Download AQUATIC MACROPHYTES IN THE TROPICS: ECOLOGY

AQUATIC MACROPHYTES IN THE TROPICS: ECOLOGY OF
POPULATIONS AND COMMUNITIES, IMPACTS OF INVASIONS
AND USE BY MAN
S. M. Thomaz
Department of Biology/Nupélia, Maringá State University, Paraná, 87020-900, Brazil
F. A. Esteves
Department of Ecology/Nupem, Federal University of Rio de Janeiro, Brazil
K. J. Murphy
Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life
Sciences, Graham Kerr Building, University of Glasgow, Glasgow G12 8QQ, UK
A. M. dos Santos
State University of Montes Claros, Minas Gerais, Brazil
A. Caliman
Department of Ecology, Federal University of Rio de Janeiro, Brazil
R. D. Guariento
Department of Ecology, Federal University of Rio de Janeiro, Brazil
Key-words
Aquatic biodiversity, nutrient cycling, food webs, nuisance species, assemblages
Summary
1. Introduction
The term ‘aquatic macrophytes’ refers to large plants visible to the naked eye and
having at least their vegetative parts growing in permanently or periodically aquatic
habitats. These plants colonize a variety of aquatic habitats and can be divided into the
following life forms: rooted submerged – plants that grow completely submerged and are
rooted into the sediment (e.g. elodea, Elodea canadensis); free-floating – plants that float
on or under the water surface (e.g. water hyacinth, Eichhornia crassipes); emergent – plants
rooted in the sediment with foliage extending into the air (e.g. cattail, Typha domingensis);
and floating-leaved – plants rooted in the sediment with leaves floating on the water surface
(e.g. water lilies, Nymphaea spp). An additional two life forms have been proposed:
epiphytes – plants growing over other aquatic macrophytes (e.g. Oxycarium cubense); and
amphibious – plants that live most of their life in saturated soils, but not necessarily in
water (e.g. Polygonum spp).
Macrophytes include macroalgae of the divisions Chlorophyta (green algae),
Xanthophyta (yellow-green algae) and Rhodophyta (red algae) and the “blue-green algae”
(more correctly known as Cyanobacteria); Bryophyta (mosses and liverworts); Pteridophyta
(ferns); and Spermatophyta (seed-bearing plants). However, most of the literature devoted
to freshwater macrophytes has investigated three major groups: the Charales (an order of
Chlorophyta comprisingy large – up to 2 m – and relatively complex multicellular algae),
together with the vascular plant groups, Pteridophyta and Spermatophyta.
Macrophytes colonize virtually all freshwater habitats, from the tiny “living ponds”
provided by Bromeliaceae (e.g. Utricularia spp), to thermal springs (e.g. Najas tequefolia)
and waterfalls (e.g. members of the Podostemaceae colonize even the giant Iguaçu Falls,
Brazil/Argentina). Most rivers, lakes, lagoons and reservoirs are colonized to differing
degrees by macrophytes, whilst wetlands are characterized as areas where macrophytes
dominate.
Studies on aquatic macrophytes, and especially their ecology, were few in number
before the 1960s. The reasons are historical because the science of limnology primarily
originated in north-temperate countries, where deep lakes are characteristic: such
freshwater systems are amongst the least favorable of habitats to support aquatic
macrophytes. Consequently, phytoplankton was considered (correctly) as the main primary
producer and pelagic food webs were prioritized in those studies. A great increase in the
literature concerning macrophytes occurred after 1960, caused probably by the recognition
that a great number, if not most, aquatic ecosystems were in fact shallow, with extensive
littoral regions favorable for supporting aquatic macrophyte communities. A second factor
was increasing recognition of the role played by macrophytes in the biodiversity-support
functioning of freshwater systems: vital for many animal communities, such as aquatic
invertebrates, fish and aquatic birds.
In this article it is not possible to cover all relevant topics in depth: the literature on
tropical macrophyte ecology and management is too large for this to be possible. We utilise
Neotropical ecosystems (which support the highest macrophyte diversity) for many of our
examples but also include data from tropical and sub-tropical Australasia, Africa and Asia.
Following the publication guidelines for this book, we cited only the 20 most used literature
items for the article. However, we also used numerous other references which are provided
in a separate table (see Appendix 1). The link of these references to each specific topic
considered in our article will be provided by the first author ([email protected])
upon request.
2. General features of macrophytes
2.1. Evolution
Although still controversial, the origin of terrestrial plants is generally agreed to be
from green algae of the order Charales, known as stoneworts. After colonizing the land,
representatives of numerous different families returned to water, colonizing both freshwater
and marine ecosystems, with good evidence for at least 211 (but probably more)
independent colonization events of this nature having occurred.
It is interesting to note that angiosperms began the return to water very early in their
evolutionary history. An analysis of the angiosperm phylogenetic tree shows the terrestrial
shrub Amborella trichopoda as the first diverging lineage from the main branch of the
angiosperm phylogenetic tree, but the families Cabombaceae, Nymphaeaceae and
Hydatellaceae, which comprise only aquatic species, occupy the second basal lineage.
Fossil material collected in the Vale de Agua locality (in a complex of clay pits situated in
the Beira Littoral, Portuguese Basin) and in Crato (Northeast Brazil) confirms that water
lilies have colonized this region since the Early Cretaceous (125-115 Mya). Thus, some
adaptations found in extant submerged species, like aerial pollination and presence of
stomata (see below), are interpreted only under an evolutionary perspective.
2.2. Main adaptations to life in water
The aquatic habitat imposes strong pressures on plant survival. Although all life
forms of macrophytes face a limiting environment, pressures on survival are strongest for
submerged plants, the ones that have most fully completed the evolutionary return of
angiosperms to the aquatic habit. Water has greater density and viscosity compared to air
and thus gases (including CO2, needed for photosynthesis) diffuse at extremely low rates in
water, compared to in air. This is even more prominent in lentic ecosystems, where the
absence of flow means that large boundary-layers may surround leaves, leading to rapid
depletion of CO2 near plant surfaces. Aquatic ecosystems also have often-anoxic sediments
which cause problems for root survival. In addition, light may be strongly reduced beneath
the water surface not only by absorption of light energy by water molecules, but also by the
presence of biogenic (e.g. algae) and abiogenic (e.g. silt and clay) suspended matter, as well
as dissolved organic matter (usually humic substances). Again, light limitation primarily
affects mainly rooted-submerged plants, which have consequently evolved a number of
adaptations to cope with light limitation and other pressures on plant survival in water.
Concerning CO2 acquisition, submerged macrophytes display an array of
physiological and exploitation strategies to ameliorate the carbon constraints within the
aquatic medium. Probably one of the most common strategies is the use of the ion
bicarbonate (HCO3-), the concentration of which in the ocean and in most fresh-waters
(except soft water ecosystems) are high compared to dissolved CO2. However, CO2 is still
the preferred form of inorganic carbon used by most aquatic plants, since the exploitation
of bicarbonate involves the expensive synthesis of a complex of enzymes, such as carbonic
anhydrase, thereby elevating the energetic cost of photosynthesis. Even so, this mechanism
is efficient in waters with high pH values (>8.0), where CO2 is scarce or even absent,
providing a competitive advantage to species able to assimilate bicarbonate. About 50% of
species so far tested show evidence of bicarbonate use (attesting the efficiency of this
strategy in carbon acquisition). Examples of species that use bicarbonate are Egeria densa,
Egeria najas (both native to South America), Elodea canadensis and Potamogeton spp.
Other physiological strategies to overcome carbon limitation involve the use of C4
enzymes or crassulacean acid metabolism (CAM). Examples of species with C4 - like
metabolism include Egeria densa, Elodea nuttallii and Hydrilla verticillata while CAM
metabolism is found in Isoetes bolanderi, Crassula, Littorella, and Sagittaria, amongst
others.
