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Lotic ecosystems

    Lotic system ecology is the study of the biotic and abiotic interactions within flowing continental waters (Angelier 2003) Together with lentic system ecology which involves less dynamic continental waters such as lakes and ponds these fields form the more general study area of freshwater or aquatic ecology
    Lotic waters can be diverse in their form ranging from a spring that is only a few centimeters wide to a major river that is kilometers in width (Allan 1995) Despite these differences Gillen and Malmqvist (1998) noted some unifying characteristics that make the ecology of running waters unique from that of other aquatic habitats: 1. Flow is unidirectional; 2. These systems are in a state of continuous physical change; 3. There is a high degree of spatial and temporal heterogeneity at all scales (microhabitats); 4. Variability between lotic systems is quite high; and 5. The biota is specialized to live with flow conditions The purpose of this page is to acquaint the reader with the basic concepts behind lotic system ecology

    Important Abiotic Factors

    Flow

    Flow is the key abiotic operator in lotic systems influencing their ecology The strength of water flow can vary between these systems involving a continuum from torrential rapids to slow backwaters that almost seem like lentic systems The speed of the water flow can be variable within and across systems It is typically based upon variability of friction with the bottom or sides of the channel sinuosity obstructions and the incline gradient (Allan 1995) In addition the amount of water input into the system from direct precipitation snowmelt and/or groundwater can affect flow rate Flowing waters can alter the shape of the streambed through processes of erosion and deposition creating a variety of habitats including riffles glides and pools (Cushing and Allan 2001)

    Light

    Light is important to lotic systems because it provides the energy necessary to drive primary production via photosynthesis and can also provide refuge for prey species in shadows it casts The amount of light that a system receives can be related to a combination of internal and external stream variables The area surrounding a small stream for example might be shaded by surrounding forests or by valley walls Larger river systems tend to be wide so the influence of external variables is minimized and the sun reaches the surface These rivers also tend to be more turbulent however and particles in the water increasingly attenuate light as depth increases (Cushing and Allan 2001) Seasonal and diurnal factors might also play a role in light availability because the angle of incidence the angle at which light strikes water can lead to light lost from reflection Known as Beer’s Law the shallower the angle the more light is reflected and the amount of solar radiation received declines logarithmically with depth (Giller and Malmqvist 1998) Additional influences on light availability include cloud cover altitude and geographic position (Brown 1987)

    Temperature

    Most lotic species are poikilotherms whose internal temperature varies with their environment thus temperature is a key abiotic factor for them Water can be heated or cooled through radiation at the surface and conduction to or from the air and surrounding substrate Shallow streams are typically well mixed and maintain a relatively isothermal temperature within an area In deeper slower moving water systems however a strong difference between the bottom and surface temperatures may develop Spring fed systems have little variation as springs are typically from groundwater sources which are often very close to ambient temperature (Giller and Malmqvist 1998) Many systems show strong diurnal fluctuations and seasonal variations are most extreme in arctic desert and temperate systems (Giller and Malmqvist 1998) The amount of shading climate and elevation can also influence the temperature of lotic systems (Allan 1995)

    Chemistry

    Water chemistry between systems varies tremendously The chemistry is foremost determined by inputs from the geology of its watershed or catchment area but can also be influenced by precipitation and the addition of pollutants from human sources (Allan 1995; Cushing and Allan 2001) Large differences in chemistry do not usually exist within small lotic systems due to a high rate of mixing In larger river systems however the concentrations of most nutrients dissolved salts and pH decrease as distance increases from the river’s source (Giller and Malmqvist 1998)
    Oxygen is likely the most important chemical constituent of lotic systems as all aerobic organisms require it for survival It enters the water mostly via diffusion at the water-air interface Oxygen’s solubility in water decreases as water temperature increases Fast turbulent streams expose more of the water’s surface area to the air and tend to have low temperatures and thus more oxygen than slow backwaters (Giller and Malmqvist 1998) Oxygen is a byproduct of photosynthesis so systems with a high abundance of aquatic algae and plants may also have high concentrations of oxygen during the day These levels can decrease significantly during the night when primary producers switch to respiration Oxygen can be limiting if circulation between the surface and deeper layers is poor if the activity of lotic animals is very high or if there is a large amount of organic decay occurring (Cushing and Allan 2001)

