EUTROPHICATION

EUTROPHICATION

Eutrophication may be dened as the inorganic nutrient enrichment of natural waters, leading to an increased production of algae and macrophytes. Many lakes are naturally eutrophic, and in some cases there is a progressive eutrophication as the lake matures (Section 3.7.3). The term eutro- phication is more widely known in relation to human activities, where the articial introduction of plant nutrients (particularly phosphorus and nitrogen) has lead to community changes and a deterioration of water quality in many freshwater systems. This aspect has become increasingly important with increases in human population and more extensive development of agriculture, and eutrophication now ranks with other major anthropogenic effects such as deforestation, global warming, depletion of the ozone layer, and large-scale environmental disturbance in rela- tion to its potentially harmful effect on natural ecosystems. In terms of aquatic microbiology, eutrophication results in changes in the biomass and taxonomic composition of all groups of microorganisms present in freshwater systems.

This ASSIGNMENT is about the origins of eutrophica- tion, the algal response to increased nutrient levels, and measures that can be taken to control the growth of deleterious microbial populations. These issues are discussed particularly in relation to standing waters (lentic systems) where high nutrient levels can lead to the development of intense algal blooms and the resulting environmental deterioration. Eutrophication of rivers and streams is also an important aspect of freshwater microbiology, but environmental effects are generally less acute than in lentic systems

Lakes and reservoirs are generally less effective at diluting pollutants than streams are, for two reasons. First, deep lakes and reservoirs often contain stratified layers (Figure 7-30, p. 147) that undergo little vertical mixing. Second, they have little or no flow. The flushing and changing of water in lakes and large artificial reservoirs can take from 1 to 100 years, compared with several days to several weeks for streams. As a result, lakes and reservoirs are more vulnerable than streams are to contamination by runoff or discharge of plant nutrients, oil, pesticides, and non- degradable toxic substances such as lead, mercury, and arsenic. Many toxic chemicals and acids also enter lakes and reservoirs from the atmosphere

ORIGINS OF EUTROPHICATION

The origin of Eutrophication can be natural as well as result of human activities i.e. artificially

Natural eutrophication

Nutrient status of freshwater environments: from oligotrophic to eutrophic systems

Variations in nutrient level affect the natural ecology of aquatic systems. The three inorganic nutrients of major importance in freshwater systems are nitrates, phosphates, and silicates. High concentrations of these nutrients in lake water promote the active growth of phytoplankton, leading to the massive development of algal biomass (high primary productivity) and the resulting growth (high secondary productivity) of all other lake organisms including bacteria, zoo- plankton, and sh. The twin aspects of nutrient concentration and productivity have been used to provide a trophic classication of lakes in temperate climates. Two major categories can be recognized:

eutrophic lakes high concentrations of nitrates and phosphates, high primary and secondary productivity, and

oligotrophic lakes low concentrations of either nitrates or phosphates (or both), low primary and secondary productivity.

These two major categories are part of a continuum in terms of water quality, and it is convenient to recognize ve main groups hypertrophic, eutrophic, mesotrophic, oligotrophic, and ultra-oligotrophic in descending order of enrichment and productivity.

Attempts have been made to dene these terms in relation to xed boundary values for both nutrient and productivity water-quality parameters. One particular scheme, developed by the Organization for Economic Cooperation and Development (OECD), provides specic criteria for temperate lakes in terms of the mean annual values of total phosphorus, chlorophyll-a, and Secchi depth

Although this scheme provides a useful concep- tual framework for dening trophic categories, there are limitations in its practical use for classifying particular lakes. Because of variability within eco- systems, some water bodies can be classied in one or another trophic category, depending on which parameter is used. In an attempt to alleviate this, a more exible open boundary system has been developed (OECD, 1982), where the status of indi- vidual water bodies is determined by statistical t to more open ranges of the above parameters. The connection between nutrient status and algal productivity implied in this system is not absolute. Lake Baikal (Russia), for example, is ultra- oligotrophic in terms of physicochemical character- istics but is close to mesotrophic on the basis of primary production and the high productivity of subsequent heterotrophic levels (Mazepova, 1998). In other cases, lakes of high nutrient levels have limited productivity due to conditions of high acidity (acidotrophic lakes), high content of brown humic acids, or high turbidity all of which may severely limit algal growth. Climatic differences also bring limitations to the classication system.

Table

Determinant of nutrient status location and morphology

Location has an important inuence on the nutrient status and productivity of lakes in relation to local geological conditions (e.g., phosphate content of their substrata), nature of the catchment area (parti- cularly soil nutrient content), and internal hydro- logical characteristics (degree of mixing, water retention time). Many oligotrophic lakes occur in mountainous areas, with a distinctive morphology (typically deep lakes) and with an infertile, undis- turbed catchment area. In contrast to this, eutrophic lakes are typically shallow lowland water bodies, surrounded by a fertile (often cultivated) catchment.

