Drought is a complex phenomenon resulting from interactions between physical and social systems that impacts both societies and ecosystems around the world yearly. Drought risks are systemic and increasing (UNDRR, 2021). In 2019, scientists from African countries urged the world to end the drought in drought research and asked for more support to better identify and prepare for drought disasters (Padma, 2019). Preparing for future drought requires a thorough understanding of the complexities of systemic drought risk and adaptation feedbacks, effective drought risk management, and good communication of the risk and potential adaptation options. In the special issue on Drought, society, and ecosystems, we aim to showcase the diverse interdisciplinary research being done on the interactions between drought, ecosystems, and people (including human-induced climate change and management).

We solicit contributions (commentaries, review articles, original research articles) from different perspectives on this interdisciplinary topic from research scientists of different fields, students, practitioners, and stakeholders. It will be a unique opportunity to further our understanding of drought risk, adaptation, and feedbacks. The co-listing of this special issue under the Copernicus journals Natural Hazards and Earth System Sciences (NHESS), Hydrology and Earth System Sciences (HESS), Geoscience Communication (GC), and Biogeosciences (BG) allows for more diversity in perspectives.

Abstracts that fall under one of the following (or related) themes will be considered:

• drought risk analysis
• drought impact attribution
• water security in diverse contexts
• drought risk management and communication
• co-creation of drought information services
• drought in (socio-)ecological systems
• drought and the food–water–energy–environment nexus
• influence of human activities on drought hazard
• socio-hydrology of human–drought interactions
• drought and vulnerability (in ecological and/or social systems)
• drought adaptation (in ecological and/or social systems)
• climate change impacts on drought/water security.

Manuscripts with diverse authorships and with case studies in different geographic regions are especially welcomed. The special issue arises from the International Association of Hydrological Sciences (IAHS) Panta Rhei working group Drought in the Anthropocene and aims to showcase the research done on drought–society–ecosystem interactions during the IAHS Panta Rhei decade (2013–2023). However, unsolicited contributions are also highly encouraged. The guest editors aim for diversity and balance in contributions and authors, encouraging researchers from countries underrepresented in science, women, and minorities to contribute to this special issue.

References:

UNDRR: GAR Special Report on Drought 2021, United Nations Office for Disaster Risk Reduction, ISBN 9789212320274, 2021.

Padma, T.V.: African nations push UN to improve drought research, Nature, 573, 319-320, https://doi.org/10.1038/d41586-019-02760-9, 2019.

This special issue investigates the fascinating interactions between the geosphere and biosphere that are active near the Earth’s surface and within the critical zone. The special issue focuses on modelling and observational studies conducted along the extreme climate and ecological gradient of the Chilean Coastal Cordillera. We welcome submissions that address the physical or chemical processes whereby micro-organisms, animals, and plants influence the shape and development of the Earth’s surface over timescales ranging from the present day to the distant geologic past. As this topic is highly interdisciplinary in scope, we encourage submissions from diverse disciplines including geology, geography, geochemistry, surface geophysics, ecology, microbiology, soil sciences, hydrogeology, geomorphology, and climatology. Integrative studies that bridge between disciplines are particularly welcome. Two types of manuscript submissions are possible. These include either original and/or new scientific full-length research articles presenting new data and interpretations or review and synthesis papers that are invited by the editors and integrate data from different studies and disciplines to address state-of-the-science questions related to Earth surface shaping by biota.

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.

Ocean alkalinity enhancement is one of several ocean-based carbon dioxide removal (CDR) approaches that are currently under evaluation. By increasing alkalinity of the seawater, dissolved carbon dioxide is converted to bicarbonate and carbonate ions, thereby allowing alkalinity-enhanced seawater to absorb more carbon dioxide from the atmosphere.

There are several different methods by which ocean alkalinity can be enhanced. These include the spreading of fine-grained natural or manufactured minerals in coastal or open-ocean settings, electrochemical production of alkaline compounds, electrochemical removal of hydrochloric acid, and a combination of the aforementioned, among others. Following the alkalinity enhancement, either carbon dioxide is absorbed passively from the atmosphere through natural air–sea gas exchange dynamics or carbon dioxide sourced from the atmosphere may be added directly to treated water prior to its release into the ocean. While ocean alkalinity enhancement could be an effective, durable, and scalable CDR strategy, the environmental impacts, both intended and unintended, are not well understood.

This special issue explores a range of biological and ecological impacts associated with alkalinity enhancement and approaches for monitoring strategies in order to safely scale scientific research in the field. The target audience of this special issue includes not only the ocean alkalinity enhancement research community but also those involved in making decisions about the funding, permitting, and monitoring of potential field trials and pilot-scale studies. In keeping with our mission to publish all valid research, we consider negative and null results.

