16 Types of Aquatic Ecosystems-I : Freshwater Ecosystem

Sharda R. Gupta

Module 30: Types of Aquatic Ecosystems-I

Objectives: To learn about freshwater aquatic and marine ecosystems

Learning Objectives

• Ø The Hydrological Cycle and the Aquatic Ecosystems
• Ø Limiting Factors in Aquatic Ecosystems
• Ø Aquatic Organisms
• Ø Freshwater Aquatic Ecosystems

30.1. Introduction

The aquatic ecosystems including freshwater and marine, are linked directly or indirectly as components of the hydrological cycle. The freshwater systems are now among the most endangered habitats in the world, due to human development, pollution and climate change. Freshwater ecosystems account for less than 0.01% of the planet’s total surface area; support more than 100,000 species, including fish. Wetlands are considered to be nature’s purification systems for cleaning the water from chemicals and other pollutants. More than half of the world’s wetlands have disappeared since 1900, which is of great concern. Human impacts on freshwater ecosystems include flow alteration, pollution, species extinctions, invasive species, thermal alterations, global climate change, and increases in ultraviolet radiation (Dodds et al. 2013).

The oceans cover 71% of the planet’s surface area. Marine and coastal environments contain diverse habitats that support a wide variety of marine life. Some examples of marine and coastal habitats include mangrove forests, coral reefs, sea grass beds, estuaries in coastal areas, hydrothermal vents, and seamounts and soft sediments on the ocean floor. The marine ecosystem provide goods and services like food provision, nutrient cycling, gas and climate regulation Plankton is the primary producer for all life in the ocean, and produce more than 50% of the oxygen. Marine fish and invertebrates provide over 2.6 billion people with about 20% of their average per capita protein intake (https://www.cbd.int/marine/important.shtml). The marine ecosystems are facing threats due to pollution, over fishing, habitat destruction, invasion of exotic species and global climate change.

30.2 The Hydrological Cycle and the Aquatic Ecosystems

Oceans cover about 71% of the planet’s surface area and are very important for the preservation of all life on earth. Oceans play an important part in the hydrological cycle, because precipitation consists of evaporated oceanic water and in the regulation of the earth’s climate. Oceans also participate in biogeochemical cycles like nitrogen and phosphorus.

All freshwater ecosystems are regulated by the hydrological cycle, the continuous process of water movement between different states. The major pathway of water cycle involves an exchange between earth surface and atmosphere via precipitation and evaporation. Water evaporated from oceans and terrestrial environments falls as precipitation. The precipitation that remains on the land surface follows a path determined by gravity and topography. Flowing-water ecosystems begin as streams. These streams, in turn, coalesce into rivers as they follow the topography of the landscape. The streams collect in basins and floodplains to form standing-water ecosystems such as ponds, lakes, and inland wetlands. Rivers eventually flow to the coastal marine systems and form estuaries, which represent the transition zone from freshwater to marine.

Out of the only 2.5% of global freshwater, about 69.5% is frozen in glaciers, polar ice caps, snow and permafrost, and more than 30% constitutes the groundwater. Only an extremely small proportion is available as habitat for living organisms on the surface of the Earth. Less than 0.3% of all freshwater on the Earth occurs in the lakes, rivers and wetlands and is utilizable by humans for their diverse needs (Shiklomanov, 2000). Rivers (including streams) comprise only 0.0002% of the total volume.

30.3Limiting Factors in Aquatic Ecosystems

There are positive effects on living organisms which live under water. However, there are also factors that limit aquatic life. Factors that determine which life forms can be sustained in an aquatic ecosystems are:

Sunlight

Temperature

Dissolved oxygen

Availability of nutrients

30.3.1. Sunlight: Light intensity at the lake surface varies seasonally and with cloud cover and decreases with increase in depth of the water column. The littoral zone is found near the shore, , light reaches up to the bottom, and rooted plants grow ( Fig.30.1). The open water zone, where light intensity is adequate and plankton are the dominant biota. Photosynthetic organisms include algae suspended in the water (phytoplankton), algae attached to surfaces (periphyton), and vascular aquatic plants (macrophytes). Producers need sunlight to produce oxygen and other organic substances that will sustain consumers. Production can only occur in the zone where sunlight can penetrate. Suspended matter may interfere with the penetration of sunlight into water. The rate at which light decreases with depth depends upon the amount of light-absorbing mostly organic carbon compounds washed in from decomposing vegetation in the watershed. Profundal zone constitutes the deep water with light very low or absent

30.3.2. Temperature: Lakes of temperate regions generally become thermally stratified due to differential heating and cooling. In the summer, the upper layers of water become warm and do not mix with cooler water of deeper layers, because of difference in the density of water (Fig. 30.2). The cool water has higher density than the warm water. The warmer part of the lake with small temperature gradient is called epilimnion and the cooler part of the lake is called hypolimnion. These two layers are separated by metalimnion, characterized by a steep decline in temperature and include thermocline. During winter, when the surface waters, rich in dissolved oxygen, attain a temperature of  4oC, it sinks to the bottom. During autumn and spring, the bottom waters, rich in nutrients and dissolved CO2, move upward resulting into mixing of two waters. This is the phenomenon is called overturn, and at this time thermocline disappears ( Fig.30.2).

