35 MACROPHYTES BOON OR BANE
Nusrat Ara, Shahnaz Jamil
Department of Botany, L. N. Mithila University, Darbhanga
Abstract - Aquatic macrophytes, often also called hydrophytes, are key components of aquatic and wetland ecosystems. This review is to briefly summarizes various macrophyte classifications and covers numerous aspects of macrophytes’ role in wetland ecosystems, namely in nutrient cycling. The most widely accepted macrophyte classification differentiates between freely floating macrophytes and those attached to the substrate, with the attached, or rooted macrophytes further divided into three categories: floating-leaved, submerged and emergent. Biogeochemical processes in the water column and sediments are to a large extent influenced by the type of macrophytes. Macrophytes vary in their biomass production, capability to recycle nutrients, and impacts on the rhizosphere by release of oxygen and organic carbon, as well as their capability to serve as a conduit for methane. With increasing eutrophication, the species diversity of wetland macrophytes generally declines, and the speciose communities are being replaced by monoculture- forming strong competitors. A similar situation often happens with invasive species.
Keywords: Aquatic Macrophytes, Bryophytes, Hydrophytes, Aquatic Ecosystems, Wetlands.
1 INTRODUCTION
Aquatic macrophytes (often referred to as hydrophytes) constitute an assemblage of taxonomically diverse macroscopic plants whose life cycle takes place completely or periodically in the aquatic environment.
Adaptive mechanisms evolved by macrophytes allow them to optimally respond to environmental heterogeneity and inhabit various types of aquatic habitats, including freshwater bodies, watercourses, wetlands, swamps, seasonally flooded areas, as well as brackish and marine environments. From a systematic point of view, aquatic macrophytic vegetation encompasses members of different groups, including green macroalgae (Chlorophyta, e.g.
Cladophora spp.), charophytes (Charophyceae, e.g. Chara and Nitella spp.) and higher aquatic plants, the latter being represented by both vascular plants (Tracheophyta) and bryophytes. According to some studies, macroalgae of the groups Rhodophyta and Xanthophyta can also be categorized as aquatic macrophytes.
Vascular plants of aquatic ecosystems are significantly inferior to terrestrial tracheophytes in their diversity and abundance, accounting for only about 1
% of the total number of vascular flora. It is estimated that aquatic tracheophytes are represented by 33 orders and 88 families, numbering about 2614 species in 412 plant genera. Most of aquatic vascular plants are represented by angiosperms (Magnoliopsida), but aquatic
vegetation also includes representatives of the classes Polypodiopsida and Lycopodiopsida. A small percentage of gymnosperms have been recorded to date as those capable of growing in an aquatic environment, among which Retrophyllum minus (Pinopsida) was identified as the only obligatory inhabitant of aquatic habitats. Among bryophytes, aquatic species make up about 0.5 % of all species, being found in all three divisions:
Bryophyta, Marchantiophyta and Anthocerotophyta (mosses, liverworts and hornworts, respectively). According to the traditional point of view, representatives of aquatic vascular vegetation evolved from terrestrial plants that subsequently made a transition back to aquatic environments [19]. However, some studies have provided evidence of the aquatic origin of several groups of aquatic angiosperms. Macrophytes, being an important part of various types of aquatic ecosystems, have a wide array of environmental functions and practical applications. The aim of this article was to analyze ecological groups and adaptation features of aquatic macrophytes, as well as their functional role in aquatic ecosystems.
2 CLASSIFICATION OF MACROPHYTES The diverse and heterogeneous group of macrophytes has posed a challenge for definition and classification. Macrophytes can be loosely defined as all forms of
36 macroscopic aquatic vegetation visible by
naked eye. This is in contrast to microphytes, i.e., microscopic forms of aquatic plants, such as planktonic and periphytic algae. The fact that macrophytes live in aquatic environments, at least seasonally, makes them different from terrestrial plants that don’t tolerate flooded environments. The macrophytes include taxonomically very diverse representatives: macroalgae (e.g., Chara and Nitella), mosses and liverworts (e.g., Sphagnum and Riccia), and vascular plants. Vascular plants represent the largest group of macrophytes including aquatic ferns (Azolla, Salvinia), Gymnosperms (rare) and Angiosperms, both mono- and dicots. Altogether, aquatic macrophytes are represented in seven plant divisions: Cyanobacteria, Chlorophyta, Rhodophyta, Xanthophyta, Bryophyta, Pteridophyta and Spermatophyta, consisting of at least 41 orders and 103 families. It is important to keep in mind that the evolution of angiosperms preceded in terrestrial environments and when some of them returned to aquatic environment they had to evolve various adaptations. There are a number of classical treaties on aquatic macrophytes some of them presenting quite elaborated classification schemes.
