Introduction production, altered abiotic conditions will have important consequences

Introduction

Recent
anthropogenic activities have caused a significant change in the turbidity of
freshwater and marine ecosystems (Kapsimalis et al., 2014). Understanding the
mechanisms through which species interactions are influenced by anthropogenic
change has come to the lead of many ecological disciplines. Turbidity is
essentially a measurement of how cloudy or clear the water is, or, in other
words, how easily light can be transmitted through it. Turbidity affects
organisms that are directly dependent on light, like aquatic plants, because it
limits their ability to carry out photosynthesis. This, in turn, affects other
organisms that depend on these plants for food and oxygen. As a consequence,
fish production is reduced. Since light and nutrients are important drivers of
phytoplankton and bacterial production, altered abiotic conditions will have
important consequences for the aquatic food web base, with implications for the
productivity of higher trophic levels (Brett et al., 2017; Lefebure et al.,
2013; Liess et al., 2016).

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As
sediments and other suspended solids increase in the water, the amount of light
that can pass through the water decreases. There are three main types of
particles which are algae, detritus (dead organic material), and silt
(inorganic, or mineral, suspended sediment). The major cause of turbidity in
the open water zone of most lakes is typically phytoplankton. As algae,
sediments, or solid wastes increase in the water, so does turbidity. The algae
grow in the water and the detritus derives from dead algae, zooplankton,
bacteria, fungi, etc. produced within the water column, and from watershed
vegetation washed in to the water. If light penetration is reduced
significantly, macrophyte growth may be reduced which would in turn impact the
organisms reliant on upon them for food and cover. Low photosynthesis can also
result in a lower daytime release of oxygen into the water. Effects on
phytoplankton growth are complex depending on too many factors to generalize. High
levels of turbidity for a brief timeframe may not be significant and may even
be to a lesser extent an issue than a lower level that endures longer.

The
importance of the two types of input will vary with the location of the water
body. Inputs from outside the system are called allochthonous while
autochthonous inputs are produced within the system. For example, streams
draining through woodlands have large allochthonous inputs, while large lakes
and oceans have large autochthonous inputs. Organic detritus comes from stream
or wastewater discharges. Organic matter comes into water from aerial or
terrestrial sources such as falling leaves, in rain, as drainage, etc. or is
generated from within the system by photosynthesis and chemosynthesis. Ingested
food is digested and the remains, together with the waste products of
metabolism, are passed out as faeces and urine. The organic composition of
faeces varies with the efficiency with which dietary parts are assimilated and
the rate at which faecal matter is produced also differs with food quality.
Animals with lower quality diets, like many detritivores, feed almost
continuously and produce large amounts of faecal material. In contrast,
carnivores have the most nutritious diet and many eat and defecate irregularly
as food is often retained within the gut to allow efficient digestion.

Sedimentation
does not usually spontaneously start increasing in a system; there are reasons
for increased. Closer to shore, particulates may also be clays and silts from
shoreline erosion, and resuspended bottom sediments. Sediment comes largely
from shoreline erosion and from the resuspension of bottom sediments due to
wind mixing. On occasion there are natural system changes such as volcanic
eruptions or earthquakes that cause debris flows, mud flows, and landslides, or
human activities such as clear cuts that cause sudden mass movement. However,
most sediment increases are gradual and are caused by changes such as land
management, instream alterations, or short-term climatic events. In assessing
sedimentation, evaluation of environmental change will help to identify other
factors such as precipitation, discharge, shear stress, or a change in channel
platform or geometry that may also accompany the sedimentation changes.

Dredging
operations, channelization, increased flow rates, floods, or even too many
bottom-feeding fish such as carp may stir up bottom sediments and increase the
cloudiness of the water. High concentrations of particulate matter can modify
light penetration, cause shallow lakes and bays to fill in faster, and smother
benthic habitats – impacting both organisms and eggs. As particles of silt,
clay, and other organic materials settle to the bottom, they can suffocate
newly hatched larvae and fill in spaces between rocks which could have been
used by aquatic organisms as habitat. Fine particulate material also can clog
or damage sensitive gill structures, decrease their resistance to disease, prevent
proper egg and larval development, and potentially interfere with particle
feeding activities. Dissolved inorganic matter results from solution and
weathering processes, either by the water body itself or by indirect inputs
like surface drainage and hydrothermal sources. Inorganic particles are common
in water which is erosive or which re-suspends sediments. Sand and other
mineral grains are swept up by waves on marine and lake shores, and lowland
rivers are characteristically turbid as they carry a heavy load of fine
inorganic particles. In addition to resulting from erosion, inorganic particles
also have biogenic origins. The siliceous frustules of diatoms and the shells
of many kinds of invertebrates and protists all result from living organisms,
as does coral sand eroded from reefs.

