Tidal Flat Habitats
Fig. 1. Indian River Lagoon tidal flat: (A) view of flat on eastern edge of Coon Island, Ft. Pierce, FL.; (B) exposed area of flat with Ft. Pierce Inlet in background; (C) protected area of flat with Tucker Cove in background.
What are Tidal Flats?
Tidal flats are intertidal, non-vegetated, soft sediment habitats, found between mean
high-water and mean low-water spring tide datums (Dyer et al. 2000) and are generally located in
estuaries and other low energy marine environments. They are distributed widely along coastlines
world-wide, accumulating fine-grain sediments on gently slopping beds, forming the basic structure
upon which coastal wetlands build. Although tidal flats comprise only about 7% of total coastal
shelf areas (Stutz and Pikey 2002), they are highly productive components of shelf ecosystems
responsible for recycling organic matter and nutrients from both terrestrial and marine sources and
are also areas of high primary productivity. In the Indian River Lagoon (IRL), tidal flats can
occupy significant areas (Schmalzer 1995) but are most prominent and abundant in the vicinity of
inlets where tidal influence is strongest.
Why are Tidal Flats Important?
Tidal flats are highly productive areas and although biological diversity may be
relatively low, tidal flats support a high biomass of micro- and infaunal organisms, support large
fin and shellfish stocks and play an important role in intertidal nutrient chemistry. Tidal flats
provide enormous water carrying capacity, protecting areas of the IRL from storm surge as well as
storm water runoff. Tidal flats along with intertidal salt marshes and mangrove forests constitute
IRL wetlands and are a vital part of the lagoon ecosystem. Tidal flats will often form the buffer
zone between deeper reaches of the lagoon thereby protecting intertidal habitats by dissipating wave
energy, thus reducing erosion of mangroves and salt marshes. Collectively these intertidal habitats
are of great importance to large numbers of invertebrates and fish, supporting complex estuarine
food webs and provide resting and feeding areas to large numbers indigenous and migratory birds.
The Physical Setting
Zonation of tidal flats may be divided into three parts (Amos 1995): 1) the supratidal
zone, located above high water; 2) the intertidal zone, located between high and low water; and 3)
the subtidal zone which occurs below low water and is rarely exposed to the atmosphere. Most studies
of tidal flats concentrate on the intertidal region.
Tidal flats are highly dynamic systems, in constant motion. The ETDC, refers to the
Erosion, Transport, Deposition and Consolidation cycle whereby sediments within intertidal flats are
transported continuously. Although the physical aspects of this cycle are well understood,
predictions of sedimentation are difficult because of site-specific differences (Black et al.
Depending on sediment grain size, tidal flats may be generally categorized as either mud
or sandflats. Generally, mudflats are located in the upper part of the intertidal zone, sandflats
are located in the lower part. Mixed sand-mud flats can occur between the two systems but this
zonation may be modified by the presence of tidal creeks and the vagaries of sediment flux. The
distribution of mud and sandflats along a shoreline is primarily due to the relative strength of
prevailing water currents.
Rapidly moving water will tend to carry larger, heavier sediment particles, washing away
smaller particles and preventing their deposition. Hence tidal flats with low energy water movement
are characterized by more muddy sediments whereas in higher energy regimes, with stronger currents
and moderate wave action, flats are generally composed of courser sandy sediments. For example, Coon
Island is located in the Indian River Lagoon along the north side of Fort Pierce Inlet. The eastern
edge of the island gives way to a relatively large tidal flat (Fig. 1A). Sediments on the southern
end of the tidal flat, in close proximity to the inlet where currents are relatively strong, are
predominantly sandy (Fig. 1B). On the northern and western portions of the flat, more protected from
inlet currents, tidal flat sediments are more muddy (Fig. 1C). Nearby, in the vicinity of Jack
Island, but further distanced and protected from inlet currents, low tide exposes a mud flat (Fig.
2) surrounding a small mangrove island.
Fig. 2. Indian River Lagoon mudflat, Ft. Pierce, FL.
Mudflats have higher organic loads than sandflats. Organic material may be derived from
in situ production or come from adjacent coastal sources (salt marshes, mangrove forests, seagrass
beds). Muddy systems are composed of sediments containing > 80 % silt and clay (particle size < 63
µm) (Dyer 1979). Silts are fine inorganic particles held in suspension by slight water
movement, while clay particles are colloids of hydrated aluminum silicate with iron and other
Sediments of sandflats are comprised of larger, independent grains, mostly quartz
(silica) derived from erosion. Sandy sediments can be divided into three size categories: 1) course
(0.5 - 1.0 mm); medium (0.25 - 0.5 mm); and fine (0.063 - 0.25 mm). In southern Florida systems,
mud-sand and combinations of coral rock (Ca CO3) soft-bottom types are common.The CaCO3 sediments of
southeastern Florida are composed primarily of codacean algal plates (Myers and Ewel 1990).
