Land use affects temporal variation in stream metabolism

Stream metabolism (gross primary production and ecosystem respiration) is increasingly used to assess waterway health because mean values are responsive to spatial variation in land use, but little is known about how human land use influences the temporal variability of stream metabolism. We investigated daily variation in dissolved O₂ (DO) concentrations and calculated mean and within-season variation in gross primary production (GPP) and ecosystem respiration (ER) rates at 13 stream sites across a landuse intensity gradient in the Auckland region, New Zealand, over 9 y. Based on generalized linear mixed models, mean daily GPP (0.1-12.6 g O₂ m^-2 d^-1) and ER (1.8-29.6 g O₂ m^-2 d^-1) and seasonal variation in stream metabolism were significantly related to landuse intensity with higher variability associated with higher values of a landuse stress score. Overall, mean daily rates and day-to-day variation in GPP and ER were greatest in summer and least in winter. We recommend summer monitoring over a minimum 5-d period to assess stream health. Our results show that human land use affects the mean and the temporal variability of DO and stream metabolism. This finding has important consequences for characterizing in-stream processes and the resilience of stream ecosystems. Only long-term temporal monitoring provides the data needed to assess fully how streams function.