Sea). Accessed 5 April 2010. Available at: https://www.biomareweb.org/3.6.html.
• Little, C. 2000. The Biology of Soft Shores and Estuaries. Oxford University Press, Oxford. 252 pp.
• MacIntyre, H. L., R. J. Geider and D. C. Miller. 1996. Microphytobenthos: the ecological role of the “Secret Garden” of unvegetated, shallow-water marine habitats. I. Distribution, abundance and primary production. Estuaries 19: 186-201.
• McIntyre, A. D. 1968. The macrofauna and meiofauna of some tropical beaches. J. Zool. 156: 377-392.
• Mees, J. and M. B. Jones. 1997. The hyperbenthos. Ocean. Mar. Biol.Ann. Rev.35: 221-255. Mitbavkar, S. and A. C. Anil. Diatoms of the microphytobenthic community: population structure in a tropical intertidal sand flat. Mar. Bio. 140: 41-57.
• Myers, R. L. and J. J. Ewel (Eds.), 1990. Ecosystems of Florida. U. of Central Florida Press, Orlando. 765 pp.
• Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. Oxford University Press, Oxford. 578 pp.
• Nybaken, J. W. and M. D. Bertness. 2005. Marine Biology: an Ecological Approach. Benjamin Cummings Publishers, San Francisco. 579 pp.
• Olafsson, E. B. , C. W. Peterson and W. G. Ambrose. 1994. Does recruitment limitation structure populations and communities of macro-invertebrates in marine soft sediments? The relative significance of pre- and post-settlement processes. Ocean. Mar. Biol. Ann. Rev. 32: 65-109.
• Orth, R. J. 1975. Destruction of eelgrass, Zostera bonasus, in Cesapeake Bay. Chesapeake Sci. 16: 205-208.
• Paterson, D. M., R. J. Aspden and K. S. Black. 2009. Intertidal flats: ecosystem functioning of soft sediment systems. In: Perillo, G. M., E. Wolanski, D. R. Cahoon and M. M. Brinson (Eds.), Coastal Wetlands An Integrated Approach. Elsevier, Amsterdam. Pp. 317-343.
• Peterson, C. H. 1979. Predation, competitive exclusion, and diversity in the soft-sediment benthic communities of estuaries and lagoons. In: Livingston, R. J. (Ed.), Ecological Processes in Coastal Marine Systems. Plenum Press, New York. 548 pp.
• Posey, M. H., B. R. Dumbauld and D. A. Armstrong. 1991. Effects of burrowing mud shrimp, Upogebia pugettensis (Dana), on abundances of macro-infauna. J. Exp. Mar. Biol. Ecol. 148: 283-294.
• Quammen, M. L. 1982. Influence of subtle substrate differences on feeding by shorebirds on intertidal mudflats. Mar. Biol. 71: 339-343.
• Rice, M. E., J. Piraino & H. F. Reicherdt. 1995. A survey of the Sipuncula of the Indian River Lagoon. Bu. Mar. Sci. 57(1): 128-135.
• Robertson, A. I. 1988 Decomposition of mangrove leaf litter in tropical Australia. J. Exp. Mar. Biol. Ecol. 116: 236-247.
• Ruiz, G. M., J. T. Carlton, E. D. Grosholz, and A. H. Hines. 1997. Global invasions of marine and estuarine habitats by non-indigenous species: mechanisms, extent, and consequences. Am. Zool. 37: 621-632.
• Schmalzer, P.A. 1995. Biodiversity of saline and brackish marshes of the Indian River Lagoon: historic and current patterns. Bull. Mar. Sci. 57(1): 37-48.
• Sibert, J. R. 1981. Intertidal hyperbenthic populations in the Nanaimo Estuary. Mar. Biol. 64: 259-265.
• Stal, L. J. 2003. Microphytobenthos, their extracellular polymeric substances, and the morphogenesis of intertidal sediments. Geomicrobio. J. 20 (5): 463-478.
• Stal, L. J. and F. C. de Brouwer. 2003. Biofilm formation by benthic diatoms and their influence on the stabilization of intertidal mudflats. Berichte -Forschungszentrum Terramare 12: 109-111.
• Stanley, S. M., 1970. Relation of shell form to life habits of the bivalve molluscs. Geol. Soc. Am. Monographs. 125: 1-296.
• Stutz, M. L. and O. H. Pilkey. 2002. Global distribution and morphology of deltaic barrier island systems. J. Coast. Res. 36: 694-707.
• Thrush, S. F., R. D. Pridmore, R. G. Bell, V. J. Cummings, P. K. Dayton, R. Ford, J. Grant, M. O. Green, J. E. Hewitt, A. H. Hines, M. T. Hume, S. M. Lawrie, P. Legendre, B. H. McArdle, D. Morrisey, D. C, Schneider, S. J. Turner, R. A. Walters, R. B. Whitlatch and M. R. Wilkinson. 1997. The sandflat habitat: scaling from experiments to conclusions. J. Exp. Mar. Biol. Ecol. 216: 1-9.
• Van der Wal, D., P. M. Herman, R. M. Forster, T. Ysebaret, F. Rossi, E. Knaeps, Y. M. Plancke and S. J. Ides. 2008. Distribution and dynamics of intertidal macrobenthos predicted from remote sensing: response to microphytobenthos and environment. Mar. Ecol. Prog. Ser. 367: 57-72.
• Virnstein, R. W. 1977. The importance of predation by crabs and fishes on benthic infauna in Chesapeake Bay. Ecol. 58: 1199-1217.
• Watzin, M. 1983. The effects of meiofauna on settling macrofauna: meiofauna may structure macrofaunal communities. Oecologia 59: 163-166.
• Winkler, G. and W. Greve. 2004. Trophodynamics of two interacting species of estuarine mysids, Praunus flexuosus and Neomysis integer, and their predation on the calanoid copepod Eurytemora affinis. J. Exp. Mar. Biol. Ecol. 308: 127-146.