Ecological Development of Reservoirs

Early environmental researchers identified three phases in the biological development of reservoirs: a breakdown of the original plant and animal communities and a recolonization by different species; a stage of increased primary and secondary production, mainly of plankton, and the establishment of temporary biotic communities; and, finally, a period of relative stabilization (Figure 4). This succession holds true for most reservoirs, but the levels of available nutrients at the more final stages of development can vary quite substantially. While some reservoirs stabilize at nutrient levels far below those of original water bodies in the reservoir area, others may experience increasing eutrophication because of extensive nutrient inputs.

The most apparent environmental alteration during the filling phase of a reservoir is that terrestrial habitats are becoming inundated and their vegetation destroyed. There are many examples of reservoir formation where forests and even villages were inundated and left to deteriorate. The decomposition of submerged vegetation and organic soils may

1. Preimpoundment stage: relative stability

2. Community breakdown and recolonization

3. 'Damming effect'

4. Relative stabilization

Damming / V

A

Oligotrophication

■ 1

2 ,, 3 ,,

4 .

Figure 4 Hypothesized ecological successions in reservoirs. A-C denote alternative scenarios where A represents a eutrophication and B-C an oligotrophication of the reservoir compared with preimpoundment nutrient levels.

Time

Figure 4 Hypothesized ecological successions in reservoirs. A-C denote alternative scenarios where A represents a eutrophication and B-C an oligotrophication of the reservoir compared with preimpoundment nutrient levels.

produce huge amounts of greenhouse gases such as methane (CH4) and carbon dioxide (CO2) - especially when reservoirs are created in tropical rainforests and boreal peat lands. It is not known how many reservoirs are net C emitters, but some of the ~1-3 billion metric tons of carbon that is sequestered in reservoirs is also converted into CH4 and CO2. It has been estimated that water reservoirs release 20% and 4%, respectively, of the world's annual anthropogenic emissions of CH4 and CO2. Other reservoir constructions are preceded by clearings of the areas to become inundated - to avoid timber and other vegetation floating around in the reservoir.

As a reservoir fills with water, the inundation and erosion of previously unflooded land and vegetation release nutrients and vegetative debris into the water. The increase in nutrient levels together with the increased light penetration in the reservoir cause the phytoplankton to multiply rapidly. As the phyto-plankton respond to the new conditions, zooplankton and macroinvertebrates react in much the same way, resulting in an overall short-term increase in the productivity of the reservoir ecosystem. But once the nutrients from the flooded soils are depleted, the plankton population might either decrease or increase, depending on the inflow of nutrients to the reservoir.

Along with the erosion on new reservoir shorelines there is recolonization of plant species that are more adapted to the new conditions. Weed species exploit this situation in large reservoirs. The development of new plant communities very much depends on the stabilization of the substrate. At sites exposed

Figure 5 Extensive shoreline erosion in the Gardiken Reservoir as a result of impoundment of former terrestrial ground in the upper Ume River in northern Sweden. The disturbance of such areas makes establishment of plant and animal communities almost impossible. The woody debris is uprooted stumps from inundated forest land. Note that lower parts of the reservoir are ice-covered. Photo: Christer Nilsson.

Figure 5 Extensive shoreline erosion in the Gardiken Reservoir as a result of impoundment of former terrestrial ground in the upper Ume River in northern Sweden. The disturbance of such areas makes establishment of plant and animal communities almost impossible. The woody debris is uprooted stumps from inundated forest land. Note that lower parts of the reservoir are ice-covered. Photo: Christer Nilsson.

to wave and ice action, new shorelines can be completely deprived of their fine sediments, leaving barren ground (a 'bathtub ring') that does not offer suitable habitat for plants (Figure 5). Therefore, new reservoirs may exhibit a gradual increase in the cover of shoreline plants during the first years, succeeded by a gradual decrease as substrates become depleted.

In contrast, reservoirs in regions with warm climates and with small fluctuations in water level can develop luxuriant vegetation. For example, eutrophicated reservoirs in the tropics may be invaded by dense mats of floating aquatic plants, such as the water hyacinth (Eichhornia crassipes), giant Salvinia (Salvi-nia molesta) and Nile cabbage (Pistia stratiotes). These mats block light from reaching submerged vascular plants and phytoplankton, and often produce large quantities of organic detritus that can lead to anoxia and emission of gases, such as CH4 and hydrogen sulfide (H2S). The material derived from these plants is usually of low nutritional quality and is not typically an important component of the food for zooplankton or fish. Accumulations of aquatic macrophytes can restrict access for fishing or recreational uses of reservoirs and can block irrigation and navigation channels and intakes of hydroelectric power plants.

