Over the last 2 decades, studies have shown that the sea-surface microlayer covers large areas of the global ocean and, under certain conditions, can even form biofilm-like (and thus unique) habitats between the ocean and the atmosphere. Due to its recurrent biofilm-like features and unique position at the atmosphere–ocean interface, the microlayer is central to a range of global biogeochemical and climate-related processes, including marine carbon cycling, air–sea gas exchange, physical surface processes, and aerosol production. Despite the fact that processes occurring in and across the microlayer have received increasing attention during recent years, our knowledge about the microlayer remains rudimentary. New approaches and technologies have been developed to understand the formation and function of the microlayer on a mechanistic level. For example, the BASS (Biogeochemical processes and Air–sea exchange in the Sea-Surface microlayer; https://uol.de/en/bass) group has been funded to explore the significance of the microlayer as a bio- and photochemical reactor in an unprecedented comprehensive and interdisciplinary approach, and BASS findings will contribute to this special issue (SI). The purpose of the SI is twofold: (1) to present outcomes of a BASS mesocosm study and (2) to present further contributions on the microlayer from the wider research community based on field, mesocosm, modelling, and laboratory studies. This SI will bring together the latest scientific studies to describe the microlayer as an interface between the ocean and atmosphere across different disciplines.

Mercury (Hg) is a chemical pollutant of human health concern worldwide; a consequence of anthropogenic activities; and the focus of the Minamata Convention on Mercury (MC; https://minamataconvention.org/en), an international treaty to protect human health and the environment from the adverse effects of mercury. The MC entered into force on 16 August 2017 and committed to limiting the use and environmental release of mercury. Also, the 1998 Protocol on Heavy Metals of the 1979 Convention on Long-Range Transboundary Air Pollution (LRTAP) commits parties to mitigating emissions of mercury (as well as cadmium and lead) from a variety of point sources and provides guidance on mitigating emissions associated with heavy metal use in manufactured products. The MC framework requires an evaluation of the effectiveness of its measures in meeting the objectives beginning no later than 6 years after the convention’s entry into force and periodically thereafter. The Protocol on Heavy Metals requires a periodic review of the progress towards meeting the obligations in the protocol and the sufficiency and effectiveness of those obligations and an evaluation of whether additional emission reductions are warranted.

This multi-journal special issue (SI) is intended to develop the required information that can be scientifically exploited to address key policy questions of the conventions: (1) what are the contributions of anthropogenic emissions and releases and other Hg sources to current Hg levels observed in air, biota, humans, and other media? (2) How have these contribution levels changed over time and over the timeline of the convention? (3) How do the contribution levels and their trends vary geographically at the global scale? (4) What are the contributions of anthropogenic emissions and releases and other drivers to the temporal trends in observed Hg levels across global regions? (5) How are observed Hg levels expected to change in the future?

The special issue aims at collecting relevant research advances arising from the design, implementation, and results of the Multi-Compartment Hg Modeling and Analysis Project (MCHgMAP) and from the scientific community on all aspects of biogeochemical mercury cycling, including primary and secondary emissions, observations, process studies, and single to multi-compartmental and statistical model development and application. A challenge of analysing the fate of emitted mercury is that it can recycle between the atmosphere, land, and ocean, and as a result, past and present emissions can continue to affect the environment on timescales of decades to centuries. MCHgMAP is an ensemble modelling initiative developed to inform the effectiveness of evaluations of the MC and LRTAP, utilizing a coordinated modelling approach between single-medium (atmosphere, land, and ocean) and multi-media mercury models to consistently simulate the changing global and regional environmental Hg cycling and analyse its drivers. The SI includes an overview paper on MCHgMAP, describing its scientific background and design (an important and crucial preparatory stage), which will be referenced by the individual papers on this project that follow.

SCOR (Scientific Committee on Oceanic Research) Working Group 167 (Reducing Uncertainty in Soluble aerosol Trace Element Deposition, RUSTED), appointed in October 2022, brings together experts from the atmospheric chemistry, ocean biogeochemistry, and modelling communities. Aiming to reduce uncertainties in soluble aerosol trace element deposition, RUSTED will quantitatively assess different aerosol leaching schemes; formulate standard operating procedures (SOPs) for frequently used aerosol leaching schemes; and develop a user-friendly, open-access database of aerosol trace element data which includes advice on the use of the data in Earth system models.

In this special issue, we propose to curate cutting-edge studies which advance our knowledge of the deposition of soluble aerosol trace elements and their impacts on marine ecosystems. We also encourage the submission of manuscripts which address challenges and/or report recent advances in the field of aerosol trace element deposition from researchers outside the working group.

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.