Print PageFlow variation in rivers change environmental gradients and cause habitats to expand and connect or contract and disconnect. More extreme discharge fluctuations scour river beds or dewater channel reaches, severely disturbing biota and resetting community succession. How do these changes alter the densities or performances of food web members at various trophic positions? How can we predict how changes in flow, due to land and water use or climate, will change short and long term dynamics in river food webs and the abundances of biota they support? Field observations and experiments combined with dynamic models of transient but repeated dynamics are needed.
Results/Conclusions
In a variety of temperate and tropical river systems, disturbances mediated by periodic floods lengthen food chains. In rivers with strongly seasonal rainfall (Panama, Papua New Guinea, the Eel of northern California), floods give large-bodied predators periodic access to smaller prey. In such systems, flow variability determines whether predators and prey can persist (sensu Huffaker 1958) in periodically divided habitats. This situation occurs in a prairie-margin stream in Oklahoma with periodically connected channel pools and probably in a floodplain river of Papua New Guinea with seasonally connected off-channel lakes. In gravel-bedded rivers, floods can also lengthen food chains by releasing high algal production from severe limitation by inedible grazers, so that it can be transferred through early successional edible prey up to predators. In the Eel River of northern California, scouring winter floods release algae from predator-resistant grazers, with major ecological consequences for biogeochemical cycling, river-watershed linkages mediated by insect emergence, and predators dependent on energy transferred from algae through soft-bodied, edible grazers. For both mechanisms, dynamic models of transient dynamics can evaluate how timing, frequency and magnitude of floods will affect algal accrual at various channel network positions, and (with less certainty) the fate of that algae. We need this knowledge to anticipate how water shortages, and more intense management, will change flow variability in ways that affect populations and food web interactions of species we value, and species we do not want to proliferate. Regional river biota have adapted to various seasonal patterns flow variability (e.g. Mediterranean vs continental), so that large changes in the timing of flow or in water temperatures regimes may be more disruptive than variation in the magnitude of discharge.
