A critical review of the N-15(2) tracer method to measure diazotrophic production in pelagic ecosystems

Dinitrogen (N-2) fixation is an important source of biologically reactive nitrogen (N) to the global ocean. The magnitude of this flux, however, remains uncertain, in part because N-2 fixation rates have been estimated following divergent protocols and because associated levels of uncertainty are seldom reported-confounding comparison and extrapolation of rate measurements. A growing number of reports of relatively low but potentially significant rates of N-2 fixation in regions such as oxygen minimum zones, the mesopelagic water column of the tropical and subtropical oceans, and polar waters further highlights the need for standardized methodological protocols for measurements of N-2 fixation rates and for calculations of detection limits and propagated error terms. To this end, we examine current protocols of the N-15(2) tracer method used for estimating diazotrophic rates, present results of experiments testing the validity of specific practices, and describe established metrics for reporting detection limits. We put forth a set of recommendations for best practices to estimate N-2 fixation rates using N-15(2) tracer, with the goal of fostering transparency in reporting sources of uncertainty in estimates, and to render N-2 fixation rate estimates intercomparable among studies

Nitrogen Fixation in Mesoscale Eddies of the North Pacific Subtropical Gyre: Patterns and Mechanisms

Mesoscale eddies have been shown to support elevated dinitrogen (N2) fixation rates (NFRs) and abundances of N2-fixing microorganisms (diazotrophs), but the mechanisms underlying these observations are not well understood. We sampled two pairs of mesoscale cyclones and anticyclones in the North Pacific Subtropical Gyre in 2017 and 2018 and compared our observations with seasonal patterns from the Hawaii Ocean Time-series (HOT) program. Consistent with previous reports, we found that NFRs were anomalously high for this region (up to 3.7-fold above previous monthly HOT observations) in the centers of both sampled anticyclones. In 2017, these elevated rates coincided with high concentrations of the diazotroph Crocosphaera. We then coupled our field-based observations, together with transcriptomic analyses of nutrient stress marker genes and ecological models, to evaluate the role of biological (via estimates of growth and grazing rates) and physical controls on populations of Crocosphaera, Trichodesmium, and diatom symbionts at the mesoscale. Our results suggest that increased Crocosphaera abundances in the 2017 anticyclone resulted from the alleviation of phosphate limitation, allowing cells to grow at rates exceeding grazing losses. In contrast, distributions of larger, buoyant taxa (Trichodesmium and diatom symbionts) appeared less affected by eddy-driven biological controls. Instead, they appeared driven by physical dynamics along frontal boundaries that separate cyclonic and anticyclonic eddies. No examined controls were able to explain our 2018 findings of higher NFRs in the anticyclone. A generalized explanation of elevated NFRs in mesoscale eddies remains challenging due to the interplay of eddy-driven bottom-up, top-down, and physical control mechanisms.This work was funded by the Simons Foundation (Award # 721252 to DMK, 721256 to AEW, 721223 to EFD, 721221 to MJC, 721244 to EVA, 721225 to STD, 329108 to SJ, and 724220 to JPZ) and expedition funding from the Schmidt Ocean Institute for R/V Falkor Cruise FK180310 in 2018.Peer reviewe

Nitrogen Fixation in Mesoscale Eddies of the North Pacific Subtropical Gyre: Patterns and Mechanisms

Mesoscale eddies have been shown to support elevated dinitrogen (N2) fixation rates (NFRs) and abundances of N2-fixing microorganisms (diazotrophs), but the mechanisms underlying these observations are not well understood. We sampled two pairs of mesoscale cyclones and anticyclones in the North Pacific Subtropical Gyre in 2017 and 2018 and compared our observations with seasonal patterns from the Hawaii Ocean Time-series (HOT) program. Consistent with previous reports, we found that NFRs were anomalously high for this region (up to 3.7-fold above previous monthly HOT observations) in the centers of both sampled anticyclones. In 2017, these elevated rates coincided with high concentrations of the diazotroph Crocosphaera. We then coupled our field-based observations, together with transcriptomic analyses of nutrient stress marker genes and ecological models, to evaluate the role of biological (via estimates of growth and grazing rates) and physical controls on populations of Crocosphaera, Trichodesmium, and diatom symbionts at the mesoscale. Our results suggest that increased Crocosphaera abundances in the 2017 anticyclone resulted from the alleviation of phosphate limitation, allowing cells to grow at rates exceeding grazing losses. In contrast, distributions of larger, buoyant taxa (Trichodesmium and diatom symbionts) appeared less affected by eddy-driven biological controls. Instead, they appeared driven by physical dynamics along frontal boundaries that separate cyclonic and anticyclonic eddies. No examined controls were able to explain our 2018 findings of higher NFRs in the anticyclone. A generalized explanation of elevated NFRs in mesoscale eddies remains challenging due to the interplay of eddy-driven bottom-up, top-down, and physical control mechanisms.This work was funded by the Simons Foundation (Award # 721252 to DMK, 721256 to AEW, 721223 to EFD, 721221 to MJC, 721244 to EVA, 721225 to STD, 329108 to SJ, and 724220 to JPZ) and expedition funding from the Schmidt Ocean Institute for R/V Falkor Cruise FK180310 in 2018.Peer reviewe

Friction factors and rheological behavior of Australian honey in a straight pipe

The pumping characteristics of four Australian honey samples were investigated in a straight pipe. Six flow rates (100-500 kg h(-1)) were studied at three temperatures (35-50degreesC). The pressure loss increased with an increase in the length of the pipe, as the low rate was increased and as the temperature was reduced. In the 25.4 mm-pipe, the Reynolds number ranged from 0.2-32.0 and are substantially less than the critica value (2040-2180) for laminar condition in the system. The relationship between the wall shear stress and shear rate approximated power-law behaviour, and the power-law index was not significantly (p>0.05) different from 1.0. The honey samples exhibited Newtonian behaviour at all the temperatures and this was confirmed by rheometric studies using Couette geometry. A friction chart was generated independent of temperature and the type of honey. An equation was developed to predict the pressure loss of the honey in a typical pipeline at any temperature once the viscosity-temperature relationship had been established