The high nutrient status of lowland lakes is closely tied to human occupation of lowland areas,

with the resulting development and enrichment of agricultural land and the disposal of nutrient, rich human effluent into the freshwater systems. Such differences between mountain lakes and those in cultivated lowlands were noted at an early stage in the development of limnology by Naumann (1919)

Artificial eutrophication: the impact of human activities

Human activities have been a major cause of eutro- phication of freshwater systems (Mason, 2002), either by direct discharge of contaminating nutrients into the aquatic system or indirectly, by such pro- cesses as deforestation or alteration of drainage patterns. Direct contamination of water sources involves three main types of pollutant domestic discharges (particularly sewage), industrial efuent, and agricultural waste. The sources of nutrient entry into the freshwater system are of two main types as follows.

Point source: where inow into the lake or stream is localized. This is typical of sewage and industrial efuent, and is also characteristic of some types of agricultural pollution.

Following are some of the point sources

Wastewater effluent (municipal and industrial)

Runoff and leachate from waste disposal systems

Runoff and infiltration from animal feedlots

Runoff from mines, oil fields, unsewered industrial sites

Overflows of combined storm and sanitary sewers

Runoff from construction sites less than 20,000 m (220,000ft)

Untreated sewage

Diffuse source: where entry of organic pollutants occurs over a wide area and includes agricultural seepage, runoff from road systems, and aerial pollution.

Following are some of the diffuse sources

Runoff from agriculture/irrigation

Runoff from pasture and range

Urban runoff from unsewered areas

Septic tank leachate

Runoff from construction sites >20,000 m

Runoff from abandoned mines

Atmospheric deposition over a water surface

Other land activities generating contaminants

Eutrophication of rivers and streams

The environmental effects of high nutrient levels are generally less acute in lotic compared with lentic systems. The main reason for this is water ow, which causes continuous displacement of suspended material (low retention time) and prevents the build- up of planktonic biomass in any particular locality. Turbulence and vertical circulation in streams and rivers also prevent stratication, limiting algal growth due to reduced overall light exposure during the circulation cycle and also due to the shading effects of suspended particulate matter. Vertical circulation in lotic systems also prevents the extremes of water column microenvironment (sur- face O2 supersaturation, anaerobic hypolimnion) which frequently develop in eutrophic lakes (Figure 5.9). In spite of these growth limitations, algal blooms (with resulting environmental problems) may develop in some rivers.

ECOLOGICAL EFFECTS OF EUTROPHICATION IN STANDING WATERS

Increased biomass of phytoplankton

Toxic or inedible phytoplankton species

Increases in blooms of gelatinous zooplankton

Increased biomass of benthic and epiphytic algae

Changes in macrophyte species composition and biomass

Decreases in water transparency (increased turbidity)

Colour, smell, and water treatment problems

Dissolved oxygen depletion

Increased incidences of fish kills

Loss of desirable fish species

Reductions in harvestable fish and shellfish

Decreases in perceived aesthetic value of the water body

THE GROWTH AND IMPACT OF ALGAL BLOOMS

Algal blooms and eutrophication

Algal blooms are simply dense populations of planktonic algae which develop in aquatic systems. They may occur in a wide range of environments, including lakes and rivers, exposed mudats, and snowpacks and are part of the normal seasonal development in many ecosystems. In all of these environments, the development of algal blooms can be seen as a balance between the processes of population increase (high growth rate, ability to out-compete other algae) and population loss (effects of grazing, parasitic attack). Increased levels of inor- ganic nutrients lead to a general increase in primary productivity (see previously) but may also promote algal blooms at different times of the year. In lentic environments, these blooms include the spring dia- tom bloom, late-spring blooms of green algae, and summer blooms of dinoagellates and blue-green algae (Figure 3.14). Most of these blooms have no adverse effects on the environment, and the increased algal biomass is transferred to other lake biota via the normal food web (Figure 10.1).

The major problems of eutrophication come with anthropogenic enrichment of the environment and the formation of dense blooms of toxic dinoagel- lates (principally marine) and colonial blue-green algae (freshwater).

Toxic dinoagellates.

These organisms are parti- cularly characteristic of marine waters. They do occur in estuaries, however, and are occasionally seen in major rivers associated with estuaries so do have some peripheral relevance to freshwater systems. Some of these dinoagellates are important in the formation of neurotoxins, and are potential hazards in terms of human water contact and food consump- tion (Silbergeld et al., 2000). Toxic dinoagel- lates include Gambierdiscus toxicus (CTX toxin), Gonyaulax Alexandrinum (STX), Gymnodinium breve (brevetoxins), Diophysis spp. (Okadaic acid), and Pesteria piscicida (unknown toxin).

Colonial blue-green algae.

