Characteristics of nighttime respiration in outdoor mesocosms

Abstract

To better understand the patterns and regulation of nighttime community respiration, dissolved oxygen (DO) and pH were simultaneously measured at 5-min intervals for 37 days in each of three outdoor mesocosms with different fish stocking levels. Nighttime decreases in community respiration rates were estimated fairly well by an exponential function of time and slightly worse by a linear one, irrespective of ecosystem differences, but smaller time coefficients were obtained for dissolved inorganic carbon (DIC) than for DO. Respiratory quotients increased significantly from nightfall to the hour before dawn. To roughly estimate gross productivity from net productivity measurements, we extrapolated nighttime respiration from various parts of the night to the daytime; among the models evaluated, that extrapolating the respiration rate averaged over the whole nighttime to the previous day led to the highest correlation between irradiance and estimated gross productivity. Significant correlations were found between estimated daytime gross production rates and respiration rates just after sunset, whereas respiration before sunrise seemed quite constant and close to minimum metabolic rates of the ecosystems. Nighttime respiration was also affected by the composition and/or metabolic state of the system, expressed here by daily net community productivity. Multiple regression analysis showed that more than 75% of daily and between-pond variation in respiration rates just after sunset was explained by daytime gross productivity, planktonic and detrital carbon concentrations, and daily net community productivity.