Riparian wetlands are intrinsically linked to both the stream and the surrounding terrestrial ecosystems of the catchment. In many places of the world, however, riparian zones have remained the only remnants of both wetland and woody habitats available for wildlife. They are surrounded by intensively used areas for either agriculture or urban colonization. The performance of riparian wetlands to provide ecological services becomes reduced by the same degree as these bordering ecosystems become degraded. However, even in degraded landscapes, the beneficial effects ofthe riparian wetland ecosystems are astonishingly high. For humans, healthy riparian wetlands are vital as filters and nutrient attenuators to protect water quality for drinking, fisheries, and recreation.
Riparian wetlands are natural traps for fine sediments and for organic matter, but they may vary from a nutrient sink to a nutrient source at different times of a year depending on high or low water levels. Particle-bound nutrients, such as orthophosphate ions, become deposited in the riparian wetlands during spates and may accumulate there. This may increase the amount of phosphate that becomes released during the following flood event. Therefore, technical plans for phosphorus retention in artificial wetlands in agricultural landscapes include a hydraulic design which hampers the release of particles from the wetland, for example, by providing continuous, and sufficiently broad wetland buffer strips along the streams. (see Riparian Zone Management and Restoration).
For the removal of nitrogen inputs from floodwater and from lateral groundwater inputs, riparian wetlands are very efficient. Generally it can be taken for granted that the slower the water flow (both ground and surface water) the higher is the nitrate uptake rate; however, the precise flow pathways in the sediments have to be considered. In anoxic soils, reduction and denitrification processes transform inorganic nitrogen forms into nitrogen gas which is then released into the atmosphere. Once the nitrate has been completely reduced, sulphate is also reduced in the anoxic sediments. Nitrogen also becomes immobilized by bacterial growth and/or condensation of cleaved phenolics during the aerobic decay of organic matter. Aquatic macro-phytes and trees growing in the riparian wetlands are very efficient in nitrogen stripping by incorporating mineral nitrogen forms into their biomass. They can represent the most important nitrogen sinks in riparian systems. Some riparian wetland plants (e.g., alder, Alnus sp., and several leguminous trees) have symbiotic bacteria associated to their roots that can fix atmospheric nitrogen when this nutrient is scarce in the soils. Thus, not all riparian wetlands exclusively remove nitrogen.
Like other wetlands, riparian wetlands are important players in the carbon cycle of the watershed. They accumulate large amounts ofcoarse particulate organic matter (CPOM) and they release dissolved organic matter into the stream and gaseous carbon compounds into the atmosphere (Figure 1).
In the boreal zone, the spring snowmelt runoff contributes to more than half of the annual total organic carbon (TOC) export. The larger the riparian wetland zone, the bigger the amount of exported TOC. On the other hand, riparian wetlands receive large amounts of dissolved carbon from litter leachates from the surrounding forests, especially during the leaf-fall period. These leachates can be an important source for phosphorous and other nutrients, as well as for labile carbon compounds. These substances enhance heterotrophic microbial (bacterial and fungal) activity.
Spring snowmelt also carries large amounts of fine particulate organic matter (POM). Riparian wetlands often provide surface structures that act like a comb to accumulate these particles (e.g., macrophytes), and enhance the production of detritivores. Additional POM is produced by riparian trees. The general trend for litter production to increase with decreasing latitude (valid in forests) is overlain by species-specific productivity and physiological constraints due to the waterloggedness in riparian wetlands. Here, the litter production is generally higher in periodically flooded, than in permanently flooded, wetlands. Depending on the oxygen content of the soils, the chemical composition of the leaves, and the activity of detritivores, more or less dense layers of 'leaf peat' can accumulate in the sediments. This organic matter stock can be increased by undecomposed tree logs and bark. A reduction of the water level in the riparian wetlands leads to an increased mineralization of the carbon stocks and enhances the release of carbon dioxide.
