We examined the relationship between potential respiration rates, as measured by electron transport system (ETS) activity, standardized at a defined temperature (ETS2O). and nutrients and sestonic particles in a set of 101 Spanish reservoirs spannin a wide range of limnological characteristics. ETS activity ranged from 0.009 to 31 31 μmol e- 1−1 h−1 Among the nutrients, it was significantly correlated only with phosphorus, and only during the stratification period. During this period, the best regressor of ETS2O was the concentration of chlorophyll a (Chla), whereas during the mixing period the best regressor was particulate organic nitrogen (PON), suggesting a greater contribution of non-algal sestonic particles to total metabolism. Both the mean ETS20:PON ratio and the mean Chla:PON ratio increased systematically with increasing PON, indicating a lower relative contnbution of detrital particles to total seston as trophic state increased. In contrast, the mean ETS20:Chla ratio was constant across the range of Chla. Vertical profiles of ETS2O. Chla and PON were more coherent during the stratification period. when subsurface and metalimnetic peaks were frequent, than during the mixing period.