Comparison of in vitro and in situ plankton production determinations

Abstract

Plankton production was measured using 8 techniques at 4 stations in the Celtic Sea, North Atlantic Ocean, in April 2002. Primary production (PP) was derived from 14 C incorporation into particulate carbon after 24 h simulated in situ, PP( 14 CSIS), and 2 h photosynthesis-irradiance incuba- tions, PP( 14 CPUR), and from 2 published satellite algorithms, PP(VGPM) and PP(M91). Gross production (GP) was calculated from O2 evolution, GP(O2), and 18 O enrichment of dissolved O2, GP( 18 O), after 24 h simulated in situ incubations, and from in situ active fluorescence measured by fast repetition rate fluorometry (FRRF). Net community production (NCP) was determined from changes in in situ dissolved oxygen, NCP(ΔO2), and from changes in oxygen during 24 h simulated in situ incubations, NCP(O2). Dark community respiration (DCR) was derived from changes in oxygen during a 24 h dark incubation, DCR(O2), and daily oxygen uptake, DOU( 18 O, O2), was calculated from the difference between GP( 18 O) and NCP(O2). Three stations were dominated by picoautotrophs and the fourth station was dominated by diatoms. While most of the comparisons between techniques fell within previously published ranges, 2 anomalies occurred only at the diatom-dominated station. Rates of PP( 14 CPUR) were oxygen uptake in the dark. The low rates of PP( 14 CPUR) in relation to PP( 14 CSIS) may have resulted from the heterogeneous nature of the bloom and differences in sampling time. However, it is also possible that dissolved organic material (DOM) released by the stressed diatom population restricted the diffusion of 14 C into the cells, thereby causing a greater underestimate of PP by techniques using short incuba- tions. The significantly higher rates of oxygen uptake in the light are difficult to reconcile, and we do not know whether the light enhanced oxygen uptake was directly linked to carbon fixation. How- ever, the release of DOM may also have provided substrate for enhanced respiration in the light. These anomalies were only revealed through the concurrent measurement of plankton production by this wide range of techniques. Further investigation of DOM excretion and light-enhanced respira- tion during diatom blooms is warranted.