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D. Optimisation of stream bank vegetation cover -  intermediate complexity hypothesis

The natural characteristics and ecological health of streams are linked to the landscape by the biotic and physicochemical properties of the riparian zone. The functions of riparian vegetation with respect to aquatic ecosystems is summarised in Table 6.2.

Riparian vegetation largely determines the input of solar energy to a river by directly influencing the primary productivity of algae and macrophytes, and by indirectly influencing the productivity of higher trophic levels like invertebrates and fish. According to the intermediate complexity hypothesis (Zalewski et al. 1994), the optimal energy pathways in river channels are achieved at an intermediate level of complexity of riparian vegetation. This hypothesis was verified by field studies in small river systems.

In the case of primary production, the opening of dense canopies over stream habitats may create better conditions for cyanophyte communities while disadvantaging diatom communities. This shift in algal populations can influence water quality. Both cyanophytes and diatoms possess an high affinity for phosphorus at concentrations below 500 µg P-PO₄ l^-1 (Figure 6.5), but, at low phosphorus concentrations (below 200 µg l^-1), cyanophytes have a three times higher ability to assimilate phosphorus (maximum uptake rate of about 770 µg P-O₄ g^-1 dry mass (d.m.) day^-1) than the diatoms (maximum uptake rate of about 302 µg P-O₄ g^-1 d.m. day^-1). This, in turn, suggests that cyanophytes have an higher capacity to buffer the stream from pollutants than do the diatoms community. This results in an higher absorption capacity for the stream ecosystem.

The level of complexity in riparian vegetation is also important for fish community biomass and diversity. It was found that, for small-sized rivers, the optimal complexity of riparian vegetation for maintaining high fish biomass and diversity is when the amount of light reaching the stream channel is between 300 to 700 µEcm^-2 s^-1 (Łapińska 1996, Zalewski et al. 2001). Fish biomass and diversity in such habitats may be up to three times higher than in habitats with higher or lower levels of light input (Figure 6.6).

A proposed design for the rehabilitation of an in-stream habitat for fish by restoring and properly maintaining riparian ecotones is summarised in Table 6.3.

The proper restoration and management of streams for fishery enhancement purposes should include the conservation or reconstruction of the natural channel morphology and its riparian zone structure. By restoring the pool-riffle-run sequences, together with the optimal degree of canopy cover by the riparian vegetation, the habitat carrying capacity of the stream for biota can be maintained, and the river self-purification ratio could be accelerated (Figure 6.7).

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