Fig. 3. Burrow openings of infaunal organisms: (A) polychaete worm; (B & C) stomatopods
Topographic features of tidal flats in different flow regimes can also differ. In low flow areas,
i.e., areas more characteristic of mudflats, surfaces are generally smooth, occasionally interrupted
by burrow openings (Fig. 3 A, B & C), pellets (Fig. 11), and fecal casts (Fig. 13) created by
infaunal organisms. Sandflat surfaces can be noticeably carved by prominent ripple structures (Fig.
4) caused by wave action on the sandy bed.
Fig. 4. Tidal flat ripples in Indian River Lagoon, FL.
Mud and sand flats also differ in their vertical concentration of oxygen content which
influences microbial activity. Microbial activity in tidal flats is significant: it stabilizes
organic fluxes by reducing seasonal variation in primary productivity thereby ensuring a more
constant food supply (Robertson 1988); and, the sheer bacterial biomass in these environments rivals
that of the animals living within the sediment. In muddy sediments, several factors contribute to
more extensive anoxic areas below the surface because of higher oxygen uptake. The lower
permeability (i.e., the amount of water flowing though sediment) in fine sediments tends to trap
detritus. Higher microbial numbers, due to the increased surface attachment area of fine grains,
leads to increased anaerobic degradation of the detrital matter, producing hydrogen sulfide, methane
and/or ammonia. The resulting black, anoxic, reducing layer, occurring < 1 cm below the surface
can be strikingly differentiated from the relatively thin oxygenated layer occurring above it.
Between the two is a grayish layer in which the redox potential (Eh) (a measure reflecting the
balance between oxidation and reduction processes) decreases rapidly. This layer is referred to as
the redox potential discontinuity layer (RPD) (Little 2000).
With increased depth, microbial activity becomes chemosynthetic (producing energy from
chemical bonds). In contrast, on sandflats, water easily percolates through the sediment,
resulting in oxygen penetrating as far as 10 to 20 cm below the surface. The relatively large sized
sand grains also provide enough void space to allow for occupancy by a group of organisms called
meiofauna (muticellular organisms < 1 mm in length) – see below. In addition, light
penetration is much deeper in sandy as opposed to muddy environments, allowing for prolonged
photosynthesis by the microphytobenthos (organism inhabiting interstitial spaces between sediment
particles) – also see below - even during tidal submersion.
Tidal Flat Organisms
Tidal flats play host to a diverse biotic assemblage ranging from microscopic organisms
found adhering to and living within interstitial spaces of sediment particles to large epibenthic
forms such as crabs, fish and wading birds. Paterson et al. (2009) classify benthic organisms
associated with tidal flats into essentially 5 categories based on size and lifestyle: 1) the
Microbenthos includes bacteria, diatoms, euglenoids and ciliates; 2) the Meiobenthos (aka Meiofauna)
are comprised of multicellular organisms less than 1 mm, occurring within the interstices of the
sediment grains; 3) the Hyperbenthos are small (a few mm in length) organisms occurring in the water
column just above the surface but may also be found within the sediment; 4) the Macrobenthos
includes organisms over 1 mm in length that move freely through soft sediments, e.g., polychaete
worms, bivalves and amphipods; and 5) the Epibenthos are comprised of large, active predatory and
grazing species such as crabs, molluscs, fish, rays, wading birds and mammals.
Paterson et al. (2009) further subdivide the microbenthos into 4 subcategories: i) the
Picoheterobenthos which includes bacteria and viruses; ii) the Picophytobenthos which includes
photosynthetic cells < 2 µm; iii) the Microphytobenthos which are comprised of unicellular
photosynthetic organisms > 2 µm; and iv) the Microheterobenthos which are unicellular
heterotrophic organisms > 2 µm.
Of these subgroups, the microphytobenthos is perhaps the most extensively studied and
represent an interesting and ecologically significant group. The microphytobenthos include
unicellular, eukaryotic algae (primarily benthic diatoms), cyanobacteria and flagellates. This
assemblage of organisms often imparts a brown, green (Fig. 5) and/or golden brown film on the
surfaces of tidal flats during daytime low-tide periods as they migrate vertically from depths of 1
to 2 mm (Little 2000). Growing within the illuminated surface tidal flat sediments, the
microphytobenthos play a significant role in system productivity, trophodynamics and sediment
stability (MacIntyre et al. 1996). In fact, the microphytobenthos can be the most important
primary producers in coastal ecosystems with large intertidal flats and can provide a substantial
food source for the meio- and macrobenthos. Dense, rigid, microbial mats on fine sand sediments
result from cyanobacterial activity whereas biofilms of epipelic diatoms are generally found on
mudflats (Stal 2003).
Fig. 5. Tidal flat surface colored green by epipelic microalgae.
