Freshwater Biology (1996) 36, 249–263 Periphyton dynamics in a floodprone prealpine river: evaluation of significant processes by modelling URS UEHLINGER, HEINRICH BU ¨ HRER AND PETER REICHERT Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-8600 Du ¨ bendorf, Switzerland SUMMARY 1. Periphyton chlorophyll a was measured at weekly or 2 weekly intervals from October 1992 to March 1994 at four sites in a Swiss prealpine gravel bed river that was frequently disturbed by unpredictable spates. 2. To evaluate the dominant processes that control periphyton biomass, measured data were compared with a set of simulations from an empirical dynamic periphyton model. Different combinations of process hypotheses were systematically activated and deactivated in order to assess their importance. 3. The simplest model leading to an acceptable agreement with measured data employs a biomass-dependent growth rate, a detachment rate directly proportional to discharge and biomass, and a catastrophic loss rate during bed moving spates. Terms describing light or temperature dependence had a minor effect on the model fit. 4. The model describes the temporal pattern of the periphyton biomass as a series of growth curves periodically truncated by spates. Within the uncertainties of the measurements, mainly caused by the spatial heterogeneity of periphyton, the biomass recovered along deterministic trajectories. 5. Sensitivity analyses with respect to model parameters and model structure showed that site-specific model parameters could not be unequivocally determined, and that the model yields similar results with slightly different formulations of processes. This indicates that the data base with respect to periphyton biomass was too small for a unique identification of model details but that the main conclusions on the significance of processes did not depend on arbitrary choices of the model formulation. Introduction A major factor limiting the accumulation of periphyton (Triska et al., 1983; Bothwell, 1986, 1988, 1989). ‘Biotic’ losses such as grazing and autogenic sloughing may (epilithon) in gravel bed rivers are flows that cause loss of biomass by increasing shear stress, physical become major constraints of biomass accrual only after long periods without spates, because recovery of the abrasion by suspended solids, and abrasion by moving substrata (Tett et al., 1978; Fisher et al., 1982; Biggs & invertebrates is usually slow compared with periphy- ton growth, and loss by autogenic sloughing is Close, 1989; Uehlinger, 1991; Biggs & Thomson, 1995). After such disturbances biomass accrual is a function expected to become important only as the community ages (Fisher et al., 1982; Biggs & Close, 1989; Power, of the growth of the surviving and colonizing algae, and of the current induced drag. The rate of coloniza- 1992). Multiple linear regression analysis has shown that a few parameters such as time since spate, mean tion depends on the number and proximity of refugia, and on the immigration properties of colonizers discharge and maximum discharge during the spate can explain  60% of the variance in periphyton (Kaufman, 1979; Stevenson, 1990; Peterson & Stevenson, 1991). The growth rate is controlled by biomass of streams (Biggs, 1988; Fisher & Grimm, 1988; Biggs & Close, 1989; Uehlinger, 1991). This has light, temperature and the availability of nutrients © 1996 Blackwell Science Ltd 249