Hutchinson (1975) attempted to consolidate various life-form based classifications (grouped by the relation of plants to water level and substratum) and growth-form based classifications (grouped by structural similarity and relations to the physical environment) into a unified “ecological” classification.
Although his product is quite detailed, its main four categories remain the most widely accepted macrophyte classification until today. It differentiates between freely floating macrophytes and those attached to the substrate, with the attached, or rooted macrophytes further divided into three categories: floating-leaved, submerged and emergent. Brief description of the 4 categories follows:
“Emergent macrophytes” typically occur in the upper littoral zone at a depth of about 1-1.5 m, their root and rhizome systems are often adapted to permanently anaerobic sediments and they have aerial reproductive organs. This group includes
rather diverse types of plants that can be further categorized into two groups:
1) Erect emergents (e.g., Typha, Phragmites) and
2) Creeping emergents (e.g., Ludwigia spp.,
Myriophyllum aquaticum, Hydrocotyle spp., Nasturtium aquaticum, Alternanthera phyloxeroides) (Rejmánková 1992). “Floating-leaved macrophytes”
represented by genera such as Nymphaea, Victoria, or Brasenia, typically occur at water depths from ~0.5 to 3 m, they have long petioles with leaves adapted to mechanical stress and reproductive organs floating or aerial. “Submersed macrophytes” (Chara, Elodea, Cabomba, Utricularia, Myriophyllum) are most adapted to the live in aquatic environment. The depth distribution of angiosperms is limited to about 10 m, the representatives of other taxonomic groups occur at all depths within the photic zone.
They often have elongated, ribbon-like or dissected leaves, and aerial, floating or (rarely) submersed reproductive organs.
“Freely floating macrophytes” (Eichhornia, Lemna, Salvinia) often have highly reduced morphology and duckweeds (former Lemnaceae, now Araceae) are the smallest angiosperm with the whole plant in genus Wolfia being only about 1 mm across (Sculthorpe 1967). Their leaves and reproductive organs are aerial and/or floating and since they are not rooted in the sediments, their nutrient absorption completely from water. Besides the Hutchinson’s ecological classification of macrophytes, several authors attempted to classify macrophytes into functional types. Plant functional types can be defined as sets of plants exhibiting similar responses to environmental conditions and having similar effects on the dominant ecosystem processes. Grouping plants based on their functional traits became increasingly important when the need for understanding the link between plant species richness and ecosystem functioning has emerged as one of the central questions in ecology during the past decade. A simplification from species to functional groups is also attractive for predictive modeling of vegetation responses to human-induced changes in the environment, e.g., climate change.
However, despite different classification
37 schemes and terminologies there is no
single commonly accepted functional type classification useful for all studies whether they are concerned with wetland or terrestrial plants. It seems that plant functional groups are being de novo defined for each individual study depending on the study aims. Previous assessments of macrophyte diversity between temperate and tropical regions indicated that the richness was similar, or even richer, in temperate regions.
However, Chambers et al. (2008) in their thorough comparison of global diversity of freshwater aquatic macrophytes among bioregions concluded that vascular macrophyte generic diversity for the tropics is greater than for temperate regions. These authors also pointed out that large gaps still exist in our knowledge of aquatic macrophyte abundance and distribution and warned that many of the threats to fresh waters such as eutrophication and alien species introductions will lead to reduced macrophyte diversity.