Submission is open to research within, but is not limited to, the following scope:

• biological and ecological impacts of ocean alkalinity enhancement, including those related to secondary abiotic changes (changes in trace metal concentrations, turbidity, etc.) as documented through manipulative lab, mesocosm, and field experiments; natural analogues; and computer models;
• mitigation of harmful biological or ecological impacts associated with ocean acidification;
• reversibility of harmful biological or ecological impacts;
• spatial differences in biological or ecological impacts across ocean regions or habitats;
• discussion and/or modelling of environmental monitoring strategies for field trials, pilot studies, or large-scale applications;
• review papers or meta-analyses on any of the above.

The following related topics are out of scope:

• research focused solely on abiotic processes (e.g. dissolution kinetics, estimates of CO₂ removal potential),
• techno-economic and life-cycle analyses,
• social impact studies.

To accelerate high-quality submissions, the first 10 accepted manuscripts (limited to two publications per research grant) will be offered financial support of up to  1800 Euro to offset either publication fees or costs for conference travel.

Financial support is provided the Ocean Alkalinity Enhancement (OAE) R&D Program, a multi-funder effort incubated by Additional Ventures and fiscally sponsored by the Windward Fund. The Program is partnering with CEA Consulting to support this Special Issue. Please direct all questions to Lydia Kapsenberg (lydia(at)ceaconsulting.com).

This special issue features a series of papers that were presented at the sixth iLEAPS–OzFlux Joint Conference in Tāmaki Makaurau / Auckland, New Zealand, in January 2023. It includes local to global reports on trace gas emissions, soil microbial interactions, eddy covariance observations, and vegetation responses under global environmental change. The purpose is to give a broad overview of some of the latest research in land surface–atmosphere interactions on various temporal and spatial scales.

Oxygen (O₂) is considered an effective indicator of ocean health and climate change. Its level, distribution and variability from sub-seasonal to multidecadal scales provide relevant information on the physical and biogeochemical functioning of the ocean and often act to constrain life in the ocean.

Current evidence indicates that the coastal (i.e., most directly influenced by land) and open-ocean areas have been losing O₂ since the middle of the last century, with consequences for living organisms and biogeochemical cycles that are not yet fully understood. In the open ocean the O₂ inventory has decreased by a few percent (i.e., 0.5–3%), and oxygen minimum zones (OMZs) are expanding, which is primarily attributed to global warming, although a quantitative understanding is still lacking. The number of hypoxic coastal sites has increased, predominantly in response to worldwide eutrophication, yet trends in deoxygenation in the global coastal zone remain ill-defined.

The development and extension of low-O₂ concentration areas degrade the living conditions and contract the metabolically viable habitat for a large number of pelagic, mesopelagic, and benthic organisms. The effects on individuals exposed to low O₂ can result in altered foodweb structures.

Deoxygenation affects many aspects of the ecosystem services provided by the ocean and coastal waters. For example, deoxygenation effects on fisheries include low oxygen affecting populations through reduced recruitment and population abundance and also through altered spatial distributions of the harvested species causing changes in fishing activity. This can lead to changes in the profitability of the fisheries and can affect the interpretation of the monitoring data, leading to misinformed management advice.

Model simulations for this century project a decrease in oxygen under both high- and low-CO₂ emission scenarios, while the projections of the coastal ocean at the land–ocean interface indicate that eutrophication will likely continue in many regions of the world. Warming is expected to further amplify deoxygenation in coastal areas influenced by eutrophication by strengthening and extending stratification.

This special issue will investigate new developments and insights related to low-oxygen environments and deoxygenation in open and coastal waters.

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.

Review process: This inter-journal special issue co-lists papers of different journals. Thereby, each paper was submitted to 1 particular journal and underwent the regular interactive peer-review process of that journal. Depending on the journal, the peer review was handled by regular members of the editorial board and/or by guest editors designated by the journal’s chief/executive editors.

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.

Review process: This inter-journal special issue co-lists papers of different journals. Thereby, each paper was submitted to 1 particular journal and underwent the regular interactive peer-review process of that journal. Depending on the journal, the peer review was handled by regular members of the editorial board and/or by guest editors designated by the journal’s chief/executive editors.

The Tropospheric Ozone Assessment Report (TOAR-II) is an official activity of the International Global Atmospheric Chemistry Project (IGAC) (https://igacproject.org/activities/TOAR/TOAR-II). The goal of TOAR-II is to build on the success of TOAR-I (2014–2019) and produce an updated assessment of tropospheric ozone’s global distribution and trends from the surface to the tropopause. This effort will include an analysis of the impacts of ozone on human health, crop and ecosystem productivity, and climate. Original tropospheric ozone research conducted by the TOAR-II community may be submitted to one of six Copernicus journals (ACP/AMT/BG/GMD/ESSD/ASCMO), and the accepted papers will be linked through the TOAR-II Community Special Issue.

