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