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 (