Assessment of river health: accounting for perturbation pathways in physical and ecological space

1. A full understanding of changes to river ecosystem structure and functio ning along the continuum from relatively pristine to profoundly perturbed r equires knowledge of physical, chemical and ecological properties at many s patial and temporal scales. Perturbations may span broad spatio-temporal sc ales or be spatially and temporally discrete. 2. The relevant scale for study depends on the manner in which perturbation s move through two kinds of pathways - through the physical space of the ca tchment hierarchy and through the ecological space of river food webs. Diff erent kinds of perturbation (e.g. inputs of sediment, nitrogen or phosphoru s, changes to shading or discharge) vary in the degree and manner in which they are propagated downstream and through food webs. 3. The fundamental importance of disturbance regimes and refugia to river h ealth is now clear and managers need to take this into account when devisin g river management schemes. 4. A comparison of the physics, chemistry and ecology of streams in catchme nts of native tussock and exotic pasture grassland in New Zealand serves to emphasize (i) the extreme complexity of interacting factors resulting from land use change, (ii) the importance of disturbance regime (not encapsulat ed in biotic indices) and biogeographic patterns (usually unmeasured) in de termining ecosystem structure and functioning, and (iii) the lack of concor dance between any single index of health and various fundamental features o f ecosystem functioning. 5. Practical considerations limit most evaluations of river health to a sma ll suite of indices, though it is important that researchers continue to ev aluate the spatial, temporal and biological limitations of these indices. T he vagaries of history and geography (extinction and colonization trajector ies in relation to natural disturbance regimes) and the complexity of inter acting physical, chemical and ecological responses to perturbation suggest that multi-scale, multi-temporal studies of river function offer the best o pportunity to evaluate river health.