The large-scale cultivation of macroalgae has been proposed as a marine carbon dioxide removal (mCDR) strategy, yet its efficiency and consequences for ocean biogeochemistry remain uncertain. Using a new macroalgae aquaculture module within an ocean biogeochemistry model, NEMO-MEDUSA, we investigate carbon removal potential and biogeochemical feedbacks under hypothetical global-scale macroalgae cultivation with varying harvest strategies, loss rates, and iron availability. Overall cultivation enhances air-sea CO2 uptake by 11.0 PgCyr-1, but only similar to 27 % of macroalgal production results in additional CO2 uptake. Furthermore, phytoplankton and zooplankton biomass is suppressed by almost 50 % and is geographically displaced by significant surface nutrient changes. Sinking of harvested biomass increases oxygen demand during remineralisation, leading to widespread oxygen depletion and the emergence of suboxic conditions at the seafloor in deposition regions. When macroalgal growth is not supplemented with iron micronutrient, its production declines sharply (-74 %), revealing a significant limitation for large-scale feasibility. Collectively, our results reveal that large-scale macroalgal cultivation offers low mCDR potential, that it is both spatially extensive and locally intensive, and its unintended biogeochemical consequences can be substantial. Our findings highlight the urgent need to assess nutrient constraints and ecological trade-offs before considering this method as a viable large-scale mCDR strategy.

We present a model describing the population dynamics of benthic biota, feeding from a common resource that is supplied by a flux of sinking organic carbon arriving on the seafloor. By using allometric relationships for the physiological processes of growth, mortality and respiration, and for food limitation, the model represents the population dynamics of organisms ranging in size from bacteria (10-14 g wet weight C) to large metazoans (103 gwwt C). The effect of temperature on physiological rates is also included. The only forcing information required is the ambient temperature and the rate of supply of sinking organic carbon. The model can be used for, and tuned to, specific locations. However, a parameter set is provided that is generally applicable. The ability of the model to simultaneously reproduce biomass size distributions at five contrasting sites is demonstrated for this parameter set. Other examples of use are also shown, using the model to explore global patterns of benthic biomass, and responding to a change in food supply.

Abstract The North Atlantic phytoplankton bloom (NAB) forms the base of the food web upon which marine ecosystem services throughout the northern North Atlantic rely. The NAB's nutrient supply is predominantly driven by deep winter mixing, which is most pronounced in the subpolar gyre where it has been projected to abruptly decline within the next few decades. Here we use idealized experiments with an Earth system model to investigate how mixed layer depth, surface nutrients and chlorophyll concentrations over the gyre may respond to positive carbon emissions followed by both zero and negative emissions mitigation efforts initiated at various Global Warming Levels (GWLs). Results show a decline in all three variables during positive emissions, with the warmer the GWL the less likely the prospect of recovery back to pre‐industrial conditions. In the scenario where carbon emissions are held at zero once reaching a GWL of 1.5°C, the NAB started to recover toward preindustrial conditions. However, zero emissions after 2.5°C and warmer showed a continued decline in the NAB. These results suggest a tipping threshold may lie between 1.5 and 2.5°C GWL, beyond which only negative emissions can achieve recovery of the NAB. Even in the lowest warming scenario examined, 1.5°C, recovery of the NAB did not occur within 5 centuries under zero emissions, nor did it occur within one century under negative emissions.

