Natural Communities of NH -- Photo Guide

Intertidal flat  (S3)


(formerly saline/brackish intertidal flat)

Sparsely vegetated intertidal flats, often referred to as simply mudflats, are exposed at low tides. They occur between salt marsh communities landward and subtidal areas seaward. There is little vascular plant growth here, but algae, worms, clams, snails, green crabs, and horseshoe crabs are all common. This community occupies a significant portion of New Hampshire’s Great Bay Estuary, as can be seen at low tide. Flats also extend up tidal rivers where they become progressively more brackish. 

The flats form in depositional environments protected from high-energy wave action in bays and rivers, or along the coast behind landforms such as rocky spits, barrier beaches, and sand bars (all of which contribute sediments to the formation of this community). The substrate, exposed completely during particularly low spring tides, ranges in composition from sand to mud and silt. Tidal creek channels exposed at low tide are included in this community.

A variety of primary foods (microalgae, phytoplankton, and detritus) supports a high level of productivity in this community, and benthic diatoms and other microalgae occurring in this environment are important contributors to the primary productivity of the entire estuarine system. Characteristic invertebrates found in New Hampshire’s intertidal mud flats include marine worms and mollusks. Arthropods are also well represented and include green crabs (Carcinus maenus), rock crabs (Cancer irroratus), flat-clawed hermit crabs (Pagurus pollicaris), and horseshoe crabs (Limulus polyphemis). During the diurnal (twice daily) tidal flooding, several species of fish and other aquatic species feed on the benthos and epibenthic algae. This community also provides important foraging habitat for shorebirds and other animals when the intertidal flat is exposed.

The substrate is composed of sand, silt, and clay rich in organic matter. Muds (comprised of silt and clay particles) contain a greater percentage of organic matter than sands. Coarser sediments are deposited in areas exposed to greater wave action and stronger currents while finer particles are deposited in lower-energy environments. Tide and wave-driven ice action during winter months can also influence sediment accretion, erosion, and transport.
Surface water salinity fluctuates widely according to seasonal variation in freshwater discharge. The upstream limit of this community occurs where salinity levels are 0.5 ppt or less during the period of annual low freshwater flow.

This community is distinguished from coastal shoreline strand/swales by the absence or very sparse cover of vascular plants and more frequent tidal flooding. It differs from intertidal rocky shores by a finer soil texture and looser (less firm) substrate.

Characteristic vegetation:
Vascular plants are sparse to (more typically) absent. Brackish flats may support populations of small spike-rush (Eleocharis parvula) and mudwort (Limosella australis). Although macroalgae is typically uncommon across the exposed substrate, New Hampshire’s intertidal flats are overall rich in seaweed species (169 seaweed species have been documented in the Great Bay estuary).  


Good examples of this community can be found throughout Great Bay, and at Hampton Marsh (Hampton/Seabrook), Sagamore Creek (Portsmouth), and Rye Harbor State Park (Rye).

Intertidal flats are one of the primary communities in the sparsely vegetated intertidal system.


Intertidal flat near the mouth of Crommet Creek at Adams Point in Durham (photo by Ben Kimball) 
Intertidal flat near the mouth of Crommet Creek
at Adams Point in Durham (photo by Ben Kimball)

Intertidal flat at Sagamore Creek (photo by Ben Kimball for the NH Natural Heritage Bureau)
Intertidal flat at Sagamore Creek in Portsmouth (photo by Ben Kimball)


Intertidal flats (center) are used as foraging ground by gulls and wading birds (photo by Ben Kimball for the NH Natural Heritage Bureau)
Intertidal flats (center) are used as foraging ground by gulls and wading birds
(photo by Ben Kimball)

strip of exposed intertidal flat at Wagon Hill Farm in Durham (photo by Ben Kimball)
strip of exposed intertidal flat at Wagon Hill Farm in Durham (photo by Ben Kimball)

(Submerged) intertidal flats between tidal creeks in Great Bay (photo by Ben Kimball for the NH Natural Heritage Bureau)
(Submerged) intertidal flats between tidal creeks
in Great Bay (photo by Ben Kimball)
 

back to Natural Communities of NH Photo Guide