Control of net community production by microbial community respiration at Station ALOHA

Net community production (NCP) constrains the amount of carbon that is available for storage within or export from, the photic zone of marine ecosystems. With the aim of studying the control exerted by microbial community respiration (MCR) and gross primary production (GPP) over NCP, short-term variability in rates of MCR, GPP and NCP were measured over a 13-day sampling period (August 23–September 4, 2012) at Station ALOHA. During this period, picoplankton abundances and concentrations of photosynthetic pigments demonstrated weak to moderate decreases coincident with the passage of a low salinity water mass through the sampling area. During the same period, rates of MCR, measured using the in vivo INT method, varied by 2.7-fold. Rates of primary production (PP) were estimated based on four different methodologies: Fast Repetition Rate Fluorometry (FRRF), 14C-bicarbonate assimilation, diel changes in mixed layer O2 concentrations based on Seaglider measurements, and isotopic composition (17ΔO2) of dissolved O2. Remarkably, these different methods for measuring microbial metabolism were consistent, suggesting that collectively these different approaches provide an accurate constraint on ecosystem metabolism, despite the different time and space scales over which these measurements integrate. FRRF-derived estimates of PP, which were similar in magnitude to 14C-based-PP, were significantly correlated with variability in mixed layer MCR. None of the other independent measurements of PP demonstrated significant relationships to MCR. Day-to-day variability in MCR and bacterial growth efficiency (BGE), derived based on measurements of bacterial production (as estimated by 3H-leucine incorporation) and MCR, were related to changes in NCP estimated from O2/Ar ratios (O2/Ar-NCP), suggesting short-term variability in the efficiency with which microorganisms consume dissolved organic matter exerts direct control on rates of NCP in this ecosystem. Our results highlight internal consistency among the derived rates of ecosystem metabolism using a suite of different methodologies to estimate productivity and respiration. Moreover, we find that rates of MCR can be as variable as PP and that variability in MCR may regulate NCP in this ecosystem.