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Tag: Marine Ecosystems

  • Atmospheric oxygen constraints revise Southern Ocean productivity estimates

    What the study found

    The study found an annual Southern Ocean net primary production of 6.5 ± 1.36 PgC yr−1, which is higher than most Coupled Model Intercomparison Project Phase 6 (CMIP6) model and satellite-based estimates. The findings also indicate that many Earth system models underestimate productivity in ways that affect estimates of air-sea carbon dioxide exchange.

    Why the authors say this matters

    The authors conclude that these productivity estimates provide quantitative benchmarks for Southern Ocean carbon uptake. The study suggests that combining these benchmarks with airborne carbon dioxide observations and surface ocean pCO2 (the partial pressure of carbon dioxide) and temperature observations reduces uncertainty in projected end-of-century Southern Ocean carbon dioxide uptake.

    What the researchers tested

    The researchers constrained Southern Ocean productivity south of about 44° S by linking CMIP6-modeled productivity to modeled air-sea O2 fluxes and by applying O2 flux estimates derived from airborne O2/N2 observations. They compared these estimates with CMIP6 models, satellite-based estimates, Argo oxygen-based estimates, and observations of carbon dioxide fluxes, pCO2, and temperature.

    What worked and what didn't

    The oxygen-based approach produced a productivity estimate that was consistent with Argo oxygen-based estimates but higher than most CMIP6 model and satellite-based estimates. The study also found that CMIP6 models with underestimated productivity tended to show weak summer carbon dioxide uptake, and some showed excessive summer temperature-driven outgassing, leading to incorrect seasonal carbon dioxide flux cycles with summer outgassing instead of the summer uptake indicated by observations. The authors further report that these constraints reduce uncertainty in model-projected end-of-century Southern Ocean carbon dioxide uptake by 53%.

    What to keep in mind

    The abstract says Southern Ocean productivity estimates remain highly uncertain because of limited observations. It also notes that the suggested source of the model errors may be inadequate representation of ocean vertical mixing, but the abstract does not provide a direct test of that explanation.

    • Annual Southern Ocean net primary production was estimated at 6.5 ± 1.36 PgC yr−1.
    • This estimate is higher than most CMIP6 model and satellite-based estimates.
    • The estimate is consistent with Argo oxygen-based estimates.
    • Many CMIP6 models with underestimated productivity showed weak summer CO2 uptake and, in some cases, summer outgassing.
    • Using these constraints reduced uncertainty in projected end-of-century Southern Ocean CO2 uptake by 53%.
  • Australia reefs face different prickly population patterns

    Australia reefs face different prickly population patterns

    What the study found

    The study found that two overabundant echinoderms, crown-of-thorns starfish and long-spined sea urchins, affect Australian reefs in different ways. Crown-of-thorns starfish show recurrent boom-and-bust outbreaks, while long-spined sea urchins can form high-density populations that create long-lasting urchin barrens.

    Why the authors say this matters

    The authors conclude that bridging research and sharing knowledge across tropical and temperate reef systems could improve understanding of these parallel problems. They also state that spatially structured management aimed at prevention is more efficient and cost effective than reactive control efforts.

    What the researchers tested

    The researchers synthesized and compared the ecology and management of crown-of-thorns starfish and Centrostephanus rodgersii, focusing on the Great Barrier Reef and the eastern Great Southern Reef. The comparison covered population dynamics, nutritional ecology, regeneration of nutritional resources, and management by culling or harvesting.

    What worked and what didn't

    For both species, the abstract reports evidence that local top-down active management by culling or harvesting can be effective. It also reports large funding differences, with funding for Great Barrier Reef crown-of-thorns starfish management being more than 40 times higher than funding for Tasmanian urchin management.

    What to keep in mind

    This is a comparative review, so the abstract does not present new experimental results. The summary also does not describe detailed limitations beyond noting that management approaches and funding levels differ across regions.

    • Crown-of-thorns starfish have recurrent boom-and-bust outbreaks on the Great Barrier Reef.
    • Long-spined sea urchins can form stable, high-density populations that create urchin barrens lasting decades or more.
    • The two species differ in nutritional ecology: crown-of-thorns starfish are obligate corallivores, while long-spined sea urchins are omnivores.
    • The abstract reports evidence that local culling or harvesting can be effective for both species.
    • Funding for Great Barrier Reef crown-of-thorns starfish management is reported to be more than 40-fold higher than funding for Tasmanian urchin management.

  • Hydrology controlled nitrate cycling in a tidal freshwater river

    What the study found

    The study found that nitrate cycling in the Hawkesbury River tidal freshwater zone was strongly shaped by hydrology, especially discharge and residence time. Internal nitrate cycling played a major role, and the balance between external nitrate inputs and in-stream processing changed between wet and dry conditions.