Understanding the nuances of the effects of nutrient limitation on oceanic primary production has been the focus of many bioassay experiments by oceanographers. A theme of these investigations is that they identify the currently limiting nutrient at a given location, or in other words they identify the proximate limiting nutrient (PLN). However, the ultimate limiting nutrient (ULN; the nutrient whose supply controls system productivity over extensive timescales) can be different from the PLN. Our motivation is to investigate the identity of the ULN. The ULN constrains oceanic primary production over extensive timescales and consequently overall ocean fertility. The rate of oceanic photosynthesis affects planetary oxygen and carbon dioxide, impacting climate. Understanding past ocean fertility is fundamental to understanding Earth system history and biological evolution.Investigations that have considered the ULN have often utilised box models for example the work of, Tyrrell (1999) and Lenton and Watson (2000). To facilitate investigation of the ULN the carbon-centric Grid Enabled Integrated Earth system model (cGEnIE) nutrient cycles have been extended to create NutGEnIE. NutGEnIE incorporates three open nutrients cycles nitrogen, phosphorus, and iron. The impacts of diazotrophs, capable of fixing nitrogen, are represented alongside those of other phytoplankton. NutGEnIE is capable of extended duration model simulations necessary to investigate the ULN while, at the same time, including iron as a potentially limiting nutrient. NutGEnIE is described here, with particular focus on the biogeochemical cycles of iron, nitrogen and phosphorus. Model results are compared to ocean observational data to assess the degree of realism. Model-data comparisons include physical properties, nutrient concentrations, and process rates (e.g., export and nitrogen fixation). The comparisons of NutGEnIE to ocean observational data are largely positive, suggesting that the dynamics of NutGEnIE are valid. The validations, allied to the ability to run an Earth System model with open nutrients cycles of nitrogen, phosphorus, and iron over extensive time periods supports the proposed use of NutGEnIE to revisit the question of the ULN for oceanic primary production.

Abstract The ocean biological carbon pump (BCP) transports organic matter from the surface to the deep ocean. Accurately quantifying the efficiency of the BCP is essential for understanding potential climate feedbacks and entails measuring the flux of organic material in and out of the mesopelagic layer (approximately 100–1,000 m). Observational estimates are often restricted to measuring the BCP efficiency over short timescales. Here we use an ocean biogeochemical model to diagnose where, and on what timescales, the mesopelagic is sufficiently in steady state that balancing the carbon budget may be possible. For the majority of the ocean the sources and sinks of organic carbon in the mesopelagic do not balance on timescales shorter than 1 year. Assuming steady state risks falsely inferring the existence of missing processes or the magnitudes of known ones to close the budget and will lead to incorrect estimates of the strength of the BCP.

Mixing of the ocean around Antarctica is a key process that exerts influences over large scales and in multiple ways. By redistributing heat in the ocean, it exerts strong influences on the Antarctic Ice Sheet, with implications for sea level rise globally. Similarly, the redistribution of ocean heat affects the production of sea ice in winter and its melt in summer, with consequences for climate. Mixing also affects the distribution of nutrients in the ocean, with direct impacts on the marine ecosystem and biodiversity and with consequences for fisheries.It was long thought that mixing of the seas close to Antarctica was predominantly caused by winds, tides and the loss of heat from the ocean especially in winter. However, we recently discovered that when glaciers calve in Antarctica, they can trigger underwater tsunamis. These are large (multi-metre) waves that move rapidly away from the coastline and when they break, they cause sudden bursts of very intense mixing. Simple calculations indicated that the net impact of these underwater tsunamis could be as strong as winds, and much more important than tides, in driving mixing. It was also argued that they are likely to be relevant everywhere that glaciers calve into the sea, including Greenland and across the Arctic. As our ocean and atmosphere continue to heat up, it is very possible that glacier calving will become more frequent and intensify, increasing further the impact of underwater tsunamis on large-scale climate, the cryosphere and ecosystems.This is an exciting new avenue of scientific investigation and many key questions remain unanswered. We need to know how widespread and frequent the generation of underwater tsunamis is, how far they travel from the coastline before breaking, and how variable this is. We need to measure what impacts the extra mixing has on ocean temperature and nutrient concentrations, and to determine what this means for the cryosphere and ocean productivity. There is a pressing need to include the effects of underwater tsunamis in the computer models that are used for projecting future ocean climate and ecosystem conditions and to determine the feedbacks between climate change and the generation of more underwater tsunamis.To answer these questions, our project will deploy innovative techniques for measuring the ocean and ice in close proximity to a calving glacier, including robotic underwater vehicles and remotely-piloted aircraft, and cutting-edge deep-learning techniques applied to satellite data. We will use advanced computer simulations to fully understand the causal mechanisms responsible for the creation and spread of the underwater tsunamis and their impacts on ocean climate and marine productivity. We will make our developments in computer simulation available to the whole community of users, for widespread uptake and future use.This project will have significant benefits for academics seeking to predict the future of Antarctica and its impacts on the rest of the world, for Governments and intergovernmental agencies seeking to understand how best to respond to climate change, and for the curious general public wanting to learn more about the extremes of the planet and why they matter. The fieldwork will be especially photo- and video-genic and will lead to outstanding outreach and impact opportunities, and we will work with media agencies seeking to tell compelling stories about the extremes of the Earth.