Fig. 2. Thermal stratification in a temperate lake ( adapted from Singh et al. 2015)

With respect to overturns, lentic water bodies are divisible into: (i) monomictic (one overturn per year, in polar regions during summer, and in subtropical regions during winter), (ii) dimictic (two overturns, one in spring and the other in fall, in temperate regions), (iii) polymictic (more or less continuous mixing, in high altitude lakes in equatorial regions),

30.3.3 Dissolved oxygen: The dissolved oxygen (DO) concentration in the epilimnion remains high throughout the summer because of photosynthesis and diffusion from the atmosphere. However, conditions in the hypolimnion vary with the extant of nutrient enrichment. In eutrophic (more productive) lakes, DO declines during the summer. In oligotrophic lakes, low algal biomass allows deeper light penetration and less decomposition. In extremely deep, unproductive lakes, DO may persist at high concentrations, near 100% saturation, throughout the water column all year.

The solubility of oxygen, or its ability to dissolve in water, decreases as the water’s temperature and salinity increase. DO levels in an estuary also vary seasonally, with the lowest levels occurring during the late summer months when temperatures are maximum? Oxygen enters the aquatic ecosystem from the atmosphere and on account of production by phytoplankton. Oxygen levels, like temperatures, also decrease with depth. In marine ecosystems, ddissolved oxygen maximum layer (OML) about 700-1000 meters and nutrient maximum at about same depths (Fig.30.3).

30.3.4. Availability of nutrients: Nutrient supplies are usually satisfactory in freshwater ecosystems. However, in open oceans nutrients are often in short supply. Nutrients are an important limiting factor for productivity in aquatic life zones. Phosphorus is the main limiting nutrient in freshwater life zones, whereas nitrogen is the main limiting nutrient in saltwater life zones.

30.4. AQUATIC ORGANISMS

The aquatic organisms are described in Table 30.1, and briefly described as follows:

Plankton: The main kinds of organisms in aquatic ecosystems are free-floating, very small organisms called plankton. Phytoplankton is the most important kind of plankton, because phytoplankton consists of producers. The other species of plankton are consumers (zooplankton).

Nekton; is a group of larger, swimming consumers, which eat plankton or other consumers. Examples of nekton are fish and turtles.

Benthos: are the bottom-dwelling organisms (Table 30.1).

Decomposers: These include bacteria, fungi, and other microorganisms, are the other major group in the food web. These organisms decompose organic material into simple inorganic substances. Some of the decayed material is subsequently recycled as nutrients, such as phosphorus (in the form of phosphate, PO43-) and nitrogen (in the form of ammonium, NH4+) which are readily available for new plant growth. Carbon is released largely as carbon dioxide that acts to lower the pH of bottom waters.

30.5. FRESHWATER AQUATIC ECOSYSTEMS

Freshwater ecosystems can be categorised into three types :

(i) Lentic or standing water, which includes lake and ponds.

(ii) Lotic or running water, which includes springs, streams and rivers.

(iii) Wetlands include marshes and swamps, where water levels frequently rise and fall, seasonally as well as annually.

30.5.1. Freshwater lentic ecosystems: (Lakes and Ponds)

Lakes and ponds are large, natural bodies of standing water. These are fed mainly by rainfall and melting snow. Communities occur in a wide range of water bodies and nutrient conditions under diverse climates. Most of them are young in geological sense. Ponds may be seasonal with life span of few weeks and months, or perennial with age of several hundred years. While lakes and ponds have much in common, lakes are larger and deeper. The greater size and depth of lakes makes for some differences in dissolved oxygen levels, plant growth, and temperature. There is a very large variety of freshwater lakes, from fishing ponds to Lake Baikal in Siberia. Lakes date back as far as the ice ages. For example, Lake Baikal of Russia and some others are very ancient. Lake Baikal is the oldest, largest, and deepest freshwater lake in the world. It contains 20% of the world’s total unfrozen freshwater reserve. The world’s richest and most unusual freshwater faunas can be seen on this link (http://whc.unesco.org/en/list/754). Lake Vostok, in Antarctica, is one of the largest sub-glacial lakes in the world. On top of the lake is an icecap four kilometers thick. The ice actually insulates the water, preventing it from freezing.