Exploitation strategies to overcome CO2 limitation involve morphological and
anatomical adaptations that allow plants to obtain alternative sources of carbon, in addition
to that present in the water medium. Floating or aerial leaves are common in several species
(e.g. Cabomba furcata, Myriophyllum brasiliense and Potamogeton amplifolius) and they
allow plants to absorb CO2 directly from the atmosphere. Other species use their roots to
exploit the high concentrations of CO2 found in sediments, usually well above the
concentrations found in water. Species using CO2 from sediments have modified transport
vessels to permit movement of CO2 from the roots to leaves and high root:shoot weight
ratios (0.5 - 2.0). This strategy is found mainly in isoetids (e.g. Littorella and Lobelia),
providing a very specialized adaptation for this group whose species mainly colonize
oligotrophic softwater habitats, where CO2 is usually scarce in the water. Highly dissected
and thinner leaves, compared to terrestrial angiosperms, found for example in Cabomba
and Ceratophyllum, increase leaf surface area and also thereby increase carbon acquisition
rates.
The problems with anoxic sediment faced by aquatic macrophytes have led many
macrophytes to aerenchyma, a tissue containing gas spaces linking roots to leaves. Oxygen
is transported from leaves toward roots by this system. In addition, some species (e.g.,
Ludwigia adscendens) develop air roots that may be interpreted as short circuits to the
atmosphere, allowing greater transport of oxygen to the submerged and underground
organs.
Concerning light limitation under water, most plants cope with this in part by
increasing pigment content in leaves and locating chloroplast-containing cells in the
superficial epidermis. Most submerged macrophytes are considered “shade plants”, since
they are very efficient photosynthesisers at low light levels. This is usually achieved by
reduced respiration rates, and by reducing the thickness of their leaves. Several submerged
freshwater macrophytes also elongate shoots and concentrate their photosynthetic tissues
close to the water surface, in a strategy known as “canopy forming” (e.g. Egeria densa and
Hydrilla verticillata). This strategy renders these species (known as elodeids) a great
competitive ability and once they colonize a habitat, they may eliminate others colonizing
only the deeper parts of the water column (especially the isoetids, such as Isoetes).
Submerged plants typically have conspicuously reduced cuticule, useful to minimize
water loss in terrestrial habitats but superfluous under water. Vascular tissues, such as
xylem, and structural tissue such as lignin are also characteristically reduced. Although
important for terrestrial plants, these tissues lose their function under water, since this
medium furnishes support for plants. By reducing the need to synthesize such tissues
aquatic macrophytes further reduce energetic costs.
Finally, most submerged species still depend on the aerial medium for reproduction,
giving that pollination is primarily by insects. Flowers of these species are usually
positioned above the water surface. The flowers of the carnivorous Utricularia foliosa, for
example, reach up to 10 cm above the water surface, while Vallisneria spp. produce long
peduncles that may reach more than 1 meter and raise the female flowers into the air.
However, some species of Callitriche, Najas and Ceratophyllum, among others, have
developed hydrophilic pollination, thereby completing the adaptation of their sexual
reproduction to the aquatic environment.
3. Importance of macrophytes for ecosystem structure and functioning
Macrophytes affect aquatic ecosystems in a variety of ways, especially the shallower
ones where they colonize large areas. These plants change the water and sediment
physicochemistry, influence nutrient cycling, may serve as food for invertebrates and
vertebrates, both as leaves and dead biomass (detritus) and, in particular, change the spatial
structure of the waterscape by increasing habitat complexity. These roles of aquatic plants
have been extensively shown in temperate regions but they also occur in tropical habitats.
Aquatic plants are not inert objects, but active organisms whose metabolism affects
the water medium. The littoral region usually differs from the limnetic one in terms of the
thermal regime, gases, concentrations of ions (including the most limiting nutrients
nitrogen and phosphorus), pH and dissolved carbon, among other features. Under the
higher temperatures of tropical waters, oxygen is usually super-saturated, CO2 is nondetectable and pH values may easily reach 9.5 inside stands of submerged plants at noon.
Nutrients are released rapidly during decomposition contributing to the inorganic and
organic nutrient pools in the water. The majority of phosphorus, calcium, magnesium and
other ions are released from decomposing detritus in the first week. Giving the high stocks
of nutrients contained in macrophytes, their release through decomposition strongly affects
the water column. In a neotropical floodplain lagoon (Mogi, Brazil), 71% of nitrogen and
phosphorus were found in the biomass of Eichhornia azurea and Scirpus cubensis, and
only 29% were in the water column. High stocks of nutrients were also measured in a
Neotropical reservoir (Lobo, Brazil): the biomass of Nymphoides indica and Pontederia
cordata together had 12 and 5 times more nitrogen, and 18 and 24 times more phosphorus,
than the water of the littoral and limnetic regions, respectively. It is interesting to note that
part of these nutrients were locked up in sediments, but through macrophyte activity they
are made available for periphyton and plankton.
In temperate ecosystems nutrients are released through decomposition during fall,
when macrophyte shoots die back, but in tropical, more temperature-constant aquatic
habitats, release can occur thorough the year. However, even in the tropics, several aquatic
ecosystems do have seasons (e.g., rainy and dry) and may have periods of greater nutrient
inputs by plant decomposition. For example, together with decomposition of the
amphibious vegetation, that covers most of river-floodplain habitats, the decomposition of
aquatic macrophytes contributes to increase nutrients and reduces oxygen in these
ecosystems during high water periods, when those plant species unable to tolerate
inundation die and decompose. In higher-latitude tropical areas also seasonal differences in
temperature, also affects decomposition. Significant effects of temperature in tropical areas
have been experimentally shown: an increase of temperature from 17 to 27o C (a range
easily found in several tropical aquatic ecosystems), increased Egeria najas decomposition
rates threefold. In addition, water oxygen decreased and ions increased from 3 to 5 times
faster at 27o C than at 17 o C.
Macrophytes also contribute indirectly to nutrient cycling by releasing dissolved
organic matter that, in turn, supports the activity of nitrogen-fixing bacteria. This semisymbiotic relationship, in which the heterotrophic bacteria are favored by organic
compounds released by macrophytes and in turn furnishes nitrogen to these plants, has been
shown for several species of macrophytes (e.g. Utricularia sp, Eichhornia crassipes,
Nymphoides indica). In general, nitrogen fixation is higher close to the rhizosphere and
thus, concentrations of nutrient in sediments may be increased by this means, with the
surplus being released to the water column. In addition to nutrient amendments in sediment,
organic matter is also increased in this compartment under macrophyte colonization.
Giving the high production and great colonization of waterbodies by macrophytes, it
is tempting to suggest that the exceptional biomass produced by these plants enters food
webs, either directly by grazing (herbivory food webs) as well as through detritus
(detritivorous food webs). In fact, macrophytes have been shown to compose the diet of
several fish species in a variety of tropical freshwater ecosystems. Maybe the most studied
example of macrophyte herbivorous in tropics is the grass carp, which feeds on several
species of macrophytes. Invertebrates also use several species of tropical macrophyte
directly as food. For example, in the northeastern Argentina, at least 23 species of
invertebrates were found feeding on 13 species of aquatic macrophytes. Not only leaves are
eaten, but pollen has also been shown to compose the main diet of bees. This was found for
several species of the emergent Ludwigia in wetlands of South America.
Despite being used by fish and invertebrates, as revealed by stomach content
analyses, the first studies carried in tropical wetlands using stable isotopes raised doubts
about the importance of macrophytes as a source of energy for higher trophic levels. An
early investigation carried with detritivorous fish in the 1980s in the Amazonian floodplain
suggested that phytoplankton, instead of higher plants, composed the base of food webs.
Although detecting a varying importance of macrophytes for higher trophic levels, the
predominance of algae (both phytoplanktonic and periphytic) in tropical floodplains as
primary sources of energy have been confirmed by several other more recent studies. These
results led to a paradox: the higher biomass and faster production of macrophytes in
tropical waters are apparently not significant as source of energy for higher trophic levels.
However, recent studies have shown different trends. Investigations carried in an
Amazonian floodplain lake indicated that C4 macrophytes may contribute up to 59% of the
carbon for two species of fish. In another study carried in Rio Grande (Mexico), fish larvae
obtained carbon predominately from algal production in early summer, but used organic
carbon derived from emergent macrophytes as river discharge decreased in mid-summer.
Thus, this area is still open, since it seems that macrophytes do play a major role as energy
sources at least in specific ecosystems or habitats, or during part of the seasonal cycle. In
addition, the massive quantity of detritus produced during macrophyte decomposition
releases dissolved organic matter which, together with particulate matter, sustains microbial
food webs. Thus, independently of being important as a basic resource for food chains
composed of large organisms, the “burning” of organic matter originated from macrophytes
may drive nutrient cycling in aquatic ecosystems. This aspect is poorly known in tropical
waters where microorganism activity is believed to be much faster than in temperate
waters.