The North Atlantic subpolar gyre is a key region for the North Atlantic phytoplankton bloom (NAB), the foundation of the regional foodweb. The NAB is dependent on nutrients seasonally introduced into the surface ocean by deep winter convection. Under climate change, this pattern is threatened by increasing water column stratification, and the NAB may “collapse” as a result, representing a potential “tipping point” in the Earth system. We investigate change in winter mixing and the impacts on the SPG and the broader northern North Atlantic using 1. a spread of future projections from a low-resolution Earth system model (UKESM) and 2. a single, high-warming projection of a high-resolution ocean-only configuration of the same model (NEMO-MEDUSA). For both models we find significant declines in the strength of the NAB during the 21st century. In UKESM, this occurred across all projections, but with low spatiotemporal coherence. In NEMO-MEDUSA, changes in upper mixed layer depth, surface nutrients and chlorophyll concentrations were noticeably abrupt and more highly spatiotemporally-correlated. We also find a large (>30 day) phenological shift in the peak of the bloom aligned with the timing of this change, which may affect foodweb dynamics. Overall, defining “collapse” as a halving of surface chlorophyll, we find that the NAB collapses by the end of the century regardless of future projection. However, the timing, abruptness and coherence of this collapse differs in high and low resolution models, suggesting the need for higher resolution for prediction of abrupt and irreversible changes, especially those involving ecosystem dynamics.

Ocean biological processes, principally the surface production, sinking and interior conversion of organic carbon to CO2 store enough carbon in the ocean interior to keep atmospheric CO2 concentration substantially lower than it would otherwise be. The size of this effect is linked to the depth at which sinking organic matter is remineralised in the ocean, with a deeper mineralisation causing a greater storage. Two prominent hypotheses regarding the control over the depth at which sinking material is lost are Temperature and Ecosystem Structure, specifically the proportion of diatoms in the surface community. These are both theoretically valid (temperature controls respiration, diatoms control density) and have some support in the literature, however to date have been considered in isolation. In this paper we firstly compute the strength of these effects in isolation from simple theory and show that they produce relationships consistent with existing literature thus suggesting that both factors may play a role. We use these relationships to produce an equation linking mineralisation depth, parameterised as remineralisation length scale, to community structure and temperature, thus uniting the two factors. An analysis of this equation suggests that community structure exerts a stronger control over remineralisation length scale than does temperature.

The export of elements (particularly carbon, nitrogen and phosphorus) from the Poles critically supports global marine biodiversity and major fisheries as well as the sequestration of atmospheric carbon to the deep ocean. Ecosystem processes regulate this export, but major uncertainties remain in terms of how and by how much. Progress on understanding key ecosystem interactions is hindered by lack of data and their representation in Earth system models is poor. The two polar regions share similarities in environmental extremes which make them sensitive to the impacts of climate change. They both receive nutrients from multiple and diverse sources and the delivery of these nutrients to other oceans is regulated by similar ecosystem processes. However, the extent to which these ecosystem processes will be modified by climate change is unclear and urgently needs to be determined. BIOPOLE will determine how polar ecosystems regulate the balance of carbon and nutrients in the world’s oceans and, through it, their effect on global fish stocks and carbon storage. It will address this challenge by integrating ambitious fieldwork campaigns and innovative modelling in a multidisciplinary and highly coordinated approach. BIOPOLE will capitalise on world-leading capabilities and infrastructure in ocean and high-latitude research, including cutting-edge land-based facilities, state-of-the-art polar research vessels and innovative autonomous instrumentation. Collaboration with national and international partners will further strengthen BIOPOLE’s multidisciplinary approach and efficient use of infrastructure. BIOPOLE’s legacy will be the first assessment of the global impact of polar ecosystems on biogeochemical cycling and fish stocks; technologically-novel approaches and strong partnerships between leading international science groups.