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.

Review process: This inter-journal special issue co-lists papers of different journals. Thereby, each paper was submitted to 1 particular journal and underwent the regular interactive peer-review process of that journal. Depending on the journal, the peer review was handled by regular members of the editorial board and/or by guest editors designated by the journal’s chief/executive editors.

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.

Review process: This inter-journal special issue co-lists papers of different journals. Thereby, each paper was submitted to 1 particular journal and underwent the regular interactive peer-review process of that journal. Depending on the journal, the peer review was handled by regular members of the editorial board and/or by guest editors designated by the journal’s chief/executive editors.

This special issue features a series of papers that were presented at the sixth iLEAPS–OzFlux Joint Conference in Tāmaki Makaurau / Auckland, New Zealand, in January 2023. It includes local to global reports on trace gas emissions, soil microbial interactions, eddy covariance observations, and vegetation responses under global environmental change. The purpose is to give a broad overview of some of the latest research in land surface–atmosphere interactions on various temporal and spatial scales.

The Tropospheric Ozone Assessment Report (TOAR-II) is an official activity of the International Global Atmospheric Chemistry Project (IGAC) (https://igacproject.org/activities/TOAR/TOAR-II). The goal of TOAR-II is to build on the success of TOAR-I (2014–2019) and produce an updated assessment of tropospheric ozone’s global distribution and trends from the surface to the tropopause. This effort will include an analysis of the impacts of ozone on human health, crop and ecosystem productivity, and climate. Original tropospheric ozone research conducted by the TOAR-II community may be submitted to one of six Copernicus journals (ACP/AMT/BG/GMD/ESSD/ASCMO), and the accepted papers will be linked through the TOAR-II Community Special Issue.

Drought is a complex phenomenon resulting from interactions between physical and social systems that impacts both societies and ecosystems around the world yearly. Drought risks are systemic and increasing (UNDRR, 2021). In 2019, scientists from African countries urged the world to end the drought in drought research and asked for more support to better identify and prepare for drought disasters (Padma, 2019). Preparing for future drought requires a thorough understanding of the complexities of systemic drought risk and adaptation feedbacks, effective drought risk management, and good communication of the risk and potential adaptation options. In the special issue on Drought, society, and ecosystems, we aim to showcase the diverse interdisciplinary research being done on the interactions between drought, ecosystems, and people (including human-induced climate change and management).

We solicit contributions (commentaries, review articles, original research articles) from different perspectives on this interdisciplinary topic from research scientists of different fields, students, practitioners, and stakeholders. It will be a unique opportunity to further our understanding of drought risk, adaptation, and feedbacks. The co-listing of this special issue under the Copernicus journals Natural Hazards and Earth System Sciences (NHESS), Hydrology and Earth System Sciences (HESS), Geoscience Communication (GC), and Biogeosciences (BG) allows for more diversity in perspectives.

Abstracts that fall under one of the following (or related) themes will be considered:

• drought risk analysis
• drought impact attribution
• water security in diverse contexts
• drought risk management and communication
• co-creation of drought information services
• drought in (socio-)ecological systems
• drought and the food–water–energy–environment nexus
• influence of human activities on drought hazard
• socio-hydrology of human–drought interactions
• drought and vulnerability (in ecological and/or social systems)
• drought adaptation (in ecological and/or social systems)
• climate change impacts on drought/water security.

Manuscripts with diverse authorships and with case studies in different geographic regions are especially welcomed. The special issue arises from the International Association of Hydrological Sciences (IAHS) Panta Rhei working group Drought in the Anthropocene and aims to showcase the research done on drought–society–ecosystem interactions during the IAHS Panta Rhei decade (2013–2023). However, unsolicited contributions are also highly encouraged. The guest editors aim for diversity and balance in contributions and authors, encouraging researchers from countries underrepresented in science, women, and minorities to contribute to this special issue.

References:

UNDRR: GAR Special Report on Drought 2021, United Nations Office for Disaster Risk Reduction, ISBN 9789212320274, 2021.

Padma, T.V.: African nations push UN to improve drought research, Nature, 573, 319-320, https://doi.org/10.1038/d41586-019-02760-9, 2019.

Ocean alkalinity enhancement is one of several ocean-based carbon dioxide removal (CDR) approaches that are currently under evaluation. By increasing alkalinity of the seawater, dissolved carbon dioxide is converted to bicarbonate and carbonate ions, thereby allowing alkalinity-enhanced seawater to absorb more carbon dioxide from the atmosphere.

There are several different methods by which ocean alkalinity can be enhanced. These include the spreading of fine-grained natural or manufactured minerals in coastal or open-ocean settings, electrochemical production of alkaline compounds, electrochemical removal of hydrochloric acid, and a combination of the aforementioned, among others. Following the alkalinity enhancement, either carbon dioxide is absorbed passively from the atmosphere through natural air–sea gas exchange dynamics or carbon dioxide sourced from the atmosphere may be added directly to treated water prior to its release into the ocean. While ocean alkalinity enhancement could be an effective, durable, and scalable CDR strategy, the environmental impacts, both intended and unintended, are not well understood.

