Based on the presence or absence of flow in its water two main types of aquatic bodies can be distinguished. These are:
(A) Lentic aquatic system.
(B) Lotic aquatic system.
A. Lentic Aquatic Systems:
Lentic aquatic systems are those which contain stagnant waters. These are usually formed in small or large depressions on earth’s surface which possess no exit for the water to flow out. Lentic systems, therefore, are closed systems. Whatever gets in it, rarely gets out. It has to decay, decompose or persist as such within the lentic body. Natural processes in the long run change such a body into a swamp or a marsh, a wetland and finally to dry land. Small puddles, ponds and lakes are familiar examples of such systems.
1. The zones of a lentic water body:
A lentic aquatic body can be divided into four major zones, each of which has its own characteristics (Fig. 1.6). The zone around the margins of the aquatic body which consists of shallow waters is known as the Littoral zone. Plenty of light is available and rooted plants grow in this zone only. Limnetic zone is the zone of open waters which are deeper as well.
Availability of plenty of light promotes active photosynthesis and growth of free floating autotrophs – the planktons. Under the limnetic zone occurs Profundal zone which receives very little light. Hence this zone may be referred to as aphotic zone in contrast to euphoric zone (limnetic and littoral regions) which are well illuminated. Under the profundal zone lies the Benthic Zone – the region near the bottom of the aquatic body. Both profundal and benthic zones are characterised by presence of heterotrophs which live on dead and decaying organic material falling down from the limnetic zone above.
2. Characteristics of a lentic aquatic system:
The environment in a lentic aquatic system differs remarkably from that of a logic system. It is mainly the absence of flow which is responsible for these differences. Important features which characterise a lentic body of water can be summarized as follows:
(a) There is a marked stability in the physicochemical characteristics of water quality. Lentic systems derive most of their waters from rains, surface runoffs or from underground sources. These undergo little change in their qualities for years and years together. So, for long durations of time the quality of water in lentic systems remains virtually the same.
(b) There is usually a positive correlation between the seasonal changes in physicochemical characteristics of water and the productivity of a lentic body. Fluctuations in water quality from season to season affect the composition of the living community. Periods of bright sunshine are usually found associated with a rich growth of phytoplankton and a remarkable lowering of the concentration of a number of plant nutrients.
(c) Total surface area of lentic water body is more important in assessment of overall productivity rather than total volume of the water or the depth of the water body. It is the extent of littoral and limnetic zones which determines the productivity. There could be enormous volume of nutrient rich water in profundal zone or the benthic zone but these waters lack enough sunshine to be productive.
(d) Lentic systems are closed systems. Substances of persistent nature, products of decay and mineralization of organic matter as well as the decomposition products of pollutants discharged in the aquatic body stay in the system. The biotic community is often considerably affected by these materials.
(e) Thermal stratification and stratification of oxygen and other nutrients. One of the most characteristic features of a lentic water body is the phenomenon of stratification or vertical donation which usually involves deeper static waters – more than 6 to 8 metres in depth. Stratification causes different layers of a lentic water to have different oxygen content and nutrient status. Such conditions develop mainly because of the fact that:
1. Light illuminates only the upper layers – the limnetic zone, where active photosynthesis and growth occur which results in plenty of oxygen and rapid consumption of nutrients. Profundal and benthic zones are dark. Some oxygen also dissolves into the surface water from the atmosphere above.
2. Nutrients are in surplus supply near the bottom strata from where they dissolve out into the water. Decomposition, mineralization and regeneration of nutrients occur actively in the profundal and benthic zones while consumption of plant nutrients is nominal in these zones. Being oxygen deficient in nature, products of anaerobic microbial activity also accumulate in these zones.
An aquatic body receives or loses almost 95% of its heat through its open surface which is in contact with atmospheric air. As a result, the temperature of its surface layers is directly influenced by atmospheric temperatures and the presence or absence of air circulation. However, deeper layers being insulated from all sides do not undergo such rapid changes in temperature which affect the density of water as well.
