ENVIS Technical Report: 25

Ecosystem is defined as ‘the complex of a community of organisms and its environmental functioning as an ecological unit’. It is a dynamic system where the biotic and abiotic components are constantly acting and reacting upon each other bringing forth structural and functional changes. An aquatic ecosystem is a group of interacting organisms dependent on one another and their water environment for nutrients (e.g., nitrogen and phosphorus) and shelter. An aquatic ecosystem is an ecosystem that is based in water, whether it is a pond, lake, river or an ocean. It involves living aquatic organisms (e.g.: fish, planktons, annelids, etc.), which constitute as the biotic factors and their relationship with their environment, which collectively can be referred to as the abiotic factor. Composing more than 70% of the earth’s surface, aquatic ecosystems are not only the dominant features of earth but are also very diverse in species and complexity of interaction among their physical, chemical and biological components.

Types of aquatic ecosystems

Aquatic ecosystems are broadly categorised based on the differences in their salt content as:

Freshwater ecosystems
Marine ecosystems (includes the ocean and the sea) and
Estuarine ecosystems (region where freshwater from a river mixes with the sea)

Familiar examples of Freshwater ecosystems include lakes, ponds, rivers, streams, and prairie potholes. They also include areas such as floodplains and wetlands, which are flooded with water for all or only parts of the year. Freshwater ecosystems are characterised as

Lotic (running waters) – streams, rivers, etc.
Lentic (still waters) – wetlands, ponds, tanks, lakes, etc.

The lotic ecosystems comprises of springs, rivulets, creeks, brooks, rivers, etc. which tread their course from being narrow, shallow and relatively rapid to increasingly broad, deep and slow moving. Waterfalls are common features of lotic ecosystems. Lentic ecosystems generally include ponds, lakes, bogs, swamps, reservoirs, pools, etc. and they vary considerably in physical, chemical and biological characteristics.

Ponds are smaller bodies of still water located in natural hollows, such as limestone sinks, or that result from the building of dams, either by humans or beavers. Ponds are found in most regions and may exist either seasonally or persist from year to year.

Rivers and streams are bodies of fresh, flowing water. The water runs permanently or seasonally within a natural channel into another body of water such as a lake, sea, or ocean. Rivers and streams are generally more oxygenated than lakes or ponds, and they tend to contain organisms that are adapted to the swiftly moving waters ⁵.

A lake is a sizable waterbody surrounded by land and fed by rivers, springs, or local precipitation. A lake's structure has a significant impact on its biological, chemical, and physical features. Lakes can be classified on the basis of a variety of features, including their formation and their chemical or biological condition, as oligotrophic and eutrophic. Oligotrophic lakes are characterised by relatively low productivity and are dominated by cold-water bottom fishes such as lake trout. Eutrophic lakes, which are relatively shallower, are more productive and are dominated by warm-water fishes such as bass. Natural processes of lake formation most commonly include glacial, volcanic, and tectonic forces while human constructed lakes are created by reservoirs or excavation of basins.

Wetlands are habitats that are partially submerged by water and include habitats like marshes, swamps, ponds, etc. they also include lakes, reservoirs and ponds. They function as ecotones, transitions between different habitats and have characteristics of both aquatic and terrestrial ecosystems. These habitats support diverse flora and fauna and are highly productive ecosystems akin to the tropical rainforest in terrestrial ecosystems (Ramachandra T.V. Ahalya N., and Rajasekara Murthy C, 2005 ⁶)