3 ECOLOGICAL FUNCTION OF AQUATIC MACROPHYTES
Macrophyte vegetation is an important component of various types of aquatic ecosystems. Together with phytoplankton species, these autotrophic organisms are the primary producers that provide the conversion of light energy into organic carbon compounds, thereby contributing to the formation of the trophic structure of aquatic ecosystems. During photosynthesis, macrophytes not only synthesize organic substances, but also release oxygen, which is necessary for the respiration of aquatic organisms and decomposition of organic matter. Aquatic macrophytic vegetation is a food resource for a wide range of herbivores, both invertebrates (snails, crayfish, insect larvae) and vertebrates; in addition, many species of aquatic and wetland macrophyte plants is consumed by humans and used for medical purposes.
Macrophytes, especially submerged species, are important for aquatic food webs and affect the interaction between predatory, planktivorous and benthivores fish, as well as between fish and invertebrates. According to available data, 37 freshwater herbivorous fish species
belonging to 24 families feed on macrophytes. Some other aquatic or semi- aquatic vertebrates, such as waterfowl, turtle, nutria, muskrat, manatee, and others, use aquatic macrophytes as food.
Freshwater and marine macrophytes also affect communities of animals and other aquatic organisms through a chain of habitat-related mechanisms, including by providing nursery, living and feeding places. Macrophytes, similarly to other aquatic organisms (e.g. phytoplankton, cyanobacteria), are producers and emitters of biologically active substances (allelochemicals), including low molecular weight volatile organic compounds, which play an important role in interspecies communication and competition.
Allelochemicals released by aquatic plants perform a variety of functions, thereby affecting the composition and development of aquatic communities.
These substances mediate allelopathic activityagainst competitors, perform a protective role or can act as attractants, exhibit antimicrobial activity and inhibit the growth of pathogenic microorganisms and harmful algal blooms. It is assumed that inhibition of phytoplankton and bacterioplankton, including cyanobacteria species, by allelochemical compounds secreted by macrophytes contributes to the stabilization of clear water states in shallow lakes. Additionally, macrophytes affect aquatic ecosystems through their influence on hydrological regime of water bodies (e.g. flow velocity, formation of surface waves, etc.), bottom sediment formation and water quality. Noticeable changes in pH values, dissolved gas concentrations and ionic composition of water may result from their metabolism.
Macrophytes that grow in near-shore areas can promote the stabilization of shores and contribute to reduction in erosion rates. Macrophytes possess a great potential to concentrate mineral elements from the aquatic environment and bottom sediments, and some species are known as metal hyperaccumulators, which is based on their metabolic features and high metal tolerance. By absorbing mineral ions, these plants participate in the processes of biogeochemical cycling of elements, nutrient turnover and water self-purification; by removing excess nutrients such as phosphates and
38 nitrogen compounds, macrophytes are
capable of counteracting the eutrophication processes in water systems. On the other hand, aquatic and wetland macrophytes are capable of accumulating non-metallic inorganic and organic pollutants; moreover, macrophyte species can detoxify some organic xenobiotics by absorbing them from the aquatic environment and transforming these compounds after absorption.
Therefore, macrophytes are increasingly used in phytoremediation processes of contaminated water bodies, as well as in engineered systems known as constructed wetlands, for the treatment and purification of domestic, agricultural, mining and industrial wastewaters.
3.1 Advantages and Disadvantages of Macrophytes
Macrophytes play an important role in the freshwater ecosystem functioning of many shallow water bodies: as primary producers, by providing structure in the habitat of many animal species and provide shelter and food to invertebrates and fish. Macrophytes, which are major primary producers in shallow freshwater systems, have been reported to contribute substantially to biodiversity at the ecosystem level. Macrophytes are also involved in ecosystem processes such as biomineralization, transpiration, sedimentation, elemental cycling, materials transformation, and release of biogenic trace gases into the atmosphere.
Recent studies have established the importance of aquatic macrophytes in regulating the nutrient availability in the water and enhancing the stability of lakeshores. Macrophyte assemblage can be influenced by geology, land use, and water and sediment chemistry.