    Substrate

    The inorganic substrate of lotic systems is composed of the geologic material present in the catchment that is eroded transported sorted and deposited by the current Inorganic substrates are classified by size on the Wentworth Scale which ranges from boulders to pebbles to gravel to sand and to silt (Giller and Malmqvist 1998) Typically particle size decreases downstream with larger boulders and stones in more mountainous areas and sandy bottoms in lowland rivers This is because the higher gradients of mountain streams facilitate a faster flow moving smaller substrate materials further downstream for deposition (Cushing and Allan 2001) Substrate can also be organic and may include fine particles autumn shed leaves submerged wood moss and more evolved plants (Allan 1995) Substrate deposition is not necessarily a permanent event as it can be subject to large modifications during flooding events (Cushing and Allan 2001)

    Lotic System Biota

    Bacteria

    Bacteria are present in large numbers in lotic waters Free-living forms are associated with decomposing organic material biofilm on the surfaces of rocks and vegetation in between particles that compose the substrate and suspended in the water column Other forms are also associated with the guts of lotic organisms as parasites or in commensal relationships (Giller and Malmqvist 1998) Bacteria play a large role in energy recycling (Allan 1995) which will be discussed in the Trophic Relationships section

    Primary Producers

    Algae comprised of phytoplankton and periphyton are the most significant sources of primary production in most streams and rivers (Giller and Malmqvist 1998) Phytoplankton float freely in the water column and thus are unable to maintain populations in fast flowing streams They can however develop sizable populations in slow moving rivers and backwaters (Allan 1995) Periphyton are typically filamentous and tufted algae that can attach themselves to objects to avoid being washed away by fast current In places where flow rates are negligible or absent periphyton may form a gelatinous unanchored floating mat (Cushing and Allan 2001)
    Plants exhibit limited adaptations to fast flow and are most successful in reduced currents More primitive plants such as mosses and liverworts attach themselves to solid objects This typically occurs in colder headwaters where the mostly rocky substrate offers attachment sites Some plants are free floating at the water’s surface in dense mats like duckweed or water hyacinth Others are rooted and may be classified as submerged or emergent Rooted plants usually occur in areas of slackened current where fine-grained soils are found (Brown 1987; Cushing and Allan 2001) These rooted plants are flexible with elongated leaves that offer minimal resistance to current (Angelier 2003)
    Living in flowing water can be beneficial to plants and algae because the current is usually well aerated and it provides a continuous supply of nutrients (Cushing and Allen 2001) These organisms are limited by flow light water chemistry substrate and grazing pressure (Giller and Malmqvist 1998) Algae and plants are important to lotic systems as sources of energy for forming microhabitats that shelter other fauna from predators and the current and as a food resource (Brown 1987)

    Insects and Other Invertebrates

    Up to 90% of invertebrates in some lotic systems are insects These species exhibit tremendous diversity and can be found occupying almost every available habitat including the surfaces of stones deep below the substratum adrift in the current and in the surface film Insects have developed several strategies for living in the diverse flows of lotic systems Some avoid high current areas inhabiting the substratum or the sheltered side of rocks In stronger current species have developed weighted cases attachments to anchored pads of silk recurved clinging claws suction cup cup devices and flattened streamlined bodies (Brown 1987; Cushing and Allan 2001) Additional invertebrate taxa common to flowing waters include mollusks such as snails limpets clams mussels as well as crustaceans like crayfish and crabs (Cushing and Allan 2001) Like most of the primary producers lotic invertebrates often rely heavily on the current to bring them food and oxygen (Brown 1987) Invertebrates especially insects are important as both consumers and prey items in lotic systems