 

The
emphasis in lake studies is different from studies of the stream environment.
Because lakes are sediment sinks and essentially closed systems (for sediment),
toxins are of great concern. Once a lake has been polluted, it is difficult to
clean. Sediment is important in these environments because many inorganic
toxins bind to fine sediments. A large percentage of lake sediment literature
is aimed towards sediment toxicity. Concern does arise on a regional or
national level when mega fauna, such as birds or deer, are affected. This is
very different from streams which express environmental changes throughout
their systems. Under normal environmental conditions, benthic invertebrates can
move quickly enough to keep ahead of fluctuations in natural sedimentation.
Artificial dumping and accelerated sedimentation introduces too much sediment
too quickly for benthic invertebrate organisms to avoid it (Herdendorf 1992).
Case-dwelling mobile macro invertebrate species can do very well in areas of
rapid sedimentation because of the decrease in competition and their ability to
escape the sediment (Thorman and Wiederholm 1984). Loss of benthic communities
may also occur if an increase in wave action erodes the substrate (Herdendorf
1992).

 

Preferred
spawning habitat in lakes can be similar to that in streams, but because of the
diverse and relatively more stable environment, spawning occurs in a large
variety of substrates. Lake trout in Lake Huron prefer cobble and rubble and do
not generally use coarse sands or gravels to spawn. Lake trout, like stream
trout, cannot successfully spawn in areas that are covered with fine sediments
(Nester and Poe 1987). Other lake species prefer sand, rocks, inshore
environments, logs, sticks, plants, or vegetative nests (Herdendorf 1992). Sediment
quality in lakes is extremely variable geographically. The introduction of
excess fine sediment can be addressed in lake tributaries or in the watershed,
but the actual sediment quality is difficult to alter because once it is in the
lake, it is hard to remove. Sediment traps such as filter dams and de-silting
basins can be used in the tributaries above a lake to reduce the amount of fine
sediment that is delivered to the lake (EPA 1973).

 

Dredging
of lake bottoms is often considered as a remedial technique to remove excess
sediment. Dredging temporarily increases turbidity in the lake and can cause
environmental degradation because of the decrease in primary productivity. The
sediment may be a nutrient sink and dredging may reintroduce the nutrients back
into the lake. The loss of shallow zones may result in the loss of large
macrophyte beds, resulting in turn in an increase in the algal population. To
further complicate the dredging issue, lakes and other bodies of water are
often used for disposal of sludge, which can contain very high levels of
toxins. Similar problems exist for river and bay dredging as well.

 

Estuaries
have been studied in depth by numerous disciplines. The use of estuaries by
fish is of concern to fisheries management specialists. Most of this knowledge
and interest has been limited to the researcher’s own professional peers and has
lacked the advantages of interdisciplinary research. The concern for estuaries
is growing, and there is a need for practical, useful data and associated
management practices. Estuaries have been recognized for their large biomass production
and pollution-filtering systems. The emphasis has been primarily on the flora
of estuaries and not the fauna, except for bird uses. Estuaries are important
for anadromous fish because it is a passage that they must make when migrating
from the streams to the ocean or on their return to spawn. Estuaries also serve
as a feeding ground and nursery for many fish and shellfish species.
Catadromous fish, such as eels, spawn at sea but spend a large portion of their
lives in coastal estuaries. Because of the physical, chemical, and biotic
diversity of estuarine systems, they are among the most biologically diverse
and richest systems found on earth.

 

Estuaries
are extremely sensitive to human action. Most large bays have associated large
estuaries and also have sizable seaport cities associated with them. A majority
of the world’s population lives along the coast line, so estuaries are
significantly impacted by land-use practices, recreation, and exploitation.
Ship traffic near estuaries can be especially heavy and affects the entire
estuarine ecosystem, because it introduces new variables including physical and
chemical alterations. Estuaries are also sites of dredging for sand and gravel
for industrial and commercial use. A characteristic of estuaries is that their
beds are constantly moving because of river inflow and tidal fluctuations. The
bedload is composed mainly of sand-sized particles which are easily entrained
and move for long distances. The bed material is not always transported in a
downstream direction. Depending on tidal influences, material may be moved up
and down the channel. Fine silts and clays flocculate in the salt water and are
deposited in tidal marshes. The dynamics of sediment transport in and through
estuaries is extremely complex.