Shipboard sampling of ocean biogeochemical properties is necessarily limited by logistical and practical constraints. As a result, the majority of observations are obtained for the spring/summer period and in regions relatively accessible from a major port. This limitation may bias the conceptual understanding we have of the spatial and seasonal variability in important components of the Earth system. Here we examine the influence of sampling bias on global estimates of carbon export flux by sub-sampling a biogeochemical model to simulate real, realistic and random sampling. We find that both the sparseness and the 'clumpy' character of shipboard flux observations generate errors in estimates of globally extrapolated export flux of up to similar to +/- 20%. The use of autonomous technologies, such as the Biogeochemical-Argo network, will reduce the uncertainty in global flux estimates to similar to +/- 3% by both increasing the sample size and reducing clumpiness in the spatial distribution of observations. Nevertheless, determining the climate change-driven trend in global export flux may be hampered due to the uncertainty introduced by interannual variability in sampling patterns.

Ocean deoxygenation due to anthropogenic warming represents a major threat to marine ecosystems and fisheries. Challenges remain in simulating the modern observed changes in the dissolved oxygen (O 2 ). Here, we present an analysis of upper ocean (0-700m) deoxygenation in recent decades from a suite of the Coupled Model Intercomparison Project phase 6 (CMIP6) ocean biogeochemical simulations. The physics and biogeochemical simulations include both ocean-only (the Ocean Model Intercomparison Project Phase 1 and 2, OMIP1 and OMIP2) and coupled Earth system (CMIP6 Historical) configurations. We examine simulated changes in the O 2 inventory and ocean heat content (OHC) over the past 5 decades across models. The models simulate spatially divergent evolution of O 2 trends over the past 5 decades. The trend (multi-model mean and spread) for upper ocean global O 2 inventory for each of the MIP simulations over the past 5 decades is 0.03 ± 0.39×1014 [mol/decade] for OMIP1, −0.37 ± 0.15×10 14 [mol/decade] for OMIP2, and −1.06 ± 0.68×10 14 [mol/decade] for CMIP6 Historical, respectively. The trend in the upper ocean global O 2 inventory for the latest observations based on the World Ocean Database 2018 is −0.98×10 14 [mol/decade], in line with the CMIP6 Historical multi-model mean, though this recent observations-based trend estimate is weaker than previously reported trends. A comparison across ocean-only simulations from OMIP1 and OMIP2 suggests that differences in atmospheric forcing such as surface wind explain the simulated divergence across configurations in O 2 inventory changes. Additionally, a comparison of coupled model simulations from the CMIP6 Historical configuration indicates that differences in background mean states due to differences in spin-up duration and equilibrium states result in substantial differences in the climate change response of O 2 . Finally, we discuss gaps and uncertainties in both ocean biogeochemical simulations and observations and explore possible future coordinated ocean biogeochemistry simulations to fill in gaps and unravel the mechanisms controlling the O 2 changes.

The North Atlantic bloom (NAB) is the massive phytoplankton biomass increase that occurs each spring in the North Atlantic Ocean and which provides the base of the North Atlantic food web. Secondary producers (e.g. calanoid copepods) and the species that feed upon them have, over evolutionary timescales, developed life cycle synchronicities to coincide with the timing of the bloom. This makes understanding the future of the NAB – both in terms of its magnitude and timing – crucial for understanding the impacts of climate change on the North Atlantic ecosystem. The NAB is driven by nutrients supplied to the surface ocean by deep winter mixing, but the North Atlantic Subpolar Gyre (SPG) has been identified as a tipping element in the climate system (Armstrong McKay et al., 2022). If a tipping element crosses a critical threshold – a tipping point – associated with a given level of climate change, non-linear, abrupt and potentially irreversible (over human timescales) changes are projected to occur, altering the tipping element into a qualitatively different state. The SPG may be on the verge of a tipping point characterised by a deep convection collapse. This could significantly impact the availability of surface nutrients in the region, and may cause an abrupt decline in the NAB. We use a coupled climate model (UKESM1) forced under different emissions scenarios, as well as a high resolution ocean-only model forced by UKESM1’s model atmosphere, to investigate whether the SPG undergoes a convection collapse and how this is linked to future changes in the NAB.