Benthic diatoms may be classified into 2 categories: 1) the epipelon; and 2) the
epipsammon. Diatoms that belong to the epipelon move actively in the surface layers while those
belong to the epipsammon are attached to sediment grains and have limited mobility. Vertical
migratory behavior of epipelic diatoms is an adaptive strategy controlled more by light than tides
(Mitbavkar and Anil 2004). In mudflats, the cohesive nature of silt particles, due to their charged
nature and organic coating, provide some surface stability to the sediment flat. Equally important
in the stability of mudflat surfaces is the production of extracellular polymeric substances (EPS)
by epipelic diatoms. EPS consist mostly of polysaccharides, are independent of photosynthesis,
and are produced by epipelic diatoms in association with their motility.
Thus extensive surface biofilms on intertidal mudflats, resulting from EPS matrix production,
produces a protective micro-environment embedded with biofilm organisms (de Brouwer and Stal 2000,
Stal and de Brouwer 2003). This biofilm has been thought to increase the sediment erosion threshold
although this relationship has been questioned (Stal 2003). Recent studies using remote sensing have
found that the microphytobenthos combined with sediment characteristics provide a reliable predictor
of the distribution and dynamics of intertidal macrobenthos (van der Wal et al. 2008).
The Meiobenhtos (or meiofauna) (from the Greek word “meio” meaning smaller) include a
host of multi-cellular organisms, less than 1 mm in length, living interstitially among sediment
particles in a wide range of marine and freshwater habitats including estuarine sand and mudflats.
Meiofauna are entirely aquatic, requiring water within interstitial spaces to survive. Average
densities of meiofaunal organisms are approximately 106 per square meter of substratum but represent
only a few grams of biomass. Higher densities usually occur in softer, muddy, sheltered areas. This
is thought to be, in part, a consequence of the increased bacterial food supply, i.e., the smaller
mud particles providing more surface area for increased bacterial attachment and growth. Predators
as well as physical disturbances can also affect population densities of meiofauna (Bell and Coull
1978), but since most meiofaunal organisms reproduce so rapidly, predators cannot significantly
reduce their abundances.
Temporary meiofauna are represented by macrofaunal larvae and juveniles and are part of
the meiobenthos only during a portion of their life history. Permanent meiofauna are part of the
meiobenthos throughout their entire life cycle (McIntyre 1968), e.g., nematodes, harpacticoid
copepods, ostracods. Nematods are usually the most abundant member of meiofaunal assemblages with
harpacticoid copepods second in abundance. Although nematodes, copepods and turbellarians (Fig. 6 A)
usually comprise more than 95% of the meiofaunal community, most phyla have meiofaunal
representatives. Minor phyla represented in the meiofauna include the gastrotrichs (Fig. 6 B),
kinorhynchs, rotifers, tardigrades, priapulids and loriciferans. See Nielsen (2001) for a taxonomic
listing of meiofaunal organisms. Grain size is important in determining the size and types of
meiofaunal organisms present. For example, coarse grain sediments have greater interstitial volume
accommodating relatively larger meiofauna as opposed to fine grain sediments where burrowing forms
(e.g., kinorhynchs) are more likely to be present.
Fig. 6. IRL meiofauna: (A) Lehardyia spp. (Phylum: Platyhelminthes); (B) Tetranchyroderma bunti (Phylum: Gastrotricha). Photos by Rick Hochberg.
In intertidal flats with a relatively high mud content, the majority of mieofauna are
found in the upper 2 cm of sediment, usually dictated by the relative depth of the RPD (redox
potential discontinuity). However, in coarse, well oxygenated sediments, meiobenthos can be found at
deeper depths. Meiofauna of upper, more exposed layers of sediment include forms with greater
tolerance to salinity and temperature fluctuations. Because of the stability and complexity of
interstitial habitats, the diversity of the meiofaunal community far outnumbers that of the
associated macrofauna. It has been shown that meiofauna can also affect the densities of macrofaunal
larvae and juveniles recruiting to the benthos (Watzin 1983).
Meiofauna play an integral role in estuarine food web dynamics. As mentioned above, by
feeding on bacteria as well as benthic diatoms and protozoans, meiofauna provide a link to higher
trophic level consumers. For example, meiofaunal copepods serve as a food source for several
predators especially juvenile fish. Copepods are high in essential fatty acids required by fish. In
turn, copepods fatty acid make up is similar to that of the microphytobenthos that they consume
(Coull 2009). Meiofauna are also important in nutrient recycling because they facilitate
biomineralization of organic matter. They are also good indicators of estuarine health because of
their high sensitivity to anthropogenic inputs. For further information on meiofauna, please see
Higgins and Thiel (1988) and Giere (2009).
