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

Stream ecosystems include both surface and subsurface components that are connected by the flow of water. Processes occurring in subsurface sediments affect those in the surface, and vice versa. A model was developed to investigate the effects of nutrient transformations occurring in subsurface sediments 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 subsurface zone. Parameters were based on information from Sycamore Creek, Arizona, a nitrogen-limited desert stream that has been extensively studied. The behavior of the model was examined both at steady state and as it approached steady state after periphyton biomass was reduced to low values, simulating the effects of a scouring flood. Previous work has shown that subsurface sediments are a source of inorganic nutrients, mainly nitrogen, to surface water and to periphyton communities in Sycamore Creek. Thus it was expected that in this model, periphyton biomass would increase when exchange between surface and subsurface zones was increased, and biomass would also increase when the rate of nutrient transformation in subsurface sediments was increased. Model results confirmed the expectations and highlighted the important role of organic nitrogen in mediating the periphyton-nutrient feedback. Post-flood recovery of periphyton biomass was particularly sensitive to elevated concentrations of inorganic nitrogen in flood water. The model shows that processes in the subsurface zone can have a large effect on surface organisms, and illustrates how surface periphyton communities interact with subsurface microorganisms through the recycling of nutrients.