Measurement of planktonic CO2 respiration in the light

Respiration is an essential physiological process for all organisms and is a significant component of the global carbon cycle. However, CO2 release rates in the light (CRR) by aquatic communities have seldom been measured directly; instead, the typical procedure is to measure oxygen consumption rates in the dark (OCR) and assume that they are approximately equivalent to CRR. Here we describe a method to measure CRR (and consequently gross photosynthesis rates) based on C-13 addition to incubation water in a closed system. We quantified planktonic CRR in situ in a lake, and also in laboratory experiments, where we tested the effects of temperature and irradiance on respiration rates. We also measured O-2 rates (respiration in the dark and net production in the light) to compare with those based on CO2, and found that in the lake, these rates were similar both for photosynthesis and respiration. In laboratory, however, they were very different, with CO2-based photosynthesis being higher, and CRR being lower than correspondent O-2 rates. These findings show that the usual approach of assuming similar OCR and CRR can be erroneous. We also found that higher temperature and irradiance caused higher CRR. The method was successful to provide CRR and photosynthetic measurements, but would greatly benefit from more precise techniques to measure the concentration of dissolved inorganic carbon in water.