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,
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 eightinch 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.
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 >7mg/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
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?
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.
Eel grass (Zostera marina) in a saline lagoon.
Source: Comhairle nan Eilean Siar Scotland
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.
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?
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
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
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
Cattail, common (Typha latifolia
Duckweed, lesser (Lemna minor)
Fragrant white water lily (Nymphaea
odorata)