Impacts of Eutrophication

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Impacts of Eutrophication
Eutrophication in the Sea of Azov. Source
SeaWiFS Project, NASA/Goddard Space Flight Center
and ORBIMAG
Developed by Richard Sandford with contributions
from Martin Bloxham and Paul Worsfold,
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Impacts of Eutrophication
3.1 Decrease in the transparency of water Light
is essential for the growth of green plants and
sunlight provides the energy for photosynthesis.
The penetration of sunlight into a body of water
determines the depth and quantity of algae and
other underwater plants. Water transparency
decreases as colour, suspended sediments and
algae increase. Transparency of water can be
measured using a Secchi disk. The Secchi disc is
a simple scientific instrument used to measure
water transparency. The Secchi disk is an
eight-inch disk painted with alternating black
and white quadrants. Which is lowered into a
water body until it can no longer be seen. This
depth of disappearance is a measure of the
water's transparency. The Secchi disc depth
indicates the water transparency and provides a
rough estimate of light penetration in the water
column. As a general rule, light can penetrate to
a depth of two times the Secchi depth. For
example, if the Secchi depth was 3m, then light
can penetrate to a depth of 6m. As light
penetration increases, so does the amount of
plant growth and oxygen produced by algae and
aquatic plants. The Secchi depth of muddy and
eutrophic lakes, estuaries and big rivers ranges
from 0 to 2 m but in oligotrophic or blue water
oceans it can be as great as 40 m. In many lakes,
the Secchi depth is approximately one-third of
the depth of the photic zone. The clarity of lake
water varies with season due to algal blooms or
suspended sediment and these are well reflected
by measurements of Secchi depth.
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Impacts of Eutrophication
3.2 Development of anoxic conditions (low oxygen
levels) The level of dissolved oxygen in
surface and near surface water is an important
measure of the state of the health of the aquatic
environment. Dissolved oxygen levels become
depressed as a result of the inability of natural
processes to supply oxygen at the rate demanded
for the oxidation of organic matter or reduced
chemical substances. Dissolved oxygen deficiency
may be particularly acute in the cases of
eutrophication, discharge of sewage and the
discharge of organic industrial, agricultural and
aquacultural effluents. Extreme oxygen
deficiencies (e.g., anoxia) can result in the
elimination of all higher life forms. Anoxic
conditions, especially in sediments, can also
lead to the liberation of less reactive forms of
metals from particles into aqueous phases. Under
anoxic conditions anaerobic bacteria flourish.
Anaerobic bacteria often produce foul smelling
compounds such as hydrogen sulphide (H2S),
thioalcohols (RSH) and ammonia (NH3).
Levels of dissolved oxygen of gt7mg/l in surface
marine and freshwaters, depending upon
temperature, represent essentially
oxygen-saturated conditions. Levels below 4 mg/l
represent serious oxygen depletion with some
species exhibiting avoidance. Species mortalities
can occur below these levels with severe
prejudice to most aerobic organisms occurring
below 3 mg/l.
Fish kill in the Baltic Sea (Source WVU
(Wissenschaftsverbund Um-Welt), Germany
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Impacts of Eutrophication
3.3 Increased algal blooms Algae are simple
plants, which contain chlorophyll a as their
primary photosynthetic pigment. Algae are found
in fresh and marine waters and vary in size from
large kelps (meters in length) to microscopic
organisms. In low numbers, most algae are
harmless and are an essential part of any healthy
ecosystem because they produce oxygen and are a
source of food for other aquatic animals.
However, excessive algae growths or blooms can
cause serious water quality problems including
unpleasant tastes and odours, unsightly scums
which significantly reduce the aesthetic and
recreational amenity of the water body and
blockages in pump valves and filters. In
addition, dead or decomposing algae utilises
oxygen in the water body which can contribute to
fish kills and the death of other aquatic
animals. Question What are harmful algal
blooms?
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Impacts of Eutrophication
3.4 Loss of habitat (e.g. Sea grass
beds) Pressures exerted on biodiversity can
generally be divided into ecosystem loss,
fragmentation, degradation and modification. This
has resulted in the decline or extinction of many
species of plants and animals. If sufficient
amounts and types of suitable habitat cannot be
maintained wildlife can be put at risk.
Eutrophication can cause serious effects to
living resources or their habitats. Marine or
estuarine systems with biogenically structured
habitat, such as coral reefs or seagrass beds,
are especially vulnerable to eutrophication.
Bays, lagoons, enclosed seas, and open coastal
waters can also be affected. The accelerated
increase in the input of nutrients to the marine
system represents a serious threat to the
integrity of marine ecosystems and the resources
they support.
