3. LIFE IN A RIVER

ECOSYSTEM

Each form of surface water (stream, river, lake, lagoon, sea, ocean) represents a different ecosystem. The same is true for a stream or a river, in which living conditions are changing through the flow of a river. An ecosystem is composed of a system of plants and animals (biocenosis) and its living environment (biotope). In a specific area, plants and animals live in a dynamic equilibrium among themselves and with the environment. All are interconnected in a complex food chain or food web. Ecosystems are not coincidental communities of plants and animals in a specific environment. They have developed slowly through long evolutionary processes of conformation to each other. They are also not fixed and unchangeable. They are constantly evolving and adapting to new natural and man-caused conditions.

An ecosystem is a total of plant and animal communities living in dynamic equilibrium within an area or habitat, connected with each other by means of complex (food) chain relations. In a river, water quality and flow dynamics play an essential role in the development and maintenance of an ecosystem. There is a physical, chemical, and biological continuum from the source to the mouth of a river.

BIOTIC AND ABIOTIC ELEMENTS OF LIVING CONDITIONS

The living conditions in a river change through the flow of a river from its source to its mouth at the sea and display a vast variety of habitats and biotopes. The plant and animal communities living in a river or on its banks are also changing. The most critical element that determines living conditions in a river is its water flow, which is turbulent, meaning that the flow of water is irregular and continuously mixing. Its velocity decreases from the water surface towards the bottom and is dependent on the quantity of water, the width of the watercourse, as well as the slope, depth, and shape of the riverbed. At the bottom next to the sediment layer, the flow velocity is only 1/10th of that at the water surface. This enables life at the bottom of a river. Near the river bank, the water flow velocity is also slower (Figure 1).

A high flow velocity in rivers is considered 2–3 m/s; in mountain streams, it is approximately 0.5 to 1 m/s. The Danube River is 2857 km long, and its mean multi-annual discharge is roughly 6500 m³/s. It is the second-largest river in Europe (the first being the Volga, which is 3740 km long and whose average discharge is 8500 m³/s). The mean velocity in the Serbian part of the Danube River is between 0.66 to 0.89 m/s (Babic-Mladenovic et al. 2010; Babic-Mladenovic and Kolarov, 2010). The flow velocity in the Djerdap gorge was 5 m/s before the construction of the Iron Gate I hydropower dam; afterwards, it was reduced in the Djerdap reservoir to 0.3 m/s. The flow velocity affects the shape of a riverbed and the structure of sediments at different parts of the bottom of a river. The differences in flow velocity result in different sediment structure that can be as small as mud, sand, pebbles and gravel or as large as rocks and boulders. Different sediments represent different living conditions (habitats), and in this way, the flow velocity defines the structure of plant and animal communities. The metabolisms of these organisms lead to the effect of self-purification in rivers (see Chapters 6 and 7).

The upper section of a river (crenon)

A relatively small number of organisms live in the upper section (spring zone and headwaters) of a river (crenon). The water current is relatively strong and fast, and it is moving gravel along the river. The water is clean, rich in oxygen, but poor in nutrients. The river bottom is an especially appropriate living environment because the flow velocity is slower there. There are good living conditions for small invertebrates that live in the sediment (hyporheic zone) and on it. They form a benthic community of animals. Their most important role is that they decompose falling leaves (e.g., stoneflies). These small animals have adapted to the conditions of the water current in different ways. The larvae of some mayflies have, for example, very flat bodies. Some caddis fly larvae weigh their bodies down with small pebbles, while others have small hooks or suckers on their abdomen so that they can attach themselves to rocks. Due to the harsh conditions and steep slopes, there are no fish in this zone of the rivers.

The middle section of a river (rhithron)

In its middle section (rhithron), where the slopes are decreasing, and the water current slows down, the dominant community is periphyton or organisms that cover a hard substrate. Periphyton inhabits gravel on places where organic substances from the river settle down. Periphyton is composed of bacteria, fungi, and algae. Algae produce oxygen and are food for other organisms. Periphyton is also a suitable living space for benthos. For this part of the river, those organisms that collect organic detritus or scrape periphyton from pebbles are characteristic. These include snails, the larvae of some beetles, mayflies, and other small invertebrates (Figure 2). At smaller flow velocities, the periphyton is thicker; at faster currents, the periphyton is thinner. Periphyton plays an important role in self-purification processes in a river. The predominant fish species in fast-running and clear waters is trout, in the transition zone between lower rhithron and potamon, grayling and nase.