Watershed – Linking aquatic and terrestrial ecosystems

Aquatic ecosystems are not simply isolated bodies or conduits but are closely connected to terrestrial environments. Further, aquatic ecosystems are connected to each other and provide essential migration routes for species. Aquatic ecosystems require sediment loads, chemical and nutrient inputs from the adjoining terrestrial ecosystems for sustenance. Even isolated lakes are linked to the land and water around them through the flow of freshwater. Many of the problems faced by freshwater ecosystems come from outside the lakes, rivers, or wetlands themselves. Watershed is all the land and water area, which drains towards a river or a lake, river or a pond. A watershed is a catchment basin that is bound by topographic features, such as ridge tops and performs primary functions of the ecosystem (Ramachandra T.V. Ahalya N., and Rajasekara Murthy C, 2005 ⁶). Thus, the watershed can constitute slopes, agricultural lands, forests, streams, waterbodies, buildings, etc. People and animals are also a part of the watershed community and all depend on the watershed and they in turn influence what happens there. Accordingly, what happens in a small watershed also affects the larger watershed. Soil, water and vegetation are the most vital natural resources for the survival of man and animals. To obtain the maximum and optimum production from all the resources, the three resources need to be managed efficiently. They need to be managed effectively, collectively and simultaneously, and all these can be conveniently and efficiently managed in a watershed.

Freshwater ecology

Aquatic ecosystems usually contain a wide variety of life forms including bacteria, fungi, and protozoan; bottom-dwelling organisms such as insect larvae, snails, and worms; free-floating microscopic plants and animals known as plankton; large plants such as, grasses and reeds; and also fish, amphibians, reptiles, and birds. Viruses are also a significant part of the microbial ecology in natural waters and have recently been shown to play an important role in the nutrient and energy cycles. Plants, animals and microbes interact with each other and their environment bringing about changes in the water quality with the performance of ecological services, such as decomposition and nutrient cycling.

The assemblages of these organisms vary from one ecosystem to another because the habitat conditions unique to each type of ecosystem tend to affect species distributions. Freshwater ecosystems like rivers are relatively oxygen-rich and fast flowing compared to lakes. The species adapted to these particular river conditions are rare or absent in the still waters of lakes and ponds. Organisms capable of adhering to exposed surfaces are found in the upper reaches of streams. Such adhering organisms are termed periphyton. The periphyton includes attached clumped and filamentous green and blue green algae and various sessile invertebrates including larvae of insects like blackflies and other midges, mayfly and stonefly nymphs and planarians. Farther downstream floating and emergent vegetation may be found along with sessile invertebrates and those that burrow in the softer substrate, such as clams and burrowing mayfly nymphs. Chemically, the upper reaches of lotic environments are rich in oxygen; as the water moves downstream and becomes sluggish, the oxygen level tends to drop. Due to the continual addition of nutrients and detritus en route, nutrient levels tend to be higher downstream. In small streams in which producers are limited or absent, the major source of nutrients is from external ecosystems. Such materials are referred to as being allocthanous.

Lentic ecosystems (still waters) can be considered to have three zones – littoral, limnetic and benthic (figure 1.3). The littoral zone is the near shore area where sunlight penetrates all the way to the sediment and extends from the shoreline to the innermost rooted plants, successively passing form the rooted species with floating leaves, such as water lilies and deeper water to various submerged but rooted species. This zone is populated by frogs, snakes, snails, clams, and considerable variety of adult and larval insects. The limnetic (pelagic) zone is the open water down to the depth of light penetrations: in shallow lentic environments the light may penetrate to the bottom. This zone contains phytoplankton (diatoms, green and blue green algae, etc.), zooplankton (protozoa, microcrustaceans, arthropods, etc.). It is also inhabited by a variety of larger swimming organisms including fish, amphibians and larger insects. The benthic zone (the bottom of the lake) is covered by fine layers of mud consisting mostly of decomposers. Euphotic zone of the lake is the layer from the surface to the depth where light levels become too low for photosynthesis. In the littoral zone, there is enough light for rooted plants to grow, but beyond this zone, there are no rooted plants as the water is too deep for light to reach them. The deepest part of the open water forms the profoundal zone, but this is relevant only in extremely deep lakes. The profoundal zone occurs below the limnetic zone and this zone may constitute the largest water volume of a lake. This zone is beyond the depth of effective light penetration.