Respiration of lipids by copepods during diapause (overwintering dormancy) contributes to ocean carbon sequestration via the seasonal lipid pump (SLP). Parameterizing this flux in predictive models requires a mechanistic understanding of how life history adaptation in copepods shapes their timing of exit from diapause. We investigate the optimal phenology of Calanus finmarchicus in the Norwegian Sea using an individual-based model in which diapause exit is represented as a trait characterized by phenotypic mean and variance. Without interannual variability, optimal exit correlated with the onset of the spring phytoplankton bloom and phenotypic variance was of no benefit. In contrast, copepods endured reduced fitness and adopted bet-hedging strategies when exposed to interannual variability in bloom timing and predation: later exit from diapause and phenotypic variance maintained adult numbers in anomalous late-bloom years. Exit nevertheless remained well before the peak of the bloom which is a favorable strategy when low predation early in the year enhances survival of eggs and early developmental stages. Our work highlights the complex interactions between C. finmarchicus and its environment and the need for improved understanding of bet-hedging strategies and the cues of diapause exit to progress the representation of the SLP in global biogeochemical models.

The Paris Agreement plans for "net-zero" carbon dioxide (CO2) emissions during the second half of the 21st century. However, reducing emissions from some sectors is challenging, and "net-zero" permits carbon dioxide removal (CDR) activities. One CDR scheme is ocean alkalinity enhancement (OAE), which proposes dissolving basic minerals into seawater to increase its buffering capacity for CO2. While modeling studies have often investigated OAE at basin or global scale, some proposals focus on readily accessible coastal shelves, with TA added through the dissolution of seafloor olivine sands. Critically, by settling and dissolving sands on shallow seafloors, this retains the added TA in near-surface waters in direct contact with atmospheric CO2. To investigate this, we add dissolved TA at a rate of similar to 29 Teq y(-1) to the global shelves (<100m) of an Earth system model (UKESM1) running a high emissions scenario. As UKESM1 is fully coupled, wider effects of OAE-mediated increase in ocean CO2 uptake -e.g. atmospheric x CO2, air temperature and marine pH- are fully quantified. Applying OAE from 2020 to 2100 decreases atmospheric xCO(2) similar to 10 ppm, and increases air-to-sea CO2 uptake similar to 8%. In-line with other studies, CO(2)uptake per unit of TA added occurs at a rate of similar to 0.8 mol C (mol TA)(-1). Significantly for monitoring, advection of added TA results in similar to 50% of CO2 uptake occurring remotely from OAE operations, and the model also exhibits noticeable land carbon reservoir changes. While practical uncertainties and model representation caveats remain, this analysis estimates the effectiveness of this specific OAE scheme to assist with net-zero planning.

AbstractChlorophyll in phytoplankton absorbs solar radiation (SR) and affects the thermal structure and dynamics within upwelling regions. However, research on this process across global‐scale coastal upwelling systems is still lacking. Here, we use a coupled ocean‐biogeochemical model to investigate differing responses to chlorophyll‐induced solar absorption between Pacific and Atlantic coastal upwelling regions. Chlorophyll‐induced solar absorption leads to colder Pacific coastal upwelling but warmer Atlantic coastal upwelling. In the Pacific, the shading effect of the surface chlorophyll maximum leads to colder subsurface water, which is then upwelled, contributing to cooling. The more stratified upper ocean leads to shallower mixed layer depth, intensifying offshore transport and upwelling. In the Atlantic, the absorption of SR by the subsurface chlorophyll maximum causes warmer and weaker upwelling. The processes described, in turn, trigger positive feedback to ocean biogeochemistry and potentially interact with climate dynamics, underscoring the necessity to incorporate them into Earth system models.

