Description: Protecting the ocean has become a major goal of international policy as human activities increasingly endanger the integrity of the ocean ecosystem, often summarized as “ocean health.” By and large, efforts to protect the ocean have failed because, among other things, (1) the underlying socio-ecological pathways have not been properly considered, and (2) the concept of ocean health has been ill defined. Collectively, this prevents an adequate societal response as to how ocean ecosystems and their vital functions for human societies can be protected and restored. We review the confusion surrounding the term “ocean health” and suggest an operational ocean-health framework in line with the concept of strong sustainability. Given the accelerating degeneration of marine ecosystems, the restoration of regional ocean health will be of increasing importance. Our advocated transdisciplinary and multi-actor framework can help to advance the implementation of more active measures to restore ocean health and safeguard human health and well-being.

Description: Highlights: • The epoch of the Anthropocene, a period during which human activity has been the dominant influence on climate and the environment, has witnessed a decline in oxygen concentrations and an expansion of oxygen-depleted environments in both coastal and open ocean systems since the middle of the 20th century. • This review paper provides a synthesis of system-specific drivers of low oxygen in a range of case studies representing marine systems in the open ocean, on continental shelves, in enclosed seas and in the coastal environment. • Identification of similar and contrasting responses within and across system types and corresponding oxygen regimes is shown to be informative both in understanding and isolating key controlling processes and provides a sound basis for predicting change under anticipated future conditions. • Case studies were selected to achieve a balance in system diversity and global coverage. • Each case study describes system attributes, including the present-day oxygen environment and known trends in oxygen concentrations over time. • Central to each case study is the identification of the physical and biogeochemical processes that determine oxygen concentrations through the tradeoff between ventilation and respiration. • Spatial distributions of oxygen and time series of oxygen data provide the opportunity to identify trends in oxygen availability and have allowed various drivers of low oxygen to be distinguished through correlative and causative relationships. • Deoxygenation results from a complex interplay of hydrographic and biogeochemical processes and the superposition of these processes, some additive and others subtractive, makes attribution to any particular driver challenging. • System-specific models are therefore required to achieve a quantitative understanding of these processes and of the feedbacks between processes at varying scales. Abstract: The epoch of the Anthropocene, a period during which human activity has been the dominant influence on climate and the environment, has witnessed a decline in oxygen concentrations and an expansion of oxygen-depleted environments in both coastal and open ocean systems since the middle of the 20th century. This paper provides a review of system-specific drivers of low oxygen in a range of case studies representing marine systems in the open ocean, on continental shelves, in enclosed seas and in the coastal environment. Identification of similar and contrasting responses within and across system types and corresponding oxygen regimes is shown to be informative both in understanding and isolating key controlling processes and provides a sound basis for predicting change under anticipated future conditions. Case studies were selected to achieve a balance in system diversity and global coverage. Each case study describes system attributes, including the present-day oxygen environment and known trends in oxygen concentrations over time. Central to each case study is the identification of the physical and biogeochemical processes that determine oxygen concentrations through the tradeoff between ventilation and respiration. Spatial distributions of oxygen and time series of oxygen data provide the opportunity to identify trends in oxygen availability and have allowed various drivers of low oxygen to be distinguished through correlative and causative relationships. Deoxygenation results from a complex interplay of hydrographic and biogeochemical processes and the superposition of these processes, some additive and others subtractive, makes attribution to any particular driver challenging. System-specific models are therefore required to achieve a quantitative understanding of these processes and of the feedbacks between processes at varying scales.

Description: Highlights: • Adriatic coastal area: simultaneous atmospheric and seawater field measurements • Nutrient variability in aerosols, rainwater and atmospheric deposition (AD) fluxes • Local open-fire biomass burning (BB) affected chemistry of atmospheric samples. • AD impacted nutrient levels and N:P ratios in the sea surface microlayer (SML). • The SML plankton development and organic matter enrichments followed BB episodes. Abstract: Atmospheric deposition (AD) of nutrients and its impact on the sea surface requires consideration of interfacial processes within the sea surface microlayer (SML), the ocean-atmosphere boundary layer of major importance for many global biogeochemical and climate-related processes. This study comprised a comprehensive dataset, including dissolved NO3−, NH4+ and PO43− in ambient aerosol particles, wet deposition and sea surface samples collected from February to July 2019 at a central Adriatic coastal site. The aerosol mean concentration of dissolved nitrogen (DIN = NO3− + NH4+) and PO43− were 48.8 ± 82.8 μmol m−3 and 0.8 ± 0.6 μmol m−3, respectively, while their total fluxes (dry + wet) ranged from 24.2 to 212.3 μmol m−2 d−1 (mean 123.2 ± 53.2 μmol m−2 d−1) and from 1.2 to 2.1 μmol m−2 d−1 (mean 1.5 ± 0.3 μmol m−2 d−1), respectively. Intensive local episodes of open biomass burning (BB) significantly increased aerosol DIN concentrations as well as DIN deposition fluxes, particularly altering the molar DIN/PO43− ratio of atmospheric samples. The DIN temporal patterns showed high variability in the SML (range 0.2–24.6 μmol L−1, mean 5.0 ± 7.1 μmol L−1) in contrast to the underlying water samples (range 0.5–4.2 μmol L−1, mean 1.9 ± 1.2 μmol L−1), with significant increases during BB periods. Variability in abundance of heterotrophic bacteria and autotrophs in the SML along with concentrations of bulk dissolved and particulate organic carbon as well as dissolved and particulate lipids and carbohydrates, gel particles and surfactants followed DIN enhancements with a two-week delay. This study showed that AD can affect the short-term scale enrichments of organic matter in the SML, especially when accompanied by BB emissions typical of the overall Mediterranean coastal environment. This could have strong implications for global air-sea exchange processes, including those of climate relevant gases, mediated by the SML.

Description: Highlights: • Higher representation of picophytoplankton in land-terminating glacier fjord. • Smaller phytoplankton cells associated with glacial retreat. • Intermediate baroclinic circulation influences phytoplankton distribution. • Glacial retreat likely to have major implications for summer productivity. Abstract: Along Greenland's coastline, the magnitude and timing of primary production in fjords is influenced by meltwater release from marine-terminating glaciers. How local ecosystems will adapt as these glaciers retreat onto land, forcing fundamental changes in hydrography, remains an open question. To further our understanding of this transition, we examine how marine- and land-terminating glaciers respectively influence fjord bloom phenology. Between spring and autumn 2019, we conducted along-fjord transects of hydrographic variables, biogeochemical properties and pico- and nanophytoplankton counts to illustrate the contrasting seasonal bloom dynamics in the fjords Nuup Kangerlua and Ameralik. These fjords are in the same climatic region of west Greenland but influenced by different glacial structures. Nuup Kangerlua, a predominantly marine-terminating system, was differentiated by its sustained second summer bloom and high Chl a fluorescence in summer and autumn. In Ameralik, influenced by a land-terminating glacier, we found higher abundances of pico- and nanophytoplankton, and high cyanobacteria growth in autumn. The summer bloom in Nuup Kangerlua is known to be coincident with subglacial freshwater discharge sustaining renewed nutrient supply to the fjord. We observe here that the intermediate baroclinic circulation, which creates an inflow at subsurface depths, also plays an important role in increasing nutrient availability at shallower depths and potentially explains the distribution of primary producers. Our observations suggest that the retreat of marine-terminating glaciers onto land, with consequent increases in surface water temperature and stratification, and reduced light availability, may alter the magnitude, composition, and distribution of summer productivity.