Climate change scenarios suggest that large‐scale carbon dioxide removal (CDR) will be required to maintain global warming below 2°C, leading to renewed attention on ocean iron fertilization (OIF). ...Previous OIF modelling has found that while carbon export increases, nutrient transport to lower latitude ecosystems declines, resulting in a modest impact on atmospheric CO2. However, the interaction of these CDR responses with ongoing climate change is unknown. Here, we combine global ocean biogeochemistry and ecosystem models to show that, while stimulating carbon sequestration, OIF may amplify climate‐induced declines in tropical ocean productivity and ecosystem biomass under a high‐emission scenario, with very limited potential atmospheric CO2 drawdown. The ‘biogeochemical fingerprint’ of climate change, that leads to depletion of upper ocean major nutrients due to upper ocean stratification, is reinforced by OIF due to greater major nutrient consumption. Our simulations show that reductions in upper trophic level animal biomass in tropical regions due to climate change would be exacerbated by OIF within ~20 years, especially in coastal exclusive economic zones (EEZs), with potential implications for fisheries that underpin the livelihoods and economies of coastal communities. Any fertilization‐based CDR should therefore consider its interaction with ongoing climate‐driven changes and the ensuing ecosystem impacts in national EEZs.
Our paper looks at the effectiveness of ocean iron fertilization (OIF) in a changing climate (CC) and the impacts of marine ecosystems. We find OIF has a limited potential to be a significant marine carbon dioxide removal mechanisms and amplifies impacts of CC on tropical and subtropical ocean ecosystems.
Although iron and light are understood to regulate the Southern Ocean biological carbon pump, observations have also indicated a possible role for manganese. Low concentrations in Southern Ocean ...surface waters suggest manganese limitation is possible, but its spatial extent remains poorly constrained and direct manganese limitation of the marine carbon cycle has been neglected by ocean models. Here, using available observations, we develop a new global biogeochemical model and find that phytoplankton in over half of the Southern Ocean cannot attain maximal growth rates because of manganese deficiency. Manganese limitation is most extensive in austral spring and depends on phytoplankton traits related to the size of photosynthetic antennae and the inhibition of manganese uptake by high zinc concentrations in Antarctic waters. Importantly, manganese limitation expands under the increased iron supply of past glacial periods, reducing the response of the biological carbon pump. Overall, these model experiments describe a mosaic of controls on Southern Ocean productivity that emerge from the interplay of light, iron, manganese and zinc, shaping the evolution of Antarctic phytoplankton since the opening of the Drake Passage.
Plain Language Summary
Because of the Southern Ocean's unique role in ocean circulation, Antarctic phytoplankton profoundly influence the global carbon cycle. For instance, an increase in the supply of iron—the main nutrient limiting Antarctic phytoplankton—is thought to have lowered CO2 during past ice ages by increasing phytoplankton photosynthesis. However, the potential for other essential elements to limit Southern Ocean productivity is not well known. By accounting for requirements of several nutrients in a global model, we have identified that manganese, an essential cofactor in photosynthesis, can limit phytoplankton growth across the Southern Ocean. The enduring role of manganese deficiency will likely influence the response of Southern Ocean ecosystems to ongoing climate change.
Key Points
Mn scarcity in the Southern Ocean limits phytoplankton growth in a global biogeochemical model, especially during austral spring
The spatial extent of Mn limitation is sensitive to phytoplankton traits governing photophysiology and metal homeostasis
Greater dust deposition to the Southern Ocean expands the role of Mn limitation and restricts carbon export from Fe fertilization
•First basin-wide measurements of plankton metal quotas in the N. Atlantic Ocean.•Fe and Mn quotas significantly higher on western side of section.•Cu and Ni quotas significantly elevated on eastern ...side of section.•Evidence for Al scavenging by biogenic silica.•Dissolved ratios not an accurate measure of cellular Fe quotas.
