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 river 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 influence 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-max = maximum oxygen production rate due to photosynthesis, alpha = the initial slope of the P-I function). The model was fitted to dissolved oxygen concentrations quasi-continuously recorded at the end of the reach. We estimated parameters for 137 three-day periods (during the years 1992-97) and subsequently explored the potential influence of season and disturbances (spates) on P-max, alpha and ecosystem respiration using stepwise regression analysis. 3. Photosynthesis-irradiance relationships and ecosystem respiration were subject to distinct seasonal variation. Only a minor portion of the variability of PI curves could be attributed to disturbance (spates), while ecosystem respiration did not correlate with disturbance related parameters. Regular seasonal variation in photosynthesis and ecosystem respiration apparently prevailed due to the absence of severe disturbances (a lack of significant bedload transport during high flow).