As the name implies, the hyperbenthos live just above the sediment and occur there in greater
densities than in either the adjacent sediment or water column. Distinctions have been made between
truly hyperbenthic organisms and immigrants that could be endobenthic (living within the sediment),
epibenthic (living on the surface of the sediment) or planktonic (drifting in the water column)
(Mees and Jones 1997). Hyperbenthic community structure can fluctuate seasonally due to temporary
immigrants. The term hyperbenthos was first used by Beyer (1958) and applies to the association of
small sized, bottom dependent animals (mainly crustaceans) that are capable of migrating daily or
seasonally above the sea floor. Hyperbenthic organisms can play a significant role in both tropical
and temperate estuarine food webs (Sibert 1981, Winkler & Greve 2004).
Terms such as "dermersal zooplankton", "benthopelagic plankton" and "benthic boundary
layer fauna" are generally applied to hyperbenthos in tropical areas. Many demersal fish and
epibenthic crustaceans feed on the hyperbenthos during at least part of their life cycle. Studies of
benthic pelagic coupling related to energy fluxes have underestimated the role of the hyperbenthic
community (Koulouri 2010) most likely due to inadequate sampling methods.
This group of organisms are often referred to as ecosystem engineers (Paterson et al. 2009) or
bioturbators because they are large (> 1 mm), infaunal organisms that affect the structure and
chemistry of their own microenvironment (Little 2000) by burrowing activity. Macrobenthic organisms
include molluscs, worms, crustaceans, echinoderms and hemichordates. The yabby pump (Fig. 7) is a
suction device often used to extract large, intact, infaunal organisms from soft sediment habitats.
Fig. 7. Yabby pump being used to extract infaunal macrobenthos.
Trophic modes of bioturbators include filter feeding, deposit feeding and predation (Bertness 1999).
Most bivalves are filter feeders and burrow into the sediment using their muscular foot. Bivalve
shell sculpturing (ribbing) is thought to increase friction and burrowing efficiency (Stanley 1970).
Filter feeding bivalves use their incurrent siphons to draw water into the body and pass it over the
gills where tiny food particles such as diatoms, small zooplankton and detritus are extracted. Cilia
then move the food toward the mouth. Water drawn in through the incurrent siphon also serves as a
source of oxygen enabling the bivalve to respire. Filtered water, waste products and gametes are
passed out into the water column through the excurrent siphon.
Filter feeding bivalves on Indian River Lagoon tidal flats include the angelwing clam,
Cyrtopleura costata (Fig. 8 A), the Atlantic giant cockle, Dinocardium robustum (Fig. 8 B),the
southern hard clam, Mercenaria campechiensis, the hard clam, Mercenaria mercenaria, and the lucinid
bivalve, Phacoides pectinata (Fig. 8 C) among others. These bivalves not only provide a vital
trophic link between the water column and benthic production, but are also an important and abundant
prey item of large, predatory, tidal flat species such as snails, crabs, fish and wading birds.
Fig. 8. Tidal flat bivalves: (A) Angelwing clam, Cyrtopleura costata; (B) Atlantic giant cockle, Dinocardium robustum; (C) the lucinid bivalve, Phacoides pectinata.
Filter feeding polychates include the parchment worm, Chaetopteris variopedatus (Fig.
9), noted for its tough membranous tube. It has developed paddles to pump water through the head and
out the tail of its u-shaped burrow, thus effectively enabling the animal to rise above the redox
potential discontinuity (RPD) (Little 2000). C variopedatus is abundant on IRL mud flats.
Fig. 9. Parchment worm, Chaetopteris variopedatus.
The southern Indian River Lagoon supports an unusually high assemblage of infaunal
decapod and stomatopod crustaceans (Felder and Manning 1986) that both filter and deposit feed. Two
genera of thalassinidean shrimp, Callianassa (Fig. 10 A) - the ghost shrimp, and Upogebia - the mud
shrimp, as well as the stomatopods Coronis excavatrix (Fig. 10 B), Lysiosquilla scabricauda and
Lysiosquilla spp. are abundant macrobenthic crustaceans in the Inidan River Lagoon and are capable
of extensive biotubation when constructing their elaborate, branching burrows. In the IRL, burrowing
thalassinidean shrimp are represented by two species of Callianassa (C. guassutinga & C. rathbunae)
and one species of Upogebia (U. affinis).
Borrows have more than one entrance (Fig. 2 B) and shrimp are often found near an
entrance pumping water into the burrow by beating their pleopods. These burrowing shrimp are
considered to be both filter and deposit feeders with often one or the other trophic mode being more
dominant, depending on the species (Coelho et al. 2000). Upogebiids generally feed on suspended
material filtered from the water, while callianassids mainly feed on sediment taken up within the
burrow by the second and third pereiopods. Both ghost and mud shrimp can have substantial effects on
the abundance of co-occurring macro-infauna (Posey 1991). Commensals in the burrows of these
infaunal shrimp may include polychaete worms, snapping shrimp and pea crabs.
Fig. 10. Burrowing crustaceans: (A) Ghost shrimp, Callianassa spp.; (B) stomatopod, Coronis excavatrix. Photos by Sabine Alshuth.