    Why the authors say this matters

    The authors conclude that combining high-resolution sampling with isotope-based mixing models helps resolve nitrogen transformation processes in tidal freshwater zones, where internal cycling strongly influences nitrate form, isotope composition, and downstream export. The study suggests this approach is useful for understanding dynamic environments with multiple nutrient sources.

    What the researchers tested

    The researchers studied nitrate sources and cycling along the Hawkesbury River tidal freshwater zone in Eastern Australia under different hydrological conditions. They used high-resolution stable isotope analysis of nitrate (δ15N-NO3 and δ18O-NO3) together with conservative mixing models, based on more than 650 isotope measurements.

    What worked and what didn't

    δ15N-NO3 values were significantly enriched beyond levels typical of agricultural and urban catchments, which indicated widespread non-conservative nitrate behavior. Mixing-model results showed deviations from conservative nitrate mixing in both wet and dry conditions; during wet periods, hydrological connectivity increased external dissolved inorganic nitrogen inputs, while high discharge and short residence times limited nitrate accumulation and isotopic enrichment.

    What to keep in mind

    The abstract does not describe detailed methodological limits beyond the fact that the work focused on one tidal freshwater zone and compared wet and dry hydrological conditions. The findings are specific to the Hawkesbury River system and the conditions sampled.

    • Nitrate cycling in the tidal freshwater zone was strongly controlled by discharge and residence time.
    • Internal nitrate cycling played a crucial role in regulating nitrate dynamics.
    • Wet conditions increased external dissolved inorganic nitrogen inputs through greater hydrological connectivity.
    • Dry conditions strengthened in-stream processing and isotope–concentration relationships, especially in the upper tidal freshwater zone.
    • Denitrification was an important nitrate removal pathway during dry-period low-flow conditions.
    • Negative Δ(15,18) values during wet periods were consistent with nitrification of groundwater-derived ammonium.

  • Glacial and biogenic particles create similar fjord color

    What the study found

    The study found that fjords influenced by glacial meltwater and by a coccolithophore bloom can have nearly indistinguishable apparent optical properties, including remote sensing reflectance and perceived color. Although the waters look similar from above, their inherent optical properties and scattering measurements show clear differences.

    Why the authors say this matters

    The authors conclude that standard satellite algorithms cannot separate glacial and biogenic particle regimes in fjord systems without prior knowledge. The study suggests that autonomous observations may help quantify primary production in high-latitude coastal systems.

    What the researchers tested

    The researchers made comprehensive measurements of inherent and apparent optical properties in two Norwegian fjords, Gaupnefjorden and Hardangerfjorden. They compared absorption, remote sensing reflectance, RGB chromaticity coordinates, backscattering ratios, and volume scattering functions.

    What worked and what didn't

    In both fjords, absorption spectra had minima near 532 nm, matching the green color seen in satellite imagery. Remote sensing reflectance values and RGB chromaticity coordinates were nearly indistinguishable between the two fjords, while inherent optical properties and scattering measurements did distinguish them. Both fjords had high backscattering ratios of 0.02–0.04, and Hardangerfjorden showed more pronounced forward scattering, while Gaupnefjorden had broader angular scattering patterns.

    What to keep in mind

    The findings are based on two fjords in Norway and on the specific water types studied there. The abstract does not describe additional limitations beyond the statement that standard remote sensing algorithms could not separate the two particle regimes without a priori knowledge.

    • Glacial meltwater and coccolithophore blooms produced nearly identical remote sensing reflectance spectra.
    • Both fjords showed green or turquoise color with absorption minima near 532 nm.
    • Backscattering ratios were similar in both systems, at 0.02–0.04.
    • Volume scattering functions and particulate absorption distinguished the two water types.
    • Standard satellite algorithms could not separate mineral and biogenic particle regimes without prior knowledge.

  • Marine biogeochemical cycles can shift into alternative regimes

    What the study found

    The study found that marine biogeochemical cycles can enter alternative dynamic regimes, meaning different stable patterns of behavior, when the environment is pushed to extreme values. It also found that these cycles commonly respond reversibly, so the original regime returns when the perturbation is removed.

    Why the authors say this matters

    The authors say this matters because alternative regimes in marine biogeochemical cycles are relevant for understanding climate change impacts and for developing mitigation and adaptation strategies. They also conclude that identifying these regimes may help reveal potentially dangerous paths for the ocean state under future climate conditions.