In addition to aquatic organisms, there are several species of terrestrial animals such
as birds and mammals, that use regularly macrophytes as food in the tropics. Good
examples are manatee (Trichechus inunguis), deer (Blastocerus dichoromus) and capybara
(Hydrochoerus hydrochaeris) in South America; hippopotamus (Hippopotamus
amphibious) in Africa; and goose (Anseranus semipalmata) in Australia. Terrestrial
invertebrates may feed heavily on macrophytes: the combined effects of the coleopteran
(adults) Neochetina bruchi and N. eichhorniae, together with the larvae of the dipteran
Thrypticus sp. may cause extensive damage to natural populations of water hyacinth in the
Neotropics. These and other herbivorous insect species are regularly used in the biological
control of water hyacinth, even in other continents.
If the role of macrophytes as energy source is still a matter of debate, there is a
consensus that this vegetation plays a major role by increasing the habitat complexity of
waterscape, at different scales. Increasing complexity (also known as habitat heterogeneity)
has direct and positive impacts upon the aquatic biota by increasing food at small scales
and by providing refuge for aquatic invertebrates, small sized, and young fish, that use the
littoral zones to feed, escape from predators or as nesting sites. Together, these mechanisms
lead to the maintenance of the high diversity found in littoral habitats (Fig. 1).
The positive effects of increased complexity provided by macrophytes have been
widely suggested in tropical streams, lakes and reservoirs. Studies in Neotropical reservoirs
show that the benefits for fish diversity and densities are recognized at different spatial
scales: at a single stand of macrophytes, in different arms of a same reservoir and among
reservoirs of different basins. The importance of aquatic plants as key components of
aquatic ecosystems raises the possibility of using macrophytes as a tool to manipulate
aquatic habitats, aiming at increasing the fish densities and diversity in tropical reservoirs,
since in many cases these man-made ecosystems are poorly structured.
COLONIZATION
BY PERIPHYTON
INCREASE THE
SURFACE FOR
COLONIZATION BY
MICROORGANISMS
AQUATIC
MACROPHYTES
INCREASE
FOOD FOR
INVERTEBRATE
AND SMALL
FISH
INCREASE
HABITAT
COMPLEXITY
INCREASE
REFUGIA
AVAILABILITY
INCREASE
SPECIES
RICHNESS
ATRACTION OF
INVERTEBRATES,
JUVENILE FISH
AND SMALL
BODY-SIZED
FISH
Figure 1. Some possible mechanisms lading to higher species richness in littoral habitats.
4. Macrophytes in populations
Tropical ecosystems were seen as different from temperate ones not only because of
their diversity (see “Biodiversity and Endemism” in this chapter), but also as places with
small seasonal variation. This assumption was largely based on the relatively constant
temperatures during the year, but it is not sustained when we consider the great variations
in rainfall. Besides producing direct effects upon aquatic ecosystems, seasonal rains cause
strong and predictable water level fluctuations, which lead aquatic populations to respond
with morphological, physiological and behavioral features. Macrophytes are no exception,
and populations are also strongly affected by such environmental changes often showing
conspicuous seasonal alterations.
The responses of the emergent Eleocharis interstincta to annual drawdowns, caused
by a combination of low rain and sand bar breaching, were studied in a Brazilian coastal
lagoon. Both stem height (R2 = 0.90; p< 0.05) and stem biomass (R2 = 0.65; p< 0.05) were
positively affected by water level (Fig. 2a). The adaptations of E. interstincta for solving
the support problem when its environment was terrestrial were: i) reduction of the mean
size of the stems and; ii) reduction of the space between transverse septa which was
characteristic for this species. The smaller space between each transverse septa provides
highest structural rigidity, and therefore, enhanced support. Reduction of the space between
transverse septa was observed in the field, as indicated by higher values of specific weight
(more biomass per unit of height indicating more lacunae) when the water level was
naturally drawn down, although this was not recorded when the drawdown was a result of
sandbar breaching (Figure 2b). A reduction in plant size and in the proportion of
specialized water-adapted tissues like aerenchyma, providing support for the plants in the
terrestrial phase, was reported by several other tropical species.
Figure 2. (a) Water level fluctuations and changes in aerial biomass and stem height of
Eleocharis interstincta. (b) The impact of “artificial” drawdown (sandbar breaching) on
this population can be observed in low values of specific weight after this event.
Responses of macrophyte populations (in terms of productivity, biomass and
densities) to water level fluctuations in seasonal tropical floodplains have been extensively
recorded. In general, growth changes (as demonstrated by biomass) with seasons is species
specific. In the Paraná River lagoons (Brazil), the biomass of Polygonum sp was positively
affected by water level, the biomass of Eichhornia azurea reached its peak during low
water level but the biomass of the free-floating Salvinia spp did not change seasonally. In
the Amazonian floodplain, a similar pattern is recorded: Hymenachne amplexicaulis
together with several species of grasses and Cyperaceae dominate during the dry phase;
during rising water, Oryza perennis and Paspalum repens populations increase very
rapidly, but decomposition of several species (and reduced shade) during floods are
followed by fast growth of free-floating populations of Salvinia, Pistia, Ceratopteris and
Eichhornia. Similar results are described from the Magela floodplain (Australia), where
Hymenachne acutigluma biomass increases after the first rains but decreased following a
large increase in water level, whilst Oryza meridionalis germinates after the first rains and
continues to grow as the plain fills with water.
Changes attributed to rain are even stronger in tropical temporary habitats, common in
arid and semi-arid regions. Investigations of the resistance and resilience of Najas marina,
a submerged species, to disturbances caused by flash floods in a permanent fluvial pool of a
Brazilian semiarid intermittent stream, showed that decreases in macrophyte biomass were
positively correlated with flood magnitude, varying from 25 to 53% when discharges were
lower than or equal to 0.5 m3sec-1 and between 70 and 100% when discharges were higher
than 1.0 m3sec-1. Macrophyte resilience was greater after floods of low magnitude. After
floods of 0.5 m3sec-1, three weeks were necessary to re-establish 88 percent of biomass lost,
and after a flood of 1.4 m3sec-1, six months were needed to initiate N. marina regrowth.
In addition to water levels and rain, light intensities, temperature and concentration of
dissolved inorganic carbon (DIC) are important variables that control the primary
production and population attributes of submerged aquatic macrophytes. Evidence about
the importance of these variables for primary production of three submerged species
(Utricularia foliosa, Egeria densa and Cabomba furcata) was found in Brazilian coastal
plain rivers where the low values of PAR, temperature and DIC in winter were limiting to
primary production of U. foliosa, and lower values of PAR, in winter, appeared to limit the
production of E. densa. On the other hand the higher values of PAR, and lower values of
DIC in winter and spring limited the production of C. furcata. The most productive species
in rivers of the target area was U. foliosa, a submerged non-rooted species. Its carnivorous
habit is an important additional source of nutrients for this species and probably for this
reason the gross primary production is not limited by the low total nitrogen and total
phosphorus concentration in water.
Despite being less productive than other life forms, submerged plants may also reach
high growth rates in tropical waters, especially when underwater light radiation is high.
Measurements taken in situ indicate that the Neotropical submerged species Egeria najas
may double its biomass at rates varying from 8.5 to 31.5 days. However, growth rates of
submerged plant populations are much more affected by underwater radiation, rather than
nutrients, which affect much more the free-floating species.
Indeed, fast growth rates fueled by nutrient inputs and high temperatures are usually
found for populations of emergent grasses and free-floating species. Extremely high
production has been found for populations of C4 grasses (especially Echinochloa
polystachya) in the Amazonian floodplain, where this species may reach c. 9 kg DM m-2
year-1. These results were recorded with two different methods, namely biomass changes
and CO2 flux measurements, and they are comparable with productivity of fertilized maize
fields in warm temperate conditions in Canada and the United States.