Abstract. The anthropogenic carbon distribution between the atmosphere, land surface and ocean varies significantly with the choice of scenario for identical changes in mean global surface temperature. Moving to a lower CO2 emissions scenario means that warming levels occur later, and with significantly less carbon in the three main carbon reservoirs. After 2 °C of warming, the multi-model mean ocean allocation can be up to 3 % different between scenarios, or 36 Pg in total with an even larger difference in some single model means. For the UKESM1 model, the difference between the minimum and maximum atmospheric fraction at the 2 °C Global Warming Level (GWL) is 3.6 %. This is equivalent to 50 Pg of additional carbon in the atmosphere, or the equivalent of five years of our current global total emissions. In the lower CO2 concentration scenarios, SSP1-1.9 and SSP1-2.6, the ocean fraction grows over time while the the land surface fraction remains constant. In the higher CO2 concentration scenarios, SSP2-4.5, SSP3-7.0 and SSP5-8.5, the ocean fraction remains constant over time while the the land surface fraction decreases over time. Higher equilibrium climate sensitivity (ECS) models reach the GWLs sooner, and with lower atmospheric CO2 than lower sensitivity models. However, the choice of scenario has a much larger impact on the percentage carbon allocation at a given warming level than the individual model's ECS.

We show that the distribution of anthropogenic carbon between the atmosphere, land surface, and ocean differs with the choice of projection scenario even for identical changes in mean global surface temperature. Warming thresholds occur later in lower-CO2-emissions scenarios and with less carbon in the three main reservoirs than in higher-CO2-emissions scenarios. At 2 ∘C of warming, the mean carbon allocation differs by up to 63 PgC between scenarios, which is equivalent to approximately 6 years of the current global total emissions. At the same warming level, higher-CO2-concentration scenarios have a lower combined ocean and land carbon allocation fraction of the total carbon compared to lower-CO2-concentration scenarios. The warming response to CO2, quantified as the equilibrium climate sensitivity, ECS, directly impacts the global warming level exceedance year and hence the carbon allocation. Low-ECS models have more carbon than high-ECS models at a given warming level because the warming threshold occurs later, allowing more emissions to accumulate. These results are important for carbon budgets and mitigation strategies as they impact how much carbon the ocean and land surface could absorb at a given warming level. Carbon budgeting will be key to reducing the impacts of anthropogenic climate change, and these findings could have critical consequences for policies aimed at reaching net zero.

Satellite observations have given us a clear idea of the changes in chlorophyll in the surface ocean on both a seasonal and interannual basis, but repeated observations at depth are much rarer. The permanently-stratified subtropical gyres in the Atlantic are highly oligotrophic, with most production centred on a deep chlorophyll maximum (DCM) just above the nitracline. This study explores the variations in this feature in the core of both gyres, considering both seasonal and interannual variations, and the linkages between changes at the surface and sub-surface. The in situ observations come from the Atlantic Meridional Transect (AMT), a long-running UK monitoring programme, and also from biogeochemical Argo floats. AMT provides measurements spanning more than 25 years directed through the centres of these gyres, but samples only 2 to 4 months per year and thus cannot resolve the seasonal variations, whereas the profiling floats give coverage throughout the year, but without the rigid spatial repeatability. These observational records are contrasted with representation of the centres of the gyres in two different biogeochemical models: MEDUSA and ERSEM, thus fulfilling one of AMT’s stated aims: the assessment of biogeochemical models. Whilst the four datasets show broadly the same seasonal patterns and that the DCM shallows when surface chlorophyll increases, the depth and peak concentration of the DCM differ among datasets. For most of the datasets the column-integrated chlorophyll for both gyres is around 19 mg m-2 (with the AMT fluorescence-derived values being much lower); however the MEDUSA model has a disparity between the northern and southern gyres that is not understood. Although the seasonal increase in surface chlorophyll is tied to a commensurate decrease in concentration at depth, on an interannual basis years with enhanced surface levels of chlorophyll correspond to increases at depth. Satellite-derived observations of surface chlorophyll concentration act as a good predictor of interannual changes in DCM depth for both gyres during their autumn season, but provide less skill in spring.