Lakes and ponds consist of different life zones. Typically, four zones are distinct in ponds and lakes:

• (i) Littoral Zone : The littoral zone is found near the shore, , light reaches up to the bottom, and rooted plants grow. It is the most productive zone of a lake, because it gets abundant sunlight and it receives nutrients from land run-off. The littoral zone sustains floating plants, surfaced plants, submerged plants and phytoplankton. There are also large quantities of decomposers and some animal species, such as frogs, fish and insects.
• (ii) Limnetic zone: The open water zone, where light intensity is adequate and plankton are the dominant biota. This is the zone on the surface of the lake, extending to the depth where sunlight penetrates. Depending on the available nutrients it contains phyto- and zooplankton, and various fish species. Phytoplankton includes desmids, diatoms, and filamentous algae, are the primary producers and form the base on which the rest of life depends. Also suspended in the water column are small grazing animals, called crustaceans that feed on the phytoplankton; these animals form an important link in energy flow in the limnetic zone.
• (iii) Profundal zone: This constitutes the deep water with light very low or absent. Life in the profundal zone depends on the supply of energy and nutrients from the limnetic zone. Onlyheterotrophic organisms occur in this zone.

(iv) Benthic Zone: The bottom zone of a lake is a region of marked biological activity. Associated with the benthic community are organisms which are called periphyton or aufwuchs. Aufwuchs on stones and debris constitute a mixture of algae, fungi and bacteria. In the benthic region are found aquatic insects, molluscs (clams, snails) other invertebrates, worms and crayfish.

A view of Renuka lake located in foothills of Siwaliks in Sirmaur district in western Himalaya and a Pond ecosystem at Kurukshetra are shown in Figure 30.4. Loktak Lake is the largest natural freshwater lake in the north-eastern region of India and plays an important role in the ecological and economic security of the region (Box 30.1)

Classification of Lakes

One system for classifying lakes is based on their productivity on the basis of their relative nutrient richness. On the basis of nutrient enrichment, the lakes are of the following types:

Oligotrophic: An oligotrophic lake is one which has a relatively low productivity due to the low nutrient content in the lake. The waters of these lakes are usually quite clear due to the limited growth of algae in the lake.    These lakes are usually found in the cold regions of the world where mixing of nutrients is rare.

Mesotrophic: Lakes with an intermediate level of productivity are called mesotrophic lakes. These lakes are usually clear water with submerged aquatic plants.

Eutrophic: Lakes have high levels of biological productivity due to high concentration of nutrients, especially nitrogen and phosphorus.

Eutrophication is the process of increased productivity of a lake as it ages. Eutrophication might occur

naturally or due to human impact. Often this process is greatly accelerated by human influence and is termed cultural eutrophication.

30.5.2. Freshwater Lotic Water systems

Flowing water freshwater systems, such as rivers and streams, are watersheds for precipitation water. The water becomes land run-off and flows with the rivers and streams to the sea. The lotic water bodies have markedly different biota as compared to that of lakes and ponds. Lotic bodies, such as streams are exceptionally open systems, exchanging nutrients and energy with much larger areas (Fig.30.5). Flowing freshwater bodies usually receive their nutrients from land run-off.

The different zones of the river system up at the top representing a mountain range, is the source of the river usually the source is a mountain headwater stream. It is characterized by being shallow and cold and swiftly flowing and highly oxygenated, this is the origin of the river is the upstream location coming down from that we have all of the different streams, As we move downstream, the river begins to meander or curve across the valley. Drainage basin, also called Catchment Area, is an area from which all precipitation flows to a single stream or set of streams.

A stream of the forested area, for example receives large amounts of leaf litter, fallen wood and dissolved organic matter, from the adjacent forests. In running water, currents are major controlling factor, and lack of thermal stratification is an additional factor which differentiates it from stagnant water. Generally, two zones are distinguishable within a stretch of stream: (i) a rapid zone, where current is fast enough to keep the bottom clear off silt and other loose material, consequently the bottom is firm; (ii) a pool zone, where current is slow, water is deeper, and sand and silt are deposited on the bottom. A view of water stream from the Great Himalayan National Park (GHNP) area in the state of Himachal Pradesh is shown in Figure 30.6. In the zone of fast currents, only specialized species that become firmly attached or cling to hard bottom (e.g. caddis fly and black fly larvae) are able to survive. Among fishes, those which can swim against the strong current exist. In the zone of soft bottom, burrowing animals, rooted plants and swimming animals flourish. However, plankton occur only where water is deep and the current is gentle.