Phytoplankton contribute significantly to global C cycling and serve as the base of ocean food webs. Phytoplankton require trace metals for growth and also mediate the vertical distributions of many metals in the ocean. We collected bulk particulate material and individual phytoplankton cells from the upper water column (<150m) of the North Atlantic Ocean as part of the US GEOTRACES North Atlantic Zonal Transect cruise (GEOTRACES GA03). Particulate material was first leached to extract biogenic and potentially-bioavailable elements, and the remaining refractory material was digested in strong acids. The cruise track spanned several ocean biomes and geochemical regions. Particulate concentrations of metals associated primarily with lithogenic phases (Fe, Al, Ti) were elevated in surface waters nearest North America, Africa and Europe, and elements associated primarily with biogenic material (P, Cd, Zn, Ni) were also found at higher concentrations near the coasts. However metal/P ratios of labile particulate material were also elevated in the middle of the transect for Fe, Ni, Co, Cu, and V. P-normalized cellular metal quotas measured with synchrotron X-ray fluorescence (SXRF) were generally comparable to ratios in bulk labile particles but did not show mid-basin increases. Manganese and Fe ratios and cell quotas were higher in the western part of the section, nearest North America, and both elements were more enriched in bulk particles, relative to P, than in cells, suggesting the presence of labile oxyhydroxide particulate phases. Cellular Fe quotas thus did not increase in step with aeolian dust inputs, which are highest near Africa; these data suggest that the dust inputs have low bioavailability. Copper and Ni cell quotas were notably higher nearest the continental margins. Overall mean cellular metal quotas were similar to those measured in the Pacific and Southern Oceans except for Fe, which was approximately 3-fold higher in North Atlantic cells. Cellular Fe quotas are in-line with those measured in laboratory cultures at comparable Fe concentrations. Particulate Zn, Cu, Ni, and Co are primarily associated with cellular material, but less than 30% of labile particulate Fe and Mn are biogenic. Particulate Al was primarily associated with lithogenic material, but the labile fraction was highly correlated with P, as well as with biogenic silica, suggesting that some particulate Al (perhaps around 20%) may occur adsorbed to biogenic material. Cellular element maps indicate that externally scavenged Fe was not a significant fraction of the metal associated with live phytoplankton, but adsorbed or precipitated phases are likely to be important in particulate detrital material. Such abiotic scavenging, along with differential remineralization of cellular nutrients in the water column, results in estimates of cellular metal/nutrient ratios from dissolved concentrations that significantly underestimate the ratios in phytoplankton. These data demonstrate the response of phytoplankton to the unique metal inputs to the North Atlantic Ocean.
Despite recent advances in observational data coverage, quantitative constraints on how different physical and biogeochemical processes shape dissolved iron distributions remain elusive, lowering ...confidence in future projections for iron-limited regions. Here we show that dissolved iron is cycled rapidly in Pacific mode and intermediate water and accumulates at a rate controlled by the strongly opposing fluxes of regeneration and scavenging. Combining new data sets within a watermass framework shows that the multidecadal dissolved iron accumulation is much lower than expected from a meta-analysis of iron regeneration fluxes. This mismatch can only be reconciled by invoking significant rates of iron removal to balance iron regeneration, which imply generation of authigenic particulate iron pools. Consequently, rapid internal cycling of iron, rather than its physical transport, is the main control on observed iron stocks within intermediate waters globally and upper ocean iron limitation will be strongly sensitive to subtle changes to the internal cycling balance.
Variation in ocean C:N:P of particulate organic matter (POM) has led to competing hypotheses for the underlying drivers. Each hypothesis predicts C:N:P equally well due to regional co-variance in ...environmental conditions and biodiversity. The Indian Ocean offers a unique positive temperature and nutrient supply relationship to test these hypotheses. Here we show how elemental concentrations and ratios vary over daily and regional scales. POM concentrations were lowest in the southern gyre, elevated across the equator, and peaked in the Bay of Bengal. Elemental ratios were highest in the gyre, but approached Redfield proportions northwards. As Prochlorococcus dominated the phytoplankton community, biodiversity changes could not explain the elemental variation. Instead, our data supports the nutrient supply hypothesis. Finally, gyre dissolved iron concentrations suggest extensive iron stress, leading to depressed ratios compared to other gyres. We propose a model whereby differences in iron supply and N
-fixation influence C:N:P levels across ocean gyres.
