Modelling nutrient-periphyton dynamics in streams: the importance of transient storage zones

The dynamics of a nutrient-limited periphyton community in a segment of stream was modeled. The stream segment was assumed to consist of two zones, free-flowing water and a boundary zone of zero water flow, which acted as a transient storage zone for nutrients. Studies with a biologically unreactive tracer solute (sodium chloride) were used to obtain parameters for diffusion of solute into the transient storage zone. Two simple alternative functional responses representing nutrient-limited periphyton growth were formulated, one with only nutrient limitation on periphyton growth and one that additionally included density-dependent limitation of periphyton growth. ts to fit the two alternative functional responses for periphyton growth and to predict nutrient levels in the transient storage zone showed that the two alternatives had very different implications for the steady state and dynamics of the storage zone. Empirical studies of periphyton biomass and nutrient turnover give support for the second alternative function. The model results suggests some additional experiments that can be performed to test the two alternatives.