Fostering the establishment of a global ocean observing system, Walter Munk defined the 20th century as the “century of undersampling”, and this is especially true for marine biogeochemical observations. Biogeochemical Argo is bringing us into a new era that is characterized by an unprecedented availability of high-resolution biogeochemical profiles, delivered throughout the year and globally distributed. Thanks to these observations, the dynamics of ocean carbon, oxygen, nutrients, primary producers, pH, and bio-optical properties can be examined in all three spatial dimensions. The increased availability of these observations allows us to unravel biogeochemical processes and to develop and validate hypotheses across a range of spatial and temporal scales. These new observations will also substantially improve the quality of biogeochemical models by allowing vigorous validation, improved parameterizations, and formal data assimilation.

In this special issue, we welcome contributions leveraging the high data availability provided by BGC-Argo floats combined with theoretical or numerical models, data assimilation, machine learning, multi-platform sensors, or other novel interpretative methodologies that expand the surface view of ocean ecosystem dynamics into the vertical dimension.

The purpose of the special issue is to collect the high-quality contributions presented at the oral and poster sessions entitled “Biogeochemistry in the BGC-Argo Era: From Process Studies to Ecosystem Forecasts” held at Ocean Sciences Meeting 2020 in San Diego. Moreover, the special issue is open to other relevant contributions.

Compound weather and climate events refer to combinations of multiple weather and climate drivers and/or hazards that lead to potentially large impacts. Compound events encompass a highly diverse set of events including concurrent climate extremes but also an array of nonstandard high-impact events. Consequently, research on compound events requires expertise from a variety of disciplines such as climate science, hydrology, impact modelling, engineering, and statistics, among others. The European COST Action DAMOCLES (http://damocles.compoundevents.org) provides a platform for compound event research and for generating synergies between the different research domains. This inter-journal special issue will serve as an outlet for the work within DAMOCLES and aims to advance our understanding of different aspects of compound events, including present-day and future risk assessments of such events, modelling of individual compound events, bottom-up approaches to identify new types of compound events, and novel model evaluation techniques. It is open for all submissions within its scope.

The Peruvian upwelling system is one of the most productive ecosystems in the ocean. Continuous upwelling of nutrient-rich water generates high primary production, supporting an efficient food web and leading to high fishery yields. An extensive oxygen minimum zone underlying the productive surface waters gives rise to high losses of fixed nitrogen via denitrification and anaerobic ammonium oxidation. This results in nutrient ratios of upwelled waters deviating strongly from Redfield stoichiometry, with a high N deficit and excess P. How this unusual nutrient stoichiometry affects plankton community production and succession and how this feeds back on biogeochemical cycling are still not well understood. To address these questions, an in situ mesocosm experiment was conducted in the coastal upwelling system off Peru in the framework of the Collaborative Research Center 754 "Climate-Biogeochemistry Interactions in the Tropical Ocean". The field experiment took place from January to April 2017 during a coastal El Niño, a rare event of El Niño conditions confined to Peruvian coastal waters. Key objectives of this mesocosm study, which involved 57 researchers from 10 nations, were to unravel the interplay of pelagic ecology and biogeochemistry under the unique environmental conditions of the Peruvian upwelling system. This special issue in Biogeosciences is intended as a collective outlet for publications resulting from this study, covering the broad range of research topics from seawater chemistry to plankton ecology and pelagic biogeochemistry.

The last 50 years have enabled significant progress in our ability to describe and model both the oceanic and atmospheric environments of the Indian Ocean and their interactions with the sea floor and coastal regimes. However, our understanding of the major processes of the Indian Ocean is still far from complete and is rudimentary in many respects. This SI compiles various synthesis articles about the recent results of the 2nd International Indian Ocean Expedition Program (IIOE-2: https://iioe-2.incois.gov.in/). The synthesis articles will review and highlight results of physical, biogeochemical, ecological, geological, coastal and atmospheric studies from all parts of the Indian Ocean. Moreover, the SI will serve to illustrate some of the key questions that are relevant for future research on the grand challenges in the Indian Ocean system.

In the vast low-nutrient—low-chlorophyll (LNLC) ocean, the vertical nutrient supply from the subsurface to the sunlit surface waters is low, and atmospheric contribution of nutrients may be 1 order of magnitude greater over short timescales. Recently, significant experimental, field and modeling work allowed us to better link atmospheric deposition with nutrient availability, ocean productivity, carbon cycling and marine emissions. By improving our understanding on how atmospheric inputs are impacting biological activity and the carbon balance in oligotrophic environments, their representation in biogeochemical models at different timescales and space scales will be better assessed. In particular, processes may be affected by ongoing environmental changes in the ocean such as a decrease in pH and an increase in temperature.

This special issue aims at gathering contributions from both experimental and modeling approaches showing how natural or anthropogenic inputs from the atmosphere can impact biogeochemical and ecological processes in the present and future oligotrophic ocean. It will present the results obtained during the PEACETIME (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea) cruise conducted in 2017 in the Mediterranean Sea, but the special issue is open to any submissions provided that the subject is consistent with the objectives defined above.