Macrophyte community composition and distribution varies with climate, hydrology, substrate type, and nutrient availability. Growth forms of aquatic macrophytes given below:
1. Emergent macrophytes (plants that are rooted in submersed soils or soils that are periodically inundated, with foliage extending into the air (e.g., Phragmites australis, Typha latifolia), 2. Floating-leaved macrophytes (plants rooted to the lake or stream bottom
with leaves that float on the surface of the water (e.g., Nuphar luteum), 3. free-floating macrophytes (plants that
typically float on or under the water surface (e.g., Eichhorniacrassipes) and
4. Submerged macrophytes (plants that grow completely submerged under the water, with roots or rootanalogues closely associated with the substrate (e.g., Myriophyllum spicatum).
In theory, their distribution in lentic systems occurs in organized zones.
Starting from the edge with emerged plants, followed by plants with floating leaves until we found rooted submerged species. However, abiotic factors (i.e., depth, water temperature, light incidence, input of nutrients, and interspecific competition) may facilitate heterogeneous distribution. Most submerged aquatic macrophytes belong to the families Ceratophyllaceae, Haloragaceae, Hydrocharitaceae, Nymphaeaceae and Potamogetonaceae. Submerged macrophytes are found in various types of water bodies, including estuaries, rivers, lakes, ponds, natural depressions, ditches, swamps and floodplains. They compete with phytoplankton for nutrients, decreasing the productivity of the water and causing hindrance to the movement of fish, irrigation and navigation.
3.2 Role of Macrophytes as Primary Producers
Aquatic macrophytes play a significant role in freshwater ecosystems as they provide food and shelter to invertebrates and stabilize sediments & shorelines thus reducing turbidity of aquatic systems.
Submerged macrophytes affect nutrient dynamics, lightattenuation, temperature regimes, hydrodynamic cycles, and substrate characteristics. The macrophytes are responsible for the regulation and stabilization of mineral cycling in the water bodies and hence they serve as indicators for the possible degree of damage in the ecosystem. The aquatic plants are the drivers of ecosystem productivity and biogeochemical cycles, in part because they serve as a critical interface between the sediments and the overlying water
39 column. Aquatic plants are an essential
part of the aquatic ecosystems.
3.3 Macrophytes and Faunal Diversity The presence of macrophytes and aquatic invertebrates in these places enhances the local species richness. Macrophytes play a central role in the control of phytoplankton and sustain a high faunal diversity. Diversity and abundance of invertebrates in lentic ecosystems are often influenced by the presence of aquatic macrophytes. The macrophytes support epiphytic algae and animals as well as a variety of associated mobile animals, including zooplankton, macrofauna and fish. Macrophytes provide habitat and refuge for zooplanktonic filter feeders to maintain top-down control on the phytoplankton.
Wetlands with their macrophyte assemblages can also contribute to the wellbeing of the community by acting as urban green spaces which provide aesthetic appeal, landscape diversity and recreational opportunities. However, macrophytes are also becoming a nuisance to the aquatic ecosystem, human health and economy when they turn out to be invasive.
3.4 Macrophytes and Structuring Communities in Aquatic Ecosystems Aquatic macrophytes play an important role in structuring communities in aquatic environments. These plants provide physical structure, increase habitat complexity and heterogeneity and affect various organisms like invertebrates, fishes and water birds.
Macrophytes generally colonize shallow ecosystems where they become important components, influencing ecological processes (e.g., nutrient cycling) and attributes of other aquatic attached assemblages (e.g., species diversity). The role of macrophytes as physical structures that increase habitat complexity or heterogeneity in aquatic ecosystems is widely recognized. The effect of macrophytes on populations and communities has been widely demonstrated for a variety of organisms, such as micro- and macro-invertebrates, fishand water birds.
3.5 Role of Macrophytes in fish Assemblages
The absence of physical structures in the littoral zone of created ecosystems (like reservoirs) implies a lack of suitable habitats and the instability of biotic relationships, which may limit the resources available. This lack of structure can be identified as a limiting factor for population growth. Thus, the presence of an intermediate level of cover of macrophytes may maintain populations and communities, ensuring support for a larger number of organisms. Aquatic macrophytes may have a strong influence on the population dynamics of these fish assemblages through structuring habitats. The important role of macrophytes to fish assemblages is considered in techniques aimed at fisheries resource management.