    What the researchers tested

    The researchers used a state-of-the-art marine biogeochemical model, including a one-dimensional water-column physical-biogeochemical model. They tested extreme changes in environmental conditions such as air temperature, wind velocity, and nutrient input, using sequential simulations, initial condition perturbation, and demographic stochasticity.

    What worked and what didn't

    The model showed that alternative dynamic regimes exist across a wide range of environments and methodologies. A single forcing, nutrient depletion, produced hysteresis, meaning the system's response depended on its prior state, in the planktonic trophic web that supports the biogeochemical cycles. The cycles were usually reversible after perturbations were removed.

    What to keep in mind

    The abstract does not describe specific numerical outcomes or effect sizes. It also does not state detailed limitations beyond noting that the findings come from simulations with a one-dimensional water-column model, though the authors say the large number of simulations and sensitivity analysis support the generality and accuracy of the model even under extreme environments.

    • Marine biogeochemical cycles can shift into alternative dynamic regimes under extreme environmental perturbations.
    • The cycles commonly return to the original regime after the perturbation is removed.
    • Nutrient depletion was the single forcing that induced hysteresis in the planktonic trophic web.
    • The study used a one-dimensional water-column physical-biogeochemical model with many simulations.
    • The authors say the results may help identify dangerous future ocean-state paths under climate change.

  • Sulfuric and nitric acids drive iron dissolution differently by air layer

    What the study found

    The study found that iron dissolution, the process by which iron becomes dissolved in air particles, differs between the upper mixing layer and ground-level air in a megacity. In the upper mixing layer, sulfuric acid was the main driver, while near the ground nitric acid was more important.

    Why the authors say this matters

    The authors conclude that these findings matter because they provide new data for testing atmospheric models that simulate dissolved iron concentration and deposition. The study suggests this may help improve the accuracy of iron solubility predictions.

    What the researchers tested

    The researchers compared iron acid dissolution between the upper mixing layer and ground-level air, focusing on the effects of sulfuric acid and nitric acid. They also examined how particle size affected iron solubility in submicron aerosols, which are particles smaller than 1 micrometer, and supermicron particles, which are larger than 1 micrometer.

    What worked and what didn't

    Air masses with higher sulfate-to-nitrate ratios were associated with greater iron solubility in the upper mixing layer after atmospheric aging. The abstract reports that sulfuric acid dominated iron acidification in the upper layer and in submicron particles, while nitric acid was more important near the ground; in supermicron particles, alkaline mineral dust neutralized nitric acid and reduced iron dissolution.

    What to keep in mind

    The abstract presents results from a megacity and distinguishes between upper mixing layer air and ground-level air near source regions of acidic gases, so the findings are scope-specific. No additional limitations are described in the available summary.

    • Iron dissolution differed between the upper mixing layer and ground-level air in a megacity.
    • Sulfuric acid was the main driver in the upper mixing layer and in submicron aerosols.
    • Nitric acid was more important for iron dissolution near the ground.
    • Higher sulfate-to-nitrate ratios were linked to greater iron solubility after atmospheric aging.
    • Mineral dust in larger particles neutralized nitric acid and suppressed iron dissolution.

  • Local human disturbances reduce coral reef climate refugia

    What the study found

    The study found that some coral reef areas that could act as climate refugia, meaning places where corals may better persist under climate stress, are being suppressed by local human disturbances. The most effective refugia were reefs with naturally moderate turbidity, or naturally cloudy water.

    Why the authors say this matters

    The authors conclude that reducing local human disturbances could create or restore climate refugia that are otherwise suppressed. The study suggests this could substantially expand the area of functioning coral reef refugia.

    What the researchers tested

    The researchers analyzed relationships among marine heatwaves, local human pressures, environmental conditions, and stony-coral cover. They used a mixed-effects spatio-temporal Bayesian model and examined 12,892 coral-reef sites while testing four refugial hypotheses based on latitude, remoteness, depth, and turbidity.

    What worked and what didn't

    Some potential refugia were not impacted by local human disturbances, but many were suppressed by them. The study reports that the strongest refugia were reefs with naturally moderate turbidity, while local human disturbances globally suppressed coral reefs, particularly inshore, turbid reefs that might otherwise function as refugia if local stressors were reduced.

    What to keep in mind

    The summary does not provide details on specific limitations beyond the scope of the analysis. The findings are based on the sites and variables described in the abstract, so claims are limited to those measures and the patterns observed in this study.

    • Some potential coral reef climate refugia are suppressed by local human disturbances.
    • Reefs with naturally moderate turbidity were identified as the most effective refugia.
    • The analysis included 12,892 coral-reef sites.
    • The researchers tested refugial hypotheses based on latitude, remoteness, depth, and turbidity.
    • Local human disturbances, including pollution and land-use change, were especially important for inshore, turbid reefs.