The burrowing, protobranch bivalve Macoma is also capable of both deposit and filter
feeding. For example, in low flow situations, Macoma will remove sediment from the surface with its
oral palps but will switch to filter feeding in high flow situations (Olafsson et al. 1994). Several
species of Macoma are present in the IRL.
Deposit feeders on tidal flats include surface deposit feeders that generally affect the
upper 2 to 3 cm of sediment and burrowing deposit feeders whose effects on the sediments have deeper
repercussions, i.e. up to 30 cm (Bertness 1999). Nonselective deposit feeders ingest both organic
and sediment particles and then digest the organic material, e.g., bacteria growing on the sediment
particles. Selective deposit feeders separate organic material from sediments prior to ingestion.
Deposit feeders are an important link between the benthos and the sediment. They enhance sediment
resuspension and nutrient exchange with the water column and increase productivity by increasing
oxygen and nutrient levels in the benthos (Bertness 1999).
Surface deposit feeders include mud snails, fiddler crabs, echinoderms, certain pelagic
fish and shrimp. An abundant, deposit feeding fiddler crab on IRL mudflats is Uca pugilator (Fig.
11). Fiddler crabs form two types of characteristic pellets affecting sediment surface topography.
The larger of the two pellets are formed during burrow excavation. Smaller pellets are formed during
deposit feeding when the crab removes organic matter then rolls the remaining sediment into small
balls and deposits them on the substratum.
Fig. 11. Fiddler crabs, Uca pugilator, on IRL mud flat.
The nine-armed starfish, Luidia
senegalensis, is a striking macrobenthic echinoderm in the IRL occurring on
intertidal flats as well as subtidally. When buried, L. senegalensis (Fig. 12) will invert
its stomach to feed on detritus (Hendler et al. 1995). Burrowing deposit feeders include polychaete
and sipunculan worms (e. g., Siphonosoma cumanense, and Sipunculus nudus) (Rice 1995) bivalves and
amphipods among others. The lugworm, Arenicola cristata, lives in extensive u-shaped tubes
excavated in muddy tidal flat habitats. After sediment ingestion, the lugworm deposits large fecal
casts at the posterior end of the burrow (Fig. 13).
Fig. 12. Nine-armed starfish, Luidia senegalensis: (A) on surface of mudflat, dorsal view; (B) ventral view.
Fig. 13. Burrow opening and fecal cast of the lugworm, Arenicola cristata.
Examples of predatory macrobenthic organisms on IRL tidal flats include the moon snail (or shark
eye), Polinices duplicatus (Fig. 14) and the onuphid polychaete, Diopatra cuprea (Fig. 15). P.
duplicatus crawls along the sediment feeding on infaunal bivalves by drilling a hole into the
bivalve with its radula. It then inserts its proboscis to rasp out the flesh from inside the bivalve
shell. Diopatra builds extensive mucous/sand tubes extending 50 - 60 cm below the sediment. The tube
cap which extends several centimeters above the sediment surface is in the form of a decorated
inverted hook thought to aid in food capture. Diopatra feed on the epibiota of its own as well as
its neighbor's tube caps.
Fig. 14. Moon snail, Polinices duplicatus
Fig. 15. Burrowing polychaete, Diopatra
The epibenthos are large, mobile, species that make up a substantial proportion of tidal flat
biomass. IRL epibenthos include horseshoe crabs, crabs, shrimp, molluscs, rays, skates, bottom fish,
gulls, terns, wading birds, reptiles and mammals. Most epibenthic organisms are predatory, some are
grazers. For example, raccoons, Procyon lotor elucus (Fig. 16) are opportunistic tidal flat
predators, foraging on fiddler and blue crabs, snails, fish, snakes and eggs of birds, turtles and
alligators and will dig into the sediment for infaunal bivales.
Fig. 16. Raccoons, Procyon lotor elucus, foraging on IRL mud flat.
The ragged sea hare, Bursatella leachii, (Fig. 17) is a grazing benthic
detritivore/herbivore that feeds primarily on cyanophytes and diatom mats and films found on sand,
mud and other benthic substrata. These epibenthic species can have enormous effects on the
abundance, diversity and distribution of microbenthic and infaunal tidal flat organisms and can also
severely disturb the substratum while foraging for prey, e.g. the cow-nosed ray (Orth 1975). Early
studies clearly demonstrated the effects of epibenthic predators on diminishing prey abundances with
predator exclusion cage experiments (Virnstein 1977; Peterson 1979). Further experimentation has
demonstrated a more complex picture of predator prey dynamics in soft bottom habitats particularly
when interactions between and among infaunal and epibenthic predators as well as physical parameters
of the habitat are considered (Quammen 1982, Ambrose 1984, Bottom 1984, Commito and Ambrose 1985,
Thrush et al. 1997).
Fig. 17. Ragged sea hare, Bursatella leachii.
