Transcript Eutrophication: managing a growing problem in aquatic

Definition 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,
Definition of Eutrophication
1.1 What is eutrophication?
Eutrophication as a phenomenon is described in different ways by limnologists and marine scientists.
Most limnologists consider eutrophication as an increase in the rate of supply of organic matter to an ecosystem.
For marine scientists, eutrophication (GESAMP, 1990) is “used simply to mean ‘enhanced nourishment’ and refers to the stimulation of
aquatic plant growth by mineral nutrients, particularly the combined forms of phosphorus or nitrogen”.
1.2 Why is there a difference in approach?
Though widely associated with pollution, eutrophication is also a natural process in lakes as they gradually accumulate carbon,
nitrogen and phosphorus. Lake ecosystems can recycle some of the nitrogen and phosphorus compounds deposited in the lake bed
and this promotes more production of organic material. The organic material builds up on the lakebed and contributes to the lake’s own
demise. Bogs are shallow lakes that have filled with detritus (often peat) in this manner. The process may take hundreds or thousands
of years and lead to new land formation when the bogs themselves are colonised by trees. Eutrophication of a lake may therefore be
considered as an increase in organic matter in the system. This may be a result of direct river or sewage discharges but is most
commonly associated with the increase in plant production caused by an increased supply of nitrogen and phosphorus compounds,
essential nutrients for plant growth. The problem is that human intervention is accelerating this natural process, leading ultimately to the
accelerated decline of lakes. For more information on the classification of lakes, please click here.
In the marine environment, eutrophication has only been recognised in the past three decades. It was first identified in coastal lagoons
of the US eastern seaboard through the presence of unusually intense blooms of phytoplankton. Since eutrophication has been
documented in coastal seas throughout the world – this is not to say that all coastal seas are eutrophied! In all cases, eutrophication is
seen as a non-natural phenomenon associated with an increase of essential nutrients, nitrogen and phosphorus compounds, utilised
for phytoplankton growth.
Reference:
GESAMP (1990) (IMO/FAO/UNESCO-IOC/WMO/WHO/IAEA/UN/UNEP Joint Group of Experts on the Scientific Aspects of Marine
Pollution) The State of the Marine Environment. Rep. Stud. GESAMP No. 39, 111pp, London.
Definition of Eutrophication
1.2.1 Oligotrophic Lakes
• Oligotrophic lakes are generally deep with steep sides and
relatively small drainage areas, although some oligotrophic
lakes are no more than shallow granite pans.
• Nutrient levels are low.
• Water is clear blue with high light penetration due to the low
number of planktonic or attached algae that can be supported
by the low nutrient levels.
• Water transparency (measured by Secchi disk) is typically
over 8 m.
• Hypolimnetic and benthic 02 levels do not vary much from
saturation throughout the year (10 + 10 %).
• Biomass at all trophic levels is low.
Question: What is Secchi disk?
Definition of Eutrophication
1.2.2 Eutrophic Lakes
• Eutrophic lakes are often shallow, usually less than
10 m deep with gradually sloping edges and a large
drainage.
• Nutrient levels are high.
• Primary productivity is also high with an abundance
of planktonic or attached algae.
• Surface blooms of blue-green algae are common.
• Large variation in 02 levels. Depression in the
hypolimnion (0-100 %) and supersaturation in the
epilimnion (100-200%).
• Low water clarity with light penetration often not
reaching the thermocline or lake bed. Water
transparency (measured by Secchi disk) is typically
less than 2 m and in extremely eutrophic conditions is
only a few centimetres.
• Sediments of eutrophic lakes become enriched with
nutrients as organic matter from the photic zone
accumulates.
• Eutrophic lakes have a biomass at all levels of the
food chain, including fish.
• Summer and winter fish kills are typical in eutrophic
waters.
Definition of Eutrophication
1.2.3 Mesotrophic Lakes
• Mesotrophic lakes are intermediate between
oligotrophic and eutrophic lakes.
• They can be defined as having a Secchi disk depth of
2 - 8 m.
Definition of Eutrophication
1.2.4 Dystrophic Lakes
• Dystrophic lakes contain humic acids leached from
decaying aquatic vegetation in the watershed.
• Humic acid stains the waters a characteristic yellowbrown colour.
• They are usually shallow and unproductive although
some are highly productive and contain high levels of
floating blue-green algae.
• Small dystrophic lakes are common in mountain
regions where leachate from pine forests and bogs
provide a continuous supply of humic acids.
Question: What is 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.
Simple and inexpensive measurements such as Secchi depth, and temperature-depth
profiles are important indicators of water quality changes.