The fast growth of free-floating plants makes species belonging to this life form among
the most troublesome macrophyte especially in the tropics (see “Macrophytes as Weeds” in
this chapter). Eichhornia crassipes is probably the floating species with the highest
competitive ability and it is has been shown that it displaces other free-floating plants when
occurring together. This conclusion comes from both laboratory experiment and field
evidence. Experiments testing the ecological interactions of E. crassipes and Pistia
stratiotes, two free-floating macrophytes, showed an aggressive competitive behavior of the
former, which even improved its biomass, inhibiting the growth and establishment of the
latter. Similar results were recorded at the Itaipu Reservoir (Brazil/Paraguay), where
occasional explosive population growths of free-floating species are recorded. In of these
events Salvinia herzogii (together with P. stratiotes and E. crassipes) covered large areas in
c. 3 weeks, but three months later, E. crassipes dominated the stands, dislodging the other
two species. The architecture of E. crassipes, which is taller and better able to capture light,
probably explains its dominance upon other species that develop more horizontally, such as
Salvinia spp.
The fast population growth of free floating species has been recorded in several
tropical waters: doubling times for biomass from 3 to 5 days have been recorded for E.
crassipes and Salvinia spp in Africa and South America, under near-optimal conditions,
although under in situ favorable conditions these rates vary from 8 to 15 days. Especially
Salvinia spp, which grows very fast horizontally, may double its colonized areas in c. 2 - 3
days under favorable conditions, transforming it into a nuisance very quickly. Together
with high temperatures and stable water table, explosive growths usually occur with inputs
of nutrients, especially phosphorus. However, the initial plant density is also an important
determinant of growth rates and time to colonize specific habitats. Simulations of the
effects of nutrients and initial plant densities upon floating plant growth under tropical
conditions showed that under similar initial populations but in waters with 5, 15 and 35 μg
P/L, the growth rates were of 0.4%, 1.0 and 1.6% day-1, respectively. When the simulation
considered a constant phosphorus concentration (35 μg P/L), and an ecosystem with 10km2
in area, increasing 10 times the initial population size, reducing the time for total ecosystem
cover by 0.43 year (c. 5.2 months). Simulations like this are lacking in tropics but they are
extremely important due to widespread problems caused by floating leaved species in
several tropical countries.
Finally, it is interesting to note that although free floating species such as E.
crassipes, Salvinia spp and P. stratiotes are the very common plants in Neotropical riverfloodplain systems, where they are native, they rarely cause problems in these ecosystems.
This is because their populations are naturally controlled by water level fluctuations,
together with damage caused by native insect and fungus species. The lack of these
controlling factors contributes to the success of these species in other continents and/or
other ecosystems.
Despite the general fast growth and decomposition of macrophytes in tropics, when
compared to temperate ecosystems, caution is necessary to generalize this conclusion
especially if we consider high altitudes. In a high altitude reservoir in Colombia, for
example, it was suggested that the submerged macrophyte Egeria densa developed
extremely high biomasses (the highest biomass for this species recorded ever since was
found in this study), as a result of a combination of low decay rates and continuous growth
throughout the year. This situation was possible because the reservoir experiences high
light income together with low water temperature throughout the year. Thus, it is tempting
to suggest that contrary to what was believed by the first ecologists who explored the
tropics, namely that tropical ecosystems were physically stable and with high temperatures
round the year, in fact this is rarely the case, with the few exceptions perhaps being high
altitude aquatic ecosystems where temperatures approach those of temperate regions.
5. Macrophyte communities
5.1. The organization of macrophyte assemblages
Macrophytes rarely occur as monospecific populations in freshwater systems but tend
to form recognizable assemblages composed of several species belonging to different life
forms (e.g., free-floating, submerged, emergent and floating-leaved). As in temperate lakes,
tropical macrophyte assemblages are commonly organized along depth gradients often
forming easily-distinguishable zones in relation to water depth. In general emergent species
dominate in shallow areas while the submerged ones colonize deeper sites within a littoral
transect, with floating-leaved species commonly intermediate between the two primary
depth zones.
As relevant to tropical macrophytes as to any other group of organisms is the
question: “are communities naturally organized in space and time or they are produced by a
random assemblage of species”? One of the ways to answer this question is to analyze plant
distribution along gradients (like depth zonation in macrophytes). It is interesting that
although macrophytes offer an excellent opportunity to test this central issue in community
ecology, relatively few studies using macrophytes have directly addressed this question. In
several temperate wetlands, gradient analyses tested against null models have provided
evidence that macrophytes are organized in clusters along depth gradients.
In a first attempt to test this question in tropical floodplain lagoons, null models to test
patterns of co-occurrence of macrophyte species were used. The results showed that indeed
assemblages are generally organized non-randomly. However random assemblages may
appear in specific circumstances and it depends on the degree of connectivity between the
lagoon and the river during the flood pulse phase. There was a tendency of floods to
disorganize macrophyte assemblages during high waters in lagoons that are not connected
to the main river. This suggests that macrophyte assemblages in the tropics may be very
dynamic. More studies focusing on these aspects in tropical freshwater ecosystems would
certainly contribute to the debate about the nature of communities.
5.2. Factors affecting assemblage composition
Local assemblages are composed of species contained in the regional species pool,
although long distance species may be brought by migrant birds, one of the main natural
causes for macrophyte dispersion. Nowadays, humans are also important vectors of species
introductions, and species brought from distant regions or even other continents by this way
may affect local assemblages dramatically if they have strong competitive ability (see
section “Macrophytes in populations” and “Macrophytes as weeds” in this chapter). In
addition to arrival from other places, local communities are also largely determined by seed
banks, especially when recovering from disturbances of flooding or drying. In any instance,
the local environment filters out species from the pool creating a community.
Environmental (physical and chemical) factors affect the physiology of individual
plants, potentially leading to consequences for whole populations. If the stress and/or
disturbance caused by these factors are long or strong enough, communities may also be
affected. Thus, in general, the same environmental factors affecting macrophyte
populations will also influence communities as a whole. As a consequence, community
attributes, such as diversity, dominance and functional traits also change according to
morphometry, sediment and water physicochemistry. We discuss below some of the most
important abiotic factors affecting community attributes, focusing mainly on examples
from tropical aquatic ecosystems.
In temperate wetlands, the relative importance of environmental filters that determine
species composition were estimated as follows: hydrology (50%), fertility, salinity and
disturbance (15% each) and competition, grazing and burial (<5% each). Although these
numbers are just estimates, the high importance given to hydrology highlights that this
factor may be particularly important in the tropics, where there are a great number of large
and medium-sized rivers with active floodplains experiencing relatively natural water level
fluctuations (quite different from temperate regions where most large rivers are regulated).
Good examples of such relatively pristine ecosystems in the tropics are the Amazon and its
main tributaries, Orinoco and Paraná-Paraguay (Gran Pantanal included) in South America,
the Congo basin in Africa, and the Magela floodplain in Northern Australia. Rivers like
those, together with their wetlands, usually have well developed, relatively pristine, aquatic
plant communities with rich associated faunas.
Hydrology is also considered a primary determinant of plant communities structure in
river channels. For example, in the Mary River (Australia), differences in discharge,
influencing intensity of disturbance pressure upon the system, largely determine plant
community composition in the channel: Myriophyllum verrucosum and M. variifolium
dominate at high discharge while Vallisneria nana, Potamogeton crispus and P. perfoliatus
occupy intermediate positions in the disturbance gradient.
Changes in hydrology are usually accompanied by several other factors that affect
macrophyte communities. Changes in discharge often produce drastic alterations in nutrient
concentrations, underwater light, and water flow, in most unregulated systems with annual
alterations in water level: all producing increased stress or disturbance impacting the
community of plants present. In the Amazon floodplain, for example, macrophyte
assemblages respond clearly to water level fluctuations which may reach up to 10 meters
within a single year (see “Macrophytes in Populations” in this chapter). In this floodplain,
following a seasonal cycle, a terrestrial community composed of several herbaceous species
of fast growth are successively substituted by another community of rooted aquatic plants,
and then by another comprising free floating species. Similarly, in the Magela wetland
(tropical Australia), the structure of 8 communities composed by aquatic herbs was strongly
affected by water level fluctuations and some of them change drastically according to it.
