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 oxygendepleted 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.

In February and March 2017, a coastal El Niño caused extraordinary heavy rains and a rise in water temperatures along the coast of northern Peru. In this work, we document the impacts of this phenomenon on the artisanal fisheries and the scallop aquaculture sector, both of which represent important socio-economic activities for the province of Sechura. Despite the perceived absence of effective disaster management and rehabilitation policies, resource users opted for a wide range of different adaptation strategies and are currently striving towards recovery. One year after the event, the artisanal fisheries fleet has returned to operating almost on a normal scale, while the aquaculture sector is still drastically impacted, with many people continuing to work in different economic sectors and even in other regions of the country. Recovery of the social-ecological system of Sechura likely depends on the occurrence of scallop seed and the financial capacity of small-scale producers to reinitiate scallop cultures. Long-term consequences of this coastal El Niño are yet to be studied, though the need to develop trans-local and trans-sectoral management strategies for coping with disturbance events of this scale is emphasized.

Highlights ► Hypoxia has become a worldwide phenomenon in the coastal ocean. ► We proposed stylized biological facts about the effect of hypoxia on marine organisms. ► Based on these facts we established assumptions in the framework of the DEB theory. ► We validated some of these assumptions on three marine species. ► We discuss the main issues that need to be dealt with in future works.

International audienceA field survey performed in Paracas Bay (Peru), a major scallop culture area, showed that the Peruvian scallop, Argopecten purpu-ratus, periodically faces severe hypoxic events. Oxygen uptake rate (VO2) of A. purpuratus juveniles facing progressive decrease of environmental oxygen saturation (from 100% to 5%) was measured at two contrasting temperatures ("normal condition" = 16°C and "warm condition" = 25°C). In normoxia, (oxygen saturation >70%) average VO2 was significantly (p < 0.001) higher in warm condition (0.20 ±0.004 mgO 2 ind −1 h −1) than in normal condition (0.12 ±0.007 mgO 2 ind −1 h −1). The shape of the VO2 response curve during increasing hypoxic conditions was evaluated using a segmented linear regression. The break points between linear segments allowed estimating the oxygen critical points (PcO2 , oxygen saturation units), while the slopes of the various segments was used to assess the VO2 regulatory capacity. In both temperature conditions at oxygen saturation lower than PcO2 , VO2 was a fourth of the values recorded in normoxic condition. This trend was more pronounced in the warm condition. Paradoxically, the estimated PcO2 was lower in warm condition (21.4% ±0.7) compared to the normal condition (24.4% ±1.9). However, the study of the slopes at oxygen saturations higher than PcO2 revealed that A. purpuratus can regulate its respiratory rate similarly and efficiently at both 16 and 25°C. Moreover, for VO2 above PcO2 , the estimated Q10 between normal and warm conditions was 1.78 (Arrhenius temperature = 4983 K), highlighting a moderate effect of temperature on VO2. These results reflect the adaptive capacity of this species to the changing environment along the Peruvian-Chilean coasts influenced by upwelling system and ENSO events. However, results from this study indicate that, at least during some periods, A. purpuratus spend more than 70% of its time exposed to hypoxic conditions below the estimated PcO2. Such conditions could have negative consequences on the species metabolism and harm the performance of A. purpuratus culture