Aquatic plants are important components of many freshwater ecosystems. In this review we examine natural and anthropogenic influences on the distribution and abundance of aquatic plants, and develop a conceptual model of those diverse interactions. Species of aquatic plants vary greatly in their anatomy, physiology, life-history traits, and ability to tolerate inorganic and biological stressors. Key examples of inorganic stressors are extreme regimes of flow velocity, irradiance, salinity, ice cover, temperature, nutrients, and pollutants. Stressors associated with competition, herbivory, and disease may also limit the ability of species to utilize otherwise suitable habitats. Some aquatic plants have a cosmopolitan distribution and display high levels of polymorphism and phenotypic plasticity in response to variations of environmental factors; these qualities allow them to occur over a wide range of conditions. Other species, however, have narrower tolerances and are potentially useful indicators of environmental conditions, in terms of either their presence or relative abundance within communities. In this review, we examine key environmental influences affecting aquatic plants, and their potential use as indicators at local, watershed, and regional scales.Key words: aquatic plants (aquatic macrophytes), environmental factors, environmental indicators, environmental stressors.

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Aquatic macrophytes are aquatic photosynthetic organisms, large enough to see with the naked eye, that actively grow permanently or periodically submerged below, floating on, or growing up through the water surface. Aquatic macrophytes are represented in seven plant divisions: Cyanobacteria, Chlorophyta, Rhodophyta, Xanthophyta, Bryophyta, Pteridophyta and Spermatophyta. Species composition and distribution of aquatic macrophytes in the more primitive divisions are less well known than for the vascular macrophytes (Pteridophyta and Spermatophyta), which are represented by 33 orders and 88 families with about 2,614 species in c. 412 genera. These c. 2,614 aquatic species of Pteridophyta and Spermatophyta evolved from land plants and represent only a small fraction (*1%) of the total number of vascular plants. Our analysis of the numbers and distribution of vascular macrophytes showed that whilst many species have broad ranges, species diversity is highest in the Neotropics, intermediate in the Oriental, Nearctic and Afrotropics, lower in the Palearctic and Australasia, lower again in the Pacific Oceanic Islands, and lowest in the Antarctic region. About 39% of the c. 412 genera containing aquatic vascular macrophytes are endemic to a single biogeographic region, with 61-64% of all aquatic vascular plant species found in the Afrotropics and Neotropics being endemic to those regions. Aquatic macrophytes play an important role in the structure and function of aquatic ecosystems and certain macrophyte species (e.g., rice) are cultivated for human consumption, yet several of the worst invasive weeds in the world are aquatic plants. Many of the threats to fresh waters (e.g., climate change, eutrophication) will result in reduced macrophyte diversity and will, in turn, threaten the faunal diversity of aquatic ecosystems and favour the establishment of exotic species, at the expense of native species.

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Physical and chemical factors were studied in 34 lentic waterbodies distributed along a steep altitudinal gradient ranging from tropical (77 m) to high alpine (up to 4,980 m) environments in Nepal. Bicarbonate and calcium were dominant among anions and cations, respectively, reflecting a strong influence of carbonate weathering and watershed area, rather than altitudinal climate. The relative patterns of dominant ions were similar among lakes in all altitudinal regions, although total concentrations increased with decreasing altitude. Total suspended solids were relatively high in the study lakes, as is also typical of rivers in the Ganges watershed. Suspended solids had a greater influence on water transparency than did algal biomass in the study lakes. In general, high-altitude waterbodies were oligotrophic, while those at low altitude were eutrophic. The productivity of high-altitude study lakes appeared to be limited by both available phosphorus and nitrogen, while lowland ones were nitrogen-limited.

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Increasing the amount of organic-carbon stored in the biomass of terrestrial ecosystems is an effective way to reduce the net anthropogenic emissions of greenhouse gases to the atmosphere. This can be done by conserving existing ecological reservoirs of fixed organic-carbon, maintaining or enhancing the rate of sequestration, and restoring stocks that have been depleted by past land-use practices. Most trading systems for greenhouse-gas offsets recognize the validity of projects that gain ecological offsets, and permit them to sell carbon credits in an emerging marketplace for these novel commodities. Although ecological carbon-offset projects have been criticized from a variety of perspectives, most of the supposed problems can be satisfactorily mitigated. In addition to offsetting emissions of greenhouse gases, ecological projects that accumulate carbon credits may have a strong cross-linkage to the conservation of natural values, which in itself is an important action for society to undertake. This is, however, less of a consideration for projects that are based on anthropogenic ecosystems, such as no-till agricultural systems and plantation forests, which provide relatively few benefits to native biodiversity and might even detract from that objective if developed on newly converted natural habitat. Moreover, the existing rules for carbon-offset systems exclude some kinds of ecological projects from the trading markets, even though they would result in avoided emissions or enhanced sequestration of organic-carbon. As the emerging marketplace for carbon offsets grows, it will be important to understand the co-benefits and side effects of offset projects on non-carbon values, including native biodiversity.

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