Figure 1.3: Sketch showing major zones of lake

The major food source in the profoundal zone comes from a detritus rain form the limnetic and thus the photosynthetic zone. A pond or a lake ecosystem is a dynamic ecosystem since the boundaries are limited. The water is retained in a trough and lined by terrestrial region. The entry nutrients, sediments through the surface runoffs enter and remain in the system causing fluctuations in the physico-chemical characteristics of these ecosystems. The organisms are subjected to pressure, changes in quality of water and are adapted to such changes. The primary producers like phytoplankton in the surface water perform photosynthesis and through the food chain and food webs transfer energy to higher trophic levels. The daily alteration of light and darkness forms a rhythm of activities of many aquatic organisms. The plants require light for the photosynthesis to prepare food from natural substance. The light penetration depends much on the turbidity of water caused by suspended particulate matter. The wind generates water currents, which in turn helps in nutrient movement and diffusion of gases. The distribution of oxygen in the dissolved form is essential to all aquatic life. Some organisms are lung breathers while others are gill breathers. The oxygen supply of lakes is reduced in various ways most significantly through respiration of organisms and decomposition of organic matter. In addition high temperatures also prevent the dissolution of atmospheric oxygen.

Freshwater organisms and food web

Freshwater habitats contain representatives of many groups of organism on earth. Archeae and bacteria are difficult to distinguish unless they can be brought into culture and metabolic characteristics can be used as taxonomic characteristics. Algae are the primary autotrophs in many aquatic ecosystems and are well represented in freshwaters. Protozoa are common in all freshwater habitats and can often be identified if a good microscope is available. All major phyla of invertebrates, with the exception of Echinodermata, (e.g. star fish) have some freshwater species. Invertebrates are the most fascinating and very important in the ecology of most aquatic habitats. Identification by non-taxonomists can be difficult at the species level, but numerous keys are available for coarser taxonomic resolution. Identification of Vertebrates is generally easier, because fewer and better-studied organisms are represented and many of these are assigned names. As with vertebrates many of the plants in aquatic systems have been well characterised. Aquatic plants are moderately diverse. Identification of the more obscure Mosses and liverworts is more difficult though.

Among the 'lower' (non-vascular) plants, the mosses and liverworts are virtually all terrestrial, although flourishing only in moist environments; but the larger algae are primarily aquatic. The larger algae comprise some 5,000 species in three major groups (the green, brown and red algae), the great majority of which are marine or brackish water forms ('seaweeds'). The green algae Chlorophyta includes one order of around 80 species (Ulotrichales) that is mainly freshwater. However, one major group sometimes associated with the green algae - the stonewort (Charophyta) – belongs almost entirely to freshwater. The stonewort includes 440 species, most of which are endemic at continental level or below; they tend to be very sensitive to nutrient enrichment and have declined in many areas.

Animal species are considerably more diverse and numerous in inland waters than plants. Most of the major groups include terrestrial or marine species as well as freshwater forms. Apart from fishes, important groups with inland water species include crustacea (crabs, crayfishes and many smaller organisms), mollusca (including mussels and snails), insects (including stoneflies Plecoptera, caddisflies Trichoptera, mayflies Ephemoptera), sponges, flatworms, polychaete worms, oligochaete worms, numerous parasitic species in various groups, and numerous microscopic forms.

Figure 1.4: A simple aquatic food chain

The sun provides the ultimate source of energy in all natural systems. If we look at a simple food chain in a freshwater ecosystem (figure 1.4), it may comprise of the primary producers phytoplankton, wherein they fix sunlight and convert the light energy to chemical energy. They form the base of the food chain, and are eaten by the zooplankton and planktivorous fishes, which make up the primary and secondary consumers in the aquatic food chain. Other small fishes in turn eat the zooplankton. Other larger fishes, birds, etc form the upper trophic levels. When plants or animals die, the chemicals that make up their bodies are broken down and released back into the system as nutrients by the decomposers. The major decomposers are bacteria and fungi, which make up the last trophic level in the food chain.

Phytoplankton comprises of aquatic microscopic plants suspended in water - many species of prokaryotic (blue green alga) and eukaryotic algae. Zooplankton comprises of small (often microscopic) aquatic animals, and non-photosynthetic protists suspended or weakly swimming in water, which feed on the phytoplankton. E.g., protozoa, bacteria, small crustaceans, rotifers, micro invertebrates and fish larvae, etc. Some species from most major groups of aquatic animals can eat phytoplankton, periphyton, macrophytes, detritus or other animals. Omnivory is common in freshwater invertebrates with many organisms representing various trophic levels.

