Parafluvial and Orthofluvial Ponds

In alluvial stream floodplains, permanent or temporary ponds develop from riverine dynamics either within the active channel (parafluvial pond) or in the riparian zone (orthofluvial pond). They are fed by both surface water and groundwater. In coarse-grained sediments, these ponds are connected to the main channel by the hyporheic interstitial zone, that is, an ecotone between groundwater and surface water that extends below and at either side of the stream channel. In fine-grained

Figure 3 Photographs of riparian wetlands (Tenente Amaral Stream, Mato Grosso, Brazil): (a) Stream channel with hygropetric zone (foreground) and floodplain forest (background), (b) Rockpool carved into the sandstone bedrock, (c) moist organic soil colonized by many aquatic invertebrate taxa. Leaf litter was removed. All photographs by K. M. Wantzen.

Figure 3 Photographs of riparian wetlands (Tenente Amaral Stream, Mato Grosso, Brazil): (a) Stream channel with hygropetric zone (foreground) and floodplain forest (background), (b) Rockpool carved into the sandstone bedrock, (c) moist organic soil colonized by many aquatic invertebrate taxa. Leaf litter was removed. All photographs by K. M. Wantzen.

sediments (including organic soils), the contribution of groundwater is much more important, and these ponds are often brownish from dissolved organic matter (humic acids and yellow substances). Para- and orthofluvial ponds contribute disproportionately to total species richness along riparian corridors.

Riparian Flood Zones

Even if no basin-like structures are present, flooding events create wetted zones on either side of the stream, independent of sediment type. Extension and permanence of the wetted zone depends on the valley shape, the porosity of the sediments, and eventual backflooding from tributary streams. In temporarily flooded forests with thick organic layers and in stranded debris dams, the moisture conditions may be long enough to bridge the gap between two flood events, so that many aquatic biota such as chironomids and other midges can complete their larval development in these semiaquatic habitats.

Riparian Valley Swamps

Swamps occur on soils that are waterlogged for most of the year. The lack of oxygen in the sediments allows accumulation of organic matter and selects for tree or herb species that have specific adaptations to these conditions, for example, pressure ventilation in the roots. The vegetation consists of either macrophytes or trees. Due to the shading and oxygen consumption during decomposition of organic matter, some of these riparian wetlands are hostile environments for aquatic metazoa that depend on dissolved oxygen. Some trees such as the Australian gum (Melaleuca sp.) shed bark which release secondary compounds that influence biota.

Hillside Wetlands

In areas where the aquiclude extends laterally from the stream, the riparian swamps can merge into hillside wetlands far above the flood level. Given that water-loggedness is permanently provided, these ecosystems tend to develop black organic soil layers from undecom-posed plant material. The anoxic conditions in these soils favor denitrification and nitrogen may become a limiting factor for plant growth. Carnivorous plants (Droseraceae, Lentibulariacea, Sarraceniaceae) that replenish their nitrogen budget with animal protein are commonly found in these habitats. At sites where drainage is better, woody plants invade these natural meadows. The soft texture of the soils and their position in hill slope gradients makes these ecosystems highly vulnerable to gully erosion.

Logjam Ponds and Beaver Ponds

Falling riparian trees are stochastic events which may have dramatic consequences for the hydraulics ofa stream system. Many tree species are soft-wooded, and tree dynamics are generally high in riparian wetlands. A fallen log blocks the current and creates a dam that accumulates fine particles. These natural reservoirs often extend far into the riparian zone.

Dams built by beaver (Castor sp.) can significantly alter the hydrological and biogeochemical characteristics of entire headwater drainage networks in Northern America and Eurasia. Fur trade led to the regional extinction of beavers. Few decades after reintroduction of beavers on a peninsula in Minnesota, they converted a large part of the area into wetlands, which led to a manifold increase in the soil nutrient concentrations. The activity ofbeavers considerably enhances the biodiversity of wetland-depending species. Beavers increase regional habitat heterogeneity because they regularly abandon impounded areas when the food supply is exhausted and colonize new ones, thereby creating a shifting mosaic of patches in variable stages ofplant succession.

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