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.

The distribution of oxygen in the ocean is controlled by physical, biogeochemical and biological processes. Both the supply and consumption of oxygen are sensitive to climate change in ways that are not fully understood. Recent observations suggest that the oxygen content of the ocean is declining (ocean deoxygenation) and the oxygen minimum zones and costal hypoxia sites are expanding with tremendous effects on the ocean’s ecosystems and living organisms. Following the EUR-OCEANS conference on “Ocean Deoxygenation” in Toulouse, France, in 2011, the 46th International Liège Colloquium on “Low Oxygen Environments in Marine, Fresh and Estuarine Waters” in 2014 and the discussion meeting of the Royal Society London on “Ocean Ventilation and Deoxygenation in a Warming World” in 2016, the purpose of the international conference on “Ocean Deoxygenation: Drivers & Consequences – Past|Present|Future” that took place in Kiel, 3–7 Sept 2018 organised by the Collaborative Research Centre 754 “Climate-Biogeochemical Interactions in the Tropical Ocean” was to − focus on the past, present and future state of oxygen in the ocean on global, regional and local scales − analyse mechanisms and feedbacks critical to identify natural and anthropogenic causes of oxygen variability − determine impacts on biogeochemical cycles and ecosystems.

This special issue is open to all submissions on ocean deoxygenation and is not limited to conference participants.

Purpose of special issue: The Collaborative Research Centre 754 “Climate-Biogeochemical Interactions in the Tropical Ocean” has been investigating the newly recognised threat of ocean deoxygenation since 2008. Before the project's end in 2019, the International Conference on Ocean Deoxygenation was organised in Kiel, Germany in September 2018 with over 300 participants (https://conference.sfb754.de/event/1/). Following the conference the proposed inter-journal special issue aims at presenting the recent research advances and highlights in the field of ocean deoxygenation. The special issue shall not be limited to conference participants but be open to all submissions.

After more than 20 years, mining of mineral resources, such as polymetallic nodules and massive sulfides, in the deep sea is back on the agenda of many countries and companies in their search for high-tech raw metals. In this course, the past 10 years have seen an immense increase of exploration licences issued by the International Seabed Authority (ISA), which organizes and controls activities at the ocean floor beyond national jurisdiction, generally termed the "area". The seven pilot investors are renewing their exploration licences for another five years, after which they will need to apply for an exploitation licence (or lose their contracted claim). Thus, ISA is currently developing the regulatory framework for the exploitation of seafloor mineral resources, which includes legislation to "ensure effective protection of the marine environment from harmful effects" according to UNCLOS article 145. In order to support decision-making for ISA's regulations, several benthic impact experiments were conducted more than 20 years ago. However, the impacts have only been studied over the following 5–7 years. Until now, no information has been available for the expected longer-term impacts induced by mining operations, i.e. in case of nodule mining the removal of the surface sediments and dispersal of the suspended particle plume and subsequent seafloor blanketing. Therefore, in 2015 the European project MiningImpact, funded through the Joint Programming Initiative Healthy Seas and Oceans (JPIO), revisited several impact scars at the seafloor in the nodule exploration licence areas of the Clarion–Clipperton Fracture Zone (CCZ) as well as the benthic impact experiment DISCOL (DIsturbance and reCOLonization) in the Peru Basin. Our scientific study provides the first results of environmental consequences on the deep-sea ecosystem that prevail for up to 40 years after a disturbance was created. Also, the impacted areas vary in size from single 1–2 nm long dredge or sled tracks to the 78 criss-crossing, 4 m wide plough harrow marks in the 11 km² large DISCOL experimental area. This special issue presents the scientific results of the biological, biogeochemical, geological, and oceanographic investigations carried out in the MiningImpact project (consisting of 25 institutes from 11 European countries) that is used to provide recommendations for the exploitation mining code of ISA (e.g. improved methods for the management and monitoring of mining operations) and to improve our understanding of the ecological functions of the deep seafloor.

NETCARE (Network on Aerosols and Climate: Addressing Key Uncertainties in Remote Canadian Environments) is a large research network focusing on aerosol–cloud–climate interactions. While Canadian-based, it operates with many international collaborations. It is comprised of scientists working in both atmospheric science and marine biogeochemistry, with particular attention given to a suite of intensive field measurements (with both atmospheric and oceanic components) and model evaluation and development. There are three major research directions within the network: 1. Carbonaceous Aerosol, 2. Arctic Clouds, and 3. Ocean–Atmosphere Interactions. A large amount of the research has an Arctic focus, it being a region especially susceptible to anthropogenic input and experiencing a large degree of biogeochemical change. The website for the network is http://www.netcare-project.ca/ . On the website, there is more information on research activities, field campaign details, modeling activities, data products, and personnel.