Colonial blue- greens, not dinoagellates, form the major nui- sance-algae of freshwater systems and have the potential to cause deterioration in water quality and adverse environmental effects. In many eutrophic environments (e.g., Rostherne Mere, UK; Figure 10.5) and mesotrophic lakes of lesser magnitude, quite dense blue-green blooms occur on an annual basis without any permanent

environmental effects. It is only when these algae form very dense accumulations and totally out-compete other algae that their inuence becomes severe.

General requirements for bloom formation

These algae tend to become a prominent feature of north- ern temperate lakes in midsummer, when their dominance increases and bloom formation occurs.Although the effects of blue-green blooms are often prolonged, their origin may appear to be rapid with the sudden appearance of dense surface scums in eutrophic lakes and reservoirs (Figure 10.5). The origin of these blooms is partly due to massive growth (population increase) but also the ability to oat to the water surface and form dense localized populations. The conditions necessary for the sud- den development of surface blooms are summarized in Table

The ability of blue-green algae to out-compete other members of the phytoplankton at a time of year when certain environmental aspects (light, temperature) are at an optimum, is a key feature of the success of these organisms in bloom formation.

Following are some of the major points due to which blue green algae compete other algae

Optimum growth at high temperature

Tolerance of low-light conditions

Ability for growth at low Nitrogen/Phosphorous ratios

Depth regulation by buoyancy

Resistance to zooplankton feeding

Tolerance of high pH and low CO2 concentrations

Environmental effects of blue-green blooms

Very high levels of algal biomass lead to extreme limitation of light penetration, preventing growth of other algae and completely suppressing growth of higher plants. Growth inhibition of other algae results in a dramatic loss of phytoplankton diversity within the water column. Oxygenation in the water column also reaches new extremes. Active photosynthesis at the water surface leads to high oxygen concentrations (often supersaturated) and high pH (frequently >pH10). In the lower part of the water column, higher levels of algal detritus and heterotrophic decom- position result in extreme reducing conditions, with oxygen concentrations very low or non-detectable throughout most of the hypolimnion. These redu- cing conditions permit the accumulation of substan- tial levels of ammonia and nitrite in the water column, and lead to greater release of phosphate from the lake sediments adding to the already existing eutrophication problems

Specific effects on water quality

In addition to these rather general environmental effects, blooms of blue-green algae also cause more specic problems in relation to water quality and human use of the water body:

1.Production of high concentrations of small mole- cular weight toxins which are poisonous to a wide range of animals these are discussed more fully in Section 10.7.3.

2.Fish kills: towards the end of an algal bloom, acute anoxia may develop throughout most of the water column due to large-scale death of the algal cells, resulting in a decrease in oxygen evolution by photosynthesis and an increase in oxygen

uptake by heterotrophic bacteria metabolizing the algal breakdown products. Under these sud- den and extreme conditions of oxygen depletion, massive loss of sh populations may occur, with long-term effects on the ecology of the freshwater environment.

3.Problems with water extraction and treatment for domestic use: the ltration systems of water- treatment works can become blocked with algae, affecting the efciency and maintenance costs of the extraction process. Mucopolysacchar- ides produced by algal breakdown can chelate the iron or aluminium coagulants added to the water during the treatment process, leading to an increase in the metal complexes entering the water supply. The presence of algae can also lead to unpleasant changes in the odour and taste of the water. Final problems come with the collapse of the bloom, which may lead to the accumulation of ammonia affecting the oxida- tion and disinfection capacity of chlorine and converting iron and manganese to soluble forms that can lead to discoloration of water.

4.Impacts on agriculture and sheries: blue-green blooms can have adverse effects on agriculture through damage to livestock (toxin consumption), with an increased risk of ooding to farmland. In the case of sh farming, blue-green blooms can cause changes in sh species composition, and result in sh kills (see above).

5.Loss of recreation amenity: development of intense algal blooms frequently leads to a ban on human access for swimming, sailing, and other recreational activities

Production of toxins

In the freshwater environment, toxins are produced and secreted in quantity by one major group of algae the blue-greens (Carmichael, 1994). These toxins are part of a wide range of bioactive second- ary metabolites produced by these organisms, including polyketones, alkaloids, and peptides. Toxins reach particularly high concentrations under conditions of bloom formation, when they contaminate the water supplies of wild and domestic animals and can also harm humans. Toxin production by other algae may occur in some situations. Toxic populations of the eukaryote alga Prymnesium parvum (Prymnesiophyceae), for example, may develop in the brackish waters of tidal or coastal freshwater systems causing dramatic changes in sh communities (Skovgaard and Hansen, 2003). In the marine environment, toxin production by dinoagellates assumes greatest signicance contaminating sea water, but also causing shellsh to become poisonous due to ingestion and bioconcentration by these organisms.

Major catageories of toxins include

Neurotoxins including anatoxin a,anatoxin(s) and saxitoxin

Hepatxins including microcystins and nodularians