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About UNEP
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United Nations Environment Programme
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Newsletter and Technical Publications
Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -


The model

The model presumes that phosphorus is the limiting factor in a reservoir. Therefore, it describes only the phosphorus cycle (see Appendix). It has eleven state variables; namely, dissolved reactive phosphorus, phosphorus in phytoplankton, phosphorus in zooplankton, phosphorus in planktivorous fish, phosphorus in their eggs, phosphorus in carnivorous fish, phosphorus in their eggs, phosphorus in detritus, exchangeable phosphorus in sediment, phosphorus in pore water, and volume. The conceptual diagram of the model is shown Figure 3.10. Figure 3.11 shows a simulation result for four important biological components: phytoplankton-P, zooplankton-P, planktivorous fish-P, and carnivorous fish-P.

The amount of water flowing into the reservoir is a forcing function giving the water flow rate as the inverse residence time, which varies between 0.005 and 0.05/year, corresponding to a residence time of between 20 and 200 days. How does this flow rate influence the biological elements in the lake ecosystem? To answer this question, relevant to the practice of ecohydrology, the model simulates two scenarios: one corresponding to a constant outflow rate, and one corresponding to an higher outflow rate when planktivorous fish are spawning and phytoplankton are blooming (resulting in a faster washout of eggs and phytoplankton). The residence time, of course, will also have an effect on the water depth. At lower water depths, the fish would spawn on sandy sediment without vegetation, while, at higher water, the fish would spawn on vegetation. The mortality of eggs would be much higher on the former than on the latter substrate. The influence of water depth on the survival of the eggs, therefore, is considered in the model.

The equations shown in Table 3.1 are based on a phosphorus concentration in the inflowing water of 0.5 mg dm-3  and an higher outflow rate during the period from 1 April to 1 August, annually, result an higher washout rate and lower water level. The two scenarios were determined by four phosphorus concentrations; namely, 0.05 mg dm-3  (oligotrophic), 0.2 mg dm-3  (mesotrophic), 0.5 mg dm-3  (eutrophic), and 1.0 mg dm-3  (almost hypereutrophic).

Table 3.1. Quantification of the ecohydrological effect on biological components
Biological
component
P in-flow
mg dm-3

Spring and summer peaks (mg P dm-3 )

constant out flow rate high out flow rate ¼-1/8 Difference
Phytoplankton 1.0 0.98  1.02 0.67  0.77 down 28%
Phytoplankton 0.5 0.76  0.70 0.40  0.42 down 44%
Phytoplankton 0.2 0.44  0.42 0.32  0.40 down 44%
Phytoplankton 0.05 0.32  0.32 0.16  0.16 down 50%
Zooplankton 1.0 0.02  0.54 0.17  0.19 up 29%
Zooplankton 0.5 0.02  0.38 0.21  0.23 up 10%
Zooplankton 0.2 0.02  0.30 0.04  0.18 down 31%
Zooplankton 0.05 0.02  0.28 0.04  0.16 down 33%
Planktivorous fish 1.0 2.50  1.90 1.10  1.60 down 39%
Planktivorous fish 0.5 2.30  1.50 1.01  1.19 down 42%
Planktivorous fish 0.2 1.15  1.01 0.50  0.72 down 44%
Planktivorous fish 0.05 0.70  0.70 0.33  0.48 down 42%
Carnivorous fish 1.0 0.62 0.53 down 15%
Carnivorous fish 0.5 0.60 0.52 down 13%
Carnivorous fish 0.2 0.58 0.50 down 14%
Carnivorous fish 0.05 0.56 0.49 down 12.5%

 

Note that the model is very general and has not been calibrated or validated. The quantitative results, therefore, cannot be used as a specific case study, but the results could be interpreted as a typical pattern of the reactions of biological components to changes in the hydrological forcing functions. The results, therefore, can be applied to demonstrating the ecohydrological possibilities of improved reservoir management. In specific cases, a specific model should be developed and the model must be calibrated and validated using real observations from the waterbody.

Model results

It is expected that an higher flow rate during the period between 1 April and 1 August (but lower during the rest of the year) would imply a faster wash out of the planktivorous fish eggs and phytoplankton, and an higher mortality of fish eggs due to low water levels, as shown in Table 3.1. The application of the structurally dynamic approach implies that the growth rate of zooplankton would decrease (it also means that zooplankton size is increasing), as the phosphorus concentration decreases, which is in accordance with the general observations (see, for instance, the consequences of biomanipulation in Jørgensen and de Bernardi 1998). The total effect,therefore, would be a decrease in phytoplankton and fish, particularly in planktivorous fish. The effect on zooplankton is difficult to predict because the predator and prey decrease at the same time. Likewise, because the entire ecosystem is working as an interrelated network, it is hardly possible to quantify the consequences of changes in hydrology on the biological components without the model. Nevertheless, it is possible to conclude that hydrology has a major impact on the biological components, suggesting that it is possible to reduce eutrophication impacts by changing the flow regime and the planktivorous fish community, in accordance with the observations of Zalewski and Wagner (2000).

 

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