Table 1. 2: The major groups of organisms in freshwater (UNEP)

Organisms	General features	Significance in freshwaters
Viruses	Microscopic; can reproduce only within the cells of other organisms, but can disperse and persist without host.	Causes diseases in many aquatic organisms, and associated with water-borne diseases in humans (eg. hepatitis).
Bacteria	Microscopic; can be numerically very abundant, eg. 1,000,000 per cm3, but less so than in soils. Recycles organic and inorganic substances. Mostly derives energy from inorganic chemical sources, or from organic materials.	Responsible for the decay of dead material. Present on all submerged detritus where a food source for aquatic invertebrates. Many cause diseases in aquatic organisms and humans.
Fungi	Microscopic. Recycles organic substances; responsible for decay of dead material; tends to follow bacteria in the decomposition processes. Able to break down cellulose in plant cell walls and chitinous insect exoskeletons.	Present on all submerged detritus where it is a food source for aquatic invertebrates. Some cause diseases in aquatic organisms and humans.
Algae	Microscopic and macroscopic; includes a variety of unicellular and colonial photosynthetic organisms. All lack leaves and vascular tissues of higher plants. Green Algae (Chlorophyta) and Red Algae (Rhodophyta) include freshwater species; Stoneworts (Charophyta) mostly freshwater.	Responsible for most primary production (growth in biomass) in most aquatic ecosystems. Free-floating phytoplankton main producers in lakes and slow reaches of rivers; attached forms important in shallow parts of lakes and streams.
Plants	Photosynthetic organisms; mostly higher plants that possess leaves and vascular tissues. Some free-floating surface species; mostly rooted forms are restricted to water margins.	Provide a substrate for other organisms and food for many. Trees are ecologically important in providing shade and organic debris (leaves, fruit), structural elements (fallen trunks and branches) that enhance vertebrate diversity, in promoting bank stabilisation, and in restricting or modulating floodwaters.
Invertebrates: protozoan	Microscopic mobile single-celled organisms. Tend to be widely distributed through passive dispersal of resting stages. Attached and free-living forms; many are filter feeders.	Found in virtually all freshwater habitats. Most abundant in waters rich in organic matter, bacteria or algae. Feed on detritus, or consume other microscopic organisms; many are parasitic on algae, invertebrates or vertebrates.
Invertebrates: rotifers	Near-microscopic organisms; widely distributed; mostly attached filter feeders, some predatory forms.	Important in plankton communities in lakes and may dominate animal plankton in rivers.
Invertebrates: myxozoans	Microscopic organisms with complex life cycles, some with macroscopic cysts. Formerly classified with protozoa but are metazoa.	Important parasites in or on fishes.
Invertebrates: flatworms	A large group of worm- or ribbon like flatworms; includes free-living benthic (Turbellaria), and parasitic forms (Trematoda, Cestoda).	Turbellaria include mobile bottom-living predatory flatworms. The Trematodes includes various flukes, such as the tropical schistosome that causes bilharzia; Cestodes are tapeworms: both these groups are important parasites of fishes and other vertebrates including humans. Molluscs are often intermediate hosts.
Invertebrates: nematodes	Generally microscopic or near-microscopic roundworms.	May be parasitic, herbivorous or predatory. Typically inhabit bottom sediments. Some parasitic forms can reach considerable size. Poorly known; may be more diverse than recognised.
Invertebrates: annelid worms	Two main groups in freshwaters; oligochaetes and leeches.	Oligochaetes are bottom-living worms that graze on sediments; leeches are mainly parasitic on vertebrate animals, some are predatory.