For example, in the “Eleocharis sedgeland”, Eleocharis spp. dominate during the wet
season, but are replaced by annual herbs during the dry season. The importance of water
level fluctuation for community structure was also experimentally demonstrated in
Australia, where water regime primarily determined which species germinated from the
seed bank and survived in the vegetation. In another detailed experiment carried in
Australia, it was shown that depth, duration and frequency of flooding all affected plant
community development in some way. However, the major factor determining plant
community composition was not the total duration, frequency or depth of flooding, but the
duration of individual flooding events. Among the attributes measured in this investigation,
we can cite species diversity, which showed a gradual decline under longer durations of
flooding in a treatment containing intermittent wetland seed banks. Overall, the effects of
water level fluctuation upon aquatic plant communities are complex, and full evaluation of
the influence of time, duration, frequency and other attributes linked to floods requires an
experimental approach, still lacking in the tropics.
Giving the importance of hydrology for macrophyte communities, changes in natural
water level regime usually lead to conspicuous changes in plant communities. Such
changes are of especial concern in tropics, where large reservoirs usually regulate the flow,
modifying the natural plant habitats existing downstream. We can cite as an example an
increase of submerged plant colonization (with dominance by the native Egeria najas and
E. densa, and more recently by the exotic Hydrilla verticillata) in response to flow
regulation and increase in underwater light in a floodplain of the Upper Paraná River,
Brazil. Similarly, following river regulation leading to the presence of water even during
dry periods, an emergent plant community dominated by Typha domingensis developed in
the in the Old Hadejia River, Nigeria.
Although water level fluctuations are typical of river floodplain systems, this factor is
not peculiar to these ecosystems. Lakes and reservoirs are also affected by water level
fluctuation, which can strongly influence the composition of macrophyte assemblages. In
the Itaipu Reservoir (Brazil/Paraguay), an assemblage composed mainly of submerged
species was abruptly changed after a water level drawdown that lasted 3 months. The
rooted submerged Egeria najas was the most frequent species before the drop in water level
but its frequency was reduced by c. 90%, taking almost 4 years for recovery after water
levels rose again. However, in some arms of this reservoir, the recover of water level was
immediately followed by fast growth of free-floating species (Eichhornia crassipes,
Salvinia herzogii and Pistia stratiotes), probably in response to increase in water nutrients.
This phenomenon resembles what happens in tropical reservoirs after first filling: inputs of
nutrients, together with high temperatures, often favor the development of a community
dominated by free-floating species, which may be considered the first macrophyte
communities in the ontogeny of a reservoir. This phenomenon has been recorded in several
tropical reservoirs, like Lake Kariba (Zimbabwe/ ZambiaAfrica), where enormous growths
of Salvinia molesta developed immediately after the reservoir’s first filling, and in Serra da
Mesa, South America, where a community dominated by S. auriculata and P. stratiotes
initially established in several arms. On the other hand where nutrient outburst is lower, for
example in the case of new reservoirs flooding a desert landscape, free-floating plants are
not favored in this way, and development of the aquatic plant community commences with
submerged species (e.g. Lake Nasser, Egypyt).
As indicated by these examples, nutrients are also key chemical factors affecting the
composition of macrophyte assemblages. Long term studies in Lake Nasser (Egypt) showed
conspicuous changes in submerged macrophyte community mediated by physicochemistry. The main factors mediating changes were conductivity, magnesium, calcium,
nitrate and nitrite, and hydrosoil calcium and phosphate, which favored the development of
the invasive Myriophyllum spicatum which, in turn, caused alterations in plant
communities. In the Neotropics, water and sediment chemistry have also been considered
key for macrophyte community structure, both in natural as well as in man-made aquatic
ecosystems. In the Upper Paraná River floodplain (Brazil), both sediment chemistry
(especially Fe), together with water phosphorus, were important determinants of
macrophyte diversity while in the Itaipu Reservoir, water phosphorus proved to be a major
driver of macrophyte community, leading to domination by several free-floating species.
The examples above, taken from very different aquatic ecosystems from Africa and South
America, highlight the importance of trophic status as a determinant of aquatic plant
composition in tropical region.
Giving that macrophytes clearly respond to water and sediment chemistry, the use of
these plants as bioindicators has been proposed. This is especially valid because
macrophytes do not actively move, are easy to see and identify and readily sampled.
Although presence-absence data can be used for such purposes, macrophytes are extremely
plastic and thus schemes incorporating macrophyte and community traits (e.g., diversity,
total biomass, shoot length, number of lateral branches, total root length, number of
reproductive structure etc.) can produce good results. Such an approach was applied in
several tropical ecosystems, where morphological traits of macrophyte populations present
in rivers provided a potentially useful means of assessing river trophic status.
Underwater light regime is another important factor affecting submerged macrophyte
communities, which rely on below surface light for photosynthesis. As largely described for
temperate lakes the maximum depth of colonization by submerged macrophytes in tropical
waterbodies is also directly related to Secchi disk depth or other measures of underwater
light attenuation. The presence of submerged plant assemblages is largely determined by
underwater light availability, in turn influenced by both inorganic and/or organic turbidity.
Giving the seasonality of rainfall in tropical regions, submerged plant communities can be
greatly reduced during periods of increasing water level and/or turbidity.
When turbidity is mainly due to the development of phytoplankton, submerged
macrophyte assemblages may disappear as a combination of competition for light and
nutrients. Shifts of an aquatic ecosystem toward a “turbid state”, dominated by
phytoplankton community, or the recovery to a “clear state” dominated by a submerged
macrophyte community, fall within the concept of “alternative stable states”. This
phenomenon has been extensively studied in temperate ecosystems but it seems that this
concept may also be applied for tropical ecosystems, and relevant evidence has been gained
for a variety of Neotropical aquatic ecosystems. The question is how fast the shifts occur in
tropical ecosystems and what are the mechanisms involved. An interesting transition
between states has been described in arid wetlands in Australia. Lakes of these wetlands
become turbid and freshwater after flooding, but they become highly saline and clear after
drying, which leads to a shift to dominance by a submerged plant community. Interesting
here is that contrary to what happens in temperate lakes, where transition between turbid
and clear water may be fairly slow, and is mediated by nutrients, in the Australian arid
wetlands this transition is sudden and mediated by increasing salt concentration.
It is important to note that understanding the mechanisms behind aquatic plant
community shifts is of considerable practical importance especially in tropical, poor
countries, where eutrophication is one of the greatest issues threatening waterbodies. The
establishment and sustainable maintenance of a stable clear water state in lakes and
reservoirs, dominated by submerged macrophytes represents the presence of “good quality”
water, contrary to the turbid state, dominated by phytoplankton. Light, together with
nutrients, is also responsible for community changes along reservoir chains. Conspicuous
changes in macrophyte community occur along the Tietê River (Brazil), which is heavily
regulated, with a cascade of 7 reservoirs. This river drains much of the São Paulo superconurbation and the first reservoirs, ca. 100 km downstream from that city, are highly
eutrophic and turbid. These reservoirs are colonized mainly by macrophyte assemblages
dominated by free-floating plants (e.g., Eichhornia crassipes, Salvinia spp and Pistia
stratiotes). On the other hand, the final reservoirs in the chain are oligo-mesotrophic, with
dominance of submerged assemblages (e.g., Egeria najas, E. densa and Ceratophyllum
demersum)(Fig. 3).
Wind disturbance (“exposure”) is another important physical factor affecting plant
community attributes, acting via production of waves which can increase environmental
disturbance intensity in exposed littoral parts, especially of large water bodies, where fetch
(the longest uninterrupted distance in an aquatic ecosystem free for wave formation) is
350
300
250
200
150
100
phosphorus
Secchi
chlorophyll-a
Secchi disk (m)
Phosphorus (μg/L), chlorophyll-a (μg
/L)
high. Shallow waters are more affected by physical effects of waves and tend as a result to
have substrates of larger particle size (less prone to being washed away by wave action).
These coarse sediments are poor in nutrients and organic matter, and hence their presence
represents an additional stress for plant communities of shallow waters, though the main
effect derived from waves is direct injury upon plants. Thus, waves represent a mixture of
stress and disturbance. Effects of winds/waves are usually driven both by “fetch”, direction
and duration of high wind speeds, and slope of the shoreline. The greater the fetch, the
higher the potential for wave exposure to be a significant factor affecting littoral
communities. The importance of fetch for plant communities in tropical regions has been
demonstrated in large man-made lakes, where fetch negatively affects the diversity of
macrophytes and positively the minimum depth of colonization of submerged assemblages.
Waves are not formed only by wind, but also by navigation (especially where water is
shallow and powered boats or ships are relatively large). Boat-generated waves have been
shown to affect community attributes in tropical areas, as for example in the R. Nile Egypt.