When lighter layers of water happen to be placed on denser waters thermal stratification develops. Water has its maximum density at 4°C. Above 4°C warmer waters are lighter than cooler waters as is the case with all other liquids. But below 4°C the situation is exactly opposite since warmer waters are heavier than cooler ones. The conditions under which stratification develops in a static water body are:
1. When a water body at temperatures below 4°C is subjected to cooling.
2. When a water body at temperatures above 4°C is subjected to heating.
Under both conditions the surface layers shall become lighter than the water underneath. They tend to stay at the top which prevents free circulation and the mixing of different layers of water. The imaginary line demarcating the two zones with different temperatures is known as the Thermocline while the zones above and below this line are known as Epilimnion and Hypolimnion respectively.
Conditions under which complete mixing of water occurs in a lentic aquatic body are:
1. When a water body at a temperature above 4°C is subjected to cooling up to 4°C
2. When a water body at temperatures below 4°C is subjected to heating up to 4°C.
Under both these conditions the upper layers of water shall become heavier. They will sink down, lighter waters shall come up to replace the heavier ones and thorough mixing of different layers of water shall occur.
Thus while uneven illumination of the mass of water causes different layers of water to have different amounts of dissolved gases, nutrients and other materials; it is mainly temperature which determines whether the two layers shall mix or stay as separate layers – the epilimnion and hypolimnion. The phenomenon of stratification is of great importance in temperate regions of the world where temperatures go below the freezing point of water. Cooler waters stay at the surface and ice formation starts from top downwards.
Aquatic life continues safely under the ice-sheet. In topical and sub-tropical regions where temperatures rarely go below 4°C and it is very hot during the day, stratification develops during the day time. Cooling during the night causes sinking of the oxygen-rich but nutrient-deficient top layers while nutrient-rich but oxygen-deficient layers come up to replace them. There is diurnal circulation of nutrients and oxygen which is responsible for such a high productivity of these waters.
Lotic aquatic systems are those systems which contain flowing waters. The mass of water in these systems is in a state of perpetual motion. Streams and rivers are familiar examples of such systems. The basic function of these lotic bodies of water is to carry the surplus rain water back to the sea. The total amount of water which streams and rivers of the world carry to the oceans amounts to almost 25 cms of rains distributed evenly over the land surface. Characteristic features of a lotic system can be summarized as follows:
1. There is a continuous uni-directional flow in a lotic system. The water usually flows in narrow channels which are not so deep as compared to lakes. It contains water derived from diverse sources. The bottom of a lotic body continuously adds and receives materials from the mass of water above.
2. The volume of water keeps changing which in turn brings about changes in the velocity of water currents. The water levels in a lotic system also exhibit a wide range of fluctuations.
3. Water in a lotic system acts as an effective agent of transfer, transport and dilution. They keep eroding materials all along their channels and depositing them elsewhere.
4. There is a thorough mixing of contents in a lotic aquatic system. Stratification and stagnation are altogether absent.
5. The physico-chemical parameters of water quality are also in a state of perpetual change. There is no stability in the environment of a lotic aquatic system as compared to that of lentic water body.
6. Plenty of oxygen is derived from air above which is evenly distributed throughout the water mass. To this is added the oxygen produced by autotrophs. Oxygen depletion is, therefore, rare in unpolluted lotic waters. As the population of plants and animals is limited, nutrient depletion which is frequent in lentic waters is also very rare.
7. Turbidity usually limits light penetration to deeper zones of lotic systems. There is little carbon dioxide present in these waters. Both these factors tend to limit productivity. Large crops of algae and other organisms rarely develop in moving waters.
8. Area and depth show little correlation with productivity in lotic waters.
9. There is little correlation between physicochemical characteristics of the aquatic environment and the productivity of the system.
10. Presence of water current is the dominant feature of a lotic system. Those organisms which have effective mechanism to stay at one place in the flowing waters usually occur in these systems. Productivity is low in rapidly flowing waters, rising proportionately as the velocity of flow slows down. Living organisms take full advantage of water currents to disseminate their seeds, spores and other reproductive structures all along the length and breadth of the channel. Similar species may occur throughout the entire course of a stream or a river.