Beyond dams: Can serial discontinuities be induced by natural processes?

Serial discontinuity may be the result of natural processes. For instance, temperature and faunal patterns of the S.F. John Day River (SFJD) are significantly influenced by tributary inputs during summer baseflow.   Between tributaries, the temperature upstream was 17oC and rose to 25oC downstream during midday.  At each tributary, discharge resets river temperature which subsequently increases downstream via isolation and advection. Species dominance corresponded with temperature shifts within each affected reach.  Redband trout (Oncorhyncus mykiss) were most abundant at 17oC; but only 2.1 % of the biomass occupied habitats above 23oC.  Northern pikeminnows (Ptychocheilus oregonensis) were first detected at 19oC and most abundant above 23oC.  Three such sequences were detected consecutively along the SFJD. Distribution anomalies can now be explained in terms of patch formation and gradient analysis.  Serial discontinuity, we suggest, is a special case of Rahel and Hubert’s observation that zonal distribution patterns occur where gradient breaks are sharp.  Where gradient change is gradual, individualistic distribution patterns result.  We suggest that several other factors might induce serial discontinuities: fluctations in stream power, stream gradient, shifts in stream aspect, and spring inputs, to name a few.   Our understanding of assembly rules governing stream fishes will require us to investigate further.