It is curious that today none of the survivors of those plant
lineages occur in mangroves, with the exception of the
fern Acrostichum aureum, which is widespread throughout the tropics (Tomlinson, 1986). There is no indication
from the pre-flowering plant record of saltmarsh analogues. One of the earliest land plants was Rhynia, which
occurred in what Channing and Edwards (2009)
interpreted as a hot mineral-rich spring similar to those
found at Yellowstone National Park today. Some at least
of the earliest land plants therefore lived in harsh environments, and the high mineral content of the springs might
suggest that tolerance of salinity would have been possible. However, there is no support from the fossil record
for early occurrence of extensive saltmarsh.
Bryophytes are likely to have been early components of
the terrestrial flora, but are not generally recognized as
components of saltmarsh floras. However, a number of
bryophyte species occur in tidal saltmarshes and in inland
saline habitats (Adam, 1976; Adam, 1990; Garbary et al.,
2008); therefore, while bryophytes are frequently absent
and are rarely abundant in saltmarshes, they demonstrate
a capacity to tolerate salinity and might have been more
abundant in the past.
The present-day vascular flora of saltmarshes is
diverse. Barbier et al. (2011) refer to saltmarshes as grasslands, but while large areas of the world’s saltmarshes are
grasslands or dominated by physiognomically similar
graminoids (sedges and rushes), there are also large areas
dominated by herbs or shrubs, particularly by genera of
Amaranthaceae (Figure 6).
Most saltmarsh communities are dominated by perennial species. Annual plants are found in the low marsh
zone in those parts of the world where Salicornia occurs
and in microhabitats in the mid and upper marsh where
disturbance creates openings in which annual plants are
able to germinate.
Algae
Algae are important components of saltmarshes.
Microalgae are not always apparent to the human eye,
but microalgal films contribute to marsh formation and
ecosystem productivity. Many microalgae have mucus
coatings, and this is important in binding sediment
(Coles, 1979; Underwood, 2000). Establishment of
microalgae on mudflats may promote development of
microtopography (van de Koppel et al., 2012), facilitate
the establishment of pioneer vascular plants (Coles,
1979), and protect the surface of the lower marsh from
erosion. Mason et al. (2003) showed that microalgae were
susceptible to even low concentrations of triazine herbicides, suggesting that runoff of herbicides from agricultural land into estuaries might initiate erosion of
saltmarshes.
Macroalgae may be conspicuous components of
saltmarshes. In western Ireland and western Scotland,
very dense swards of dwarf fucoids (only 1 or 2 cm tall)
dominate lower marshes (Adam, 1981), and on Atlantic
saltmarshes in North America and numerous localities in
Europe, larger fucoids contribute substantially to marsh
productivity (Roman et al., 1990) creating protective
microclimates and providing habitat for intertidal invertebrates (Tyrrell et al., 2012). Eutrophication of waterways
can promote overgrowth and smothering of saltmarshes
by green macroalgae (McComb and Lukatelich, 1995),
particularly by species of Ulva.
Microorganisms
Microorganisms play essential roles in ecosystem processes in saltmarshes, playing important roles in nitrogen,
sulfur, and other biogeochemical cycles and in breakdown
of detritus. Microbial films on detritus are an important
food source for browsing invertebrates.
Recently, there has been considerable interest in the
interactions between halophytes and microorganisms in
the rhizosphere (Ruppel et al., 2013). Bacteria increasing
the availability of phosphorus and iron have been isolated
from the rhizosphere of several halophytes (Ruppel et al.,
2013). Arbuscular mycorrhizae, which enhance nutrient
uptake by plants, were first reported from the roots of halophytes early in the twentieth century (Adam, 1990; Davy
et al., 2000), but only recently has there been investigation
of their functions (Fuzy et al., 2008; Evelin et al., 2009).
Studies so far have concentrated on individual species;
placing mycorrhizae in the context of the functions of
saltmarsh ecosystems is a research challenge for the
future.
Fauna
The majority of animal phyla can be found in saltmarshes,
although many groups have been little studied. A review
of the fauna of saltmarshes was provided by Daiber
(1982). The majority of the literature assessed in that study
was from North America; the number of studies outside
America has subsequently increased, but there are still
extensive geographic regions and many faunal groups
for which few data are available.
Marine fauna
Saltmarsh creeks are an extension of the estuarine water
body, and for nekton (species within the water column)
movement between the main estuary and creeks is readily
possible. On high tides when creeks are bank full and
overflow to flood the marsh surface, nekton can move
from the creeks onto the marsh surface. There are, however, differences in the age and size structure of nekton
assemblages in marshes compared with those in the main
estuary. Saltmarsh creeks are important nursery habitats
for many fish and crustacean species and are thus essential
for supporting the estuary ecosystem. As a consequence,
they also sustain many commercial fisheries upon which
local communities may depend for employment and from
which the larger human community gains part of its food
supply.
