Comparison of bacterial carbon production estimates from dilution and ³H‐leucine methods across a strong gradient in ocean productivity

The uptake of 3 H‐labeled leucine into proteins, a widely used method for estimating bacterial carbon production (BCP), is suggested to underestimate or overestimate bacterial growth in the open ocean by a factor of 40 uncertainty. Meanwhile, an alternative BCP approach, by the dilution method, has untested concerns about potential overestimation of bacterial growth from dissolved substrates released by filtration. We compared BCP Dil and BCP Leu estimates from three cruises across a broad trophic gradient, from offshore oligotrophy to coastal upwelling, in the California Current Ecosystem. Our initial analyses based on midday microscopical estimates of bacterial size and a priori assumptions of conversions relationships revealed a mean two‐fold difference in BCP estimates (BCP Dil higher), but no systematic bias between low and high productivity stations. BCP Dil and BCP Leu both demonstrated strong relationships with bacteria cell abundance. Reanalysis of results, involving a different cell carbon‐biovolume relationship and informed by forward angle light scatter from flow cytometry as a relative cell size index, demonstrated that BCP Dil and BCP Leu are fully compatible, with a 1 : 1 fit for bacteria of 5 fg C cell −1 . Based on these results and considering different strengths of the methods, the combined use of 3 H‐labeled leucine and dilution techniques provide strong mutually supportive constraints on bacterial biomass and production.