Most aquatic animals are confined to the surface layers of reservoirs because of the general decrease in oxygen with depth. The relative productivity of a reservoir is therefore proportional to its area rather than to its volume. As for plants, the abundance of animals depends on the range of water level fluctuations; reservoirs with large fluctuations having a much poorer fauna. For example, many benthic invertebrates have little motility and die as a result of rapid water-level drawdown. This has direct negative consequences for benthic-feeding fish. In reservoirs without large aquatic plants along the margins, the basic productivity will rely on plankton, and fish communities will be dominated by planktivorous species. The value of such reservoirs for waterfowl will be seriously curtailed because food resources will be poor and nesting places few.

Just as disturbance makes a landscape susceptible to invasion by alien plant species, the construction of water reservoirs around the globe is also considered to contribute to the accelerating spread of exotic aquatic species. One reason is that reservoir habitats are more homogeneous than those of streams, more disturbed and often more connected to other water bodies. Another is that reservoirs typically contain unstable, recently assembled communities of stocked fish. There are several reasons for introducing fish in reservoirs, such as utilizing ecological niches to which none of the existing species are adapted, increasing fishing success, providing more food fish and controlling aquatic weeds. Linked systems of reservoirs have faced increased invasion of exotic species such as the cladoceran Daphnia lumholtzi and the zebra mussel (Dreissena polymorpha). Reservoirs have also been linked to parasitic disease. For example, reservoirs have facilitated the spread of the Schistosoma parasite (including five species of flat-worms) by greatly expanding its habitat, thus causing an increased incidence of deadly schistosomiasis or bilharzia throughout the tropics, where it affects about 200 million people.

When reservoirs serve as sources for the spread of exotic or generally invasive species into surrounding landscapes they are having landscape ecological effects, i.e., effects beyond the reservoirs themselves. Reservoirs also have other such effects. One example is their ecological fragmentation of rivers, e.g., by stopping the downstream drift of plant propagules and by providing obstacles to animal migration, thus fostering discontinuities in the riverine flora and fauna. For example, while free-flowing rivers in northern Sweden show continuous downstream changes in species composition of their riparian plant communities, chains of reservoirs and run-of-river impoundments demonstrate series of distinct assemblages with shifts from one to another at each dam (Figure 6). Otherwise, the most well-known ecological fragmentation effect dams and reservoirs have on rivers is the hindering of fish runs. Fish ladders, bypass channels, and detouring of fish by trucks are examples of measures to get the fish past the dam. If the fish are anadromous, it is required that there is spawning ground upstream of the dam and reservoir for the bypass to have an effect. Yet another landscape ecological impact of reservoirs is their retention of silicon that modifies the silicon: nitrogen:phosphorus ratio and causes dramatic shift in phytoplankton communities in the sea.

Dispersal vectors Animals

Plants Riverbed profile

Communities

Free-flowing river

Rapids

10 km

40 m

Dispersal vectors Animals Plants

Riverbed profile Communities

Impounded river

10 km

40 m

Figure 6 Hypothesized relationships between vectors for animal and plant dispersal and migration, riverbed profile, and riverine communities in free-flowing vs. impounded rivers. The organisms in the free-flowing river are hypothesized to describe a gradual change downstream, whereas in the regulated river, each impoundment or reservoir is expected to develop individual organism communities (denoted 1-4). Note that animals in general have better capacities than plants do to move upstream rapids and downstream through dams. Scales are approximate. From Jansson etal. (2000a) Effects of river regulations on river-margin vegetation: a comparison of eight boreal rivers. Ecological Applications 10: 203-224, with permission from Ecological Society of America.

10 km

40 m

Figure 6 Hypothesized relationships between vectors for animal and plant dispersal and migration, riverbed profile, and riverine communities in free-flowing vs. impounded rivers. The organisms in the free-flowing river are hypothesized to describe a gradual change downstream, whereas in the regulated river, each impoundment or reservoir is expected to develop individual organism communities (denoted 1-4). Note that animals in general have better capacities than plants do to move upstream rapids and downstream through dams. Scales are approximate. From Jansson etal. (2000a) Effects of river regulations on river-margin vegetation: a comparison of eight boreal rivers. Ecological Applications 10: 203-224, with permission from Ecological Society of America.

A reservoir does not only affect its surroundings but is also a product of its catchment. Practically all kinds of activities in the catchment can be transferred to the reservoir, in most cases through materials (such as sediments and various 'pollutants') carried by moving water or through a change in the hydrograph of the water draining the catchment. For example, a catchment that is cleared from forest or that has impervious surfaces or trenches shows more dramatic responses to rainstorms, the overflowing water is erosive and the catchment thus increases its footprint in the reservoir. For obvious reasons, there are cases when this collecting effect can be the primary motive for reservoir creation, and as mentioned above it is also a major obstacle when dams are planned for removal.

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