Basic Processes In Lakes PDF

BASIC PROCESSES IN LAKES LU3 STK2953  LEARNING OBJECTIVES At the end of this learning unit, students should be able to: Describe the major zones of a lake State different classes of lakes Explain and distinguish the types of eutrophication Explain the factors that contr...

BASIC PROCESSES IN LAKES LU3 STK2953  LEARNING OBJECTIVES At the end of this learning unit, students should be able to: Describe the major zones of a lake State different classes of lakes Explain and distinguish the types of eutrophication Explain the factors that control the eutrophication process Discuss ways to control eutrophication Explain the processes involved in nitrogen, phosphorus and sulphur cycles  INTRODUCTION TO LAKES Lakes are described in terms of their physical (light penetration, temperature, sediment, and morphology), chemical (chemical constituents, plant nutrients:, and dissolved gases) and biological (plankton, benthos, and nekton) properties. These properties are all interrelated, and are important variables to measure to evaluate water quality.  INTRODUCTION TO LAKES Light penetration - the littoral zone is the shallow-water area where light reaches the bottom. At times, wave action may cause shoreline erosion and bring materials into suspension in the water. These suspended materials may cause a "turbid" condition and reduce the penetration of light and thus the depth of the littoral zone. The limnetic zone is the open-water area of a lake that extends from the surface to the depth where light intensity is reduced to about one percent of the surface light. The lower boundary of this zone is the compensation level, which is the depth at which oxygen uptake by bacteria, plant, and animal respiration equals photosynthetic oxygen release by green plants. The depth of the compensation level depends upon the light-scattering and absorbing material in the water. The limnetic zone is inhabited by free-floating and swimming organisms, such as plankton and fish. The combined littoral and limnetic zones are referred to as the euphotic zone, the area where there is sufficient light for photosynthesis.  INTRODUCTION TO LAKES The profundal zone is the deep water area where only respiration and decomposition· occur. light intensity is too low in the profundal zone for photosynthesis process to occur. Temperature – The thermal properties of a lake are controlled by the length of exposure and intensity of the sun on the lake, by materials in the water which scatter and absorb light and by wind mixing. Thermal stratification is most pronounced during the summer in lakes of the warmer latitudes, and water-quality conditions often become critical in the deep water because the water is stagnant. Water reaches its greatest density at 4oC. At temperature above or below 40C, the density of water decreases. This property of water results in a regular pattern of seasonal mixing and stratification in many lakes.  PROCESSES IN LAKES PHOTOSYNTHESIS AND RESPIRATION ❑Photosynthesis is carried out only by green (chlorophyll-containing) plants and some bacteria. Respiration is carried out by both plants and animals. Both processes are essential to life and have a profound effect upon water quality in lakes. ❑Photosynthesis and respiration result in daily and seasonal fluctuations of dissolved oxygen and carbon dioxide levels in lakes. As a result, the carbon dioxide and dissolved oxygen concentrations are not always in balance. These imbalances have a significant effect upon the quality of water. ❑ For example, all lakes contain some algae, but in the presence of sufficient sunlight and plant nutrients, photosynthesis by phytoplankton may result in an excessive production of oxygen. At this time, the phytoplankton may increase to concentrations of a million cells per litre, a condition known as an algal bloom. ❑Respiration removes dissolved oxygen from the water. For example, in a thermally stratified lake during the summer, respiration by bacteria may oxidize organic materials at such a rapid rate in the bottom water that the dissolved oxygen becomes drastically depleted. Thus, when large quantities of decomposable organic wastes are introduced into natural water, severe oxygen depletion can occur.  PROCESSES IN LAKES EUTROPHICATION Eutrophication is the enrichment of lakes by nutrients and filling by sediment. although eutrophication is a natural phenomenon, recent concern has been with the acceleration of this process by man's activities. presently, it is a major problem of many lakes and reservoirs. The aging process of a lake begins with its formation in a basin. As the lake accumulates plant nutrients and sediment deposits, plant production increases and shoreline vegetation invades the lake. As the depth of the lake decreases and emergent vegetation increases, the lake becomes a bog and is referred to as a dystrophic system. Eventually the original lake becomes land.  