Riparian area is a transitional semiterrestrial/semiaquatic biome, the interface between land and river or stream extending from the edges of water bodies to the edges of upland regions. This area includes flood plains and landscapes closely surrounding streams, rivers, vernal pools, ephemeral streams, ponds and lakes.

Plant habitat along water bodies is riparian vegetation, consisted of hydrophilic plants. They contain highly diverse floral and faunal communities due to the diverse structure of these habitats, which change continually due to variable water flows above and below ground. The soil of these areas is more fertile, because of its lower position in the landscape, which enables it to hold moisture over longer periods of time. Riparian areas are important natural biofilters that protect aquatic environments from excessive sedimentation, surface runoff pollution and erosion.

Riparian areas are among the biosphere’s most complex ecological systems.

Main influences

Riparian areas are most important in water quality improvement for both surface and groundwater flow. In natural conditions, large animals influence nutrient and energy flows by consuming and redistributing energy and nutrients within and across ecosystems. Such connectivity is fundamental for maintaining healthy ecosystems.

Riparian areas and their vegetation support a wide range of important ecological functions. These include bank stabilisation, provision of living and dead organic matter, provision of habitat for aquatic and terrestrial biota, capture of sediment, retention and processing of nutrients and limiting extreme temperatures by shading. The presence of surface and groundwater enables the growth of herbs, shrubs and trees, which provide key habitat and resources for a rich fauna, from insects to mammals. Thus these areas are hot spots of biodiversity. Increased productivity, favorable microclimate and flooding regimes combined create a much higher biological diversity than the surrounding uplands.

Threats to such ecosystems

Many disturbances impact the riparian areas, such as clearance of riparian vegetation and floodplains, livestock grazing and the spread of invasive introduced plants. If livestock has access to the riparian areas and streambeds, vegetation can be lost through grazing, compromising bank stability, it destroys aquatic habitats by trampling and by adding excess nutrients by defecation. Long-term grazing can eliminate the woody vegetation that provides essential bird breeding habitat. The removal or loss of riparian vegetation results in loss of habitat, food sources, loss of shade and worse water quality due to decreased dissolved oxygen, which strongly affects the aquatic biota. All this leads to the loss of biological diversity and almost irreversible detrimental effects on the ecosystem and its functions. Cleared areas that are also affected by flooding can quickly erode water banks, taking with them valuable grass and soil. This enables for the areas and entire lands to become dry.

Restoration

Because of the high level of biodiversity and ecosystem importance of riparian areas, their restoration in case of human influence, is crucial. Riparian areas are natural environments with frequent disturbances (floods), because of this, the biota is adapted and resilient, but in case of land uses that cause degradation or that prevent recovery, it can be irreversibly damaged. This most often includes so called bank stabilisation, clearance of vegetaton and abusive grazing. For lasting riparian restoration, a large effort must be put in the recovery of the entire watershed.

The influence of vegetation on biotic and abiotic factors

An important function of riparian areas is reducing the energy of currents due to vegetation and root systems, reducing flood damage and soil erosion, trapping the sediment. Pollutants are filtered from surface runoff, enhancing water quality via biofiltration.

Riparian areas provide wildlife habitat and corridors, enabling aquatic and riparian organisms to move along water systems. Riparian vegetation also provides forage for wildlife and livestock. Nutrients from terrestrial vegetation (e.g. plant litter and insect drop) are transferred to aquatic food webs and are a vital source of energy in these webs. Vegetation helps to shade the water, reducing water temperature changes.

Effects of vegetation on temperature and light state:

• moderate temeratures enable growth and development of most aquatic organisms (bacteria, algae, invertebrates, fish, amphibians, reptiles);
• illumination depends on the proximity and size of the treetops and on the clarity of water;
• diatoms thrive better at lower light intensities and are also a better source of food;
• filamentous chlorophytes and macrophytes grow better at higher light intensities and are a poor source of food. Through respiration they reduce the oxygen level and the breakdown of accumulated organic material;
• higher temperature increases bacterial decomposition of organic material, thus increasing oxygen decomposition, leading to a further decrease in the amount of oxygen in the water;
• groups of plants that thrive in the shade (bryophytes, etc.) are an important part of the riparian system. They provide better water quality compared to other forms of organic carbon;
• without shading, certain plant species (especially invasive) would grow excessively.

Removal of pollutants

Sediments and nutrients are brought to water bodies from hillsides by surface and groundwater and slope erosion. There are 3 ways riparian areas prevent the introduction of sediments and nutrients in water bodies:

1. riparian area is more flat, which reduces the surface water runoff, limits sediment runoff and causes sediment deposition;
2. due to the reduced speed of surface water, infiltration into the soil and deposition of sediments increases;
3. vegetation and soil of the riparian area absorb and change the sediments in the shallow groundwater.

Sediments and nutrients are inseparable, as nutrients are bound with sediment particles, especially clay. The main nutrients that are in excess and cause damage are phosphorus and nitrogen. Otherwise necessary elements for living organisms, in excessive amounts they cause excessive growth of algae and macrophytes, thus removing oxygen. The main nutrient that enters the water bodies with groundwater is nitrate, since, unlike phosphorus and ammonia, it does not bind to sediment particles in the soil. Fast-growing trees, shrubs and grass are filters that consume nitrate from the groundwater for their growth, thus protecting the water bodies from excessive nitrogen inflows. Nitrate contamination in surface runoff originates from manure and fertilisers from agricultural fields. Mechanisms of phosphorus and nitrogen removal include biotic uptake, physical adsorption and microbial denitrification. Denitrification is the process of transforming nitrate into nitrogen gas (NO₃⁻ to N₂). Riparian area is the optimal place for denitrification, as it meets all the conditions for a successful course:

• it retains water, which is necessary for dentirifying bacteria;
• the supply of organic carbon is high due to litter and root secretions;
• locally stagnant water enables anaerobic conditions in which denitrification can take place.

