Planktonic primary production in estuaries: comparison of 14C, O2 and 18O methods

Rates of primary production were measured in 2 estuaries (Randers Fjord, Denmark, and the Scheldt estuary, Belgium/The Netherlands) using 3 different incubation methods: (1) the oxygen light-dark method (O 2 -LD), (2) 14 C incorporation and (3) 18 O labeling. Estimates based on the 14 C incorporation technique were not significantly different from those obtained using the O 2 -LD technique. The 18 O approach provided rates significantly lower than the 2 other techniques. Ratios of O 2 -LD to 18 O-based rates (range: 0.99 to 3.54) were often statistically significantly higher than 1 and increased with decreasing salinities and/or lower oxygen concentrations. The underestimation of gross primary production by the 18 O method may be due to an intracellular recycling of labeled oxygen which increased in magnitude with decreasing external oxygen conditions. These results suggest that the 18 O method must be used with extreme care in nutrient-rich, low oxygen systems.