The GEOTRACES program has greatly increased basin‐scale concentration measurements for a large number of elements in the ocean, both constraining external sources and internal sinks and exposing ...complex internal cycles of trace elements. Our conceptual frameworks for marine trace element cycling, however, often remain simplified as the production and remineralization of phytoplankton biomass. Despite their complexity, or perhaps because of it, trace element cycles are often predominantly considered as an extension of traditional Redfield macronutrient ratios to C or P. Here we utilize extensive data sets of particulate trace element concentrations from GEOTRACES section cruises in the South Pacific and North Atlantic Oceans to look for evidence of the internal cycles of multiple trace elements without requiring normalization to phytoplankton biomass. Using both traditional and expanded power law regression analyses and multi‐element factor analysis, we expose the internal distributions of six authigenic, biogenic, and lithogenic particulate phases and their multi‐element associations. Critically, no particulate trace element is observed to behave identically to P. Observations include a scavenged Fe phase with a slight surface maximum, which increases linearly with depth below ~ 300 m and which appears to co‐scavenge Cu, V, and La. Particulate Co is found to be associated with phytoplankton, Mn‐biooxides just below the mixed layer, and with a putative heterotrophic phase observed in the surface and at depth. We present an expanded conceptual framework for particulate trace element cycling that has explicit roles for these multiple particulate phases.
Key Points
Particulate trace element data sets contain evidence of multiple biomass, scavenged, and lithogenic phases that can be statistically revealed
Scavenged Fe and Mn phases have unique internal distributions and co‐scavenge several bioactive elements
Phytoplankton and putative heterotrophic biomass phases have unique elemental associations and internal distributions
We present a new approach for quantifying the bioavailability of dissolved iron (dFe) to oceanic phytoplankton. Bioavailability is defined using an uptake rate constant (kin‐app) computed by ...combining data on: (a) Fe content of individual in situ phytoplankton cells; (b) concurrently determined seawater dFe concentrations; and (c) growth rates estimated from the PISCES model. We examined 930 phytoplankton cells, collected between 2002 and 2016 from 45 surface stations during 11 research cruises. This approach is only valid for cells that have upregulated their high‐affinity Fe uptake system, so data were screened, yielding 560 single cell kin‐app values from 31 low‐Fe stations. We normalized kin‐app to cell surface area (S.A.) to account for cell‐size differences.
The resulting bioavailability proxy (kin‐app/S.A.) varies among cells, but all values are within bioavailability limits predicted from defined Fe complexes. In situ dFe bioavailability is higher than model Fe‐siderophore complexes and often approaches that of highly available inorganic Fe′. Station averaged kin‐app/S.A. are also variable but show no systematic changes across location, temperature, dFe, and phytoplankton taxa. Given the relative consistency of kin‐app/S.A. among stations (ca. five‐fold variation), we computed a grand‐averaged dFe availability, which upon normalization to cell carbon (C) yields kin‐app/C of 42,200 ± 11,000 L mol C−1 d−1. We utilize kin‐app/C to calculate dFe uptake rates and residence times in low Fe oceanic regions. Finally, we demonstrate the applicability of kin‐app/C for constraining Fe uptake rates in earth system models, such as those predicting climate mediated changes in net primary production in the Fe‐limited Equatorial Pacific.
Plain Language Summary
In many oceanic regions, iron exerts strong control on phytoplankton growth, ecosystem structure, and carbon cycling. Yet, iron bioavailability and uptake rates by phytoplankton in the ocean are poorly constrained. Recently, Shaked et al. (2020) established a new approach for quantifying the availability of dissolved Fe (dFe) in natural seawater based on its uptake kinetics by Fe‐limited cultured phytoplankton. Here, we extend this approach to in situ phytoplankton, establishing a standardized proxy for dFe bioavailability in low‐Fe oceanic regions.
Bioavailability is estimated through single cell Fe uptake constants, calculated by combining measured Fe contents of individual phytoplankton cells collected from multiple regions with concurrently measured dFe concentrations, as well as modeled growth rates. We then utilize this proxy for: (a) comparing dFe bioavailability among organisms and regions; (b) calculating dFe uptake rates and residence times in low‐Fe oceanic regions; and (c) constraining Fe uptake parameters of earth system models to better predict ocean productivity in response to climate change.
