LIMNOLOGY

            

Introduction

Freshwater systems – lakes, wetlands, rivers and streams, have been critical to the establishment of civilizations throughout human history. From ancient times, civilizations have been established based on their proximity to water. Water bodies are essential to humans not only for drinking but also for transportation, agriculture, energy production, industry and waste  disposal. Contaminated runoff from expanding urban and agricultural areas, airborne pollutants and hydrologic modifications such as drainage of wetlands are just few of the many factors that continue to degrade surface waters. Determining which of these factors has the most significant influence on the quality of a water body requires knowledge about the interaction of the water body with its watershed and how the various inputs affect its physical, chemical and biological characteristics. One of the critical sciences required to understand these aquatic ecosystem interactions is called limnology.

Limnology is the study of fresh or saline water covering all aquatic ecosystems, including lakes, ponds, reservoirs, streams, rivers, and oceans. It is a multidisciplinary science that integrates biology, chemistry, physics and geology in order to study inland waters as complex ecological systems.

Formation of lakes

Lakes are formed as a result of different natural and artificial processes, which are often interlinked resulting in lakes of similar origin, physical and biological characteristics.

Lakes formed by glaciers:

The gouging and scraping actions of glaciers have led to the formation of lakes in the north temperate zones. Glaciers followed existing valleys, deepening and widening them. When the mass of ice melted, it left piles of moraine, which blocked valleys, damming streams and rivers.

Ice-scour lakes:

Where ice sheets move over relatively flat surfaces of hard jointed or fractured rock, hollow basins are formed and subsequently filled with water.

Lakes formed by movements of the earth’s crust:

When a portion of the earth’s surface subsides in relation to its surroundings or conversely the sides are uplifted, a lake basin may be formed. The area and depth of water will depend on the drainage from the surrounding land, which depends on the amount of rainfall and the size of the area from where water drains into the lake.

Lakes formed by volcanoes:

Many lakes are formed due to volcanic eruptions called as crater lakes. These lakes are fed only by rainfall and run-off. They are found where a series of minor eruptions have taken place fairly close to each other, rather than one massive eruption. In some cases, where the volcano was big and erupted long ago, the floor of the enormous crater is relatively flat and water accumulates in the lowest places. Volcanic activity also forms lakes when the ash or lava from an eruption blocks natural drainage.

Lakes formed by rivers:

When the flow of water is suddenly slowed by a decrease in gradient, it takes a longer route and flows at a slower rate. Sediments are eroded from the outside and deposited on the inside so that the channel becomes more twisted. Sometimes the river breaks through a narrow isthmus between two succeeding curves. This    may leave a loop of the river to one-side of the new watercourse. The isolated portion may retain enough water to form an “oxbow lake”.

Other lakes may be formed from hollows in the valley floor, filled by the river when it overflows its banks during floods. The flat floor of a wide valley, which is periodically flooded by the river, is known as floodplain. In many parts of the world, coastal impoundment results in freshwater lagoons adjacent to the sea. The freshwater lagoons are formed because a single strip of sand barricades the flow of water from river to sea.

Lakes are constantly changing and follow the natural stages of transition and death. Starting out pristine and deep, they slowly fill in due to natural causes, becoming shallower until the wetland turns into a bog and finally ceases to exist. This is a very slow process and takes thousands of years to complete. This phenomenon called natural eutrophication is inevitable. Human activity accelerates this process and changes the water quality within few years. This is referred to as cultural or anthropogenic eutrophication, due to which clear nutrient-poor water changes to green, nutrient rich water. The overall quality and quantity of the water is dependant on the physical dimensions of the lake and its basin.

Lentic and Lotic ecosystems

Inland water bodies can be classified as either lotic (running-water) or lentic (standing-water). Lotic habitats include rivers, streams, and brooks, and lentic habitats include lakes, ponds, and marshes. The major difference between them is the persistent flow of water in a lotic ecosystem.  Large deep basins usually characterize lentic ecosystems with little or no flow existing within the basin.  Characteristic of lentic ecosystems is the development of vertical differences (vertical stratification) of several important features, which often display marked seasonal variation as well.  However, the decisive criterion is the length of time a given mass of water resides within a certain part of an aquatic ecosystem, a concept clearly related to flow rates. Some large rivers with only a slight gradient have low rates of discharge and flow and extensive floodplains with many interconnected bodies of lentic waters.