7.3 CONCEPTUAL MODELS OF ENERGY,
NUTRIENTS & COMMUNITY ORGANIZATION

The serial discontinuity concept / The flood pulse / Putting things together: nutrient spiraling

The sources and fate of energy and the pathways of nutrients in rivers interact to influence the structure of aquatic communities. These interactions have been summarised in four, related conceptual models. Of these, the first and most important is the River Continuum Concept.


The River Continuum Concept--
The importance of the longitudinal dimension in river systems is obvious from the fact that there are changes in habitat and community structure along the course. This needs to be linked to our understanding of the lateral dimension of riverine environments; especially the transfer of materials from land to water. The two ideas have been combined in the River Continuum Concept (RCC), which was developed in North America and links riparian vegetation, aquatic productivity and the structure of aquatic communities (Vannote et al. 1980; Minshall et al. 1985). The RCC is a model describing a continuous series of biological changes occurring along the longitudinal gradients within a river, from headwaters to mouth. Processes in downstream reaches are connected to those upstream, and predictable longitudinal variations in community organisation (especially representation of macroinvertebrate functional feeding groups) occur in response to varying hydrological conditions and changes in food resources. The RCC includes the assumption that river communities are structured in such a way that they make efficient use of energy inputs. This feature is not a result of co-operation among river organisms, but arises from the tendency of individual species to maximise energy intake whenever possible.

What does the RCC include? As a first step, river communities are grouped according to channel size into headwaters, medium-sized streams and large rivers. Many headwater streams are influenced strongly by riparian vegetation, which reduces aquatic primary production by shading, and contributes large amounts of allochthonous leaf litter. Shredders are predicted to be co-dominant with collectors in such streams, reflecting the importance of the riparian zone and the detritus derived from it. As stream width increases and shading decreases, the reduced importance of allochthonous inputs coincides with a greater significance of autochthonous primary production, and the import of FPOM from upstream.

In medium-sized streams, grazer-scraper biomass is maximised, but collectors are numerous also. The transition from headwaters, dependent on allochthonous inputs, to medium-sized rivers, relying on autochthonous primary production, is associated with a change in the ratio of primary production (P) to community respiration (R). (This ratio is calculated by summing all of the primary production by plants in the community and dividing that estimate by the sum of all of the respiration by consumers.) The position at which the stream shifts from heterotrophic (P/R <1) to autotrophic (P/R >1) is largely dependent upon the degree of shading and amount of the riparian vegetation.

Large rivers receive FPOM from upstream, but little CPOM. As a result, collector-gatherers and filter-feeders dominate macroinvertebrate river communities downstream. Although the shading effect of riparian vegetation is insignificant, water depth and turbidity in the lower course may limit primary production and so community metabolism once again becomes heterotrophic (P/R <1).

Limitations in the RCC are evident if it is applied to a river with an extensive floodplain. For instance, there may be an increase in autochthonous production by phytoplankton or macrophytes, and there is a lateral transfer of material across the main channel and floodplain described by the flood pulse cycle (read Section on 'Flood Pulse').

Another limitation of the RCC is that the model does not match the reality of tropical Asian river communities (Dudgeon 1995; Dudgeon and Bretschko 1995). Even in forested streams, shredders tend to be rather scarce (making up around 5% of the macroinvertebrates) and do not approach the abundance seen in North American streams. It is not clear why this is the case, but there are three, non-exclusive possibilities.

• Microbes are more important in CPOM processing in tropical Asia than in temperate streams.

• The role of some animals as shredders (e.g., freshwater crabs and shrimps) has been overlooked in Asia.

• Leaves in tropical latitudes contain chemicals that protect them from herbivorous insects on land; these same chemicals could make CPOM unpalatable to shredders in rivers.

Regardless of these limitations, the RCC has served as an aid to thinking about rivers. In particular, the way in which energy is used by aquatic communities, and the transfer of material between water and land, and between headwaters and river mouth. These are issues that must be included in effective river management strategies.


The Serial Discontinuity Concept--
The RCC is a model that might apply to pristine rivers, but few rivers remain unchanged or unaffected by human activities. Dams are certain to have an impact on the organization of aquatic communities, since the flow is blocked and the longitudinal transition of conditions along the river is altered. The dam creates a 'serial discontinuity' in the river because the gradual downstream transition in conditions is disrupted, and the longitudinal transfer of material is prevented (Ward & Stanford 1979).

The main effects of dams, in relation to the RCC, are as follows:

• Discontinuity in flow conditions is introduced: i.e., standing water behind a dam in what was formerly a flowing-water habitat.

• Movement of aquatic animals is impeded or prevented and populations are fragmented.

• Allochthonous organic matter transported by the river is deposited in the impoundment behind the dam. The availability of food downstream is therefore reduced.

• Suspended sediments are deposited behind the dam. Water released from the dam will pick up a 'normal' sediment load downstream where it may erode the riverbed and banks.

• The downstream transition of water temperature is altered, and water released from the dam may be either warmer (if it is taken from the surface) or cooler (if it is taken from the depths) than natural conditions. Concentrations of dissolved oxygen may be changed also.

• Phytoplankton that develops behind the dam may be released downstream providing a food resource for filter-feeders that would be unavailable under natural conditions.

• The seasonal patterns of flow will be altered, especially if the function of the dam is to provide water for irrigation (in which case dry-season flows downstream will be reduced) or to control flooding (in which case wet-season flows and floodplain inundation will change).

The consequences of the introduction of a serial discontinuity depend on its position on the river. Clearly a dam on the river main stream will have a greater impact than one on a minor tributary. If the discontinuity is far upstream, then the river will be able to recover from the loss of allochthonous inputs if it passes through forest further downstream. However, if the discontinuity is in the middle course of the river, it will have a dramatic effect on the transport of material to downstream reaches, and may affect the flood pulse (see Section next). If the dam is in the lower course, the effects may be smaller as most of the river course lies upstream of the impoundment. The deposition of suspended sediments in the standing water can reduce turbidity, allowing proliferation of phytoplankton that raises the P/R ratio.

Dam construction has important effects on river communities, and the RCC allows us to predict what types of impact result from the introduction of a serial discontinuity. The effects of dams on river ecology are considered in more detail in Section 13 (see also Dudgeon, 2000).

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