SALTMARSHES
523
lineages occur in mangroves, with the exception of the
fern Acrostichum aureum, which is widespread throughout the tropics (Tomlinson, 1986). There is no indication
from the pre-flowering plant record of saltmarsh analogues. One of the earliest land plants was Rhynia, which
occurred in what Channing and Edwards (2009)
interpreted as a hot mineral-rich spring similar to those
found at Yellowstone National Park today. Some at least
of the earliest land plants therefore lived in harsh environments, and the high mineral content of the springs might
suggest that tolerance of salinity would have been possible. However, there is no support from the fossil record
for early occurrence of extensive saltmarsh.
Bryophytes are likely to have been early components of
the terrestrial flora, but are not generally recognized as
components of saltmarsh floras. However, a number of
bryophyte species occur in tidal saltmarshes and in inland
saline habitats (Adam, 1976; Adam, 1990; Garbary et al.,
2008); therefore, while bryophytes are frequently absent
and are rarely abundant in saltmarshes, they demonstrate
a capacity to tolerate salinity and might have been more
abundant in the past.
The present-day vascular flora of saltmarshes is
diverse. Barbier et al. (2011) refer to saltmarshes as grasslands, but while large areas of the world’s saltmarshes are
grasslands or dominated by physiognomically similar
graminoids (sedges and rushes), there are also large areas
dominated by herbs or shrubs, particularly by genera of
Amaranthaceae (Figure 6).
Most saltmarsh communities are dominated by perennial species. Annual plants are found in the low marsh
zone in those parts of the world where Salicornia occurs
and in microhabitats in the mid and upper marsh where
disturbance creates openings in which annual plants are
able to germinate.
Algae
Algae are important components of saltmarshes.
Microalgae are not always apparent to the human eye,
but microalgal films contribute to marsh formation and
ecosystem productivity. Many microalgae have mucus
coatings, and this is important in binding sediment
(Coles, 1979; Underwood, 2000). Establishment of
microalgae on mudflats may promote development of
microtopography (van de Koppel et al., 2012), facilitate
the establishment of pioneer vascular plants (Coles,
1979), and protect the surface of the lower marsh from
erosion. Mason et al. (2003) showed that microalgae were
susceptible to even low concentrations of triazine herbicides, suggesting that runoff of herbicides from agricultural land into estuaries might initiate erosion of
saltmarshes.
Macroalgae may be conspicuous components of
saltmarshes. In western Ireland and western Scotland,
very dense swards of dwarf fucoids (only 1 or 2 cm tall)
dominate lower marshes (Adam, 1981), and on Atlantic
saltmarshes in North America and numerous localities in
Europe, larger fucoids contribute substantially to marsh
productivity (Roman et al., 1990) creating protective
microclimates and providing habitat for intertidal invertebrates (Tyrrell et al., 2012). Eutrophication of waterways
can promote overgrowth and smothering of saltmarshes
by green macroalgae (McComb and Lukatelich, 1995),
particularly by species of Ulva.
Microorganisms
Microorganisms play essential roles in ecosystem processes in saltmarshes, playing important roles in nitrogen,
sulfur, and other biogeochemical cycles and in breakdown
of detritus. Microbial films on detritus are an important
food source for browsing invertebrates.
Recently, there has been considerable interest in the
interactions between halophytes and microorganisms in
the rhizosphere (Ruppel et al., 2013). Bacteria increasing
the availability of phosphorus and iron have been isolated
from the rhizosphere of several halophytes (Ruppel et al.,
2013). Arbuscular mycorrhizae, which enhance nutrient
uptake by plants, were first reported from the roots of halophytes early in the twentieth century (Adam, 1990; Davy
et al., 2000), but only recently has there been investigation
of their functions (Fuzy et al., 2008; Evelin et al., 2009).
Studies so far have concentrated on individual species;
placing mycorrhizae in the context of the functions of
saltmarsh ecosystems is a research challenge for the
future.
Fauna
The majority of animal phyla can be found in saltmarshes,
although many groups have been little studied. A review
of the fauna of saltmarshes was provided by Daiber
(1982). The majority of the literature assessed in that study
was from North America; the number of studies outside
America has subsequently increased, but there are still
extensive geographic regions and many faunal groups
for which few data are available.
Marine fauna
Saltmarsh creeks are an extension of the estuarine water
body, and for nekton (species within the water column)
movement between the main estuary and creeks is readily
possible. On high tides when creeks are bank full and
overflow to flood the marsh surface, nekton can move
from the creeks onto the marsh surface. There are, however, differences in the age and size structure of nekton
assemblages in marshes compared with those in the main
estuary. Saltmarsh creeks are important nursery habitats
for many fish and crustacean species and are thus essential
for supporting the estuary ecosystem. As a consequence,
they also sustain many commercial fisheries upon which
local communities may depend for employment and from
which the larger human community gains part of its food
supply.
SALTMARSHES
523