The fact that always holds true is when the velocity of ground- and surface water is lower, plants can take up nutrients easier and bacteria have more time for denitrification. Removal of nitrates also occurs on at the border between the water body and groundwater. Denitrifying bacteria and aquatic plants generally have the same role of reducing nitrogen.

The influence of the riparian area on water bodies’ bank erosion

Water bank erosion is a constant and natural process, but dangerously accelerated by humans. Channels with strong and dense growth on the bottom and along the banks erode more slowly than channels without it or with only grass vegetation. Bank erosion depends on several factors; riparian vegetation, bank geometry, hydrology and stratigraphy.

Vegetation retains water in the case of higher water levels, prevents the soil from freezing and strengthens the bank. Willows contribute a great deal to stopping erosion as they grow quickly. Willow banks are 80 % more resistant to material drift than grass banks and 30 % more than banks with other higher growth.

Structure and diversity of vegetation

The vegetation can be very different from and of higher variety than the flora and fauna in the surrounding resource-poor hinterland. There are 2 basic vegetation gradients that differ spatially:

1. longitudinal – from the source to the mouth of the river. There are climatic, altitudinal, hydrological and geomorphological differences between the source and mouth of the river;
2. cross-section of the transition area between aquatic and terrestrial environments: aquatic à semi-aquatic species à terrestrial species in higher areas. This gradient is dependent on local factors; topographic diversity, flooding, channel dynamics, etc.

Riparian areas change over time. Past and present floods, erosion and deposition of sediments have a large impact on vegetation, especially on the flood plains of large lowland rivers.

Riparian vegetation over the world includes different types of plants. In Central Europe the most characteristic are alder, ash, willows and poplars, in America it consists of shrubs, ferns and trees such as yew, wild cherry and redwood, in Asia it is sedges and in Australia tea tree.

Water and microclimate

Moisture is an important part of the microclimate of riparian areas and occurs in several forms:

• water in the stream;
• groundwater, including subsurface flow along a drained bed;
• soil moisture.

Surface water evaporation and plant transpiration create a microhabitat with more humid conditions and less extreme temperatures. The part of the landscape with a body of water is more resistant to drought and provides refuge for animals in adverse conditions (drought, heat, cold).

Food and primary production

Riparian areas are among the most productive ecosystems. This is a result of water availability and soil richer in nutrients than soil at higher elevations. The soil receives nutrients after floods and from higher lying areas after rain. High primary production provides a supply of plant materials, e.g. leaf litter and a variety of flowering and fruiting plants. It is a favorable habitat for many terrestrial invertebrates, which are then food for insectivores.

Animal habitat and corridor

Riparian areas occupy a small part of the landscape, but often have greater biodiversity than neighboring habitats. They are of high importance for birds, reptiles, amphibians and insects. They provide water, food, shelter, nesting and hiding places.

Larval and adult stages of many aquatic insects stay close to the riverbed in the riparian area. All species are very sensitive to changes in this ecosystem.

Leaf litter, decaying wood and river debris provide feeding and hiding places for invertebrates, amphibians, reptiles and small mammals. Loose soils are favourable for dens and nesting ground fauna, from insects to mammals. Tall riparian trees are nesting sites for birds, bats and arboreal animals.

Many terrestrial animals spend at least a part of their life in the riparian area. Some use it daily (drinking, feeding, sleeping), others seasonally (food, breeding) or stay during a certain life period. For these animals, the riparian area provides shelter during drought, or they move there often for food.

Aquatic food webs

• organic material from aquatic and terrestrial sources supplies food webs with energy in the form of carbon;
• only a small proportion of the carbon is actually useful for aquatic animals, as part of it is mineralized by bacteria and another part is bound in molecules that cannot be taken up by animals;
• carbon is available in the form of decaying trunks, litter (leaves, fruits), dead animals and other wood material, which is washed into the water bodies;
• available carbon is also produced by microalgae and some aquatic plants;
• driftwood trunks and branches are the habitat for algae colonisation;
• fruits and arthropods are a nutritious food source for vertebrates;
• most small water bodies are heterotrophic, as they depend on external carbon sources. Heterotphonic waters have respiration greater than the total primary production.

Further research

Riparia are plant-animal communities, ecosystems of detritus, soils, fungi and animals. There is considerable information about riparian plants, but little is known about the belowground components – roots and their associated organisms. Thus there remain many research opportunities on riparian topics and biotic groups.

Conclusion

Riparian areas influence the water balance of landscapes and are crucial ecosystems for landscape ecology and biogeography. They influence species exchange, biodiversity, trophic chains, biochemical processes and reduce hydrological extremes. Water bodies are interdependent with surrounding terrestrial areas, thus any fragmentation or ecological degradation of riparian areas and wetlands directly impacts the terrestrial landscapes and vice versa. The longitudinal ecological connectivity and dependency over large geographic ranges combined with transversal connectivity of adjacent uplands results in highly complex ecological interactions. This shows that riparian areas are very vulnerable to human activities and land-use change. These areas are one of the most biodiverse ecosystems in most climate areas and and are of vital ecosystem importance in terms of water presence, the regulation of biogeochemical cycles and the reduction of climatic changes. It is highly important they receive special attention in conservation efforts.

Sources:

• https://en.wikipedia.org/wiki/Riparian_area
• https://www.sciencedirect.com/topics/earth-and-planetary-sciences/riparian-area