In terms of understanding ecological interactions, tidal flats have been contrasted
with rocky intertidal habitats (Bertness 1999, Little 2000, Nybakken and Bertness 2005). The three
dimensionality of soft bottom habitats as opposed to hard, rocky, mostly two dimensional substarta
affords soft bottom infaunal organisms several advantages: they can retreat into deeper sediments or
burrows when threatened by predation and, in addition, many infaunal bivalves can survive partial
predation, i.e., siphon nipping; having the ability to move around in the sediment, many infaunal
organisms can avoid direct competition with neighbors and escape other predatory burrowers;
desiccation does not pose the threat to infaunal organisms during low tide as it does on rocky
shores, particularly in fine sediments that retain moisture; and finally, organic materials
collecting on sediments provide a ready, constant food source. Perhaps the biggest draw back to the
infaunal lifestyle is lack of a securing "anchor" in the sediment. For example, in the rocky
intertidal, organisms are securely attached to the rock surface by utilizing such mechanisms as
cement, byssal threads, a muscular foot, etc.. During periods of severe storm erosion (Little 2000),
larger infauna in soft bottom habitats may become easily dislodged and subsequently displaced.
Threats to Tidal Flats
Water and sediment quality characteristics are important factors in maintaining healthy
lagoon habitats. Tidal flat areas face a number of anthropogenic and natural threats including
predicted sea level rise, loss of habitat, salinity fluctuations, pollution, erosion and invasive
Of major concern to all coastal areas worldwide and particularly threatening to coastal
wetlands, including tidal flats, are predicted sea level rises in response to global climate change.
Current estimates put predicted levels of sea rise at 60 cm in the next one hundred years. Although
over geological time, estuaries are thought of as ephemeral, most present day estuaries have been
relatively stable for approximately 6,000 years. Past changes in sea level have greatly affected
estuarine outlines and could have rapid and significant present-day effects (Holligan and Reiners
1992). Rising sea levels could render intertidal flats into subtidal habitat and inundate adjoining
mangrove and salt marsh areas (Little 2000).
Ever increasing population growth and development along Florida's coastline, coupled
with alterations caused by mosquito impoundments, have led to changes and degradation in Florida's
wetland and coastal areas. It is estimated that since the 1950's, 75% of the mangrove forests and
salt marshes bordering the Indian River Lagoon have been destroyed, altered or functionally
isolated. These changes in mangrove and salt marsh areas have direct repercussions for bordering
sand and mudflats.
Excessive freshwater flows from storms or the construction of agricultural and urban
drainage projects can lead to extreme salinity fluctuations in the IRL estuary and can affect
community structure and stability of tidal flats. Continuous exposure to lower salinity regimes can
compromise stenohaline, shallow burrowing infaunal organisms as well as the microphytobenthos,
resulting in deleterious effects on food web dynamics.
Point and non-point sources of pollution pose direct threat to IRL habitats including
tidal flats. Excessive nutrients can increase the proliferation of cyanobacterial mats covering IRL
tidal flats as well as promote excessive phytoplankton growth that can interfere with normal filter
feeding processes of infaunal oragnisms. In addition, excessive nutrients can cause the appearance
and proliferation of macroalgal species such as Ulva and Enteromorpha interfering with the normally
unvegatated status of the tidal flat.
Storm water runoff (non-point pollution), draining both urban and agricultural areas,
contains suspended sediments as well as industrial, automotive and household chemicals, pesticides,
and animal wastes. Turbidity levels are affected by the amount of total suspended organic and
inorganic solids (TSS) in the water column. Increased turbidity reduces light penetration and can
affect the photosynthetic capacity of tidal flat epipelic microalgae. Chemical pollutants are
incorporated into benthic sediments and adhere to sediment grains. Although the high bacterial
biomass associated with tidal flats, particularly mudflats, can break down these pollutants
somewhat, when excessive, these contaminants can accumulate in tidal flat/estuarine food webs.
IRL sediments are mostly made up from sands, silts and shell fragments. However, about
10% of the lagoon bottom is covered with muck - a loose, black, organic-rich mud. Although most muck
occurs in deeper areas of the lagoon, e.g., the intracoastal waterway, it is also found at the
mouths of most of the IRL major tributaries. When disturb, for example during intentional removal or
by storms or boat activity, etc., muck particles can be carried with currents and deposited in
shallower, near shore areas such as tidal flats. Muck displacement can potentially interfere with
infaunal filter and deposit feeding, as well as change the depth of the redox potential
discontinuity (RPD) layer.
Sources of IRL tidal flat erosion are many. Storms, wind induced waves, hurricanes,
epibenthic bioturbation, prop scarring, etc., can singly and sometimes synergistically contribute to
the erosion of tidal flats. Because most of IRL tidal flat areas are located in the vicinity of
inlets, they are further subjected to fluctuations in tidal current velocities. As mentioned above,
since most infaunal organisms burrowing on the tidal flat lack an anchoring structure, severe rapid
erosion, i.e. that which outpaces the ability of these organisms to burrow more deeply, can lead to
substantial changes in infaunal abundance.
