Annual cycle and inter-annual variability of gross primary production and ecosystem respiration in a floodprone river during a 15-year period

1. Temporal variation in ecosystem metabolism over a 15-year period (1986-2000) was evaluated in a seventh order channelised gravel bed river (mean annual discharge 48.7 m(3) s(-1)) of the Swiss Plateau. The river is subject to frequent disturbance by bed-moving spates. Daily integrals of gross primary production (GPP) and ecosystem respiration (ER) were calculated based on single-station diel oxygen curves. 2. Seasonal decomposition of the time series of monthly metabolism rates showed that approximately 50% of the variation of GPP and ER can be attributed to season. Annual GPP averaged 5.0 +/- 0.6 g O-2 m(-2) day(-1) and showed no long-term trend. 3. Ecosystem respiration, averaging 6.2 +/- 1.4 g O-2 m(-2) day(-1), declined from 8.8 to 4.1 g O-2 m(-2) day(-1) during the 15-year period. This significant trend paralleled a decline in nitrate and soluble reactive phosphorus concentrations, and the biochemical oxygen demand discharged by sewage treatment facilities upstream of the study reach. The ratio of GPP to ER (P/R) increased from 0.53 to about 1 as consequence of ER reduction. 4. Bed moving spates reduced GPP by 49% and ER by 19%. Postspate recovery of GPP was rapid between spring and autumn and slow during winter. Recovery of ER lacked any seasonal pattern. Annual patterns of daily GPP and to a minor extent of daily ER can be described as a sequence of recovery periods frequently truncated by spates. 5. The study showed that disturbance by frequent bed-moving spates resulted in major stochastic variation in GPP and ER but annual patterns were still characterised by a distinct seasonal cycle. It also became evident that stream metabolism is a suitable method to assess effects of gradual changes in water quality.