The rate of primary production (PP) in the ocean is a significant and highly variable component of the global carbon cycle which also drives the biogeochemical cycling of major chemical elements such as nitrate, phosphate, and silicate. The magnitude of this rate sets the carrying capacity of the ocean, the upper boundary of export production and drives air-sea CO₂ exchange. Yet, despite its importance in the earth’s carbon cycle, the rate of PP in the ocean is not particularly well constrained; uncertainties are most significant when comparing rates measured in the sea (in situ) versus in a bottle (in vitro) and /or comparing currencies (e.g. oxygen to carbon).  The present uncertainty in PP measurements must be addressed by the oceanographic community as accurate estimations of primary and community production are essential to understanding the ecological consequences of climate change on the base of the oceanic food chain.

To address this issue, we used high precision measurements of the diel cycles of dissolved O₂, O₂/Ar and particulate carbon to estimate gross PP, community respiration (CR) and net community production (NCP) at several sites across the nutrient depleted and nutrient replete regions of the N. Pacific Ocean.  In Henderikx Freitas, White, and Quay (2020) published in Global Biogeochemical Cycles (https://doi.org/10.1029/2019GB006518), we describe daily in situ rate measurements of gross production and community respiration estimated from high frequency diel cycles in oxygen (O₂) and optically derived particulate carbon from several platforms (both ship-based and via profiling floats)  across an ecological gradient in the North Pacific spanning the high nutrient/low chlorophyll subarctic to the oligotrophic subtropical gyre. Both oxygen and carbon-based gross primary production and respiration rates indicated a ~3× increase between subtropical and subpolar stations. We consistently found that gross production and community respiration rates were in approximate balance at all stations across the full ecological gradient, implying that community respiration is fueled by recently produced organic matter and that recycling efficiency (~90%) is similar along the gradient. We determined that phytoplankton turnover time doubles (from 2 to 4 days) between subtropical and subpolar regimes whereas biomass increases by ~10-fold. We found a consistent photosynthetic quotient (1.4 ± 0.2 mol O₂ mol C^-1), respiratory quotient (1.0 ± 0.2 mol O₂ mol C^-1), and gross to net production ratio (2.0 ± 0.3) at all stations which underscores the surprising similarity of fundamental ecological characteristics despite the transition from nutrient deplete to replete conditions.

These findings have two major implications for future work: (1) they present an entirely novel means for constraining spatial and temporal changes in the photosynthetic and respiratory quotients and (2) given that the float- and ship- based estimates of in situ production and respiration generally agreed well suggests that float-based diel O₂ and POC measurements have the potential to greatly expand our knowledge of spatial and temporal variability of productivity and respiration in the ocean.

In Henderikx Freitas, White et al. (2020) published in the journal Applied Optics (https://doi.org/10.1364/AO.394123), we expanded on our understanding of the diel periodicity of optical proxies for carbon (beam attenuation, c_p and particulate backscattering, b_bp) by pairing these measurements with in situ flow cytometric measurements of microbial diversity and particle size distributions in the North Pacific Subtropical Gyre. The aim was to understand how community composition may alter coherence of the diel cycles we have measured. We observed clear and coherent diel cycles in all bulk and size-fractionated optical proxies for particle biomass. We show evidence linking diurnal increases in particulate beam attenuation c_p and b_bp to daytime particle growth and division of cells, with particles < 7 μm driving the daily cycle of particle production and loss within the mixed layer. Quantitative analysis revealed that particle concentrations in the 2-7 μm size range were sufficient to reconstruct c_p at the time of sampling, whereas Prochlorococcus dynamics were essential to reproduce temporal variability in b_bp. This study is a significant step towards improved characterization of the particle size range represented by in-situ bulk optical properties and a better understanding of  how microbial diversity impacts diurnal variability in particle production in the oligotrophic open ocean.

Last Modified: 10/02/2020
Modified by: Angelicque E White