eutrophication. eutrophic waters have an enriched supply of nutrients and are highly productive...
TRANSCRIPT
Eutrophication
Eutrophication
Eutrophic waters have an enriched supply of nutrients and are highly productive oligotrophic waters are much less productive
because they have a restricted nutrient supply mesotrophic waters are intermediate in
productivity & fertility some waterbodies occur in inherently fertile
watersheds and are naturally eutrophic “cultural eutrophication” is caused by
anthropogenic nutrient additions through dumping of sewage or runoff of agricultural fertilizer
both fresh and marine waters can become eutrophic, but the problem is usually more severe in affected inland waters
Symptoms of Eutrophication
The most conspicuous symptom of eutrophication is a large increase in primary productivity, especially of phytoplankton, which can develop an algal “bloom”
shallow eutrophic waterbodies may also have vigorous beds of aquatic plants (or “macrophytes”)
the increased productivity of algae and macrophytes allows invertebrates, fish, and waterfowl to be abundant in many eutrophic waterbodies
but extremely eutrophic (or hypertrophic) waters may have severely degraded water quality
Eutrophic waters may develop noxious blooms of cyanobacteria (blue-green bacteria) and algal phytoplankton during the summer cyanobacteria may cause an off-flavour of drinking
water they may also release toxic organic compounds into
water also, the decomposition of dead algal biomass creates a
large O2 demand, depleting its concentration in water
the anoxic conditions are extremely stressful to aquatic animals, such as fish
anoxia also facilitates the production and release of hydrogen sulphide (H2S) and other noxious gases
cultural eutrophication represents a degradation of water quality and ecological conditions, and it is an important environmental problem in many areas
it affects the ability of waterbodies to:
be a source of drinking water support commercial and sport fisheries and be used for recreation
Causes Of Eutrophication
Principle of Limiting Factors: certain ecological processes are controlled by whichever environmental factor is present in the least supply relative to demand according to this theory, primary production can
be limited by the nutrient present in least supply relative to the biological demand (assuming that light, temperature, & O2 are adequate)
in almost all freshwaters, primary production is limited by the availability of inorganic phosphorus, occurring as the ion phosphate (PO4
-3)
marine coastal waters, however, are usually limited by the availability of inorganic nitrogen, particularly of nitrate (NO3
-)
Phosphorus is Limiting
In addition to inorganic P, other potentially limiting nutrients include inorganic N (NO3
- and/or NH3
+) and dissolved inorganic C (as bicarbonate, HCO3
-)
however, various lines of evidence suggest the role of P as the limiting nutrient for eutrophication of freshwaters:
P has the smallest ratio of supply : demand “supply” is the concentration in freshwater “demand” is the concentration in plants and
algae this ratio suggests that P is the most likely
candidate to be the primary limiting factor
Phosphorus is limiting (cont’d)
Critical experiments were done in the Experimental Lakes Area (ELA) in nw Ontario: these were “whole-lake experiments” in which
nutrients were added at various rates and combinations to selected lakes, followed by monitoring of the ecological responses
Lake 304 was fertilized with P, N, & C and it became eutrophic, but then recovered after P addition (only) was stopped
the hourglass-shaped Lake 226 was divided into two basins, which were fertilized with C+N+P or with C+N; only the +P basin had algal blooms
this is why we should not P in lakes
The top basin of Lake 226 in the ELA was made eutrophic by an experimental whole-lake addition of phosphate; the bottom basin, separated from the top by a vinyl curtain, remained oligotrophic
Sources and Control of P Loading
During the 1960s and 1970s, the average discharge of P to inland waters was about 2 kg/person-year about 84% of the P loading involved the
dumping of municipal sewage, and the rest was livestock sewage and agricultural fertilizer
the total N discharge was 12.5 kg/person-year; 36% from municipal sources and 64% agricultural
because P is the primary limiting nutrient for eutrophication, control strategies have focussed on reducing rates of input of P from large, discrete sources, such as sewage works
Phosphorus in Detergent
In the 1960s and early 1970s, detergent contained 50-65% of sodium tripolyphosphate (12-16% as P) the TPP was added to detergent as a "builder", to reduce
the activity of Ca, Mg, & Na in wash water and allow the cleaning agents (surfactants) to work more efficiently
~3 x 106 kg/yr of high-P detergent were being used in NA
because all was flushed into the sewer system, detergents accounted for ~ 1/2 of the P in wastewater discharges during the early 1970s
because, detergent use is a discrete activity, and good substitutes are available for phosphate in the builder function, it was relatively easy to rapidly decrease P loading by regulating the use of high-P detergents
in 1970, detergent sold in Canada could contain 16% P, but this became limited to 2.2% by 1973
some areas have banned the sale & use of detergent containing P
Sewage Treatment
Usually, the principal objective of sewage treatment is to reduce inputs of microbial pathogens and O2-consuming organic matter to receiving waters but where surface water is vulnerable to eutrophication,
sewage may also be treated to reduce P in the effluent all towns and cities in Canada have facilities to collect
sewage effluent from homes, businesses, institutions, and factories
the infrastructure consists of webs of underground pipes and other collection devices
some municipalities have separate systems to collect domestic and industrial wastes (the latter contains toxic & hazardous chemicals that should be treated separately)
many municipalities also have a separate system of pipes to handle the large volumes of “storm flow” from the surface runoff of rainwater and snow meltwater
Eventually, large quantities of waste water mut be discharged into the ambient environment, usually into a nearby lake, river, or ocean