    Fishes and Other Vertebrates

    Fishes are probably the best-known inhabitants of lotic systems The ability of a fish species to live in flowing waters depends upon the speed at which it can swim and the duration that its speed can be maintained This ability can vary greatly between species and is tied to the habitat in which it can survive Continuous swimming expends a tremendous amount of energy and therefore fishes spend only short periods in full current Instead individuals remain close to the bottom or the banks behind obstacles and sheltered from the current swimming in the current only to feed or change locations (Angelier 2003) Some species have adapted to living only on the system bottom never venturing into the open water flow These fishes are dorso-ventrally flattened to reduce flow resistance and often have eyes on top of their heads to observe what is happening above them Some also have sensory barrels positioned under the head to assist in the testing of substratum (Brown 1987)
    Lotic systems typically connect to each other forming a path to the ocean (spring → stream → river → ocean) and many fishes have life cycles that require stages in both fresh and salt water Salmon for example are anadromous species that are born and develop in freshwater and then move to the ocean as adults Eels are catadromous and are born and develop in the ocean and then move into freshwater as adults (Giller and Malmqvist 1998)
    Other vertebrate taxa that inhabit lotic systems include amphibians such as salamanders reptiles (eg snakes turtles crocodiles and alligators) various bird species and mammals (eg otters beavers hippos and river dolphins) With the exception of a few species these vertebrates are not tied to water as fishes are and spend part of their time in terrestrial habitats (Giller and Malmqvist 1998) Many fish species are important as consumers and as prey species to the larger vertebrates mentioned above

    Trophic Relationships

    Energy Inputs: autochthonous and allochthonous sources

    Autochthonous energy sources are those derived from within the lotic system During photosynthesis for example primary producers form organic carbon compounds out of carbon dioxide and inorganic matter The energy they produce is important for the community because it may be transferred to higher trophic levels via consumption Additionally high rates of primary production can introduce dissolved organic matter (DOM) to the waters (Cushing and Allan 2001) Another form of autochthonous energy comes from the decomposition of dead organisms and feces that originate within the lotic system In this case bacteria decompose the detritus or coarse particulate organic material (CPOM; >1mm pieces) into fine organic particulate matter (FPOM; <1mm pieces) and then further into inorganic compounds that are required for photosynthesis (Allan 1995; Cushing and Allan 2001) This process is discussed in more detail below
    If energy sources are from the terrestrial environment they are termed allochthonous Leaves twigs fruits etc are typical forms of terrestrial CPOM that have entered the water by direct litterfall or lateral leaf blow (Giller and Malmqvist 1998) In addition terrestrial animal-derived materials such as feces or carcasses that have been added to the system are examples of allochthonous CPOM The CPOM undergoes a specific process of degradation Allan (1995) gives the example of a leaf fallen into a stream First the soluble chemicals are dissolved and leached from the leaf upon its saturation with water This adds to the DOM load in the system Next microbes such as bacteria and fungi colonize the leaf softening it as the mycelium of the fungus grows into it. The composition of the microbial community is influenced by the species of tree from which the leaves are shed (Rubbo and Kiesecker 2004) This combination of bacteria fungi and leaf are a food source for shredding invertebrates which leave only FPOM after consumption These fine particles may be colonized by microbes again or serve as a food source for animals that consume FPOM Organic matter can also enter the lotic system already in the FPOM stage by wind surface runoff bank erosion or groundwater Similarly DOM can be introduced through canopy drip from rain or from surface flows (Giller and Malmqvist 1998)

