RESEARCH ARTICLE Shift in a Large River Fish Assemblage: Body- Size and Trophic Structure Dynamics

As the intensity and speed of environmental change increase at both local and global scales it is imperative that we gain a better understanding of the ecological implications of commu-nity shifts. While there has been substantial progress toward understanding the drivers and subsequent responses of community change (e.g. lake trophic state), the ecological im-pacts of food web changes are far less understood. We analyzedWabash River fish assem-blage data collected from 1974-2008, to evaluate temporal variation in body-size structure and functional group composition. Two parameters derived from annual community size-spectra were our major response variables: (1) the regression slope is an index of ecologi-cal efficiency and predator-prey biomass ratios, and (2) spectral elevation (regression mid-point height) is a proxy for food web capacity. We detected a large assemblage shift, over at least a seven year period, defined by dramatic changes in abundance (measured as catch-per-unit-effort) of the dominant functional feeding groups among two time periods; from an assemblage dominated by planktivore-omnivores to benthic invertivores. There was a con-current increase in ecological efficiency (slopes increased over time) following the shift as-sociated with an increase in large-bodied low trophic level fish. Food web capacity remained relatively stable with no clear temporal trends. Thus, increased ecological efficien-cy occurred simultaneous to a compensatory response that shifted biomass among func-tional feeding groups