Invasive species pose yet another threat to estuarine tidal flats. Since invasive
species do not normally occur in an area, they may lack natural predators and pathogens, allowing
them to proliferate and out-compete native species. Estuaries and shallow-water muddy sediments have
proportionately more invasive species than rocky shores and open coast sandy shores. This difference
probably results from the fact that most introductions, intentional or not, take place within the
estuary (Ruiz et al. 1997, Little 2000).
The following table is an abbreviated list of tidal flat organisms. Select available
links to learn more. Additional species reports can be found in the alphabetized lists of this site.
|Avicennia germinans||Black Mangrove
|Cladium mariscus ssp. jamaicense||Jamaica Swamp Sawgrass
|Juncus roemerianus||Black Needle Rush
|Laguncularia racemosa||White Mangrove
|Rhizophora mangle||American Mangrove, Mangrove, Red Mangrove
|Spartina alterniflora||Smooth Cordgrass
|Spartina bakeri||Sand Cordgrass
|Spartina patens||Salt Meadow Cordgrass
ALGAE & OTHER PROTISTS
|Acetabularia calyculus||Umbrella Alga
|Enteromorpha spp.||Green algae
|Nitzschia frustulum var. subsalina||
|Padina pavonica||Peacock's Tail , Peacock's Tail Alga
|Aedes sollicitans||Salt marsh mosquito
|Anodontia alba||Buttercup Lucine
|Aplysia brasiliana||Sooty Seahare
|Arenicola cristata||Lugworm, Southern Lugworm
|Batillaria minima||West Indian False Cerith
|Bursatella leachii||Ragged Sea Hare
|Callinectes ornatus||Ornate Blue Crab
|Callinectes sapidus||Blue Crab
|Callinectes similis||Lesser Blue Crab
|Chaetopterus variopedatus||Parchment Tube Worm
|Cirratulus grandis||Orange Fringed Worm
|Clibanarius vittatus||Striped Hermit Crab, Thinstripe Hermit, Thinstripe Hermit
|Cyrtopleura costata||Angel Wing Clam, Angelwing Clam
|Dinocardium robustum||Atlantic Giant Cockle
|Diopatra cuprea||Plumed Worm
|Fasciolaria tulipa||True Tulip, Tulip Snail
|Geukensia demissa||Ribbed Horsemussel, Ribbed Mussel
|Glycera americana||Blood Worm
|Limulus polyphemus||Atlantic Horseshoe Crab, Horseshoe Crab, King Crab
|Littorina irrorata||Marsh Periwinkle
|Luidia clathrata||Gray Sea Star, Grey Sea Star
|Luidia senegalensis||Nine-arm Sea Star, Nine-armed Sea Star
|Lysiosquilla scabricauda||Scaly-tailed Mantis Shrimp, Scaly-tailed Squilla,
Spring-tailed Mantis Shrimp
|Macoma brevifrons||Short Macoma
|Macoma tenta||Elongate Macoma
|Mercenaria campechiensis||Southern Hard Clam, Southern Quahog
|Mercenaria mercenaria||Cherrystone, Hard Clam, Littleneck, Northern Quahog
|Neanthes succinea||Clam Worm, Pileworm, Ragworm
|Nereis succinea||Clam Worm
|Palaemonetes spp.||Grass shrimp
|Pectinaria gouldii||Ice Cream Cone Worm, Trumpet Worm
|Petrochirus diogenes||Giant Hermit Crab
|Pinnixa chaetopterana||Tube Pea Crab
|Polinices duplicatus||Shark Eye
|Polymesoda caroliniana||Carolina marsh clam
|Prunum apicinum||Common Atlantic Marginella
|Scoloplos fragilis||Polychaete worm
|Sesarma spp.||Marsh crabs
|Tagelus plebeius||Stout Tagelus
|Uca minax||Redjointed Fiddler
|Uca pugnax||Atlantic Marsh Fiddler
|Upogebia affinis||Coastal Mud Shrimp
|Balanoglossus aurantiacus||Golden Acorn Worm
REPTILES & AMPHIBIANS
|Alligator mississippiensis||American Alligator
|Caretta caretta||Loggerhead Sea Turtle
|Crocodylus acutus||American Crocodile
|Lepidochelys kempii||Kemp's Ridley Sea Turtle
|Malaclemys terrapin||Diamondback terrapin
|Nerodia clarkii||Salt Marsh Snake
|Nerodia clarkii compressicauda||Mangrove salt marsh snake
|Nerodia clarkii taeniata||Atlantic Salt Marsh Snake
|Acanthurus bahianus||Ocean Surgeon
|Achirus lineatus||Lined Sole
|Acipenser brevirostrum||Lake Sturgeon (gilbert, 1989), Pinkster, Salmon Sturgeon,
|Anchoa mitchilli||Bay Anchovy
|Archosargus probatocephalus||Convict Fish, Sheepshead, Sheepshead Bream, Sheepshead
|Carcharhinus leucas||Bull Shark
|Carcharhinus limbatus||Black Tip Shark, Blacktip Shark
