Misconceptions of the marine biological carbon pump in a changing climate: Thinking outside the "export" box.

The marine biological carbon pump (BCP) stores carbon in the ocean interior, isolating it from exchange with the atmosphere and thereby coregulating atmospheric carbon dioxide (CO2 ). As the BCP commonly is equated with the flux of organic material to the ocean interior, termed "export flux," a change in export flux is perceived to directly impact atmospheric CO2 , and thus climate. Here, we recap how this perception contrasts with current understanding of the BCP, emphasizing the lack of a direct relationship between global export flux and atmospheric CO2 . We argue for the use of the storage of carbon of biological origin in the ocean interior as a diagnostic that directly relates to atmospheric CO2 , as a way forward to quantify the changes in the BCP in a changing climate. The diagnostic is conveniently applicable to both climate model data and increasingly available observational data. It can explain a seemingly paradoxical response under anthropogenic climate change: Despite a decrease in export flux, the BCP intensifies due to a longer reemergence time of biogenically stored carbon back to the ocean surface and thereby provides a negative feedback to increasing atmospheric CO2 . This feedback is notably small compared with anthropogenic CO2 emissions and other carbon-climate feedbacks. In this Opinion paper, we advocate for a comprehensive view of the BCP's impact on atmospheric CO2 , providing a prerequisite for assessing the effectiveness of marine CO2 removal approaches that target marine biology.

Can Top-Down Controls Expand the Ecological Niche of Marine N2 Fixers?

The ability of marine diazotrophs to fix dinitrogen gas (N2) is one of the most influential yet enigmatic processes in the ocean. With their activity diazotrophs support biological production by fixing about 100-200 Tg N/year and turning otherwise unavailable dinitrogen into bioavailable nitrogen (N), an essential limiting nutrient. Despite their important role, the factors that control the distribution of diazotrophs and their ability to fix N2 are not fully elucidated. We discuss insights that can be gained from the emerging picture of a wide geographical distribution of marine diazotrophs and provide a critical assessment of environmental (bottom-up) versus trophic (top-down) controls. We expand a simplified theoretical framework to understand how top-down control affects competition for resources that determine ecological niches. Selective mortality, mediated by grazing or viral-lysis, on non-fixing phytoplankton is identified as a critical process that can broaden the ability of diazotrophs to compete for resources in top-down controlled systems and explain an expanded ecological niche for diazotrophs. Our simplified analysis predicts a larger importance of top-down control on competition patterns as resource levels increase. As grazing controls the faster growing phytoplankton, coexistence of the slower growing diazotrophs can be established. However, these predictions require corroboration by experimental and field data, together with the identification of specific traits of organisms and associated trade-offs related to selective top-down control. Elucidation of these factors could greatly improve our predictive capability for patterns and rates of marine N2 fixation. The susceptibility of this key biogeochemical process to future changes may not only be determined by changes in environmental conditions but also via changes in the ecological interactions.

Loss of fixed nitrogen causes net oxygen gain in a warmer future ocean.

Oceanic anoxic events have been associated with warm climates in Earth history, and there are concerns that current ocean deoxygenation may eventually lead to anoxia. Here we show results of a multi-millennial global-warming simulation that reveal, after a transitory deoxygenation, a marine oxygen inventory 6% higher than preindustrial despite an average 3 °C ocean warming. An interior-ocean oxygen source unaccounted for in previous studies explains two thirds of the oxygen excess reached after a few thousand years. It results from enhanced denitrification replacing part of today's ocean's aerobic respiration in expanding oxygen-deficient regions: The resulting loss of fixed nitrogen is equivalent to an oceanic oxygen gain and depends on an incomplete compensation of denitrification by nitrogen fixation. Elevated total oxygen in a warmer ocean with larger oxygen-deficient regions poses a new challenge for explaining global oceanic anoxic events and calls for an improved understanding of environmental controls on nitrogen fixation.

Global Marine N2 Fixation Estimates: From Observations to Models.

Fixed nitrogen (N) limits productivity across much of the low-latitude ocean. The magnitude of its inventory results from the balance of N input and N loss, the latter largely occurring in regionally well-defined low-oxygen waters and sediments (denitrification and anammox). The rate and distribution of N input by biotic N2 fixation, the dominant N source, is not well known. Here we compile N2 fixation estimates from experimental measurements, tracer-based geochemical and modeling approaches, and discuss their limitations and uncertainties. The lack of adequate experimental data coverage and the insufficient understanding of the controls of marine N2 fixation result in high uncertainties, which make the assessment of the current N-balance a challenge. We suggest that a more comprehensive understanding of the environmental and ecological interaction of marine N2 fixers is required to advance the field toward robust N2 fixation rates estimates and predictions.

Patterns of deoxygenation: sensitivity to natural and anthropogenic drivers.

Observational estimates and numerical models both indicate a significant overall decline in marine oxygen levels over the past few decades. Spatial patterns of oxygen change, however, differ considerably between observed and modelled estimates. Particularly in the tropical thermocline that hosts open-ocean oxygen minimum zones, observations indicate a general oxygen decline, whereas most of the state-of-the-art models simulate increasing oxygen levels. Possible reasons for the apparent model-data discrepancies are examined. In order to attribute observed historical variations in oxygen levels, we here study mechanisms of changes in oxygen supply and consumption with sensitivity model simulations. Specifically, the role of equatorial jets, of lateral and diapycnal mixing processes, of changes in the wind-driven circulation and atmospheric nutrient supply, and of some poorly constrained biogeochemical processes are investigated. Predominantly wind-driven changes in the low-latitude oceanic ventilation are identified as a possible factor contributing to observed oxygen changes in the low-latitude thermocline during the past decades, while the potential role of biogeochemical processes remains difficult to constrain. We discuss implications for the attribution of observed oxygen changes to anthropogenic impacts and research priorities that may help to improve our mechanistic understanding of oxygen changes and the quality of projections into a changing future.This article is part of the themed issue 'Ocean ventilation and deoxygenation in a warming world'.