Modelling nutrient-periphyton dynamics in streams with surface-subsurface exchange

Stream ecosystems include both surface and subsurface components that are c onnected by the flow of water. Processes occurring in subsurface sediments affect those in the surface, and vice versa. A model was developed to inves tigate the effects of nutrient transformations occurring in subsurface sedi ments on the growth of surface-dwelling periphyton. In the model the stream is divided into three zones: free-flowing surface water, a surface storage zone where flow is minimal and where periphyton growth occurs, and a subsu rface zone. Parameters were based on information from Sycamore Creek, Arizo na, a nitrogen-limited desert stream that has been extensively studied. The behavior of the model was examined both at steady state and as it approach ed steady state after periphyton biomass was reduced to low values, simulat ing the effects of a scouring flood. Previous work has shown that subsurfac e sediments are a source of inorganic nutrients, mainly nitrogen, to surfac e water and to periphyton communities in Sycamore Creek. Thus it was expect ed that in this model, periphyton biomass would increase when exchange betw een surface and subsurface zones was increased, and biomass would also incr ease when the rate of nutrient transformation in subsurface sediments was i ncreased. Model results confirmed the expectations and highlighted the impo rtant role of organic nitrogen in mediating the periphyton-nutrient feedbac k. Post-flood recovery of periphyton biomass was particularly sensitive to elevated concentrations of inorganic nitrogen in flood water. The model sho ws that processes in the subsurface zone can have a large effect on surface organisms, and illustrates how surface periphyton communities interact wit h subsurface microorganisms through the recycling of nutrients. (C) 1999 El sevier Science B.V. All rights reserved.