50
0
Reservoirs cascade
Figure 3. Upper: reservoir cascade along the Tietê River Brazil with dominance of free
floating species in the first reservoirs and submerged species in the lowest ones. Lower:
phosphorus, chlorophyll-a and Secchi disk values recorded in the reservoirs (mean values
and standard deviations are shown). In the lower figure, B. Bonita is on the right and Três
Irmãos on the left side of axis x.
Although we deal above only with abiotic factors, biotic interactions can be important
as additional drivers of tropical macrophyte community structure. For example while water
regime played a major role in species germination and initiation of reproduction in
Australian wetlands, the subsequent reproductive output of macrophytes was controlled by
grazing (simulated experimentally by clipping). Maybe the most heavily studied example of
macrophyte herbivory is the grass carp (Ctenopharyngodon idella), introduced to many
tropical countries as a biocontrol agent against submerged aquatic weeds. Grass carp feed
on several species of macrophytes and can change completely the plant community in
aquatic ecosystems where it is introduced. However, there are several other large
herbivores that feed on macrophytes (see “Importance of macrophytes for ecosystem
structure and functioning” in this chapter). These animals potentially affect macrophyte
assemblages by preferentially predating specific species, but their effects are poorly known.
In addition to these native herbivores, buffalo are cause of conspicuous changes in
macrophyte communities in tropical waterbodies. The elimination of buffalo caused an
increase in plant diversity in the Brazilian Pantanal lagoons and changed communities
toward increased abundances of Eleocharis spp in Australian floodplains. In the Amazon
floodplain, areas subject to buffalo grazing had a decrease in herbaceous plant species
richness. It is of special concern that the expectation that once the buffalo were removed the
floodplains would re-establish a natural vegetation has not been met. Instead of Phragmites
vallatoria (the species recorded in remote times) re-colonizing areas where buffalo were
removed, the perennial grass Hymenachne acutigluma has spread and dominated large
areas. Giving the strong effects of these animals on macrophyte communities, as shown by
these examples, and their presence in many tropical countries, environmental agencies
should have special policies concerning their control.
Animals may also affect the structure of plant communities at smaller scales
(centimeters or meters) not only by grazing. Fish and other animal activities may cause
disturbances that affect macrophyte community structure at the scale of small patches. It
has been demonstrated in temperate lakes that a dynamic patch structure in submerged
vegetation occurs as a response to disturbance caused by nesting fish. Although we do not
know of any study using this approach in tropical aquatic ecosystems, this is a potential
area of inquiry in tropics, giving the great number of animal species (e.g. waterfowl and
fish) utilizing littoral areas for nesting or refuge.
5.3. Biodiversity and Endemism
Information about global patterns of macrophyte diversity is extremely scarce. In part,
this limitation is due to the definition of “aquatic macrophyte”, since many species are
amphibious and colonize transitional zones. In early investigations, comparisons among
regions surveyed in neotropics with others in temperate regions suggested that the highest
diversity was found at cool temperate latitudes. In a comparison of highland streams in the
Ecuadorian Andes with physically and chemically similar lowland streams in Denmark, for
example, it was found that, at least for submerged species, the tropical habitats supported
lower richness. If true, these results would indicate aquatic macrophytes as the only group
of organisms for which the normal latitudinal gradient of diversity (i.e. declining with
increasing latitude) was not valid.
However, an extensive survey carried using several international data-bases (primarily the
Royal Botanical Garden, Kew – UK) has recently changed this picture. According to this
survey, vascular aquatic macrophytes are represented by 33 orders and 88 families, with
about 2613 species in c. 412 genera. Contrary to what was shown previously, vascular
macrophyte species diversity is highest in the Neotropics (984 species), intermediate in the
Orient, Nearctic and Afrotropics (664, 644 and 614 species, respectively), lower in the
Palearctic and Australasia (498 and 439 species, respectively), and lower again in the
Pacific region and Oceanic islands (108 species), whilst only very few vascular macrophyte
species have been found in the Antarctica bioregion (12 species). The greatest diversity in
the tropics is in part due to the high number of species belonging to the family
Podostemaceae (180), which supports solely aquatic species. Considering that temperate
regions have been much more heavily investigated than tropical ones, where taxonomic
studies are still scarce, it is expected that this difference will increase with higher sampling
and taxonomic efforts in the tropics. A good example of this conclusion comes from a
survey carried in Neotropics, where plotting the number of studies against the number of
species showed a clear tendency of increasing species richness, which is still far from
reaching an asymptote (Figure 4). This strongly suggests that the number of macrophyte
species will increase with new surveys in the tropics.
Cumulative number of species
900
800
700
600
500
400
300
200
100
0
0
1
2
3
4
5
6
7
8
9
10
11
12
Studies
Figure 4. Cumulative number of species found in 12 studies made in large spatial scales in
Neotropical ecosystems.
Aquatic plant endemism seems also to be higher in tropical than other bioregions of
the world. A recent survey found that 395 aquatic macrophyte species (64% of total
present) were endemic to the Afrotropical region whilst 604 species (61%) are endemic to
the Neotropics. Lower degrees of endemicism are found in the Nearctic (268 species; 42%),
Palaearctic (139 species; 28%), Pacific (8 species; 7.4%) and Antarctic (no endemic
macrophyte species). Tropical areas also have high endemism at smaller spatial scales. For
example, 114 endemic macrophyte and wetland species, sub-species or varieties (about
20% of the total) were recorded in Southern Africa (South Africa, Lesotho, Swaziland,
Namibia, Botswana); and 100 species (again, c. 20% of the total) in a region including
South Brazil, Uruguay, Paraguay and North Argentina.
Concerning alpha (local) diversity, macrophytes can also exhibit high richness in
small areas in the tropics. Within waterbodies of the Upper Paraná River (Brazil), for
example, up to 14 species have been recorded inside a 1m2 quadrat (P. Carvalho,
unpublished). In small, shallow streams of the Upper Congo Basin, such as the Musola
River (Northern Zambia), surveys during 2006 found macrophyte diversity not as high as
this, but still reaching 6 – 7 species per m2 (typically in mixed communities supporting
submerged, floating-leaved, and emergent species: including Potamogeton nodosus,
Aponogeton vallisnerioides, Nymphaea caerulea, Phragmites australis, Ludwigia repens,
Cyperus alopecuroides and Panicum obtusifolium).
A more complete analysis of diversity should encompass different spatial scales and
include also beta (or between habitats) diversity, since the gamma diversity (diversity
within a large region) is a function of both “between” and “within” habitat diversity. There
are few data concerning beta diversity in tropical waterbodies. In the Itaipu Reservoir
(Brazil), macrophyte beta diversity was shown to be linearly and positively correlated with
habitat heterogeneity, which is in accordance with the general ecological principle that the
more different habitats exist along a gradient, the more species will colonize that gradient.
A tentative attempt at integrating diversity at different spatial scales was made using studies
carried out in a floodplain in Brazil (Fig. 4). More studies using this approach would be
useful to compare different tropical and temperate regions in terms of macrophyte diversity,
and help to prioritize areas of special interest for biodiversity conservation. This is
especially valid for macrophytes, given the important role that these plants have in
structuring the waterscape and providing habitat for other biota.
Paraná Basin
Species richness undetermined
Upper Paraná River Floodplain
62 species
Paraná River
34 species
n = 6 lagoons
turnover (Beta 1) = 0.23
Ivinheima River
16 species
n = 6 lagoons
turnover (Beta 1) = 0.12
Canal do Meio Lagoon
13 species
turnover (Beta 1) = 0.14
Garças Lagoon
18 species
turnover (Beta 1) = 0.21
Clara Lagoon
22 species
turnover (Beta 1) = 0.30
Turnover among rivers
CA 2
Baia River
40 species
n = 8 lagoons
turnover (Beta 1) = 0.10
Baia
Ivinheima
Paraná
CA 1
Figure 4. Studies of macrophyte alpha, beta and gamma diversity carried out in the Upper
Paraná River floodplain at different spatial scales. Upper level indicates gamma diversity
for the Upper Paraná floodplain; intermediate level shows gamma diversity for lagoons of
each river independently and beta diversity among lagoons of each river; lower level shows
alpha diversity of three lagoons connected to the Paraná river and beta diversity among
stands of each of these lagoons. Coordinate Analysis (right hand side) provides evidence
that beta diversity is higher among lagoons connected to the Paraná River.
