Verification of 14C and O2 derived primary organic production measurements using an enclosed ecosystem

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Abstract
The stimulus for these experiments came from a recent series of papers which have suggested that the 14 C technique may underestimate primary production by as much as 10-fold. We evolved the following strategy to attempt to verify the 14 C technique in nearshore waters: (i) to examine the validity of in vitro (i.e., bottle incubation) measurements by comparing observed in situ oxygen changes in a large enclosed natural ecosystem against those determined in vitro and if no evidence of containment effects were indicated, (ii) compare 14 C and oxygen measurements in simultaneous in vitro incubations. The first step essentially tests the containment problem, the second the physiological and calibration problems of the 14 C technique. The first experiment was run with nitrate as the main source of inorganic nitrogen, the second with ammonia. PQs for converting the 14 C measurements to oxygen values were calculated from the equation PQ = PQ c + 2/(C/NO 3 ) where PQ c is the ‘carbon PQ’ (taken as 1.25) and (C/NO 3 ) is the molar carbon to nitrate assimilation ratio. Although there appear to be some minor residual problems in the interpretation of the data when nitrate was the dominant nitrogen source, the overriding conclusions were: first, that the close agreement between the changes in in situ and in vitro dissolved oxygen concentration during the photoperiod gave no evidence for any notable containment effect upon photosynthesis. Secondly, the in vitro rates of 14 CO 2 -determined photosynthetic production and gross photosynthetic oxygen production agreed, within the precision of the two techniques. The experiment further demonstrated the need to determine soluble as well as particulate organic production and to pay attention to the potential effect of the nitrogen nutrient upon the PQ. Thus it was concluded that our data give no evidence for marked errors in the 14 C-technique of measuring primary organic production for coastal waters.

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