    Invertebrates

    Invertebrates can be organized into many feeding guilds in lotic systems Some species are shredders which feed on non-woody CPOM and their associated microorganisms Others are suspension feeders which use their setae filtering aparati nets or even secretions to collect FPOM and microbes from the water These species may be passive collectors utilizing the natural flow of the system or they may generate their own current to draw water and also FPOM in (Allan 1995) Members of the gatherer-collector guild actively search for FPOM under rocks and in other places where the stream flow has slackened enough to allow deposition (Cushing and Allan 2001) Grazing invertebrates utilize scraping rasping and browsing adaptations to feed on periphyton and detritus Finally several families are predatory capturing and consuming animal prey Both the number of species and the abundance of individuals within each guild is largely dependent upon food availability Thus these values may vary across both seasons and systems (Allan 1995)

    Fishes

    Fishes can also be placed into feeding guilds Planktivores pick plankton out of the water column Herbivore-detritovores are bottom-feeding species that ingest both periphyton and detritus indiscriminately Surface and water column feeders capture surface prey (mainly terrestrial and emerging insects) and drift (benthic invertebrates floating downstream) Benthic invertebrate feeders prey primarily on immature insects but will also consume other benthic invertebrates Top predators consume fishes and/or large invertebrates Omnivores ingest a wide range of prey These can be floral faunal and/or detrital in nature Finally parasites live off of host species typically other fishes (Allan 1995) Fishes are flexible in their feeding roles capturing different prey with regard to seasonal availability and their own developmental stage Thus they may occupy multiple feeding guilds in their lifetime The number of species in each guild can vary greatly between systems with temperate warm water streams having the most numbers of benthic invertebrate feeders and tropical systems having large numbers of detritus feeders due to high rates of allochthonous input (Cushing and Allan 2001)

    Community Patterns and Diversity

    Local Species Richness

    Large rivers have comparatively more species than small streams Many relate this pattern to the greater area and volume of larger systems as well as an increase in habitat diversity Some systems however show a poor fit between system size and species richness In these cases a combination of factors such as historical rates of speciation and extinction type of substrate microhabitat availability water chemistry temperature and disturbance such as flooding seem to be important (Giller and Malmqvist 1998)

    Resource Partitioning

    Although many alternate theories have been postulated for the ability of guild-mates to coexist (see Morin 1999) resource partitioning has been well documented in lotic systems as a means of reducing competition The three main types of resource partitioning include habitat dietary and temporal segregation (Giller and Malmqvist 1998)
    Habitat segregation was found to be the most common type of resource partitioning in natural systems (Schoener 1974) In lotic systems microhabitats provide a level of physical complexity that can support a diverse array of organisms (Vincin and Hawknis 1998) The separation of species by substrate preferences has been well documented for invertebrates Ward (1992) was able to divide substrate dwellers into 6 broad assemblages including those that live in: coarse substrate gravel sand mud woody debris and those associated with plants showing one layer of segregation On a smaller scale further habitat partitioning can occur on or around a single substrate such as a piece of gravel Some invertebrates prefer the high flow areas on the exposed top of the gravel while others reside in the crevices between one piece of gravel and the next while still others live on the bottom of this gravel piece (Giller and Malmqvist 1998)
    Dietary segregation is the second-most common type of resource partitioning (Giller and Malmqvist 1998) High degrees of morphological specializations or behavioral differences allow organisms to use specific resources The size of nets built by some species of invertebrate suspension feeders for example can filter varying particle size of FPOM from the water (Edington et al 1984) Similarly members in the grazing guild can specialize in the harvesting of algae or detritus depending upon the morphology of their scraping apparatus In addition certain species seem to show a preference for specific algal species (Giller and Malmqvist 1998)
    Temporal segregation is a less common form of resource partitioning but it is nonetheless an observed phenomenon (Giller and Malmqvist 1998) Typically it accounts for coexistence by relating it to differences in life history patterns and the timing of maximum growth among guild mates Tropical fishes in Borneo for example have shifted to shorter life spans in response to the ecological niche reduction felt with increasing levels of species richness in their ecosystem (Watson and Balon 1984)