Key Points
A proxy for dissolved Fe bioavailability in low‐Fe regions is established from Fe quotas, dissolved Fe concentrations, and modeled growth rates
In situ phytoplankton cells record high and relatively uniform dissolved Fe bioavailability across many low‐Fe oceanic regions
The new proxy is applicable for calculating in situ Fe uptake rates and biological Fe residence times and for validating global model output
Iron is important in regulating the ocean carbon cycle
. Although several dissolved and particulate species participate in oceanic iron cycling, current understanding emphasizes the importance of ...complexation by organic ligands in stabilizing oceanic dissolved iron concentrations
. However, it is difficult to reconcile this view of ligands as a primary control on dissolved iron cycling with the observed size partitioning of dissolved iron species, inefficient dissolved iron regeneration at depth or the potential importance of authigenic iron phases in particulate iron observational datasets
. Here we present a new dissolved iron, ligand and particulate iron seasonal dataset from the Bermuda Atlantic Time-series Study (BATS) region. We find that upper-ocean dissolved iron dynamics were decoupled from those of ligands, which necessitates a process by which dissolved iron escapes ligand stabilization to generate a reservoir of authigenic iron particles that settle to depth. When this 'colloidal shunt' mechanism was implemented in a global-scale biogeochemical model, it reproduced both seasonal iron-cycle dynamics observations and independent global datasets when previous models failed
. Overall, we argue that the turnover of authigenic particulate iron phases must be considered alongside biological activity and ligands in controlling ocean-dissolved iron distributions and the coupling between dissolved and particulate iron pools.
Antarctic Bottom Water has been warming in recent decades throughout most of the oceans and freshening in regions close to its Indian and Pacific sector sources. We assess warming rates on isobars in ...the eastern Pacific sector of the Southern Ocean using CTD data collected from shipboard surveys from the early 1990s through the late 2010s together with CTD data collected from Deep Argo floats deployed in the region in January 2023. We show cooling and freshening in the temperature‐salinity relation for water colder than ∼0.4°C. We further find a recent acceleration in the regional bottom water warming rate vertically averaged for pressures exceeding 3,700 dbar, with the 2017/18 to 2023/24 trend of 7.5 (±0.9) m°C yr−1 nearly triple the 1992/95 to 2023/24 trend of 2.8 (±0.2) m°C yr−1. The 0.2°C isotherm descent rate for these same time periods nearly quadruples from 7.8 to 28 m yr−1.
Plain Language Summary
Cold winds blowing over polynyas (areas of ice‐free water) on the Antarctic continental shelf create sea ice, forming very cold and somewhat salty, hence very dense, waters. These dense shelf waters descend the continental slope to the abyss, mixing with adjacent waters to form Antarctic Bottom Water (AABW). AABW spreads northward from there, filling much of the global abyssal ocean as it mixes with warmer, lighter waters above. AABW has been warming on pressure surfaces, freshening and cooling on density surfaces, and reducing in volume (contracting). These changes are likely a result of melting Antarctic ice sheets, which freshen the shelf waters, making them less dense, hence less able to sink to the bottom. We compare profiles of ocean temperature and salinity in the eastern Pacific sector of the Southern Ocean collected in 2023 and 2024 by robotic freely drifting profilers to data collected from ships from the early 1990s to the late 2010s. We find all of the above listed changes, but also acceleration of the warming, with the rate from 2017/18 to 2023/24 being nearly triple the rate from 1992/95 to 2023/24. The contraction rate has nearly quadrupled. This acceleration has been predicted by high‐resolution climate model simulations.
Key Points
Antarctic Bottom Water (AABW) changes in the east Pacific sector of the Southern Ocean are assessed using Deep Argo and ship‐based CTD profiles
Bottom water warming rates from 2017/18 to 2023/24 nearly triple compared to 1992/95 to 2023/24 rates, contraction rates nearly quadruple
AABW cooling and freshening on isopycnals is also observed in the region, relative to older Circumpolar Deep Water
Diatoms are prominent eukaryotic phytoplankton despite being limited by the micronutrient iron in vast expanses of the ocean. As iron inputs are often sporadic, diatoms have evolved mechanisms such ...as the ability to store iron that enable them to bloom when iron is resupplied and then persist when low iron levels are reinstated. Two iron storage mechanisms have been previously described: the protein ferritin and vacuolar storage. To investigate the ecological role of these mechanisms among diatoms, iron addition and removal incubations were conducted using natural phytoplankton communities from varying iron environments. We show that among the predominant diatoms, Pseudo-nitzschia were favored by iron removal and displayed unique ferritin expression consistent with a long-term storage function. Meanwhile, Chaetoceros and Thalassiosira gene expression aligned with vacuolar storage mechanisms. Pseudo-nitzschia also showed exceptionally high iron storage under steady-state high and low iron conditions, as well as following iron resupply to iron-limited cells. We propose that bloom-forming diatoms use different iron storage mechanisms and that ferritin utilization may provide an advantage in areas of prolonged iron limitation with pulsed iron inputs. As iron distributions and availability change, this speculated ferritin-linked advantage may result in shifts in diatom community composition that can alter marine ecosystems and biogeochemical cycles.
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