 

The
relative effects of land-use practices or changes were evaluated on the basis
of soil erosion and the possible effect that this would have on downstream
estuaries. Phillips (1989) found that estuarine sediment is derived from
fluvial sediment input, shoreline erosion, and migration of marine sediments
inland. Phillips (1989) also indicates that sediment storage is much more
environmentally sensitive than basin sediment yield and concludes that dramatic
changes in the watershed would be required to alter the sediment budget in the
estuary. However, processes that mobilize stored sediment would have a large effect
on the sediment budget. Even though sediment delivery may be low, total
sediment input can be high. Stopping sediment before it reaches the stream
channel is important because once it becomes stored in the channel it can be
easily remobilized. Efforts to reduce sedimentation rates will be long-term
because large quantities of sediment are already in stream channels due to
agricultural and land-use practices. If sediment is in long-term storage in
estuaries, rather than en route to the continental shelf, then sedimentation
rates should be of great importance. Increased fluvial sediment in estuaries
may result in extended tidal marshes, shoaling, infilling of navigation channels,
reduction of benthic and aquatic habitat, and reduced primary productivity due
to turbulence and limited light penetration (Phillips 1991).

 

Estuaries
are utilized by specialized organisms that have adapted to fine sediments, high
sedimentation rates, and mobile substrate. The macro invertebrates that are
found in the substrate of estuaries are much smaller than those found in
streambeds with larger particle sizes. Within the estuary, the density of fauna
is commonly greater in the freshwater tidal areas than in other parts of the
estuary (Schaffner et al. 1987). The species diversity of macro invertebrates
is usually lower in fine-sediment substrates than that in coarser particle substrates.
The diversity and evenness of species decline with an increasing percentage of silt/clay
and organic matter (Junoy and Vieitez 1990). However, fine-sediment beds are
important for burrowing tube-making invertebrates and other.

 

Sediment
quality is another widespread problem in freshwater and marine systems (EPA
1992). Sediment quality problems can occur throughout stream types, but tend to
occur where there are fine textural stream bottoms and at the lower end of the
stream system such as estuaries and deltas. The contaminated sediments can have
both direct adverse impacts on bottom fauna, and indirect effects as the toxic
substances move up the food chain. Because of the variability of conditions
encountered in stream systems, lake systems, estuaries, and oceans, a variety
of tests may be needed to characterize the physical, chemical, and biological
systems that may be affected. In addition, microbial and benthic species will
likely reflect sediment contamination that is not revealed by sampling only
fish (Burton 1988). In other words, toxic impacts may be occurring in a river,
lake, estuary, or ocean, even though sampling in the water column over the
sediments may show water that meets water quality standards. Because there is
no single method that captures all the spatial and temporal impacts of
contaminated sediment upon all organisms, a compendium has been developed to
present several complementary methods to assess sediment contamination (EPA
1992).

 

Conclusion

Streams,
lakes, and estuaries are all vulnerable to sedimentation and erosion problems. The
more intensively the land is used, the greater is the potential for erosion and
sedimentation problems. Erosion and sedimentation can adversely affect aquatic
habitat and the species that depend on it. Each system responds in a different
way to accelerated sedimentation, so each system should be evaluated
independently of the others, recognizing that hydrologically these systems may
be closely connected.

Not
all streams respond to sedimentation in the same way, depending on the stream
characters. Acceleration of erosion and sedimentation will have varying
effects. By knowing the basic characteristics of certain types of streams
through a classification system, some generalizations and predictions can be
made about channel response. This does not replace a thorough stream
investigation but it provides information for planning purposes. While lake
sedimentation requires a different method of assessment because treating the
watershed problem may not be enough. Sediment removal may be required to
restore aquatic habitat as lakes do not flush their systems of fine sediment.
For this reason lakes are much more sensitive to sedimentation than are
streams. Estuary sedimentation is very complex because sediment transport is
not always unidirectional. Tidal fluxes and stream fluxes are combined, making
sediment yield estimates very difficult, and effects along shorelines are as
important as effects in the watershed.

Significant
human influences and flocculation can further exaggerate the problems. Streams,
lakes, and estuaries seem to be very different, yet they are all part of a
larger, even more complex ecosystem. The importance of ecosystem-based
assistance must be emphasized in planning conservation management systems that
integrate the effects on soil, water, air, plants, animals, the land user, and
the community. The interrelationship must be recognized and addressed when
planning any type of basin or watershed projects. The greatest reduction of
sediment impacts on aquatic habitat will occur when conservation management
systems are planned and installed on a whole-watershed basis.

 

 

 

 

 

 

 

 

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