Within a biotic community, the total density of organisms in each size class decreases roughly log-linearly with increasing body size. These size spectra are intimately tied to ecosystem function and dynamics, since the slope of the log-linear relationship between density and ataxonomic size-class is positively correlated to the relative energy fluxes passing through the system (or with different production/biomass ratios).

In this study, we hypothesize that the response of fish communities size spectra to hydrological alteration might be dependent both on the succesional stage of the community and the type of flow alteration. Flow alteration schemes increasing the flow variability (e.g. hydropeaking) will drive mature fish communities (generally in lower reaches) to earlier succesional stages, thus causing steeper slopes than expected. Inversely, alterations that artificially stabilize the natural disturbances regimes (e.g. irrigation schemes) will drive pioneer fish communities with steep size-spectra slopes (generally in upper reaches) towards more mature communities, and producing smoother slopes than expected. To test this hypothesis, size spectra were calculated in 398 river fish communities in Spain. Expected slopes according to an index of rheophily (FRI, Wolter et al. 2013) were compared to observed slopes, in the presence of hydropeaking and irrigation schemes.

This approach will support size spectra as a candidate of universal indicators of ecological status, as proposed by some authors (e.g. Petchey & Belgrano 2010).