(Kuenen, 1942; van Straaten and Kuenen, 1957; Bosence,
1979; Dittmann, 1990; Nehls and Thiel, 1993; Eisma,
1998). Where hard surfaces are developed (such as mussel
beds, oyster reefs, serpulid reefs, shell-lag pavements, or
scattered shells on the sediment surface), encrusting
organisms and (micro-scaled) hard-surface communities
such as oysters, hydrozoans, anthozoans, bryozoans, and
algae can develop (Albrecht, 1998).
The invading or temporary fauna of tidal flats, i.e., fauna
that is not permanently resident on the tidal flat, include fish
(including rays), crabs, reptiles, and octopus that invade the
tidal flat on a high tide to feed; birds, reptiles, and mammals
that invade the tidal flat on a low tide to feed; and the birds
that use the saltmarsh vegetation and mangrove canopy for
nesting, feeding, roosting, and loafing.
Figure 6 illustrates in generalized form the contrast
between biota across a tropical macrotidal sandy to muddy
tidal flat and that of a tropical macrotidal wholly muddy
tidal flat, and the contrast between biota in various
temperate-climate mesotidal and microtidal sandy tidal
flats. Figure 7 illustrates various habitats and ecological/
biological aspects of tidal flats. Figure 8 shows some of
the biological aspects of the tidal flats.
Tidal flats as ecological systems – complexity, and
trophic and ecologic function
Tidal flats are complex systems – from the large scale of
geomorphology, habitats, ecosystems, geochemistry,
hydrology, and hydrochemistry to the smaller scales of
autoecology and microbial processes. Occurring in the
coastal zone, as the low-gradient interface between ocean,
land, atmosphere, and groundwater, they support
a diversity of biota and a diversity of physical, chemical,
and biological processes and, as such, function as
a storage system for living and decaying biomass and for
biochemical/geochemical products. As a coastal system
bridging the environments of ocean and land, and in estuarine settings bridging the environments of freshwater and
marine water, tidal flats also provide a multiplicity of ecological functions. As a major zone of primary and secondary productivity, they are an important food source for
marine and terrestrial animals.
In the first instance, there is primary production by benthic plant life such as mangroves, saltmarsh vegetation,
seagrasses, algae, and diatoms and, during the high tide,
primary production in the water column by phytoplankton.
Plant biomass is then cycled through the tidal ecosystem by
herbivory and/or nectivory by insects, gastropods, crabs,
turtles, bats, fish, and mammals, leaf litter fall and death
and decay of plant matter, and filter feeding by bivalves
and other invertebrates. Plant material in decayed or comminuted form accumulates as detritus where it is either
ingested by detritus feeders (gastropods, polychaetes, crustacea) or transformed by microbiota to microbial slime and
grazed by gastropods, polychaetes, and crustacea, or
(if mobilized into the water column) extracted by filter
feeders, or where it is incorporated on and into the
sediments becomes involved in complex geochemical and
biochemical transformations and recycled by microbiota
as part of the nitrogen, sulfur, phosphorus, and carbon cycle
(see “Mineralization”) and transformed into inorganic
products. Following incorporation of biomass and detritus
(in various stages of decay) into herbivores, nectivores,
and filter feeders, these primary consumers are preyed on
by predators such as gastropods, echinoderms, nektonic
fish, demersal fish, stingrays, octopus, and terrestrial
invaders such as snakes, other reptiles, mammals, and wading birds.
As such, tidal flats are major feeding grounds for nekton and demersal fish on the high tide and major feeding
grounds for reptiles, mammals, and avifauna on the low
tide. Figure 9 diagrammatically shows the ecological
function of mangroves on a tidal flat, with blocks of sediment showing different types of biota and their activity
commensurate with substrate type and tidal level setting,
and the invasion of terrestrial animals on the low tide
and marine animals on the high tide to feed on the resident
biota of the tidal flat.
Examples specifically of bird life utilizing the tidal flats,
their dependence on the ecosystem and effects on the ecosystem, and their nesting in mangroves and saltmarsh are
provided by Semeniuk et al. (1978), Goss-Custard (1980),
Zwarts (1981), Smit (1984), Johnstone (1990), Dann
(1991), Beukema et al. (1993, 2010), Nehls and Tiedemann
(1993), Piersma et al. (1993), Ens et al. (1994), GossCustard et al. (1996), Backwell et al. (1998), Raines
et al. (2000), Connolly and Colwell (2005), Granadeiro
et al. (2007), and Spalding et al. (2010).
Tidal flats and estuarine shallow waters also are considered to be nurseries and sanctuaries for fauna (McHugh,
1967; Blaber and Blaber, 1980; Staples, 1980; Haedrich,
1983; Boehlert and Mundy, 1988; Beukema, 1992; Maes
et al., 1998; Morrison et al., 2002; Francis et al., 2005;
Reise et al., 2010) – for contrasting discussions of the
importance of shallow-water habitats for fish nurseries,
see Able (2005), Baker and Sheaves (2005, 2007), and
Johnston et al. (2007). While shallow waters are considered to be nurseries for fish and juvenile crabs and
shrimps, nurseries also are specifically provided on tidal
flats themselves by pools of water that are left stranded
on the low tide. During periods of recruitment, the numerous small water pools on uneven surfaces of open tidal
flats, in mangroves, and in saltmarsh are abundant with
small fish. Sanctuaries are provided in the mangroves,
saltmarsh, and seagrass.
Tidal flats provide variable habitats for organisms
related to sediment types, tidal levels, and hydrochemistry
and, as such, for a given biogeography and climate setting,
provide the basis for biodiversity.
