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. 2002).
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 impurities.
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
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
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
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
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 cuprea.
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 species.
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).
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.
& 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.
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Report by: Joseph Dineen,
Smithsonian Marine Station at Fort Pierce
Photos by: Joseph Dineen, unless otherwise specified.
Photographic assistance in the field was graciously provided by Sherry Reed.
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