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Abstract. Across the Southern Ocean in winter, nitrification is the dominantmixed-layer nitrogen cycle process, with some of the nitrate producedtherefrom persisting to fuel productivity during the subsequent growingseason. Because this nitrate constitutes a regenerated rather than a newnutrient source to phytoplankton, it will not support the net removal ofatmospheric CO2. To better understand the controls on Southern Oceannitrification, we conducted nitrite oxidation kinetics experiments insurface waters across the western Indian sector in winter. While allexperiments (seven in total) yielded a Michaelis–Menten relationship withsubstrate concentration, the nitrite oxidation rates only increasedsubstantially once the nitrite concentration exceeded 115±2.3 to245±18 nM, suggesting that nitrite-oxidizing bacteria (NOB) require aminimum (i.e., “threshold”) nitrite concentration to produce nitrate. Thehalf-saturation constant for nitrite oxidation ranged from 134±8 to403±24 nM, indicating a relatively high affinity of Southern OceanNOB for nitrite, in contrast to results from culture experiments. Despitethe high affinity of NOB for nitrite, its concentration rarely declinesbelow 150 nM in the Southern Ocean's mixed layer, regardless of season. Inthe upper mixed layer, we measured ammonium oxidation rates that were two-to seven-fold higher than the coincident rates of nitrite oxidation,indicating that nitrite oxidation is the rate-limiting step fornitrification in the winter Southern Ocean. The decoupling of ammonium andnitrite oxidation, combined with a possible nitrite concentration thresholdfor NOB, may explain the non-zero nitrite that persists throughout theSouthern Ocean's mixed layer year-round. Additionally, nitrite oxidation maybe limited by dissolved iron, the availability of which is low across theupper Southern Ocean. Our findings have implications for understanding thecontrols on nitrification and ammonium and nitrite distributions, both inthe Southern Ocean and elsewhere. 

Measuring plankton and associated variables as part of ocean time-series stations has the potential to revolutionize our understanding of ocean biology and ecology and their ties to ocean biogeochemistry. It will open temporal scales (e.g., resolving diel cycles) not typically sampled as a function of depth. In this review we motivate the addition of biological measurements to time-series sites by detailing science questions they could help address, reviewing existing technology that could be deployed, and providing examples of time-series sites already deploying some of those technologies. We consider here the opportunities that exist through global coordination within the OceanSITES network for long-term (climate) time series station in the open ocean. Especially with respect to data management, global solutions are needed as these are critical to maximize the utility of such data. We conclude by providing recommendations for an implementation plan. 

Phytoplankton photosynthetic physiology can be investigated through single-turnover variable chlorophyll fluorescence (ST-ChlF) approaches, which carry unique potential to autonomously collect data at high spatial and temporal resolution. Over the past decades, significant progress has been made in the development and application of ST-ChlF methods in aquatic ecosystems, and in the interpretation of the resulting observations. At the same time, however, an increasing number of sensor types, sampling protocols, and data processing algorithms have created confusion and uncertainty among potential users, with a growing divergence of practice among different research groups. In this review, we assist the existing and upcoming user community by providing an overview of current approaches and consensus recommendations for the use of ST-ChlF measurements to examine in-situ phytoplankton productivity and photo-physiology. We argue that a consistency of practice and adherence to basic operational and quality control standards is critical to ensuring data inter-comparability. Large datasets of inter-comparable and globally coherent ST-ChlF observations hold the potential to reveal large-scale patterns and trends in phytoplankton photo-physiology, photosynthetic rates and bottom-up controls on primary productivity. As such, they hold great potential to provide invaluable physiological observations on the scales relevant for the development and validation of ecosystem models and remote sensing algorithms.