    Persistence and Succession of Communities

    Over long time scales there is a tendency for species composition in pristine systems to remain in a stable state (Hildrew and Giller 1994) This has been found for both invertebrate and fish species (Giller and Malmqvist 1998) On shorter times scales however flow variability and unusual precipitation patterns decrease habitat stability and can all lead to declines in persistence levels The ability to maintain this persistence over long time scales is related to the ability of lotic systems to return to the original community configuration relatively quickly after a disturbance (Townsend et al 1987) This is one example of temporal succession a site-specific change in a community involving changes in species composition over time Another form of temporal succession might occur when a new habitat is opened up for colonization In these cases an entirely new community that is well adapted to the conditions found in this new area can establish itself (Giller and Malmqvist 1998)

    The River Continuum Concept

    The River Continuum Concept (RCC) was an attempt to construct a single framework to describe the function of temperate lotic ecosystems from the source to the end and relate it to changes in the biotic community (Vannote et al. 1980; Allan 1995) The physical basis for RCC is size and location along the gradient from a small stream eventually linked to a large river Stream order (see stream : ranking) is used as the physical measure of the position along the RCC
    According to the RCC low ordered sites are small shaded streams where allochthonous inputs of CPOM are a necessary resource for consumers As the river widens at mid-ordered sites energy inputs should change Ample sunlight should reach the bottom in these systems to support significant periphyton production Additionally the biological processing of CPOM inputs at upstream sites is expected to result in the transport of large amounts of FPOM to these downstream ecosystems Plants should become more abundant at edges of the river with increasing river size especially in lowland rivers where finer sediments have been deposited and facilitate rooting The main channels likely have too much current and turbidity and a lack of substrate to support plants or periphyton Phytoplankton should produce the only autochthonous inputs here but photosynthetic rates will be limited due to turbidity and mixing Thus allochthonous inputs are expected to be the primary energy source for large rivers This FPOM will come from both upstream sites via the decomposition process and through lateral inputs from floodplains
    Biota should change with this change in energy from the headwaters to the mouth of these systems Namely shredders should prosper in low-ordered systems and grazers in mid-ordered sites Microbial decomposition should play the largest role in energy production for low-ordered sites and large rivers while photosynthesis in addition to degraded allochthonous inputs from upstream will be essential in mid-ordered systems As mid-ordered sites will theoretically receive the largest variety of energy inputs they might be expected to host the most biological diversity (Vannote et al. 1980; Allan 1995; Giller and Malmqvist 1998)
    Just how well the RCC actually reflects patterns in natural systems is uncertain and its generality can be a handicap when applied to diverse and specific situations (Allan 1995) The most noted criticisms of the RCC are: 1. It focuses mostly on macroinvertebrates disregarding that plankton and fish diversity is highest in high orders; 2. It relies heavily on the fact that low ordered sites have high CPOM inputs even though many streams lack riparian habitats; 3. It is based on pristine systems which rarely exist today; and 4. It is centered around the functioning of temperate streams Despite its shortcomings the RCC remains a useful idea for describing how the patterns of ecological functions in a lotic system can vary from the source to the mouth (Allan 1995; Giller and Malmqvist 1998)

    Human Impacts

    Pollution

    Today the direct pollution of lotic systems is virtually non-existent in the United Stated under the government’s Clean Act] Contaminants from diffuse non-point sources however remain a large problem (Cushing and Allan 2001) These pollutant sources are hard to control because they derive often in small amounts over a very wide area and enter the system at many locations along its length Agricultural fields often deliver large quantities of sediments nutrients and chemicals to nearby streams and rivers Urban and residential areas can also add to this pollution when contaminants are accumulated on impervious surfaces such as roads and parking lots that then drain into the system Elevated nutrient concentrations especially nitrogen and phosphorous which are key components of fertilizers can increase periphyton growth which can be particularly dangerous in slow moving streams (Cushing and Allan 2001) Another pollutant acid rain forms from sulfur dioxide and nitrous oxide emitted from factories and power stations These substances readily dissolve in atmospheric moisture and enter lotic systems through precipitation This can lower the pH of these sites affecting all trophic levels from algae to vertebrates (Brown 1987) Mean species richness and total species numbers within a system decrease with decreasing pH (Giller and Malmqvist 1998)

