HOW DOES FLOODPLAIN WIDTH AFFECT FLOODPLAIN RIVER ECOLOGY - A PRELIMINARY EXPLORATION USING SIMULATIONS

Hydraulic food chain models allow us to explore the linkages of river discharge regimes and river-floodplain morphology to the structure and dynamics of modeled food webs. Physical conditions (e.g. depth, width , velocity) that vary with river discharge affect the performance (bir th, growth, feeding, movement, or death rates) of organisms or trophic groups. Their performances in turn affect their impacts on food webs and ecosystems in channel and floodplain habitats. Here we explore the impact of floodplain width (modeled as 1X, 10X and 40X the channel wi dth) on a food web with two energy sources (detritus and vegetation), invertebrates that consume these, a size structured fish population wh ich consumes invertebrates and in which larger fish cannibalize small fish, and birds which feed on large fish. Hydraulic linkages to trophi c dynamics are assumed to be mediated in three ways: birds feed effici ently only in shallow water; plant carrying capacity varies non-linear ly with water velocity, and mobile and drifting organisms are diluted and concentrated with spillover of river discharge to the floodplain, and its reconfinement to the channel. Aspects of this model are based on field observations of Junk and Bailey from the Amazon, of Sparks fr om the Mississippi, and on our observations of the Fly River in Papua New Guinea. The model produced several counter-intuitive results, Biom ass of invertebrates and fish increased with floodplain width, but muc h more rapidly from 1X to 10X floodplains than from 10X to 40X floodpl ains. For birds, maximum biomass occurred on the 10X floodplain. Initi ally high bird biomass on the 40X floodplain declined to extinction ov er time, because although favorable fishing conditions (shallow water) were most prolonged on the widest floodplain, this advantage was more than offset by the greater dilution of prey after spillover. Bird pre dation on large fish sometimes increased their biomass, by reducing ca nnibalism and thereby increasing the abundance of small fish available to grow into the larger size class. Sensitivity analyses indicated th at model results were relatively robust to variation in parameter valu es that we chose, but much more exploration and calibration with field data are needed before we know how specific our results are to the st ructure and other assumptions of this model, We share with others the opinion that progress towards understanding complex dynamic systems li ke floodplain river ecosystems requires frequent feedback between mode ling and field observations and experimentation. This understanding is crucial for river management and restoration. Organisms in real river s have adapted to track and quickly exploit favorable conditions, and to avoid or endure adverse conditions. It is when we engineer away thi s environmental variability that we threaten the long term persistence of river-adapted biota.