Invertebrates: molluscs	Two main groups in freshwaters; Bivalvia (mussels, etc) and Gastropoda (snails, etc). Very rich in species; tend to form local endemic species.	Snails are mobile grazers or predators; bivalves are attached bottom-living filter-feeders. Both groups have specialised profusely in certain freshwater systems. The larvae of many bivalves are parasitic on fishes. Due to its feeding mode, bivalves can help maintain water quality but tend to be susceptible to pollution.
Invertebrates: crustaceans	A very large class of animals with a jointed exoskeleton often hardened with calcium carbonate.	Include larger bottom-living species such as shrimps, crayfish and crabs of lake margins, streams, alluvial forests and estuaries. Also, larger plankton: filter-feeding Cladocera and filter-feeding or predatory Copepoda. Many isopods and copepods are important fish parasites.
Invertebrates: insects	By far the largest class of organisms known. Jointed exoskeleton. The great majority of insects are terrestrial, because they breathe air.	In rivers and streams, grazing and predatory aquatic insects (especially larval stages of flying adults) dominate intermediate levels in food webs (between the microscopic producers, mainly algae, and fishes). Also important in lake communities. Fly larvae are numerically dominant in some situations (eg. in Arctic streams or low-oxygen lake beds), and are vectors of human diseases (eg. malaria, river blindness).
Vertebrates: fishes	More than half of all vertebrate species are fishes. These are comprised of four main groups: hagfishes (marine), lampreys (freshwater or ascend rivers to spawn), sharks and rays (almost entirely marine), and ray-finned 'typical' fishes (>8,500 species in freshwaters, or 40% of all fishes).	Fishes are the dominant organisms in terms of biomass, feeding ecology and significance to humans, in virtually all aquatic habitats including freshwaters. Certain water systems, particularly in the tropics, there is a good number of species. Many species are restricted to a few lakes or river basins. They are the basis of important fisheries in inland waters in tropical and temperate zones.
Vertebrates: amphibians	Frogs, toads, newts, salamanders, caecilians. Require freshwater habitats.	Larvae of most species need water for development. Some frogs, salamanders and caecilians are entirely aquatic; generally in streams, small rivers and pools. Larvae are typically herbivorous grazers, adults are predatory.
Vertebrates: reptiles	Turtles, crocodiles, lizards, snakes. All crocodilians and many turtles inhabit freshwaters but nest on land. Many lizards and snakes occur along water margins; a few snakes are highly aquatic.	Because of their large size, crocodiles can play an important role in aquatic systems, by nutrient enrichment and shaping habitat structure. They, as well as freshwater turtles and snakes are all predators or scavengers.
Vertebrates: birds	Many birds, including waders and herons, are closely associated with wetlands and water margins. Relatively few, including divers, grebes and ducks, are restricted to river and lake systems.	Top predators. Wetlands are often key feeding and staging areas for migratory species. Likely to assist passive dispersal of small aquatic organisms.
Vertebrates: mammals	Relatively few groups are strictly aquatic (eg. River Dolphins, platypus), several species are largely aquatic but emerge onto water margins (eg. otters, desmans, otter shrews, water voles, water oppossum, hippopotamus).	Top predators and grazers. Large species widely impacted by habitat modification and hunting. Through damming activities, beavers play an important role in shaping and creating aquatic habitats.