Summary
Tidal flats are tidally exposed surfaces, underlain by
a variety of substrates, and variably vegetated depending
on climate and tidal level. The entire surface between
TIDAL FLAT
671
1979; Dittmann, 1990; Nehls and Thiel, 1993; Eisma,
1998). Where hard surfaces are developed (such as mussel
beds, oyster reefs, serpulid reefs, shell-lag pavements, or
scattered shells on the sediment surface), encrusting
organisms and (micro-scaled) hard-surface communities
such as oysters, hydrozoans, anthozoans, bryozoans, and
algae can develop (Albrecht, 1998).
The invading or temporary fauna of tidal flats, i.e., fauna
that is not permanently resident on the tidal flat, include fish
(including rays), crabs, reptiles, and octopus that invade the
tidal flat on a high tide to feed; birds, reptiles, and mammals
that invade the tidal flat on a low tide to feed; and the birds
that use the saltmarsh vegetation and mangrove canopy for
nesting, feeding, roosting, and loafing.
Figure 6 illustrates in generalized form the contrast
between biota across a tropical macrotidal sandy to muddy
tidal flat and that of a tropical macrotidal wholly muddy
tidal flat, and the contrast between biota in various
temperate-climate mesotidal and microtidal sandy tidal
flats. Figure 7 illustrates various habitats and ecological/
biological aspects of tidal flats. Figure 8 shows some of
the biological aspects of the tidal flats.
Tidal flats as ecological systems – complexity, and
trophic and ecologic function
Tidal flats are complex systems – from the large scale of
geomorphology, habitats, ecosystems, geochemistry,
hydrology, and hydrochemistry to the smaller scales of
autoecology and microbial processes. Occurring in the
coastal zone, as the low-gradient interface between ocean,
land, atmosphere, and groundwater, they support
a diversity of biota and a diversity of physical, chemical,
and biological processes and, as such, function as
a storage system for living and decaying biomass and for
biochemical/geochemical products. As a coastal system
bridging the environments of ocean and land, and in estuarine settings bridging the environments of freshwater and
marine water, tidal flats also provide a multiplicity of ecological functions. As a major zone of primary and secondary productivity, they are an important food source for
marine and terrestrial animals.
In the first instance, there is primary production by benthic plant life such as mangroves, saltmarsh vegetation,
seagrasses, algae, and diatoms and, during the high tide,
primary production in the water column by phytoplankton.
Plant biomass is then cycled through the tidal ecosystem by
herbivory and/or nectivory by insects, gastropods, crabs,
turtles, bats, fish, and mammals, leaf litter fall and death
and decay of plant matter, and filter feeding by bivalves
and other invertebrates. Plant material in decayed or comminuted form accumulates as detritus where it is either
ingested by detritus feeders (gastropods, polychaetes, crustacea) or transformed by microbiota to microbial slime and
grazed by gastropods, polychaetes, and crustacea, or
(if mobilized into the water column) extracted by filter
feeders, or where it is incorporated on and into the
sediments becomes involved in complex geochemical and
biochemical transformations and recycled by microbiota
as part of the nitrogen, sulfur, phosphorus, and carbon cycle
(see “Mineralization”) and transformed into inorganic
products. Following incorporation of biomass and detritus
(in various stages of decay) into herbivores, nectivores,
and filter feeders, these primary consumers are preyed on
by predators such as gastropods, echinoderms, nektonic
fish, demersal fish, stingrays, octopus, and terrestrial
invaders such as snakes, other reptiles, mammals, and wading birds.
As such, tidal flats are major feeding grounds for nekton and demersal fish on the high tide and major feeding
grounds for reptiles, mammals, and avifauna on the low
tide. Figure 9 diagrammatically shows the ecological
function of mangroves on a tidal flat, with blocks of sediment showing different types of biota and their activity
commensurate with substrate type and tidal level setting,
and the invasion of terrestrial animals on the low tide
and marine animals on the high tide to feed on the resident
biota of the tidal flat.
Examples specifically of bird life utilizing the tidal flats,
their dependence on the ecosystem and effects on the ecosystem, and their nesting in mangroves and saltmarsh are
provided by Semeniuk et al. (1978), Goss-Custard (1980),
Zwarts (1981), Smit (1984), Johnstone (1990), Dann
(1991), Beukema et al. (1993, 2010), Nehls and Tiedemann
(1993), Piersma et al. (1993), Ens et al. (1994), GossCustard et al. (1996), Backwell et al. (1998), Raines
et al. (2000), Connolly and Colwell (2005), Granadeiro
et al. (2007), and Spalding et al. (2010).
Tidal flats and estuarine shallow waters also are considered to be nurseries and sanctuaries for fauna (McHugh,
1967; Blaber and Blaber, 1980; Staples, 1980; Haedrich,
1983; Boehlert and Mundy, 1988; Beukema, 1992; Maes
et al., 1998; Morrison et al., 2002; Francis et al., 2005;
Reise et al., 2010) – for contrasting discussions of the
importance of shallow-water habitats for fish nurseries,
see Able (2005), Baker and Sheaves (2005, 2007), and
Johnston et al. (2007). While shallow waters are considered to be nurseries for fish and juvenile crabs and
shrimps, nurseries also are specifically provided on tidal
flats themselves by pools of water that are left stranded
on the low tide. During periods of recruitment, the numerous small water pools on uneven surfaces of open tidal
flats, in mangroves, and in saltmarsh are abundant with
small fish. Sanctuaries are provided in the mangroves,
saltmarsh, and seagrass.
Tidal flats provide variable habitats for organisms
related to sediment types, tidal levels, and hydrochemistry
and, as such, for a given biogeography and climate setting,
provide the basis for biodiversity.
Summary
Tidal flats are tidally exposed surfaces, underlain by
a variety of substrates, and variably vegetated depending
on climate and tidal level. The entire surface between
TIDAL FLAT
671