  • Atlantic Ocean δ13CDIC reconstructed from sparse historical data

    What the study found

    The study reconstructed stable carbon isotope composition of marine dissolved inorganic carbon, or δ13C DIC, across the Atlantic Ocean. The authors report that the resulting dataset expands coverage, improves spatial and temporal continuity, and includes a gridded three-dimensional product for the Atlantic.

    Why the authors say this matters

    The authors say these reconstructed datasets provide new opportunities to resolve regional carbon cycle dynamics, validate Earth system models, refine estimates of ocean carbon uptake on decadal timescales, and extend climate reanalysis records. They also suggest the data can support assessment of decadal trends.

    What the researchers tested

    The researchers used a probabilistic machine learning method called Gaussian Process Regression, or GPR, to reconstruct δ13C DIC in the Atlantic Ocean. They compiled data from 51 historical cruises, applied secondary quality control using crossover analysis, retained 37 cruises for model training, validation, and testing, and used GLODAPv2.2023 Atlantic data as predictors. They also applied the validated framework to a global interior ocean mapped climatology to produce a gridded Atlantic dataset.

    What worked and what didn't

    The trained GPR model achieved an average bias of −0.007 ± 0.082 ‰ and an overall uncertainty of 0.11 ‰, with uncertainty attributed to measurement, mapping, and input-variable errors. The reconstruction increased acceptable δ13C DIC samples from 8,941 to 68,435, a factor of 7.65, and improved resolution in longitude, latitude, depth, and time. The authors also report that numerical model-based validation confirmed the model's robustness.

    What to keep in mind

    Validation was limited by sparse observations, which is why the authors supplemented it with numerical model-based validation. The abstract does not describe other major limitations beyond the available observational coverage and the fact that the work is focused on the Atlantic Ocean.

    • δ13C DIC was reconstructed for the Atlantic Ocean using Gaussian Process Regression.
    • The study used data from 51 historical cruises, with 37 retained after quality control.
    • The reconstruction increased acceptable samples from 8,941 to 68,435.
    • Reported model performance included an average bias of −0.007 ± 0.082 ‰ and overall uncertainty of 0.11 ‰.
    • The authors produced both an expanded observational reconstruction and a gridded 3D Atlantic dataset.

  • Lake Mai Ndombe is a strong greenhouse gas source

    What the study found

    Lake Mai Ndombe, a humic tropical lake in the Democratic Republic of Congo, was found to be highly supersaturated with major dissolved greenhouse gases across all seasons, sampling stations, and water depths. The authors also report that greenhouse gas concentrations generally increased with water depth, and that the lake emits substantial amounts of carbon dioxide, methane, and nitrous oxide.

    Why the authors say this matters

    The authors conclude that carbon inputs, total water depth, and/or dissolved oxygen saturation are major drivers of the flux and composition of greenhouse gases emitted from Lake Mai Ndombe. They suggest these observations may also help explain greenhouse gas emissions from other tropical humic lakes.

    What the researchers tested

    The researchers measured seasonal concentrations and isotopic compositions of the major dissolved greenhouse gases in Lake Mai Ndombe during high-water, falling-water, and low-water periods. They combined those field measurements with temperature and wind speed data to estimate gas emissions from the lake.

    What worked and what didn't

    The lake water column was weakly to non-stratified, meaning it was only weakly layered or not layered at all, and it remained highly supersaturated for all measured gases. Carbon-isotope signals suggested that one gas was largely sourced from respiration of bioavailable organic carbon, while methane reflected sedimentary methanogenesis followed by aerobic methanotrophy in the water column. Nitrogen-isotope measurements indicated a nitrogen cycle dominated by sedimentary denitrification, with near-quantitative reduction to nitrogen gas before upward diffusion and outgassing.

    What to keep in mind

    The abstract does not describe experimental limitations in detail. The study focuses on one lake in central Africa, so the results are directly shown for Lake Mai Ndombe and are only suggested to be potentially relevant to other tropical humic lakes.

    • Lake Mai Ndombe was highly supersaturated with major dissolved greenhouse gases in every season, station, and depth sampled.
    • Greenhouse gas concentrations increased with water depth.
    • The study estimated lake emissions of carbon dioxide, methane, and nitrous oxide from field measurements and model-based extrapolation.
    • Isotope evidence suggested one gas came largely from respiration of bioavailable organic carbon, while methane involved sedimentary methanogenesis and aerobic methanotrophy.
    • Nitrogen-cycle measurements pointed to sedimentary denitrification and strong reduction to nitrogen gas before release to the water column.