Variability of photosynthesis-irradiance curves and ecosystem respiration in a small river

1. We investigated photosynthesis-irradiance relationships (P-I curves; P = oxygen production rate due to photosynthesis, I = light irradiance rate at the water surface) and ecosystem respiration in a 9 km long reach of a riv er that is characterised by light conditions favouring primary production, high ambient nutrient concentrations, a high reaeration rate, and frequent spates. We addressed the question of how disturbances (spates) and season i nfluence photosynthesis and ecosystem respiration. 2. We used an oxygen mass-balance model of the river to identify ecosystem respiration rates and the two parameters of a hyperbolic P-I function (P-ma x = maximum oxygen production rate due to photosynthesis, alpha = the initi al slope of the P-I function). The model was fitted to dissolved oxygen con centrations quasi-continuously recorded at the end of the reach. We estimat ed parameters for 137 three-day periods (during the years 1992-97) and subs equently explored the potential influence of season and disturbances (spate s) on P-max, alpha and ecosystem respiration using stepwise regression anal ysis. 3. Photosynthesis-irradiance relationships and ecosystem respiration were s ubject to distinct seasonal variation. Only a minor portion of the variabil ity of PI curves could be attributed to disturbance (spates), while ecosyst em respiration did not correlate with disturbance related parameters. Regul ar seasonal variation in photosynthesis and ecosystem respiration apparentl y prevailed due to the absence of severe disturbances (a lack of significan t bedload transport during high flow).