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| - PCO2 method for measuring photosynthesis and respiration in freshwater lakes
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| Abstract
| - A method for measuring net community photosynthesis and respiration based on changes in PCO2 is described. Samples were incubated in serum stoppered bottles and successive pCO2 measurements taken from the bottle headspace. At the end of the incubation, samples were acidified with concentrated phosphoric acid and total dissolved inorganic carbon (DIC) was determined. Alkalinity was calculated from PCO2 and DIC. Alkalinity was assumed to remain constant for the duration of the experiment so that DIC could be calculated from PCO2 and alkalinity for each measurement. Uptake rates were calculated from the slope of Δ DIC over time. The assumption of alkalinity change being <15% of photosynthesis was tested using axenic chemostat cultures of Chlamydomonas reinhardtii at different nitrogen:phosphorus ratios. For all cultures, alkalinity change represented <10% of particulate carbon turnover. Alkalinity-adjusted rates were found to be not significantly different from non-alkalinity-adjusted rates. Incubations from Lakes 227 and 240 in the Experimental Lakes Area and from Station 928 in Lake Malawi typified photosynthesis-irradiance curves. 14C uptake in Lake Malawi was found to be closer to gross community photosynthesis than to net photosynthesis. Field incubations using this method were 6-11 h and required the constant presence of a researcher; however, the method can be automated.
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