wherever possible, it is highly desirable to treat the waste water to reduce concentrations of pollutants
unfortunately many municipalities in Canada continue to dump raw, untreated sewage into a nearby aquatic environment
this is especially true of cities and towns located beside an ocean, because well-flushed marine ecosystems have a huge capacity for diluting and biodegrading organic pollutants
examples: Atlantic Coast: Halifax, Saint John, St. John's St Lawrence: Montreal Quebec City Pacific Coast: Victoria
Places like Halifax Harbour are severely degraded by the smell, aesthetic, health-risk, and ecological damages caused by large amounts of raw sewage the worst of the damage is restricted to the
vicinity of sewage outfalls compared with many oceanic environments,
inland waters (lakes & rivers) have a much smaller capacity for diluting and biodegrading sewage wastes
consequently, Canadian municipalities located beside inland waters treat their sewage before discharging the effluent
of all Canadian cities, Calgary has the highest standard of sewage treatment
it uses advanced tertiary treatment to remove most inorganic P and N before discharging into the Bow River, a relatively low-flow waterbody
Sewage-treatment facility for Calgary
Sewage Treatment (cont'd)
Primary treatment is relatively simple P it involves screening to remove larger materials, then settling to reduce suspended organic matter
the resulting effluent may then be discharged into the environment
it may also be treated with a chlorine disinfectant to kill pathogenic microorganisms
primary treatment removes about: 60% of suspended solids, 25% of biological oxygen demand (BOD), and 5-15% of P
Sewage Treatment (cont'd)
Secondary treatment may be applied to the effluent of 1y treatment, mostly to further reduce BOD it usually involves a biological technology
(biotechnology) in which aerobic microbial decomposition is enhanced
two processes commonly used are: activated sludge, involving vigorous aeration of
sewage water to enhance decomposition of its organic content
trickling filters, in which sewage waste passes slowly through a complex physical substrate supporting large populations of microorganisms
these biotechnologies produce large amounts of a humus-like sludge, which is usually disposed of onto agricultural land as an organic-rich conditioner, or incinerated, or dumped into a secure landfill
secondary treatment removes about 80% of the BOD and 30-50% of the P content
Sewage Treatment (cont'd)
Tertiary treatment involves processes that remove most dissolved nutrients from the effluent P removal may be achieved by adding Al,
Fe, or Ca to develop insoluble compounds with PO4
-3, which settle from the water this can remove >90% of the phosphate
in wastewater tertiary treatment is also sometimes used
to remove NH4+ and NO3
- from the effluent because 3y treatment is expensive, it is
only used in cities located beside waterbodies that are highly vulnerable to eutrophication
Sewage Treatment (cont'd)
Artificial wetlands may be constructed to provide an ecosystem in which vigorous microbial communities decompose organic waste of sewage, while productive algae & macrophytes decrease nutrients in the water
their efficiency depends on climate, flow-through rate, and nature of the engineered wetland, but they can remove 90% of BOD and 30% of P
this ecotechnology works well for relatively small communities, but is difficult to scale up for large cities
Because tertiary treatment requires expensive investments in technology & operating costs, it is mostly used by communities beside inland waters the Great Lakes, rivers, and other inland waters are
relatively small and vulnerable to eutrophication and other kinds of water pollution
in the Great Lakes, bilateral agreements negotiated between Canada and the U.S. regulate the loading of P and other pollutants; this has required 3y treatment systems in most communities
in many other places in Canada, especially coastal ones, less attention is generally paid to the removal of P from municipal sewage effluents
such places may release raw sewage, or at most use 1y or 2y systems to reduce BOD and pathogens in effluents
even more damaging is the fact that it is uncommon that the sewage effluent of agricultural livestock is treated; livestock on factory farms produce much more fecal material than do humans in Canada
Several Case Studies
Arctic lakes Meretta and Char are two small lakes in arctic Canada Char is a typical, oligotrophic lake, with extremely
clear, nutrient-poor water, a low rate of primary production, and a small productivity of zooplankton and fish
Meretta receives sewage and is moderately eutrophic its P loading is 13-times larger than Char and N 19-
times higher
during the growing season Meretta Lake develops a phytoplankton biomass averaging 12-times greater than in Char Lake, and 40-times larger during the bloom
during winter, bottom waters of Meretta Lake become anoxic, stressing arctic char (Salvelinus alpinus)
even polar lakes, which are simple ecosystems because of severe climate, will become eutrophic if fertilized
Lake Erie ‑ Eutrophication and Other Stressors
Lake Erie has been severely stressed by a complex of environmental stressors: nutrient loading contamination by potentially toxic chemicals commercial and recreational fisheries conversion of most natural ecosystems in its
watershed into agricultural and urban land-use
deforestation has been severe deliberate & accidental introductions of
numerous species of non-indigenous, invasive plants, animals, and microbes
Lake Erie (cont’d)
This complex of environmental stressors has greatly degraded the water quality and ecosystems of Lake Erie
the damage was most acute during the 1960s and 1970s
some of the damage has since been abated because of:
actions to reduce P loading actions to reduce toxic chemicals clarification of water by zebra mussels
Drain Lake
This is a rare case of an acidified, eutrophic lake it demonstrates that even an extremely
acidic waterbody (pH 4.0) can become eutrophied by the addition of P
in some respects, the higher productivity enhanced some ecological qualities of Drain Lake:
higher primary productivity lush macrophytes a large population of breeding ducks
Drain Lake is a rare example of an acidic, eutrophic lake