Species losses may be short-term or even
permanent in localised areas (e.g., the northern
Gulf of Mexico, the Baltic Sea shelf, or the
northwestern shelf of the Black Sea). There are
no documented cases of a species extinction due
to eutrophication, but there are many examples of
localized or temporary loss of biodiversity,
shifts in community structure in both pelagic and
benthic systems, and many examples of degraded
habitats, such as coral reefs, seagrass beds, and
productive continental shelves with important
commercial fisheries, that become unsuitable for
the usual inhabitants.
Eel grass (Zostera marina) in a saline lagoon.
Source Comhairle nan Eilean Siar Scotland
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Impacts of Eutrophication
3.5 Change in dominant biota Changes in plankton
and macrophyte community structure The
production of aquatic macrophytes and algae is an
important component of wetland food chains.
Aquatic macrophytes provide structural habitat
for invertebrate and vertebrate life and also
provide substrates for colonization by epiphytic
algae and microbes that are important foods of
aquatic invertebrates. Once macrophytes
senescence, they contribute litter for
colonization by microbes which provide additional
food resources for aquatic invertebrates. In
addition to epiphytic algae, phytoplankton and
epibenthic algae are also major sources of carbon
in wetlands and are important food resources of
aquatic invertebrates. Anthropogenic
sedimentation has potential to suppress primary
production and alter natural food chain
interactions. Increased sediment in the water
column generally reduces the depth of the photic
zone and hence reduces the light available for
primary production by aquatic macrophytes and
algae. As sediment falls out of suspension,
deposition may be adequate to bury epibenthic
algae, macrophytic photosynthetic substrates, and
seed. Question What are the basic classes of
macrophyte?
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Question What are Harmful algal blooms? Among
the thousands of species of microscopic algae
there are a number that produce potent toxins.
Under the appropriate conditions of nutrients and
temperature these species can multiply at high
rates causing red tides''. Such events can
cause detrimental effects on marine and estuarine
ecosystems through reduced light, oxygen and
occasionally the production of toxins. The
impacts of these phenomena include mass
mortalities of wild and farmed fish and
shellfish, human intoxications or even death from
contaminated shellfish or fish. Alterations of
marine trophic structure through adverse effects
on larvae and other life history stages of
commercial fisheries species and death of marine
mammals, seabirds, and other animals can also
occur. Given the confusion surrounding the
meaning of red tide,'' the scientific community
now prefers the term harmful algal bloom (HAB).
This new descriptor applies not only to
microscopic algae but also to benthic or
planktonic macroalgae which can proliferate in
response to anthropogenic nutrient enrichment,
leading to major ecological impacts such as the
displacement of indigenous species, habitat
alteration, or oxygen depletion. The causes and
effects of macroalgal blooms are thus similar in
many ways to those associated with harmful
microscopic phytoplankton species. The cause of
marine algal blooms is not always clear. The key
trigger could be appropriate conditions of
nutrients and temperature, although incidents of
algal blooms triggered by pollution have occurred
in a number of countries. However, in tracking
this indicator, we may be alerted to
anthropogenic activities which are influencing
the incidence of algal blooms. For pictures of
HABs, click Here References Harmful Algae Page,
National Office for Marine Biotoxins and Harmful
Algal Blooms, Woods Hole Oceanographic
Institution, Woods Hole, MA 02543
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Impacts of Eutrophication
Dinoflagellate algae Alexandrium tamarense
Gulf of Maine Sea Surface Temperature image of an
Alexandrium sp. Bloom
1999 Hong Kong Red Tide (Unidentified species)
Blue-green algae can occur in both urban and
rural locations
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Question What are the basic classes of
macrophytes? There are 4 basic classes of
macrophytes Submerged macrophytes Grow
primarily under water, although some can resist
and respond to exposure. Reproductive organs can
be submerged, emergent or floating. Examples
include teridophytes (mosses and charophytes) and
angiosperms. Floating leafed macrophytes Possess
at least some leaves floating at the surface
attached by stems to the substrate. Many also
have submerged leaves. Examples include lilies
such as Nuphar spp., Nymphaea spp or Potamogeton
natans. Emergent macrophytes Include plants
whose aerial structures are produced in a similar
fashion to terrestrial plants. Many species grow
well on exposed substrate as well as surviving
completely submerged. Examples include
Phragmites, Scirpus spp. (bulrushes), Typha spp.
(cattails). Free floating macrophytes Grow
primarily unattached to the substrate. Some float
on the surface with much of the emergent
structure growing clear of the water, whilst
others lie under the water surface. Examples
include Lemna spp. (duckweed). Intertidal
algae (seaweeds) are often consdered as a
separate class because they do fit into the other
macrophyte classes. For pictures of macrophytes,
click Here
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Cattail, common (Typha latifolia
Fragrant white water lily (Nymphaea odorata)
Duckweed, lesser (Lemna minor)