Invited Review Paper J. Ecol. Field Biol. 34(4): 333-345, 2011 Journal of Ecology and Field Biology The role of macrophytes in wetland ecosystems Eliška Rejmánková* Department of Environmental Science and Policy, University of California Davis, One Shields Avenue, Davis, CA 956 16, USA Abstract Aquatic macrophytes, often also called hydrophytes, are key components of aquatic and wetland ecosystems. This re- view 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 be- tween 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 produc- tion, 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 mac- rophytes generally declines, and the speciose communities are being replaced by monoculture-forming strong competi- tors. A similar situation often happens with invasive species. The roles of macrophytes and sediment microorganisms in wetland ecosystems are closely connected and should be studied simultaneously rather than in isolation. Key words: eutrophication, habitat, invasive species, macrophyte, methane, nitrogen, phosphorus, resorption, wetland INTRODUCTION Aquatic macrophytes, often also called hydrophytes, for their direct contributions to human societies by pro- are key components of aquatic and wetland ecosystems. viding food, biomass, and building materials (Costanza et As primary producers, they are at the base of herbivorous al. 1997, Engelhardt and Ritchie 2001, Egertson et al. 2004, and detritivorous food chains, providing food to inver- Bornette and Puijalon 2011). tebrates, fish and birds, and organic carbon for bacteria. Good knowledge of the functions of aquatic macro- Their stems, roots and leaves serve as a substrate for pe- phytes in wetlands and shallow lake ecosystems is critical riphyton, and a shelter for numerous invertebrates and for understanding the basic ecosystem processes. It is also different stages of fish, amphibians and reptiles (Timms important for numerous applied issues such as wetland and Moss 1984, Dvořák 1996). Biogeochemical processes restoration, wastewater treatment, and management of in the water column and sediments are to a large extent invasive species (Lavoie 2010, Casanova 2011). influenced by the presence/absence and a type of mac- The goal of this review is to briefly summarize various rophytes, and macrophytes can also have a profound im- macrophyte classifications, and cover in more details nu- pact on water movement and sediment dynamics in wa- merous aspects of the macrophytes’ role in aquatic and ter bodies. Some macrophytes are of major importance wetland ecosystems. Open Access http://dx.doi.org/10.5141/JEFB.2011.044 Received 24 October 2011, Accepted 25 October 2011 This is an Open Access article distributed under the terms of the Creative *Corresponding Author Commons Attribution Non-Commercial License (http://creativecommons. org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, E-mail: [email protected] distribution, and reproduction in any medium, provided the original work Tel: +1-530-752-5433 is properly cited. pISSN: 1975-020X eISSN: 2093-4521 Copyright © The Ecological Society of Korea 333 http://jefb.org J. Ecol. Field Biol. 34(4): 333-345, 2011 DEFINITION AND CLASSIFICATION Ludwigia spp., Myriophyllum aquaticum, Hydrocotyle spp., Nasturtium aquaticum, Alternanthera phyloxeroi- The diverse and heterogeneous group of macrophytes des) (Rejmánková 1992). has posed a challenge for definition and classification. “Floating-leaved macrophytes” represented by genera Macrophytes can be loosely defined as all forms of mac- such as Nymphaea, Victoria, or Brasenia, typically occur roscopic aquatic vegetation visible by naked eye. This is at water depths from ~0.5 to 3 m, they have long petioles in contrast to microphytes, i.e., microscopic forms of with leaves adapted to mechanical stress and reproduc- aquatic plants, such as planktonic and periphytic algae. tive organs floating or aerial. The fact that macrophytes live in aquatic environments, “Submersed macrophytes” (Chara, Elodea, Cabomba, at least seasonally, makes them different from terrestrial Utricularia, Myriophyllum) are most adapted to the live plants that don’t tolerate flooded environments. The in aquatic environment. The depth distribution of an- macrophytes include taxonomically very diverse repre- giosperms is limited to about 10 m, the representatives sentatives: macroalgae (e.g., Chara and Nitella), mosses of other taxonomic groups occur at all depths within the