Causes and effects of pollution on aquatic ecosystems

When pollutants enter lakes, streams, rivers, oceans, and other waterbodies, they get dissolved or lie suspended in water or get deposited on the bed. The system is able to withstand the pollutants up to a certain threshold, beyond which the quality of the water deteriorates, affecting aquatic ecosystems. The most common problems associated with various pollutants are discussed below.

Oxygen demanding wastes are substances that oxidise in the receiving body of water, reducing the amount of dissolved oxygen (DO) available. As DO drops, fish and other aquatic life are threatened and, in the extreme case, get killed. In addition to the fall in DO levels, undesirable odours, tastes, and colours reduce the acceptability of the water as a domestic supply and its attractiveness for recreational purposes. Oxygen demanding wastes are usually biodegradable organic substances contained in municipal wastewaters or in effluents from industries such as food processing and paper production.
Contaminated water is responsible for the spread of many contagious diseases. Pathogens associated with water include bacteria responsible for cholera, dysentery, typhoid, etc., viruses cause hepatitis and poliomyelitis, protozoa are responsible for amoebic dysentery and giardiasis, and helminthes or parasitic worms cause diseases like schistosomiasis, etc.
Nutrients, when present in concentrations that can stimulate the growth of algae can be considered pollutants. The discharge of waste from industries, agriculture, and urban communities into waterbodies generally stretches the biological capacities of aquatic systems. Chemical run-off from fields also adds nutrients to water. Excess nutrients cause the waterbody to become choked with organic substances and organisms. When organic matter exceeds the capacity of the microorganisms in water that break down and recycle the organic matter, it encourages rapid growth, or blooms of algae. When they die, the remains of the algae add to the organic wastes already in the water; eventually, the water becomes deficient in oxygen. Anaerobic organisms (those that do not require oxygen to live) then attack the organic wastes, releasing gases such as methane and hydrogen sulphide, which are harmful to the oxygen requiring (aerobic) forms of life. The result is a foul-smelling, waste-filled body of water. This artificial supplementation of nutrients, and consequent abnormal increase in the growth of water plants is often referred to as eutrophication. This is a growing problem in freshwater lakes all over India. Eutrophication can produce problems such as bad tastes and odours as well as green scum algae. Also, the growth of rooted plants increases, which decreases the amount of oxygen in the deepest waters of the lake. It also leads to the death of all forms of life in the waterbodies.
Organic inputs from the food industry, i.e., carbohydrates, lipids, and proteins, all impact lakes and rivers by increasing the biological oxygen demand. The worst-case scenario is the total loss of oxygen from the water as a result of microbial activity. Lipids create the greatest oxygen demand but carbohydrates (more easily biodegradable) also result in unsightly ‘sewage fungus’. Protein waste can be degraded to produce ammonia and sulphide, both of which produce toxicity problems (Gwynfryn Jones J, 2001 ¹⁰).
Acid precipitation is caused mainly by humans burning fossil fuels, which leads to increased sulphuric, and nitric acid in the atmosphere. Acidification of aquatic ecosystems impacts all aquatic organisms. Acid rain has major effects on biological systems ranging from altered microbial activity to the ability of fish to survive and reproduce. (Table 1.3).

Table 1.3: Influences of decreasing pH on several groups of aquatic organisms (Modified from Jeffries M., and Mills D, 1990 ¹¹)

Organism of process	Approximate pH value
Most mayflies disappear	6.5
Phytoplankton species decline/green filamentous periphyton dominate	6
Most molluscs disappear	5.5 – 6
Waterfowl breeding declines	5.5
Bacterial decomposition slows/fungal decomposition predominates	5
Salmonoid reproduction fails/aluminium toxicity increases	5
Most amphibians disappear	5
Caddisflies, stoneflies, and megaloptera disappear	4.5 – 5
Beetles, bugs, dragonflies, and damselflies disappear	4.5
Most adult fish harmed	4.5

Metals and other inorganic pollutants act as toxic pollutants in aquatic ecosystems. Metals can bioaccumulate in many organisms and can be bioconcentrated in trophic food chains. Bioconcentration has led to problems such as excessive lead and mercury contamination in fish. Atmospheric deposition and industrial waste releases, particularly mining are common sources of metallic contamination. Such mining activities have had extensive negative impacts in aquatic habitats. The inorganic inputs, particularly of phosphorus, stimulate undesirable algal growths, some of which may produce particularly dangerous toxins. Arsenic can cause problems because it can be present in high concentrations naturally or as runoff form industrial uses. Historically arsenic was also used as pesticide and subsequently contaminated aquatic ecosystems. In a particularly terrible case, thousands of drinking water wells in West Bengal, India, are contaminated by naturally occurring arsenic. Radioactive compounds can be contaminants in water. They usually occur naturally in water. The primary contaminants are isotopes of radium, radon, and uranium. The effects of natural radioactive materials on aquatic habitats are difficult to gauge.

More than 10,000 organic pollutants have been created and used by man. Several hundred new chemicals are created each year and discharged by humans into the aquatic habitats, including pesticides, oil, and materials in urban runoff. Only a few of them have been tested for toxicity. In some cases microbes can break down these compounds through bioremediation in a given time. The effects of unregulated release of pollutants into large ecosystems are exemplified by the experiences in the Great Lakes of North America. Worldwide, where, about 2.3 million metric tons of pesticides are used yearly. (Walter K. Dodds, 2002 ¹²).