3.6 Macrophytes as Bio-Indictors
Plants are sensitive tools for prediction and recognition of environmental stresses.
Ecological health can be viewed in terms of ecosystems, in which structural and functional characteristics are maintained.
Ecological health has an effect on human health and well-being. The depth, density, diversity and types of macrophytes present in a system are indicators of water body health. Aquatic vegetation can influence the water quality too. The presence or absence of certain plant or other vegetative life in an ecosystem can provide important clues about the health of the environment. Phytoplankton’s are of great importance in bio-monitoring of pollution. The distributions, abundance, species diversity, species composition of the phytoplankton are used to assess the biological integrity of the water body.
Aquatic macrophytes in the littoral zones of lakes have two fundamental properties, which make them useful as limnological indicators:
3.7 Macrophytes as Invasive Species Aquatic and wetland habitats are especially vulnerable to plant invasions due to high disturbance and often high nutrients that facilitate rapid expansion of invading species. Macrophytes affect nutrient cycling, for example through transference of chemical elements from sediment to water, by both active and
40 passive processes. Macrophytes have
adaptations that enable their rapid spread and growth and increase their invasive potential. Aquatic macrophytes are often dispersed and introduced around the world for ornamental objectives and other anthropogenic interests. Effects due to invasion could affect habitat heterogeneity provided to associated organisms. Other effects of macrophytes invasion include changes in the composition of the macrophytes assemblage itself.
Considering that aquatic macrophytes exert an important role in structuring habitats, their invasion could change the waterscape, impacting other taxonomic groups. Freshwater ecosystems are highly impacted by human beings (e.g., through eutrophication), which increases the invisibility of these systems by macrophytes. Invasive macrophytes can cause serious ecological and economic damage worldwide.
4 ROLE OF MACROPHYTES IN THE TREATMENT OF EUTROPHIC WATER Surface water eutrophication can lead to algal and cyanobacterial blooms, die-off of indigenous vegetation, and a serious decrease in biodiversity. Recovery of water quality and the repair of ecosystems damaged by increased nutrient runoff is a research area of importance. Excess loading of phosphorus (P) and nitrogen (N) from domestic, agricultural and industrial wastewaters is the main cause of eutrophication of aquatic ecosystems, damaging their ecological quality and functioning. Of all the biological treatments for controlling lake eutrophication, aquatic vegetation, especially submerged macrophytes, has been recognized as being the most effective. The establishment of macrophytes stands in shallow systems can increase nutrient retention and recycling. During the growing season, macrophytes act as a sink by accumulating nutrients in developing tissues. Aquatic floating macrophytes take up inorganic nutrients mainly by the roots, although uptake through the leaves may also be significant. Members of free floating duckweeds (Lemnaceae), namely Lemna minor, L. gibba, Wolffia arrhiza, and Azollapinnata have shown potential usefulness in the treatment of
eutrophicated water system. Aquatic macrophytes are unchangeable biological filters and they carry out purification of the water bodies by accumulating dissolved metals and toxins in their tissue. Many of the macrophytes found to be the potential scavengers of heavy metals from aquatic environment and are being used in wastewater renovation systems.
5 CONCLUSION
Aquatic macrophytes comprise a systematically heterogeneous group of macroscopic plants including representatives of both vascular and non- vascular higher plants, as well as green macroalgae and charophytes.
Macrophytes inhabit various types of freshwater, brackish-water, and marine ecosystems and are characterized by various growth forms and plasticity of metabolism depending on environmental conditions. Macrophytic vegetation plays an important role in the trophic, structural and functional aspects of aquatic ecosystems, contributing to the formation of organic carbon compounds and oxygen release, influencing the hydrological regime of water bodies, providing food and shelter for ichthyofauna and aquatic invertebrates, as well as by influencing water quality.
Macrophyte species are widely used by humans for bioindication and phytoremediation of polluted water bodies, treatment of various types of wastewater, for medical purposes, etc.
However, the diversity of aquatic macrophytes in freshwater and marine ecosystems is currently threatened due to anthropogenic and climatic factors such as destruction of natural habitats, water pollution, eutrophication and global warming.
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