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Abstract Marine phytoplankton biomass and chlorophyll-a concentration are often estimated from pigment fluorescence measurements, which have become routine despite known variability in the fluorescent response for a given amount of chlorophyll-a. Here, we present a near-global, monthly climatology of chlorophyll-a fluorescence measurements from profiling floats combined with ocean color satellite estimates of chlorophyll-a concentration to illuminate seasonal biases in the fluorescent response and expand upon previously observed regional patterns in this bias. Global biases span over an order of magnitude, and can vary seasonally by a factor of 10. An independent estimate of chlorophyll-a from light attenuation shows similar global patterns in the chlorophyll-fluorescence bias when compared to biases derived from satellite estimates. Without accounting for these biases, studies or models using fluorescence-estimated chlorophyll-a will inherit the seasonal and regional biases described here. 

The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project has deployed 194 profiling floats equipped with biogeochemical (BGC) sensors, making it one of the largest contributors to global BGC-Argo. Post-deployment quality control (QC) of float-based oxygen, nitrate, and pH data is a crucial step in the processing and dissemination of such data, as in situ chemical sensors remain in early stages of development. In situ calibration of chemical sensors on profiling floats using atmospheric reanalysis and empirical algorithms can bring accuracy to within 3 μmol O 2 kg –1 , 0.5 μmol NO 3 – kg –1 , and 0.007 pH units. Routine QC efforts utilizing these methods can be conducted manually through visual inspection of data to assess sensor drifts and offsets, but more automated processes are preferred to support the growing number of BGC floats and reduce subjectivity among delayed-mode operators. Here we present a methodology and accompanying software designed to easily visualize float data against select reference datasets and assess QC adjustments within a quantitative framework. The software is intended for global use and has been used successfully in the post-deployment calibration and QC of over 250 BGC floats, including all floats within the SOCCOM array. Results from validation of the proposed methodology are also presented which help to verify the quality of the data adjustments through time. 

Abstract Sensors that use ultraviolet (UV) light absorption to measure nitrate in seawater at in situ temperatures require a correction to the calibration coefficients if the calibration and sample temperatures are not identical. This is mostly due to the bromide molecule, which absorbs more UV light as temperature increases. The current correction applied to in situ ultraviolet spectrophotometer (ISUS) and submersible ultraviolet nitrate analyzer (SUNA) nitrate sensors generally follows Sakamoto et al. (2009, Limnol. Oceanogr. Methods 7, 132–143). For waters warmer than the calibration temperature, this correction model can lead to a 1–2 μmol kg⁻¹positive bias in nitrate concentration. Here we present an updated correction model, which reduces this small but noticeable bias by at least 50%. This improved model is based on additional laboratory data and describes the temperature correction as an exponential function of wavelength and temperature difference from the calibration temperature. It is a better fit to the experimental data than the current model and the improvement is validated using two populations of nitrate profiles from Biogeochemical Argo floats navigating through tropical waters. One population is from floats equipped with ISUS sensors while the other arises from floats with SUNA sensors on board. Although this model can be applied to both ISUS and SUNA nitrate sensors, it should not be used for OPUS UV nitrate sensors at this time. This new approach is similar to that used for OPUS sensors (Nehir et al., 2021, Front. Mar. Sci. 8, 663800) with differing model coefficients. This difference suggests that there is an instrumental component to the temperature correction or that there are slight differences in experimental methodologies. 

Abstract Measurements of pH and nitrate from the Southern Ocean Carbon and Climate Observations and Modeling array of profiling floats were used to assess the ratios of dissolved inorganic carbon (DIC) and nitrate (NO₃) uptake during the spring to summer bloom period throughout the Southern Ocean. Two hundred and forty‐three bloom periods were observed by 115 floats from 30°S to 70°S. Similar calculations were made using the Takahashi surface DIC and nitrate climatology. To separate the effects of atmospheric CO₂exchange and mixing from phytoplankton uptake, the ratios of changes in DIC to nitrate of surface waters (ΔDIC/ΔNO₃) were computed in the Biogeochemical Southern Ocean State Estimate (B‐SOSE) model. Phytoplankton uptake of DIC and nitrate are fixed in B‐SOSE at the Redfield Ratio (RR; 6.6 mol C/mol N). Deviations in the B‐SOSE ΔDIC/ΔNO₃must be due to non‐biological effects of CO₂gas exchange and mixing. ΔDIC/ΔNO₃values observed by floats and in the Takahashi climatology were corrected for the non‐biological effects using B‐SOSE. The corrected, in situ biological uptake ratio (C:N) occurs at values similar to the RR, with two major exceptions. North of 40°S biological DIC uptake is observed with little or no change in nitrate giving high C:N. In the latitude band at 55°S, the Takahashi data give a low C:N value, while floats are high. This may be due to a change in CO₂air‐sea exchange in this region from uptake during the Takahashi reference year of 2005 to outgassing of CO₂during the years sampled by floats.