Petroleum products are another source of aquatic contamination. Urban runoff is a significant source of oil contamination. Chlorinated hydrocarbons such as poly-chlorinated biphenyl (PCBs) have carcinogenic properties. In addition to this many sewage treatment plants treat their final effluents with chlorine to kill all pathogens and this forms chlorinated hydrocarbons. Bioremediation is one possible way of cleaning up the spills of organic materials. (Walter K. Dodds, 2002 ¹²).

Turbidity and suspended solids are natural parts of all freshwater environments. Some are naturally highly turbid but human activities have increased the levels of suspended solids in many habitats. Agricultural and urban runoff, watershed disturbance such as logging, construction of roads, etc., removal of riparian vegetation, alteration of hydrodynamic regimes can all lead to anthropogenically attributed increment in the total suspended solids. Sediments can have different biological and physical effects depending on the type of suspended solids. High values of suspended solids can lower the primary productivity of systems by covering the algae and macrophytes, at times leading to almost their complete removal (Walter K. Dodds, 2002 ¹²)

Thermal pollution can cause shifts in the community structure of aquatic organisms. This may allow for the establishment of exotic species and local extinction of native species. As water temperature increases, it makes it more difficult for aquatic life to get sufficient oxygen to meet its needs.

Aquatic systems respond on a much shorter time scale than their terrestrial counterparts. For example, species invasions take 10 to 1000 years in terrestrial systems but only a few weeks to a few months in aquatic systems. Similarly, population changes can take 10 to millions of years on land but can occur within a few months to a few years in water. Therefore, any pressure placed on a freshwater system can result in a very rapid and deleterious response. The driving forces involved (some of which are under man’s control) can be divided into the physical and the chemical, but the response is, almost entirely, biological (Gwynfryn Jones J, 2001 ¹⁰).

Human encroachment on aquatic ecosystems is increasing at an unprecedented rate. The impacts of human pollution and habitat alteration are most evident and of greatest concern at the microbial level, where a bulk of the production and nutrient cycling takes place. Aquatic ecosystems are additionally affected by natural perturbations, including droughts, storms, and floods, where its frequency and extent may be increasing.

More than 70% of the world’s human population live in watersheds that drain to the coast (Vitousek P.M et al, 1997 ¹³ ). Further population growth will be centred in these regions, exerting unprecedented pressure on riverine, estuarine and coastal habitats receiving human pollutants (Vitousek P.M et al, 1997¹³, Peierls B.L et al, 1991 ¹⁴). Multiple negative ecological impacts on these fragile habitats (i.e., loss of biodiversity, increasing frequencies of harmful algal blooms, hypoxia, disease and declines in fisheries) have been documented (Jørgensen B.B and Richardson K. 1996 ¹⁵, Nixon S.W, 1995 ¹⁶, Paerl H.W, 1997 ¹⁷, Conley D.J, 2000 ¹⁸). Most evident are water quality and habitat changes attributable to nutrient over-enrichment, leading to excessive primary production or eutrophication (Jørgensen, B.B and Richardson, K. 1996, Nixon S.W, 1995, Paerl H.W, 1997 ^{15, 16, 17}). Eutrophication has caused significant changes in coastal nutrient (C, N, P, Si) cycling, water quality, biodiversity, fisheries, and the overall ecosystem health (Jørgensen, B.B and Richardson, K. 1996, Nixon S.W, 1995, Paerl H.W, 1997, Conley D.J, 2000, Diaz, R.J. and Solow, A., 1999 ^{15, 16, 17, 18, 19}).

Natural perturbations such as droughts, storms and floods additionally impact aquatic ecosystems. Like human disturbances, these events are predicted to increase in the foreseeable future (Goldenberg S.B et al, 2001 ²⁰). Among aquatic biota, microorganisms are generally highly sensitive to and profoundly affected by environmental perturbations. Microbes comprise the majority of aquatic biomass and are responsible for the bulk of productivity and nutrient cycling in aquatic systems. They have fast growth rates, and respond to low levels of pollutants as well as other physical, chemical, and biotic environmental changes. From detection and effect perspectives, they provide sensitive, meaningful, and quantifiable indications of ecological change.