Optode use to evaluate microbial planktonic respiration in oligotrophic ecosystems as an indicator of environmental stress

The suitability of O2 optodes to resolve the effects of global-change stressors on respiration of microbial planktonic assemblages from oligotrophic ecosystems was tested. With this aim, we first evaluated how O2 measurements with optodes on closed flasks depended on delayed temperature equilibration (hysteresis), which can bias actual measurements on samples not subjected to constant temperature. This must be addressed when using optodes for in situ measurements. Thus, we provide the mathematical tools to correct the effects of hysteresis on O2 measurements hence removing the constraints of maintaining a constant temperature over the long incubations required to measure respiration in oligotrophic ecosystems. Optodes proved suitable to resolve the effects of stressors such as CO2, temperature, thermal stratification, nutrient input, and ultraviolet radiation in different oligotrophic aquatic ecosystems. These experiments resulted in significant differences in microbial planktonic respiration for all stressors tested. From these results, we conclude that (1) optodes constitute a useful tool to make realistic measurements on samples subjected to natural (or experimental) ranges of temperature variability, and (2) microbial planktonic respiration measured with O2 optodes has the potential to be used as an ecological indicator for assessing the effects of environmental stress, even in oligotrophic aquatic ecosystems, where higher sensitivities are needed.