• Hydraulic food-chain models predict the response of river biota to hydraulic parameters such as width, depth, and velocity.

• Interaction between or among trophic levels is modulated by the flood pulse.

• Four functional groups exist in a river ecosystem:

  • Detritus, comprising dead plant material, imported or local.

  • Vegetation, or living aquatic plants or algae.

  • Herbivores-detritivores, or grazers, which feed on vegetation and detritus.

  • Predators, large fish which consume animal prey.

• Hydraulic food-chain models explore how the fundamental temporal and spatial features of floodplains influence the dynamics of the food chain.

HYDRAULIC RELATIONSHIPS

• Consider three cases:

  • A natural river with access to its floodplain

  • A leveed river cutoff from its floodplain

  • A river with artificially stabilized flow which does not exceed the top of its bank (regulated upstream).

• The hydraulic geometry of rivers has been described by Leopold and Maddock (at-a-station and downstream relations).

• Equations for biomass dynamics of each of the four trophic elements are tied to channel hydraulics (width, depth and velocity).

• Detritus increases as litter falls.

• Detritus is lost to grazers at a rate determined by their densities (detritus and grazers).

• Detritus diminishes as carbon is respired to the atmosphere as carbon dioxide.

• Outwash (outflow) is assumed to be equal to inwash (inflow).

• Vegetation renews by logistic growth until it becomes self-limiting.

• Vegetation that dies without being grazed increases the detritus.

• Grazers convert vegetation or detritus into offsprings.

• Grazers are killed by predators or die from other causes.

• Predators suffer only non-predatory mortality.

• Human fishing may add another trophic level.

• One-dimensional model portrays large-river hydraulic and trophic dynamics at a single cross-section.

• The model emphasizes temporal dynamics rather than spatial heterogeneity.

• An overview of causal linkages is shown in Fig. 4.

• Local geomorphology determines floodplain width.

• Climate and land use (and geomorphology) govern discharge amount.

• Width, depth, and velocity vary with discharge.

• Width, depth amd velocity influence trophic dynamics by affecting key parameters in the biomass balance equations.

• Table 1 shows biomass balance equations.

• Mobile grazers and predators occupy the floodplain only when water is deeper than 0.2 m.

• This depth has been found to be a critical threshold below which larger prey are vulnerable to fishing birds.

• As hydrograph recedes, mobile grazers and predators return to the main channel except for that fraction left stranded in the flood plain.

• Mortality from stranding can be high.

• In the Parana river, Argentina, mortality by stranding is four times greater than fishing.

• Trophic parameters can be linked to hydraulic parameters.

• Loss rate of detritus decreases with increasing depth because water temperature and microbial concentrations decrease as depth increases.

• Vegetation carrying capacity should decrease with depth if vegetation is light-limited.

• Above a certain velocity, local growth is reduced by sloughing, or by light limitation if high flows are turbid.

• Predator attack rates on grazers might decrease with velocity because of constraints on prey encounter or handling.

• Prey refuges in the main channel can be dislodged by high flows.

• Prey are washed away when flows exceed 2 m/s.

PRELIMINARY SIMULATIONS

• Results from simulations are shown in Fig. 5.

• The unmodified flood plain (a) maintains the most stable populations at higher trophic levels.

• In the leveed channel (b), predators initially increase only to bust later to to unsustainable feed rates.

• In regulated channels with low flow (c), grazers thrive, but they are not able to sustain a viable predator population.

• In regulated channels with average flow (d), flows are chronically too high for the nonhydrodynamic grazers to feed effectively, and they starve, following by the crash in their predator's population.

• Simulations suggest that the longest food chains are maintained only when the environment fluctuates (the flood pulse).

• Biota do not tract hydrologic cycles.

• Longer, biologically driven cycles can be superimposed on the hydrologic cycle.

FUTURE NEEDS AND DIRECTIONS

• There is a tendency for temporal and spatial variations to promote the persistence of longer food chains.

• Large rivers are defined as "those large enough to intimidate research workers."

• Levees or upstream regulation (dams) alter seasonal changes in discharge.

• Ecological paradox of rivers: Large, frequent hydrologic perturbations are crucial for long-term maintenance of biodiversity, productivity, and the higher trophic levels, which are the most prized by humans.