|Centropomus parallelus||Fat Snook
|Centropomus pectinatus||Tarpon Snook
|Centropomus undecimalis||Common Snook, Robalo, Thin Snook
|Cynoscion nebulosus||Spotted Seatrout
|Cyprinodon variegatus||Sheepshead Minnow
|Dasyatis sabina||Atlantic Stingray
|Etropus crossotus||Fringed Flounder
|Eucinostomus gula||Silver Jenny
|Eucinostomus harengulus||Tidewater mojarra
|Fundulus confluentus||Marsh Killifish
|Fundulus grandis||Gulf Killifish
|Gambusia affinis||Mosquitofish, Western Mosquitofish
|Gerres cinereus||Yellowfin Mojarra
|Ginglymostoma cirratum||Nurse Shark
|Gobionellus oceanicus||Highfin goby
|Gobiosoma bosc||Naked Goby
|Halichoeres poeyi||Blackear Wrasse
|Harengula clupeola||False Herring, False Pilchard
|Harengula humeralis||Red-ear Herring, Red-ear Sardine, Redear Herring
|Leiostomus xanthurus||Chub, Flat Croaker, Golden Croaker, Goody, Jimmy, Roach,
Silver Gudgeon, Spot, Spot Croaker
|Lobotes surinamensis||Atlantic Tripletail, Tripletail
|Lucania parva||Rainwater Killifish
|Lutjanus analis||King Snapper, Mutton Fish, Mutton Snapper
|Lutjanus apodus||Schoolmaster, Schoolmaster Snapper
|Lutjanus griseus||Black Snapper, Gray Snapper, Lowyer, Mango Snapper, Mangrove
|Menidia beryllina||Tidewater Silverside
|Microgobius gulosus||Clown Goby
|Mugil cephalus||Black Mullet, Flathead Mullet, Gray Mullet, Sea Mullet,
|Mugil curema||Silver Mullet, White Mullet
|Negaprion brevirostris||Lemon Shark
|Ocyurus chrysurus||Yellowtail Snapper
|Opsanus tao||Oyster Toadfish
|Paralichthys lethostigma||Doormat And Halibut, Mud Flounder, Southern Flounder
|Poecilia latipinna||Sailfin Molly, Topote Velo Negro
|Pogonias cromis||Black Drum, Corvina Negra, Tambor Negro
|Rhinoptera bonasus||Cara De Vaca, Cowfish, Cownose Ray, Skeete
|Sciaenops ocellatus||Corvineta Ocelada, Red Drum
|Sphyrna lewini||Scalloped Hammerhead Shark
|Sphyrna tiburo||Bonnethead Shark
|Trachinotus carolinus||Florida Pompano, Palorneta Común, Pámpano Amarillo
|Trachinotus falcatus||Pámpano, Pámpano Palometa, Permit
|Ajaia ajaia||Roseate Spoonbill
|Anas fulvigula||Mottled Duck
|Ardea herodias||Great Blue Heron
|Bubulcus ibis||Buff-backed Heron, Cattle Egret, Elephant Bird, Hippopotomus
Egret., Rhinoceros Egret
|Butorides striata||Green-backed Heron
|Calidris canutus rufa||Red Knot, Robin Snipe
|Calidris fuscicollis||White-rumped Sandpiper
|Calidris mauri||Western Sandpiper
|Calidris pusilla||Black-legged Peep, Little Peep, Peep, Sand Peep,
|Charadrius melodus||Piping Plover
|Cistothorus palustris||Marsh Wren
|Egretta rufescens||Muffle-jaw Egret, Peale’s Egret, Plume Bird, Reddish
|Egretta thula||Snowy Egret
|Egretta tricolor||Louisiana Heron, Tricolored Heron
|Falco peregrinus||Peregrine Falcon
|Haematopus palliatus||American Oystercatcher
|Haliaeetus leucocephalus||Bald Eagle
|Limosa fedoa||Marbled Godwit
|Nyctanassa violacea||Yellow-crowned Night-heron
|Nycticorax nycticorax||Black-crowned Night-heron
|Pelecanus occidentalis||American Brown Pelican, Brown Pelican, Common Pelican
|Rynchops niger||Black Skimmer
|Sterna dougallii||Roseate Tern
|Lontra canadensis||River Otter
|Procyon lotor elucus||Raccoon
|Trichechus manatus latirostris||Florida Manatee
|Tursiops truncatus||Bottlenose Dolphin, Bottlenosed Dolphin, Common Bottlenose
Dolphin, Delfín Tonina
& 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.
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.
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: http://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.
Mees, J. and M. B. Jones. 1997. The hyperbenthos. Ocean. Mar. Biol.Ann. Rev.35:
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:
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 Sipunculaof 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:
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:
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.