and liverworts (e.g., Sphagnum and Riccia), and vascu- photic zone. They often have elongated, ribbon-like or lar plants. Vascular plants represent the largest group of dissected leaves, and aerial, floating or (rarely) submersed macrophytes including aquatic ferns (Azolla, Salvinia), reproductive organs. Gymnosperms (rare) and Angiosperms, both mono- and “Freely floating macrophytes” Eichhornia( , Lemna, Sal- dicots. Altogether, aquatic macrophytes are represented vinia) often have highly reduced morphology and duck- in seven plant divisions: Cyanobacteria, Chlorophyta, weeds (former Lemnaceae, now Araceae) are the smallest Rhodophyta, Xanthophyta, Bryophyta, Pteridophyta and angiosperm with the whole plant in genus Wolfia being Spermatophyta, consisting of at least 41 orders and 103 only about 1 mm across (Sculthorpe 1967). Their leaves families (for details see Chambers et al. 2008). It is impor- and reproductive organs are aerial and/or floating and tant to keep in mind that the evolution of angiosperms since they are not rooted in the sediments, their nutrient proceeded in terrestrial environments and when some of absorption completely from water. them returned to aquatic environment they had to evolve Besides the Hutchinson’s ecological classification of various adaptations. macrophytes, several authors attempted to classify mac- There are a number of classical treaties on aquatic rophytes into functional types (Boutin and Keddy 1993, macrophytes (Gessner 1955, Sculthorpe 1967, Hutchin- Brock and Casanova 1997, Weiher et al. 1998). Plant func- son 1975, Cook 1996) some of them presenting quite elab- tional types can be defined as sets of plants exhibiting orated classification schemes (Hejný 1960, Den Hartog similar responses to environmental conditions and hav- and Segal 1964). Hutchinson (1975) attempted to consoli- ing similar effects on the dominant ecosystem processes date various life-form based classifications (grouped by (Diaz and Cabido 1997, Lavorel et al. 1997). Grouping the relation of plants to water level and substratum) and plants based on their functional traits became increas- growth-form based classifications (grouped by structural ingly important when the need for understanding the similarity and relations to the physical environment) into link between plant species richness and ecosystem func- a unified “ecological” classification. Although his product tioning has emerged as one of the central questions in is quite detailed, its main four categories remain the most ecology during the past decade (Bouchard et al. 2007). A widely accepted macrophyte classification until today simplification from species to functional groups is also (Wetzel 1975, Denny 1985). It differentiates between freely attractive for predictive modeling of vegetation respons- floating macrophytes and those attached to the substrate, es to human-induced changes in the environment, e.g., with the attached, or rooted macrophytes further divided climate change (Nygaard and Ejrnaes 2004). However, into three categories: floating-leaved, submerged and despite different classification schemes and terminolo- emergent. Brief description of the 4 categories follows: gies there is no single commonly accepted functional “Emergent macrophytes” typically occur in the upper type classification useful for all studies whether they are littoral zone at a depth of about 1-1.5 m, their root and rhi- concerned with wetland or terrestrial plants (Sieben et al. zome systems are often adapted to permanently anaero- 2010). It seems that plant functional groups are being de bic sediments and they have aerial reproductive organs. novo defined for each individual study depending on the This group includes rather diverse types of plants that can study aims (Rejmánková et al. 2011). be further categorized into two groups: 1) erect emergents Previous assessments of macrophyte diversity between (e.g., Typha, Phragmites) and 2) creeping emergents (e.g., temperate and tropical regions indicated that the rich- http://dx.doi.org/10.5141/JEFB.2011.044 334 The role of macrophytes in wetland ecosystems ROLE: SUN ENERGY PRIMARY PRODUCTION CO2 uptake H2O O2 Nutrient uptake (resorption) PHOTOSYNTHESIS Transpiration RESPIRATION O2 release CO2 Org. C exudates Conduit for gases DECOMPOSITION CH4 Nutrient release Rhizosphere related processes N, P Enzyme production HABITAT N, P ORGANIC MATER O2 C CH4 Fig. 2. Examples of annual aboveground