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CHAPTER 1. ENVIRONMENTAL ASPECTS OF EUTROPHICATION

1.2. Eutrophication as an Environmental Problem

1.2.1. Limnological Background

Natural lakes and reservoirs are distributed worldwide and exhibit much variety in their limnological characteristics. From the perspective of eutrophication, several limnological aspects are of particular importance. While it is difficult or inappropriate to divide lakes into discrete categories, broad distinctions help guide understanding and management of eutrophication. Physical factors of importance are size and depth, flushing rate, and patterns of stratification and mixing.

Shallow lakes usually have a set of conditions that enhance nutrient recycling, commonly called internal nutrient loading. Large areas of sediments deposited on the lake bottom are able to exchange nutrients with regions of the lake where plants can grow. Activities of microbes and burrowing animals and resuspension of sediments further increase the release of nutrients into the water. In addition, light levels tend to be higher throughout the water column in shallower basins than deep basins. Conditions of adequate nutrient and light levels typical of shallow lakes can lead to high levels of phytoplankton or macrophyte biomass. In general, there is a tendency for productivity to be correlated negatively with the depth of a lake.

Flushing rate, or hydraulic residence time, can have a significant influence on the responses of a lake to enrichment. Reservoirs and floodplain lakes can experience especially strong riverine flushing, at least in certain seasons. Shallow lakes with stream inflows and outflows can flush rapidly. Conversely, lakes, which exchange water via seepage or those with large volumes, have much longer residence times. While inflows often supply nutrients that enhance eutrophication, rapid flushing can reduce the time available for plant growth and result in less accumulation of biomass.

Physical processes determine the extent of stratification and mixing in lakes, a fundamental aspect of ecosystem structure and function and response to enrichment. Mixing and circulation in lakes are driven by momentum and energy exchanges with the atmosphere, inflows and discharges. Limnologists divide lakes, based on vertical density profiles, into an epilimnion or upper mixed layer, metalimnion, the region with a strong gradient in density, and hypolimnion, the region below the metalimnion. Turbulent mixing is often active only near the surface, in plunging river underflows, and sometimes at boundaries. Much of a lake is quiescent with turbulence suppressed by buoyancy forces derived from stratification. Turbulent mixing is characterized by being intermittent, discontinuous and confined to localized patches.

Physical processes operating in many lakes are illustrated in Figure 1.2. Solar radiation is attenuated exponentially as a function of depth, and the depth of penetration depends on the clarity of the water. The depth of surface mixing is determined by the balance between the buoyancy caused by surface heating and cooling and the rate of production of turbulence. Turbulence is generated by wind stirring, convective overturns, and shear instabilities. Other mechanisms, which generate motions, are differential heating and cooling between littoral and offshore waters, which can cause buoyancy-driven horizontal flows, and uneven mixed-layer deepening which can lead to gravitational adjustments driving flows. Important types of motions, which occur below the upper mixed layer, are internal waves and intrusions. One consequence of internal waves can be an oscillating turbulent boundary layer. While internal waves do not cause mixing merely by their existence, a variety of mechanisms do exist to generate localized overturns, which lead to turbulent mixing. All of these processes can redistribute nutrients within a lake and influence eutrophication.

Figure 1.2. Physical processes in inland waters.

Mixing within the surface layer occurs in all lakes and often has a daily pattern. In shallow lakes the diel cycle of stratification and mixing usually includes a period with uniform temperature from top to bottom. In deeper lakes with seasonal stratification, the depth of daily mixing is confined to the upper portion of the water column. Many lakes throughout the world are sufficiently deep to remain thermally stratified from several to many months each year. In deep, tropical lakes of Africa, Asia and South America, the general tendency is for these lakes to mix deeply during one interval each year in coincidence with their hemispheric winter or, if equatorial, when clouds reduce the sunlight and winds are high.

Chemical conditions in lakes and reservoirs are a result of biogeochemical and hydrological processes in the watersheds as well as ecological and chemical processes within the waters and sediments of the lakes and reservoirs. Complex interactions can occur. For example, alterations in the inputs of phosphorus to aquatic habitats can have important effects on the chemical cycles of other elements, such as carbon, nitrogen, sulfur and iron. Increased rates of photosynthesis associated with phosphorus-enhanced plant growth can increase carbon dioxide invasion from the atmosphere. Phosphorus enrichment can reduce the nitrogen to phosphorus ratio, which can favor growth of nitrogen-fixing cyanobacteria. Greater amounts of plant biomass resulting from phosphorus enrichment can lead to augmented respiration rates and development of waters with low or no dissolved oxygen in deeper portions of lakes. Low dissolved oxygen favors generation of methane and sulfide, production of ammonium, and release of ferrous iron from sediments. Additional information about the role of sediments in eutrophication is provided in section 1.2.4.

Biotic communities in lakes can be divided into those in the open water, or pelagic region, those in deep-water sediments, or the profundal zone, and those in near-shore habitats, or the littoral zone. Responses to eutrophication vary among these areas, and physical processes and movements of organisms link the three regions. Pelagic organisms include phytoplankton, zooplankton, free-living and particle-attached bacteria, and fish. The biota inhabiting the profundal sediments includes a wide variety of invertebrates and microbes, and their abundance and species composition is influenced strongly by the extent to which the sediments are oxygenated or anoxic. Emergent, submerged and floating vascular plants often are conspicuous in the littoral zone. These plants provide habitat for attached animals, algae and bacteria, and for free swimming fish and invertebrates.

Interactions among trophic levels can modulate impacts of nutrient additions. Piscivorous fish consume planktivorous fishes while zooplankton graze on phytoplankton and bacteria. If piscivores are substantially reduced by changes in limnological conditions or intense fishing, planktivores often increase and exert strong predation on the larger zooplankton. Hence, grazing pressure on phytoplankton declines and algal blooms may increase in severity. Furthermore, the size distribution of the phytoplankton may shift to larger species, which sink faster and may decompose at different rates than smaller algae. The likelihood of such a trophic cascade depends on the relative magnitudes of the changes in predation and grazing pressures, the availability of refuges from predation, and the degree of eutrophication.

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