FPOM Fine Particulate Organic Matter
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; >1
mm pieces) into fine organic particulate matter (FPOM; <1 mm 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, the subsidies are termed
allochthonous energy. 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.
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 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
Characteristics of streams) 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 (Coarse Particulate
Organic Matter - larger than 1 mm) inputs at upstream sites is expected to
result in the transport of large amounts of FPOM (Fine Particulate Organic
Matter - smaller than 1 mm) 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 Water 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 phosphorus
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 U.S. 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 (e.g. stocking game and food species) as well as unintentional events
(e.g. 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.