Productivity - Primary Production

	Emergent tidal freshwater wetland, Chickahominy River, Virginia. Photo credit: W. Priest, NOAA.

Plants photosynthesize and input energy into riverine ecosystems. These plants include algae, phytoplankton, and vascular plants (most land-based and wetland flowering plants and mosses) (Cummins 1974).  For lower-order streams, most organic matter in the streams originates as leaves, twigs, and other woody debris that is derived from riparian vegetation adjacent to the streams. This organic material that is produced externally from the stream and later falls into the stream is defined as allochthonous material. Conversely, primary production that originates within the stream is the source of autochthonous organic material (Minshall 1978). Larger rivers often have significant amounts of autochthonous material derived from phytoplankton assemblages, macro-algae production, and expansive wetland plant zones.

Primary production supports secondary stream production - vertebrate and invertebrate animals that inhabit or are dependent on streams and rivers during at least part of their life cycles. Vannote et al. (1980) developed the River Continuum Concept (RCC) to help explain longitudinal changes in river ecosystems focusing on how biological communities develop and change throughout a river's length. The RCC model hypothesizes that organisms in small streams with a dense riparian canopy depend more on external energy sources and are thus heterotrophic systems. In contrast, medium- (3rd to 5th order) and large- (6th and higher order) sized rivers have less canopy cover, increased potential for daily in-stream temperatures variation (especially medium-sized streams), and increased primary production, and thus function more as autotrophic systems. Processed organic material transported to downstream reaches helps to balance stream autotrophic and heterotrophic production.

Biodiversity

	Example of a bypass channel on a second-order stream with undisturbed, forested, riparian plant community. East Haddam, Connecticut. Photo credit: J. Turek, NOAA.

Streams and rivers support a variety of biota - plants and animals that live in-stream, inhabit riparian habitats, or are animals migrating to or through or visiting stream habitats. Both in-stream and riparian plant communities contribute to the biodiversity of riverine ecosystems. Plant species presence, diversity, abundance, and dominance depend on factors such as substrate type, in-stream flow velocities, frequency and duration of flooding (defining hydrologic regimes), and competition amongst plant species for light, water, and nutrients. Bayley (1995) identified the dynamic interaction of water and land within floodplains where the advance and retreat of floodwaters enhances biological productivity and sustains biodiversity.

The RCC hypothesizes that in-stream macroinvertebrate species diversity increases as new habitats and food sources occur within the system, with each species adapting to allochthonous and autochthonous food resources (Vannote et al. 1980).  In-stream biodiversity tends to increase in mid-order streams where thermal fluctuations increase causing a greater number of thermal niches for macroinvertebrate and fish species to fill. Mid-order river biodiversity is also generally higher since there is increased number of biotic colonization sources or links between larger drainages. In larger rivers, the influence of storm events become less significant, thermal fluctuations are less, fine sediment load and turbidity increase, and heterotrophy again becomes more important. Larger rivers are, thus, in general more physically stable, resulting in higher competition and predation in biota that eliminate less competitive species and limit biodiversity (Vannote et al. 1980). 

Plant communities are important biological features of stream corridors with structure and species composition helping define stream and river biodiversity.  Permanently flooded floodplain habitats are typically dominated by emergent (e.g., cattails, rice cutgrass, wild rice), floating emergent (e.g., water lilies), and/or submerged aquatic (e.g., water celery, water mosses) plants (Cowardin et al. 1978). Flood-tolerant woody species including tree, shrub, and vine (liana) layers often dominate riparian communities that are influenced by seasonal to semi-permanent flooding or saturated soils. Maples (Acer rubrum, A. saccharinum), gums (e.g., Nyssa, Liquidumbar spp.), ash (Fraxinus pennsylvanica, F. nigra), Atlantic white cedar (Chamaecyparis thyoides), willows (Salix spp.) and certain flood-tolerant oaks (e.g., Quercus bicolor, Q. phellos) are tree species typically dominating East Coast riparian habitats and wetland swamps. Bald cypress (Taxodium distichum) is a dominant species of floodplain swamps and river sloughs from Maryland south through Florida. Coastal pepperbush (Clethra alnifolia), viburnums (Viburnum recognitum, V. dentatum), alders (Alnus spp.), and highbush blueberry (Vaccinium corybosum) are common shrub species, while catbriers (Smilax spp.) and poison ivy (Toxicodendron radicans) are common liana species of East Coast riparian habitats.

In general, plant species diversity and the number of cover types in native, undisturbed riverine habitats increase with greater number of the hydrologic regimes present. Plant communities inhabiting areas more frequently affected by floods and human disturbances generally have lower species diversity or may be dominated by non-native, invasive species such as common reed (Phragmites australis). Other non-native species such as purple loosestrife (Lythrum salicaria) are establishing in many Northeastern and Mid-Atlantic riparian habitats by out-competing and displacing native emergent and other hydrophytic plants.

Animal diversity generally goes hand-in-hand with plant diversity and the number of in-stream and riparian habitat types. Intermittent streams often have a low diversity of in-stream animal species since water is only present on a seasonal basis. Low-order, perennial streams with steep gradients have bedrock, boulder, and gravel substrates, and high flow velocities. These conditions often limit the number of in-stream faunal species such as macroinvertebrates (e.g., stoneflies, mayflies) and fish that inhabit these habitats.

Higher-order rivers that have not been physically disturbed (presence of dense plant canopy cover and moderate to high in-stream dissolved oxygen levels) or contaminated with excess nutrients or other pollutants have a high diversity of zooplankton, macroinvertebrate, and fish assemblages. In mid-order river systems, a greater number of habitat types or zones (e.g., near even ratio of pools and riffles, seasonally-flooded waters, well-oxygenated cold and cool water zones) and increased structural complexity (e.g., high frequency of downed trees and other large woody debris) results in greater in-stream habitat diversity and species richness. Well-developed, mature, undisturbed riparian habitats often have a high diversity of riparian-dependent herptofauna (e.g., wood turtle, leopard frog, water snake), birds (e.g., bald eagle, osprey, yellow warbler), and mammals (e.g., river otter, muskrat, manatee, moose).  Fully functioning riverine ecosystems result in a diverse food web with primary, secondary, and tertiary (including humans) consumers interacting to provide a balance in riparian and in-stream animal populations including resident and non-resident fish species. 

In summary, streams and rivers, together with their riparian and wetland borders, are important ecosystems contributing to primary production and biodiversity.  By protecting, conserving, and minimizing direct physical alterations (e.g., dams, road crossings, and vegetation clearing) and secondary water quality impacts (e.g., excess nutrients, increased runoff temperatures and deceased dissolved oxygen levels due to watershed urbanization), these ecosystems provide substantial ecological functions or services (e.g., foraging habitat, production export - as discussed below), societal values (e.g., recreational and commercial fishing, wildlife viewing), and high ecological integrity translating to high biodiversity and linkages to other ecosystems - uplands and estuaries.