This special issue explores a range of biological and ecological impacts associated with alkalinity enhancement and approaches for monitoring strategies in order to safely scale scientific research in the field. The target audience of this special issue includes not only the ocean alkalinity enhancement research community but also those involved in making decisions about the funding, permitting, and monitoring of potential field trials and pilot-scale studies. In keeping with our mission to publish all valid research, we consider negative and null results.

Submission is open to research within, but is not limited to, the following scope:

• biological and ecological impacts of ocean alkalinity enhancement, including those related to secondary abiotic changes (changes in trace metal concentrations, turbidity, etc.) as documented through manipulative lab, mesocosm, and field experiments; natural analogues; and computer models;
• mitigation of harmful biological or ecological impacts associated with ocean acidification;
• reversibility of harmful biological or ecological impacts;
• spatial differences in biological or ecological impacts across ocean regions or habitats;
• discussion and/or modelling of environmental monitoring strategies for field trials, pilot studies, or large-scale applications;
• review papers or meta-analyses on any of the above.

The following related topics are out of scope:

• research focused solely on abiotic processes (e.g. dissolution kinetics, estimates of CO₂ removal potential),
• techno-economic and life-cycle analyses,
• social impact studies.

To accelerate high-quality submissions, the first 10 accepted manuscripts (limited to two publications per research grant) will be offered financial support of up to  1800 Euro to offset either publication fees or costs for conference travel.

Financial support is provided the Ocean Alkalinity Enhancement (OAE) R&D Program, a multi-funder effort incubated by Additional Ventures and fiscally sponsored by the Windward Fund. The Program is partnering with CEA Consulting to support this Special Issue. Please direct all questions to Lydia Kapsenberg (lydia(at)ceaconsulting.com).

This special issue investigates the fascinating interactions between the geosphere and biosphere that are active near the Earth’s surface and within the critical zone. The special issue focuses on modelling and observational studies conducted along the extreme climate and ecological gradient of the Chilean Coastal Cordillera. We welcome submissions that address the physical or chemical processes whereby micro-organisms, animals, and plants influence the shape and development of the Earth’s surface over timescales ranging from the present day to the distant geologic past. As this topic is highly interdisciplinary in scope, we encourage submissions from diverse disciplines including geology, geography, geochemistry, surface geophysics, ecology, microbiology, soil sciences, hydrogeology, geomorphology, and climatology. Integrative studies that bridge between disciplines are particularly welcome. Two types of manuscript submissions are possible. These include either original and/or new scientific full-length research articles presenting new data and interpretations or review and synthesis papers that are invited by the editors and integrate data from different studies and disciplines to address state-of-the-science questions related to Earth surface shaping by biota.

Oxygen (O₂) is considered an effective indicator of ocean health and climate change. Its level, distribution and variability from sub-seasonal to multidecadal scales provide relevant information on the physical and biogeochemical functioning of the ocean and often act to constrain life in the ocean.

Current evidence indicates that the coastal (i.e., most directly influenced by land) and open-ocean areas have been losing O₂ since the middle of the last century, with consequences for living organisms and biogeochemical cycles that are not yet fully understood. In the open ocean the O₂ inventory has decreased by a few percent (i.e., 0.5–3%), and oxygen minimum zones (OMZs) are expanding, which is primarily attributed to global warming, although a quantitative understanding is still lacking. The number of hypoxic coastal sites has increased, predominantly in response to worldwide eutrophication, yet trends in deoxygenation in the global coastal zone remain ill-defined.

The development and extension of low-O₂ concentration areas degrade the living conditions and contract the metabolically viable habitat for a large number of pelagic, mesopelagic, and benthic organisms. The effects on individuals exposed to low O₂ can result in altered foodweb structures.

Deoxygenation affects many aspects of the ecosystem services provided by the ocean and coastal waters. For example, deoxygenation effects on fisheries include low oxygen affecting populations through reduced recruitment and population abundance and also through altered spatial distributions of the harvested species causing changes in fishing activity. This can lead to changes in the profitability of the fisheries and can affect the interpretation of the monitoring data, leading to misinformed management advice.

Model simulations for this century project a decrease in oxygen under both high- and low-CO₂ emission scenarios, while the projections of the coastal ocean at the land–ocean interface indicate that eutrophication will likely continue in many regions of the world. Warming is expected to further amplify deoxygenation in coastal areas influenced by eutrophication by strengthening and extending stratification.

This special issue will investigate new developments and insights related to low-oxygen environments and deoxygenation in open and coastal waters.

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.