6. Macrophytes as weeds
Several species of macrophytes have a variety of characteristics that make them
potential aquatic weeds all over the world. These nuisance species usually reproduce
vegetatively, grow very fast (see “Macrophytes in Populations” in this chapter), and
disperse easily by seeds or vegetative propagules, transported by water, birds or, more
recently, by boats, or accidental dispersal via the aquarium trade. Small fragments of
submerged macrophytes may travel long distances and establish new populations. In a
study involving the Neotropical submerged plant Egeria najas, fast sprouting was found in
fragments and even after 3 days in complete dryness, fragments could sprout and emit roots
after re-wetting. Indeed, successful invasions by alien submerged species may initiate from
fragments or vegetative propagules and may be entirely based on asexual propagation.
Some, if not all of the above-mentioned features, are found in free-floating (e.g. Eichhornia
crassipes and Salvinia spp) and submerged (e.g. Hydrilla verticillata and Egeria densa)
species. The transport of these and other tropical species between basins and mainly
between continents make them major nuisance species all over the world. For example,
although Hydrilla verticillata is probably native to some countries in Africa, this species
was first recorded in South Africa only in 2006 and RAPD analysis showed that invasion
originated from Indonesian and Malaysian populations.
Excessive growth of macrophytes may impact aquatic ecosystems in a variety of
ways, threatening ecosystem functioning, biodiversity and ecosystem multiple uses by man.
Free-floating species like Eichhornia crassipes or Salvinia spp may completely cover the
water surface in short time periods in tropical ecosystems, reducing light below the water
surface and gas exchange between water and atmosphere. These mechanisms may eliminate
submerged macrophytes, and change plankton and fish communities. Submerged species,
like Hydrilla verticillata and Egeria densa may also grow and dominate less turbid waters,
leading other submerged species to become locally extinct.
Impacts are usually much stronger when plants are introduced to regions where they
are not native, since the biological and physical controlling factors may not be present in
the new environment. Although mechanisms behind macrophyte invasions in tropical
ecosystems are poorly known, the literature about plant invasion records and their impacts
upon freshwater ecosystems is abundant for this region. We will explore some of these
examples below.
Natural habitats have been severely threatened in all tropical areas. Introduced species
may affect other native ones and change the assemblage structure. Examples in South
America are the Gran Pantanal (one of the largest wetlands in the world), and the Paraná
River floodplain in Brazil. In the first ecosystem the Australian grass Panicum repens and
the African Brachiaria subquadripara are invading several areas, especially those suffering
great disturbances by human interference. These two species are dominant over native
macrophytes and their expansion is a great concern for the extremely rich aquatic and
terrestrial biodiversity of this ecosystem. More recently (c. 2005), the submerged Hydrilla
verticillata was recorded in South American waters, specifically in the Tietê River (Brazil).
A few months later, this plant was found in natural areas of the Paraná floodplain,
downstream from Tietê and in less than two years this became the most frequent submerged
species, developing high biomasses (up to 500 g DM m-2) in the Paraná River channel and
its floodplain backwaters. Similarly to the Pantanal, the expansion of this submerged
species in this floodplain is of great concern, given its extreme competitiveness and
potential to dislodge other native species. Other examples come from tropical Australian
floodplains, which have also changed in recent decades due, in part, to invasions of species
such as Salvinia molesta and Urochloa mutica, and Hymenachne amplexicaulis (this one
reducing plant diversity and abundance of several macroinvertebrate orders); and from
Lake Nasser (Egypt), where Myriophyllum spicatum invasion changed submerged plant
assemblages.
One of the biggest problems affecting a natural lake in Africa in recent years was the
mass invasion of Lake Victoria (Uganda, Kenya, Tanzania) and its upstream catchment of
the Kagera River (Rwanda) by Eichhornia crassipes during the 1990s. This caused major
problems for fisheries and water transport on the lake, and severely threatened hydroelectricity generation in Uganda. The problem was brought under control by a major
biological control operation, coordinated by the Lake Victoria Environmental Management
Program, in all three riparian countries. An inspection report in 2003, for the World Bank,
which funded the program concluded that the operation :
“...has in general reduced water hyacinth coverage by 80 – 90% in most of the formerlyinfested areas. This outcome is a biologically-self sustaining one, based primarily on the
successful introduction of two host-specific natural enemies of water hyacinth (the weevils
Neochetina eichhorniae and Neochetina bruchi), by means of a rearing and release
programm with major community involvement. However, there remains a residual problem
of waterhyacinth hotspot areas, both external and internal to the Lake, which requires to be
addressed. Most of these hotspots (which occupy relatively small areas of the Lake) are
associated with nutrient-polluted areas of the lake, or inflowing rivers”.
Together with changes in macrophyte assemblages, invasions also affect other aquatic
communities that depend on macrophytes for shelter or feeding. However, not always is it
possible to show effects of invasive species upon other aquatic communities and
contradictory results after invasion are also been recorded. In a tropical floodplain in
Australia, the invasion of para grass (Urochloa mutica) had no significant detrimental
effects on aquatic macroinvertebrates living among or beneath vegetation and this finding
was attributed to similarity in growth form and structure of this species with other native
ones. An investigation about water hyacinth Eichhornia crassipes in a Zimbabwe
impoundment showed that fish density and diversity were higher in areas colonized by
these plants but diversity of zooplankton was higher in unvegetated areas. However, most
studies assessing the impact of invasions usually lack data from before invasions and/or are
time limited, which may impair definite conclusions about the role of invasive species in
tropical ecosystems.
Together with these more “ecological” impacts, excessive growth may impair water
uses by man. Fishing, navigation, tourism and, more recently, energy production, are
activities severely impacted by excessive growth of submerged and free-floating species in
the tropics. In addition, water plant habitats may be the favorite habitat for several disease
vectors (e.g. malaria) bringing an additional concern in tropical countries.
It is interesting to note that in contrast to most tropical and temperate areas, where it is
non-native species that typically become nuisance aquatic weeds, in South American
reservoirs native plants are the most troublesome species. In the Jupiá Reservoir (Brazil),
the most troublesome species are the submerged Egeria densa, E. najas and Ceratophyllum
demersum, although the emergent Typha domingensis and the free floating Eichhornia
crassipes and Pistia stratiotes also cause problems. Massive quantities of these plants
impair energy production especially during the rainy season: from 1990 to 1999, 1016
protection screens were damaged and had to be changed, at the Jupiá Dam, and from 1994
to 2001, 54,044 m3 of plants were removed from these screens. Energy production has to be
stopped during screen exchanges and even before these plants clogging screens reduce
water entering turbines, which also contributes to reduced electricity generation. Together
with costs associated with plant removal, this means that millions of dollars are lost by the
electricity company. In Ecuador, several reservoirs are almost completely covered by
Eichhornia crassipes, which may reach up to 30 kg FW m-2. Differently from the Brazilian
reservoirs, navigation with small boats by local people, who use the reservoirs as main
transport routes, is the most important activity affected in Ecuador. Still in Africa, concern
about blockage of intake of turbines has been arisen in Koka Reservoir (Ethiopia).
7. Potential use of water macrophytes:
In spite of its limited economic value in the modern world, the high number of species
and biomass distribution of aquatic macrophytes in tropical inland aquatic ecosystems have
motivated their use in a vast array of commercial, technological and cultural activities.
Such activities have important ramifications in many formal and informal economic sectors
of tropical developing countries. Aquatic macrophytes are used in the control of
eutrophication and environmental pollution, provisioning of food, soil fertilization,
pharmaceutical products and aesthetical and spiritual services such as ornamentation and
cultural manifestations. Their benefits to man also resides in wild-life conservation
practices, where they provide food for animals, protections for spawning fish and other
species (see “Importance of macrophytes for ecosystem structure and functioning” in this
chapter). In the following we attempt to highlight the many possibilities and the success of
the use of aquatic macrophytes in the tropical region.
7.1. Cultural and economic use
The use of aquatic macrophytes by humans is widespread in many parts of the world.