PROCESSES IN LAKES THE BIOGEOCHEMICAL CYCLE Materials that enter a lake from tributaries or from the atmosphere may settle in the lake basin, be removed through the outlet, or remain in solution. Those that remain in solution, and that are required for plant production, may be incorporated into living tissue. The major pathway for the flow of plant nutrients and other elements into a lake is sedimentation. Sedimentation is the process whereby fragmental material originates from the disintegration of rocks and from other sources, is transported, and is deposited in layers or remains suspended in the water. Lakes receive sediment from both inflowing water and from the wind. The sediment accumulates permanently in areas little affected by wave action. Because most lake bottom sediments are relatively permanent deposits, sediment deposition is a major cause of the aging and extinction of lakes.  PROCESSES IN LAKES Many lakes retard inflow sufficiently to trap all incoming coarse sediment, as well as part of the fine sediment. The trap efficiency of a lake depends in part upon the fall velocities of the sediment particles, the density of the water, and how much the water is slowed as it passes through the reservoir (brune, 1953). As a lake fills with sediment, its trap efficiency usually decreases. Besides being a mechanism for lake filling, sedimentation introduces plant nutrients and dissolved materials into lakes. The quantity of plant nutrients sorbed by sediment particles may be high, and the nutrients can become more soluble upon entering the different chemical environment of a lake. Once in the lake, plant nutrients are subjected to cycling (Nutrient cycling). Some elements, such as phosphorus, nitrogen, silicon, and perhaps soluble carbon, may undergo such extensive use by plants that these nutrients are virtually depleted in the water. Thus, they may become limiting nutrient elements to plants and restrict plant production until the elements are recycled or new supplies enter the lake.  PROCESSES IN LAKES Many elements are cycled through a number of chemical forms. For example, nitrogen may be taken up by plants as the stable nitrate ion (N03-). in the plant cell it is reduced and makes up an important part of plant and animal protein. Upon death of the organism, the nitrogen may be released either as ammonia gas (NH3), or the ammonium ion (NH4+ ). further oxidation of these forms by specific bacteria results in the unstable oxidized form, nitrite (N02-). further oxidation of the nitrite by other bacteria results in the stable nitrate form, thus completing the cycle. Phosphorus (p), inorganic phosphate (P04-3) , and organic phosphorus (OP--the fraction bound in plant and animal cells) follow a cycle similar to nitrogen. Carbon also occurs in several forms. In water it is found as carbon dioxide gas (C02), as the inorganic ions bicarbonate (HC03-) and carbonate {C03-2 ), or as the solid compound calcium carbonate (CaC03 ). organic carbon is that fraction of total carbon within plants and animals. The sorption {absorption and adsorption) of chemical elements and compounds by sediment is well known. Lake bottom sediment is often a rich source of plant nutrients. The release of these nutrients from the sediment is regulated by various factors, such as temperature, pH, and dissolved oxygen.  Lakes can be classified according to the degree of enrichment of nutrient and organic matter. 3 classes are generally recognized: Oligotrophic lakes: are nutrient poor, have low levels of algae and organic matter, good transparency and abundant O2. Eutrophic lakes: are nutrient rich, have high levels of algae and organic matter, poor transparency and are often oxygen-depleted LAKES Mesotrophic lakes: any intermediate, often with and abundant fish life because they have both elevated levels of organic matter and adequate supply of oxygen.  EUTROPHICATION The process of nutrient enrichment in a lake with increase in organic matter This is considered to be a natural aging process in lakes, that may take thousands of years (natural eutrophication) However, it is possible to greatly accelerate the rate of change through human activities (e.g discharge of domestic and industrial wastewater (its called cultural eutrophication) NATURAL EUTROPHICATION ❑ Eutrophication that is caused by the normal processes of the nature not interfered by human activities is termed as natural eutrophication. ❑ Takes many years to affect the water bodies in a full force. ❑ Water bodies not near the industrial or human habitat places are affected by such sort of eutrophication. CULTURAL EUTROPHICATION Caused by human activities. It is a rapid process as it takes only decades to eutrify and ultimately pollutes the water. It is more alarming than the natural eutrophication. EFFECTS OF EUTROPHICATION What are the effects of eutrophication? DIFFERENT STAGES OF THE LIFE CYCLE OF A LAKE PRINCIPLE FACTORS CONTROLLING EUTROPHICATION 1. Availability of sunlight – amount of light available is related to transparency- which in turn is related to the level of eutrophication 2. Concentration of nutrients- most important nutrients: C, N, O 3. Important terms: ✓ Euphotic zone – top layer of water in a lake, where plants produce more oxygen by photosynthesis than they remove by respiration ✓ Profundal zone: bottom zone ✓ Light compensation level: transition between two zones CASE STUDY – RED TIDES OR HARMFUL ALGAE BLOOM  SIMPLIFIED REPRESENTATION OF ALGAL PHOTOSYNTHESIS 106CO2 + 16NO3- + HPO42- + 122H2O + 18H+ →C106H263O110N16P + 138O2 Usually if N/P>20, P is limiting nutrient If N/P