The lowest section of a river (potamon)

In the lowest section (potamon), the water current becomes very slow, and the river meanders broadly and deposits fine sediments. The river bottom is muddy, and the living conditions become similar to those in lakes. Riverbanks are filled with a rich variety of vegetation that often turns into floodplains and wetlands. Plankton appears in the water. The life from the river bottom moves into the whole water space. In the water, floating and rooted plants can be found. Among small invertebrates, we can find those that float or hover in water and collect organic debris by filtering water. Some representatives are Oligochaeta, Mollusca, Odonata, and Diptera. The dominant fish species are barbel and common bream (Cyprinids), and pike. This part of the river has the richest biodiversity, and it has an extremely high density of organisms. Such conditions are also characteristic in reservoirs when a river is damned.

The hyporheic zone of a river

In addition to the riverbed, the hyporheic river zone is also a very important part of a river. A river hyporheic zone is a water-saturated area under and along the sides of a river’s bottom – a subsurface biotope (See Fig. 3). It is connected with the groundwater. In a hyporheic habitat, i.e., the interstitial spaces between substrate particles (usually in gravel beds), where temperature and pH are changing very little, abundant life can be found. The most critical element in this zone is oxygen.

Along the river course, there are zones of exfiltration (flow into the groundwater) and infiltration (flow from the groundwater) The hyporheic zone provides an ecotone between stream water and groundwater environments, which is excellent habitat for microbes as well as for aquatic macroinvertebrates and, in some cases, the biomass of the hyporheic zone could exceed the biomass of the river channel. This ecotone supports the ecosystem’s metabolism and influences river biota and chemistry.

CYCLING OF ORGANIC MATTER IN ECOSYSTEM (BIOLOGICAL CYCLE)

The basic characteristic of the biological cycle is changing inanimate (inorganic) matter into living (organic) matter. In this process, we can roughly set apart three groups of organisms, which are connected in a food network throughout a river. These are producers, consumers, and decomposers of organic matter. Some experts also call the biological cycle a self-purification system because whenever organic waste substances are introduced into a biological cycle, water quality is gradually improved by these organisms.

Producers produce living nature from inanimate matter. These are plants that take nutrients, i.e., mineral (inorganic) material from their living environment and with use of light in the process of photosynthesis (CO₂-assimilation) produce oxygen and organic carbon – they integrate inorganic matter into their cells, tissues, and bodies. Through this process, new organic substances are formed as well as all plants. In aquatic ecosystems, producers are primarily blue-green algae, diatoms, and green algae, as well as aquatic plants (macrophytes). Because they produce organic material, they are called “autotrophs”.

Consumers use plant organic matter as their food because they cannot produce it by themselves. The energy that is stored in plants passes through consumers in a food chain. Herbivorous animals are animals that eat plants. They are food for carnivorous animals, which are animals that eat other animals (predators). Omnivorous animals are animals that eat plants and other animals, and scavengers feed on dead animals. The organic matter that consumers ingest is used for their growth or energy to sustain themselves. In this process, they need oxygen, which they get from water by breathing with gills. They cannot produce organic matter by themselves, so they use it from their environment. Therefore, they are called “heterotrophs”.

Decomposers transform dead organic material into inorganic material (detritus). Dead organic material is the dead bodies of plants and animals, their secretions, and the remains of their food. This process is called mineralization. The most important decomposers in water ecosystems are bacteria and fungi and some types of insects.

The biological cycle is the basis of all the self-purification abilities of a water system, because the organic pollution that human beings discharge with wastewater into a river is integrated into a biological cycle. Along the river course, this is evidenced by a typical sequence of sensitive taxa (bioindicators, see Saprobic System, chapter 5, water quality; and chapter 6, self-purification processes, Figure 1).

Rivers are home to numerous non-pathogenic bacteria called decomposers, which consume the dead organic matter present in water as a food source. During decomposition, these microorganisms require oxygen for their respiration, which is present in water as dissolved oxygen. In this process, organic matter is broken down and reduced to its original elements. Decomposition releases the chemicals that are critical for life, such as phosphorus, nitrogen and carbon. Decomposition of organic carbon yields carbon dioxide which is released into the atmosphere. Carbon dioxide will be used by plants, to build a new life. Thus, bacteria convert organic waste into recycled materials which will be used by other organisms and maintain the river health. Besides bacteria, fungi and some types of insects represent significant decomposers. Fungi, in particular aquatic hyphomycetes, play an essential role in the breakdown of organic matter in rivers, because their extracellular enzymes break down leaf tissue, which makes it more palatable to invertebrates.

The ability of a river to clean itself has served as an inspiration to establish wastewater treatment plants (WWTPs). “Sewage-eating” (decomposing) microbes are used in activated sludge in a special basin of WWTPs (See Chapter 6, Self-purification processes in rivers, WWTPs)) for cleaning wastewater before it is discharged into a river. In this type of sewage treatment, microbes use contaminants such as oil (petroleum) and organic matter as their food source, degrade them, and release carbon dioxide and water.