    Flow Modification

    Tens of thousands of dams have been constructed in the US alone The type of dam and its mode of operation are important determinants of its effects but in general these structures alter the flow temperature and sediment regime of lotic systems (Giller and Malmqvist 1998) Additionally many rivers are dammed at multiple locations amplifying the impact Dams can cause enhanced clarity and reduced variability in stream flow which is due to an increase in periphyton abundance Invertebrates immediately below a dam can show reductions in species richness due to an overall reduction in habitat heterogeneity (Cushing and Allan 2001) Also thermal changes can affect insect development with abnormally warm winter temperatures obscuring cues to break egg diapause and overly cool summer temperatures leaving too few acceptable days to complete growth (Allan 1995) Finally dams fragment river systems isolating previously continuous populations and preventing the migrations of anadromous and catadromous species (Cushing and Allan 2001)

    Invasive Species

    Invasive species have been introduced to lotic systems through both purposeful events (eg stocking game and food species) as well as unintentional events (eg hitchhikers on boats or fishing waders) These organisms can affect natives via competition for prey or habitat predation habitat alteration hybridization or the introduction of harmful diseases and parasites (Giller and Malmqvist 1998) Once established these species can be difficult to control or eradicate particularly because of the connectivity of lotic systems Invasive species can be especially harmful in areas that have endangered biota such as mussels in the southeast or those that have localized endemic species like lotic systems west of the Rocky Mountains where many species evolved in isolation

    References

    Allan JD 1995 Stream : structure and function of running waters Chapman and Hall London Pp. 388
    Angelier E. 2003 Ecology of Streams and Rivers Science Publishers Inc Enfield Pp. 215
    Brown AL 1987 Freshwater Ecology Heinimann Educational Books London Pp. 163
    Cushing CE and JD Allan 2001 Streams: their ecology and life Academic Press San Diego Pp. 366
    Edington JM Edington MA and JA Dorman 1984 Habitat partitioning amongst hydrophyschid larvae of a Malaysian stream Entomologica 30: 123-129
    Giller S. and B. Malmqvist 1998 The Biology of Streams and Rivers Oxford University Press Oxford Pp. 296
    Hildrew AG and PS Giller 1994 Patchiness species interactions and disturbance in the stream benthos In Aquatic : scale pattern and process (PS Giller AG Hildrew and DG Rafaelli eds) Blackwell Oxford Pp. 21-62
    Morin PJ 1999 Community Ecology Blackwell Science Oxford Pp. 424
    Rubbo M. J. and J. M. Kiesecker 2004 Leaf litter composition and community structure: translating regional species changes into local dynamics Ecology 85:2519-2525
    Schoener TW 1974 Resource partitioning in ecological communities Science 2:369-404
    Townsend CR Hildrew AG and K. Schofield 1987 Persistence of stream invertebrate communities in relation to environmental variability Animal Ecology 56:597-613
    Vannote RL Minshall GW Cummins KW Sedell JR and CE Cushing 1980 The river continuum concept Canadian journal of Fisheries and Aquatic Sciences 37:130- 137
    Vinson MR and CP Hawknis 1998 Biodiversity of stream insects: variation at local basin and regional scales Annual Review of Entomology 43:271-293
    Ward JV 1992 Aquatic Insect : biology and habitat Wiley New York Pp. 456
    Watson DJ and EK Balon 1984 Ecomorphological analysis of fish taxocenes in rainforest streams of northern Borneo Journal of Fish Biology 25:371-384

    See Also

    USGS real time stream flow data for gauged systems nation-wide: http://waterdatausgsgov/nwis/rt