Their aesthetic and material importance to the peoples of the orient, the Near East and the
early European civilizations is revealed in Sanskrit, Chinese, Greek and Roman literature
and in the appearance of several species as ornamentation in ancient architecture, painting,
and metalwork. However, in the tropical region Indians were certainly the first who utilized
aquatic plants in their subsistence activities. The high number of different life forms of
aquatic macrophytes found in tropical inland waters allowed Indians to use them to distinct
purposes. Since some aquatic plants match flexibility and resistance they allowed produce
useful and durable handicraft products. For example, broad leaved aquatic plants such as
Typha spp were routinely used to manufacture a variety of artifacts such as sleeping mats
and baskets where Indians could store food and other utensils. Such plants were also used
to construct roofs for the tents and ropes which could be used in a variety of ways. The
petiole of slender plants such as Nymphaea spp were used to produce wears and adorns
which are used both routinely and in folkloric and cultural manifestations.
Most of the uses of aquatic macrophytes discovered by Indians perpetuated over the
time to modern civilizations and nowadays configure an important contribution to the
economy of tropical developing countries. For example, aquatic plants-made artifacts such
as bracelets, wears and sculptures are nowadays commercialized in the internal but mainly
in the external market. Live or died macrophytes are also used as decorative and aesthetical
products, such as macrophyte flowers arranges. Macrophytes handicrafts have been
increasingly exported for developed countries and are also an important cultural item
commercialized to tourists who visited tropical countries. Such commercial activities are
important both because they constitute an important economic subside but also because
they disseminate the cultural richness of tropical developing countries worldwide.
Some macrophyte species also have been used with industrial purposes. In Brazil
many species of Typha spp are used in the production of cellulose, which subsidize the
paper industry and many other activities.
7.2. Water Gardering
The cultivation of decorative aquatic plants may have arisen as an incidental feature
of the ancient arts of pisciculture and landscape horticulture, activities that can be traced
back to at least 2500 B. C. Since formal lotus pools became an essential feature of the
gardens of Buddhist temples, an emphasis was placed on the aesthetic, contrasting the
utilitarian, value of aquatic macrophytes. Water has always been the image of garden
designs, once water provides the irrigation and due to the pleasure of the calm
magnificence of lakes and pools, enhancing the visual impact of building and trees and
creating a peaceful environment.
Despite all life forms can be commonly observed in water gardens, more decorative
and imposing types received a greater emphasis. Notably emergent plants are those from
the genus Typha, Cyperus and numerous aroids; floating-leaved species such as Nymphaea
spp, Nymphoides spp; and free-loating plants like Eichhornia spp, Pistia stratiotes and
Salvinia spp are also extensively used. Tropical macrophytes are used to decorate water
gardens and fish aquariums both in tropics and temperate countries. Due to this practice, the
increasing exportation of macrophytes collected in the tropics to European countries,
redirected the attention of temperate botanists to poorly understood macrophyte genera, and
incidentally helped to reveal several previously undescribed species.
7.3. Medical Use
One of best-known species, accorded a therapeutic value by several races, is Acorus
calamus, the sweet flag, of which the rhizome has been used medicinally since at least the
time of Hippocrates (c. 460-377 B.C). Since then, many other references to the medicinal
use of macrophytes can be found in ancient literature. The medicinal application of aquatic
plants is still common in many places where native customs persists. In the tropics, Indians
also used aquatic plants as medicine for a large variety of illnesses. For example Pistia
stratiotes was used for Indians as a treatment for skin hurts and infections and until
nowadays have been used in the central region of Brazil in the treatment of urinary tract
infections. The rhizome of the species Acorus calamus was used to combat eyes illness and
toothache.
The use of plants in popular medicine is very common in tropical developing
countries at least for more isolated populations living in rural areas where the heritage of
the knowledge of popular medicine has been maintained and the access to ordinary
medicines are more difficult. Consequently, the use of aquatic plants for this purpose is also
evident diversified. Although still scarce, there is a growing body of studies in tropical
countries which have attempted to improve the potential use of aquatic macrophytes in
human medicine. Aquatic macrophytes are particularly interesting for this purpose because
they harbor a vast chemical arsenal against herbivory, such as phenol and anti-oxidants
which are also chemical substances of interest to human medicine. The study of such
phytotherapycs has shown they may have important antimicrobial properties. Such studies
have started to be carried out in tropical countries and are a promising field in tropical
medicine. However, the lack of communication among limnologists, botanists,
pharmaceutics and medicine scientists are responsible for the slow advances in the use of
macrophytes in human medicine are important obstacles to be overcame in tropical regions.
7.4. Source of food
Aquatic macrophytes present organs which due to their accumulated food reserves are
of potential nutritional value to man: among then seeds, fruits and swollen vegetative
perennating organs are the most important. A variety of fruits and seeds are rich in oil,
starch or protein and can be eaten raw, or dried and ground to flour which can be baked
with water or milk to give a kind of bread or cake. Numerous rhizomes and tubers are
similarly rich in carbohydrates, especially starch, sugar and mucilage, and are wholly edible
when raw or cooked. The foliage of many macrophytes provides acceptable salad
ingredients or cooked vegetable dishes. In Amazon, Indians used to utilize water hyacinth
ashes as salt and in the Northeast region of Brazil, the leaves of the macrophyte Typha
domingensis are often used in salads due to its high nutritional value and good palatability.
The rhizome of many other macrophytes is also used in the production of cookies, cakes
and other products. In Brazil, one of the most used macrophyte with nutritional value to
man is the watercress (Nasturtium sp), and it is often used in fresh or cooked dishes.
Due to its high nutritional value, aquatic macrophytes are used as food for animals as
well. Leaves of Typha domingensis are often used to feed the cattle in periods of drought in
many Brazilian States and thus, there is a great potential in use aquatic plants to substitute
ordinary animal feed. In pisciculture aquatic macrophytes can be used as fertilizers to the
water, enhancing fish production as ultimate effect, or as an alternative source of protein.
In this context, there is a general agreement that the use of aquatic macrophytes as food
source in aquiculture activities must be intensified worldwide.
7.5. Eutrophication and Pollution control
Although aquatic macrophytes have been used for a variety of ways their use in the
eutrophication and pollution control are among the most disseminated in tropical countries.
The high growth rates of aquatic macrophytes have long been viewed as a problem in the
management of impacted aquatic inland ecosystems, since the accumulation of plant
biomass can diminish the multitude of the aquatic ecosystem services (see “Macrophytes as
weeds” in this chapter). It has been a critical environmental problem in tropical developing
countries where the growth of aquatic macrophytes is high during the whole year and
sewage treatment is still deficient with the most of sewage entering tropical inland waters
without treatment. However, the traits that made tropical aquatic macrophytes an
environmental problem in eutrophic systems are the same which made them useful tools in
the management of eutrophication and pollution. The high growth rates and fast nutrient
assimilation allow aquatic macrophytes to be used as environmental filters in the treatment
of wastewater. Several studies have emphasized the use of aquatic macrophytes for this
purpose and the results have demonstrated that there are significant differences among plant
species, plant biotope (e.g., submerged, floating leaves, emergent) and plant diversity. In
general floating macrophytes such as Eichornia spp, Salvinia spp and Pistia stratiotes are
the most used species in sewage treatment because their photosynthetic activity and thereby
their nutrient assimilation are not affected by the water turbidity which is generally higher
in wastewater. Furthermore, the management of floating macrophytes is easier to perform
when plants need to be harvested. However, emergent plants such as Typha spp. and
Juncus spp are also efficient in nutrient removal, but their affects are indirect, since the
most of nutrient is assimilated by periphytic algae and bacteria which grow attached to
plant leaves. In addition, emergent plants are more efficient in adsorb heavy metals due the
fast adsorption of metals to organic matter and their post precipitation in the sediment in the
particulate form. Another important aspect is the ability of aquatic macrophytes to reduce
the number of thermo-tolerant bacteria, ubiquitous in domestic sewage and potentially
pathogen. In a Brazilian coastal lagoon (Rio de Janeiro State), aquatic macrophytes can
reduce in more than 99% the number of thermo-tolerant bacteria present in the domestic
sewage. Therefore, there is a great potential in use aquatic macrophytes to diminish the
concentration of nutrients, heavy metals and pathogens bacteria in the sewage, reducing
substantially the budget to construct ordinary wastewater treatment stations and the
ecological and sanitary damage of sewage discharge in aquatic ecosystems.
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