30.5.3. Freshwater Wetlands (Marshes and Swamps)

The freshwater wetland communities occur in shallow areas of freshwater bodies or such areas which are inundated by water for at least part of the year.  Soil is saturated with water continually.

They lie at the interface between the land and open waters and include a wide variety of habitats, subjected to water level changes of such large magnitude and frequency that are not seen in terrestrial or true aquatic ecosystems.

Generally, wetlands are also very open systems; and can be connected with rivers (riverine wetlands, located on low lying depressions and floodplains and lakes). There are palustrine wetlands that are located in depressions not directly connected with lakes or rivers, variously called marshes, bogs, fens, and temporary ponds.    A marsh is a type of wetland, where water covers ground for long periods of time, dominated by grasses and other herbaceous plants. The Okavango Delta in Botswana is probably the largest freshwater marsh in the world. Okavango marshes are made up of dense beds of papyrus,   water    lilies,     and    underwater    plants     such     as      bladderworts (https://www.nationalgeographic.org/encyclopedia/marsh/), Box 30.2.

A bog is a freshwater wetland of soft, spongy ground consisting mainly of partially decayed plant matter called peat. Bogs are generally found in cool, northern climates. Fens are peat-forming wetlands, receive nutrients from groundwater or mineral-rich surface water; also are major habitat for carnivorous plants.

Gopal (1987) has grouped Indian fresh water wetlands into two major types depending upon the duration of water logging.  (i) The perennial wetlands, with water logging throughout the year include habitats, such as river banks and margins of large lakes or reservoirs, and (ii) seasonal wetlands which dry up completely for varying periods of time depending upon the monsoon rainfall.        Examples are numerous, village ponds, fish ponds, paddy fields, river flood plains, etc.       Wetlands support a large variety of flora and fauna.  Planktonic and periphytic algae which are common in the wetland systems, more than 300 species of macrophytes are found in Indian wetlands.

SUMMARY

• Less than 0.3% of all freshwater on the Earth occurs in the lakes, rivers and wetlands; rivers comprise only 0.0002% of the total volume. Oceans cover about 71% of the land are very important for the preservation of all life on earth.
• The abiotic factors that limit aquatic life include Sunlight, Temperature, and Availability of nutrients. Nutrients are an important limiting factor for productivity in marine aquatic life zones.
• The main kinds of organisms in aquatic ecosystems are plankton, Nekton, Benthos.
• Flowing water freshwater systems also called lotic systems , such as rivers and streams.

References

• Dodds W.K. Perkin, Joshuah S, and Gerken J.E. 2013.Human impact on freshwater ecosystem services: a global perspective. Environ Sci Techno l47: 9061–9068
• Gopal, B. (ed.) 1987. Ecology and Management of Aquatic Vegetation of the Indian Subcontinent. W. Junk bv. The Hague
• Gross, M. G., and E. Gross. 1995. Oceanography: A view of Earth, 7th ed. Englewood Cliffs, NJ: Prentice-Hall.
• Herring,P.(2001).The Biology of the Deep Ocean. Oxford University Press Oxford,UK:
• SCBD (Secretariat of the Convention on Biological Diversity) 2012. Marine Biodiversity—One Ocean, Many Worlds of Life. Montreal. pp. 77.
• Shiklomanov, I. A. 2000. Appraisal and assessment of world water resources. Water International 25: 11–32.
• Singh JS, Singh SP, Gupta SR (2015). Ecology, Environmental Science and Conservation. S. Chand, New Delhi, India.
• Smith, R.L. 1996. Ecology and Field Biology. Fifth Edition. Harper Collins College Publishers Inc., New York.
• Spalding MD, et al. (2007).Marine ecoregions of the world: A bioregionalization of coast and shelf areas. BioScience 57: 573–583.
• Trujillo , A.P. and Thurman H. V. 2011 . Essentials of oceanography. Pearson Education, Inc.
• Trujillo , A.P. and Thurman H. V. 2014 . Essentials of oceanography. Pearson Education, Inc.
• Venkataraman, K. 2011. Coral reefs in India. DOI: 10.1007/978-90-481-2639-2_64
• Venkataraman, K., Raghunathan, C., Raghuraman, R. and Sreeraj, C.R. 2012. Marine Biodiversity. Published by the Director, Zoological Survey of India, Kolkata. pp. 1-164