Riparian wetlands have an equilibrating effect on hydro-logical budgets. Riparian vegetation dissipates the kinetic energy of surface flows during spates. Riparian wetlands store stormwater and release it gradually to the stream channel or to the aquifer between rainstorm events. Moreover, they are important recharge areas for aquifers. Several current restoration programmes try to increase this recharge function of riparian wetlands in order to stabilize the groundwater stocks for drinking water purposes.
Riparian wetland trees and macrophytes contribute considerably to evapotranspiration and to local and regional climate conditions. The rate ofvapor release depends on the plant functional group which needs to be considered for basin-scale water budgets.
Riverine wetlands represent a web of ecological corridors and stepstones. In intense agricultural areas they can be considered as 'green veins' that maintain contact and gene flow between isolated forested patches. Providing shadow, balanced air temperatures and moisture, shelter, resting places, food and water supply, they cover the requirements of a great deal of amphibian, reptile, bird, and mammal species. These not only use the longitudinal connection but also migrate laterally and thus reach the next corridor aside. Moreover, longrange migrating birds use the green corridors ofriparian zones in general as landmarks for migration. Networks of riparian corridors also facilitate the movement of non-native species. In some US riparian zones, their richness was about one-third greater in riparian zones than on uplands and the mean number and the cover of non-native plant species were more than 50% greater than in uplands.
During flood, drought, and freezing events, but also during pollution accidents in the stream channel, connected riparian wetland habitats represent refugia for riverine animals. In extreme cases, residual populations from the wetlands may contribute to the recolonization of defau-nated stream reaches. Riparian wetlands also act as traps and storage sites for seeds both from the upstream and from the uphill areas. The seed banks contain propagules from plants that represent a large range of moisture tolerances, life spans, and growth forms. These seeds may also become mobilized and transported during spate events.
Riparian wetlands offer a large variety of food sources. Connected wetland water bodies 'comb out' fine organic particles including drifting algae from the stream water, they receive aerial and lateral inputs of the vegetation, and they have a proper primary productivity which profits by the increased nutrient input and storage from the surroundings. Many riverine fish and invertebrate species are known to migrate actively into the riparian wetlands in order to profit by the terrestrial resources that are available during flood periods. In analogy to the 'floodpulse advantage' of fish in large river floodplains, stream biota that temporarily colonize riparian wetlands have better growth conditions than those that remain permanently in the stream channel. For example, the macroinvertebrate community of riparian sedge-meadows in Maine (USA) is dominated by detritivorous mayfly larvae (over 80% of the invertebrate biomass) during a 2-month period in spring. The larvae use the stream channel as a refuge and use the riparian wetland as feeding ground where they perform over 80% of their growth.
Reciprocal Subsidies between Aquatic and Terrestrial Ecosystems
Many aquatic species profit by the terrestrial production and vice versa. Apart from leaf litter, large quantities of fruits, flowers, seeds, as well as insects and feces fall from the tree canopies into the streams where they represent important energy and nutrient sources for the biota. In Amazonian low-order rainforest streams, terrestrial invertebrates make up a major portion of the gut content of most fish species. Fruits and seeds are preferred food items for larger fish species that colonize medium- and high-order rivers. Riparian wetlands increase the area of this active exchange zone, and they retain these energy-rich resources for a longer period than a stream bank alone would do.
Aquatic organisms also contribute to the terrestrial food webs. For example, bats are known to forage on the secondary production of emerging insects in riparian wetlands, and the shoreline harbors a large number of terrestrial predators, such as spiders, tiger beetles, and riparian lizards. Experimental interruption of these linkages (e.g., by covering whole streams with greenhouses) has shown that the alteration of riparian habitats may reduce the energy transfer between the channel and the riparian zone.
The sound of the nearby stream, the equilibrated climate, and the occurrence of attractive animal and plant species render riparian wetlands highly attractive for recreation purposes such as hiking, bird-watching, or meditation. These can be combined with 'in-channel' recreation activities such as canoeing, rafting, or fishing, and represent an economically valuable ecosystem service, that should be considered in management and conservation plans (see Riparian Zone Management and Restoration).
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