| What is a Lotic Ecosystem?
A lotic ecosystem
is any spring, stream, or river viewed as
an ecological unit of the biotic community and the physiochemical
environment. Lotic ecosystems are characterized by the
interaction between flowing water with a longitudinal gradation in
temperature, organic and inorganic materials, energy, and the organisms
within a stream corridor. These interactions occur over space and
time. A stream corridor usually consists of three major elements 1)
stream channel 2) floodplain 3) transitional upland fringe.
Within the stream there
are two major zones: riffles, shallow water where currents are strong
enough to keep the bottom clear and firm; and pools, deeper waters where
currents are reduced and silt and other debris collect on the bottom. Each
zone has its specially adapted life forms. |
| Lotic vs. Lentic Ecosystems?
On
the surface of the land, free water habitats can be classified as either
lotic (running-water) or lentic (standing-water). Lotic habitats include
rivers, streams, and brooks, and lentic habitats include lakes, ponds, and
marshes. The major difference between a lotic ecosystem and a
lentic ecosystem is the persistent flow of water in a lotic
ecosystem. Lentic ecosystems are usually characterized by large deep
basins with little or no flow existing within the basin. Characteristic
of lentic systems is the development of vertical differences (vertical
stratification) of several important features, which often display marked
seasonal variation as well.
The difference between lentic and lotic habitats is not always clear-cut.
The decisive criterion is the length of time a given mass of water resides
within a certain part of an aquatic ecosystem, a concept clearly related
to flow rates. Some large rivers with only a slight gradient have low
rates of discharge and flow and extensive floodplains with many
interconnected bodies of lentic waters. |
| Why
are Lotic Ecosystems Important?
Although rivers and
streams contain only a small percentage of the world's freshwater, they
are a vital component to the hydrologic cycle. Annually, rivers and
streams transport approximately 32-37 km3
of water to the oceans each year (Allan, J.D. 1995). Adaptations to
life in flowing water and other physicochemical features combine to create
an aquatic environment very different from the lentic environment.
Therefore, most biological organisms that originate within a lotic system,
and the ecosystems associated ecological processes, are so specialized
that they that are confined to this type of environment. |
| The
Hydrologic Cycle (From
Stream Corridor Restoration)
The
hydrologic cycle describes the continuum of the transfer of water from
precipitation to surface water and ground water, to storage and runoff,
and to the eventual return to the atmosphere by transpiration and
evaporation.
Precipitation returns water to the earth’s surface. Although most
hydro-logic processes are described in terms of rainfall events (or storm
events), snowmelt is also an important source of water, especially for
rivers that originate in high mountain areas and for continental regions
that experience seasonal cycles of snowfall and snow-melt.
The type of precipitation that will occur
is generally a factor of humidity and air temperature. Topographic relief
and geographic location relative to
large water bodies also affect the frequency
and type of precipitation. Rainstorms occur more frequently along
coastal and low-latitude areas with
moderate temperatures and low relief.
Snowfalls occur more frequently at
high elevations and in mid-latitude areas
with colder seasonal temperatures. Precipitation can do one of three things
once it reaches the earth. It can return
to the atmosphere, move into the soil,
or run off the earth’s surface into
a stream, lake, wetland, or other water
body. All three pathways play a role
in determining how water moves into,
across, and down the stream corridor. |
| River
Continuum Concept
The
river continuum concept (RCC) is an attempt to construct a synthetic
framework to describe the function of lotic ecosystems from source to
mouth, and to accommodate
variation among sites that results from
differences in their terrestrial settings (Vannote R.L, Minshall, G.W.,
Cummins K.W. at. al. 1980.)
The RCC emphasizes the idea that communities and ecosystems are in
equilibrium with their external environment. The RCC assumes that
lotic systems are structured in a predictable manner along a continuum of
physical or resource gradients (i.e. small streams to large rivers) (Statzner
& Higler 1985). In other words, as stream proceeds along its
course, it grows increasingly large as it gathers tributaries and drains
an ever increasing catchment area.
Some streams, however, may differ from the idealized RCC. Some
headwater systems may originate in meadows or on mountains or in some
cases land that has been developed, plowed, deforested, etc.. The results would be
a significant a change in the nutrient sources and flow. For
instance, agricultural practices may have denuded the surrounding
landscape and increased nonpoint nutrient loading (fertilizers).
Consequently, the stream is dominated by autotrophy as opposed to coarse
particulate matter (CPOM) inputs.
The RCC has been successful in promoting an understanding of general
principles that exist as stream systems proceed in a downstream
direction. The basic premise, that nutrient (energy) inputs follow
in a basic predictable manner and have predictable consequences on the
biotic assemblage and ecosystem processes is sensible and supported by
data (Allan, J.D. 1995).
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