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A global summary of seafloor topography influenced by internal-wave-induced turbulent water mixing
No full textTurbulent water motions are important for the exchange of momentum, heat, nutrients, and suspended matter, including sediments in the deep sea. The motions occur in a deep sea that is generally stably stratified in density. To maintain ocean–density stratification, an irreversible diapycnal turbulent transport is needed. The geological shape and texture of marine topography are important for water mixing, as most deep-sea turbulence is generated via internal waves breaking at sloping seafloors. For example, slopes of semidiurnal internal tidal characteristics can “critically” match the mean seafloor slope. In this paper, the concept of critical slopes is revisited from a global internal-wave turbulence viewpoint using seafloor topography and moored high-resolution temperature sensor data. Observations suggest that turbulence generation via internal-wave breaking at 5 % ± 1.5 % of all seafloors is sufficient to maintain ocean–density stratification. However, most, >90 %, turbulence contributions are found at supercritical, rather than the more limited critical, slopes measured at 1′ scales that cover about 50 % of seafloors at water depths <2000 m. Internal tides (∼60 %) dominate over near-inertial waves (∼40 %), which is confirmed by comparison of northeastern Atlantic data and eastern Mediterranean data (weak tides) at the same mid-latitude. Seafloor elevation spectra show a wavenumber (k) falloff rate of k−3, which is steeper than what was found previously. The falloff rate is even steeper, resulting in less elevation variance in a 1-order-of-magnitude bandwidth around kT=0.5 cycle km−1. The corresponding length is equivalent to the internal wave excursion length. The reduction in seafloor elevation variance seems to be associated with seafloor erosion by internal wave breaking. The potential robustness of the seafloor and internal wave interaction is discussed.</span
Reconstructing historical catch trends of threatened sharks and rays based on fisher ecological knowledge
No full textSmall-scale fisheries often lack historical shark and ray catch information, hampering their management. We reconstructed historical catch trends and current fishing pressure by combining local ecological knowledge, satellite-based vessel counts, and a short-term landing-site survey. To test the effectiveness of this method, we focused on the Bijagós Archipelago (Guinea-Bissau, West Africa), where historical fisheries data are lacking. Benthic rays (stingrays [Dasyatidae] and butterfly rays [Gymnura spp.]), benthopelagic rays (duckbill eagle rays [Aetomylaeus bovinus] and cownose rays [Rhinoptera marginata]), guitarfish (Glaucostegus and Rhinobatos spp.), requiem sharks (Carcharhinidae), and hammerhead sharks (Sphyrna spp.) declined in abundance by 81.5–96.7% (species dependent) from 1960 to 2020. Fishing effort increased annually: fishing trip duration by 42.0% (SE 3.4), numbers of fishing vessels at sea as perceived by fishers by 36.3% (1.0) (1960–2020), and number of vessels by 12.0% (1.1) (2007–2022). We estimated that in 2020, fishing vessels collectively captured 61–264 sharks and 522–2194 rays per day in the archipelago, depending on the proportion of the fishing fleet that was active (i.e., low fleet activity of 18% and high fleet activity of 80%). We advocate for reducing shark and ray catches by regulating fleet size, reinforcing boundaries of protected areas, and collecting fisher-dependent information on shark and ray landings to safeguard these vulnerable species and coastal livelihoods. We demonstrated the effectiveness of using this 3-pronged approach to provide baseline data on shark fisheries, a common challenge in areas with small-scale fisheries and limited research capacity.</span
Restoration of native saltmarshes enhances carbon sequestration and mitigates warming effects following <i>Spartina alterniflora</i> removal
No full textThe rapid expansion of exotic Spartina alterniflora has significantly threatened native coastal ecosystems\u27 structure and function, prompting global control efforts. Consequently, native saltmarshes restoration has emerged as a nature-based solution following invasive species removal. However, given that S. alterniflora is a high-carbon invasive species, the impacts of native saltmarshes restoration on coastal blue carbon benefits following its removal remain uncertain.Here, we quantified atmospheric carbon uptake and organic carbon storage among restored native saltmarshes (Phragmites australis and Bolboschoenoplectus mariqueter communities), unrestored bare mudflat following S. alterniflora removal and uncontrolled S. alterniflora communities to assess whether native saltmarshes can compensate for the carbon sinks and the climate effects after invasive species eradication.The results showed that S. alterniflora removal drastically reduced carbon sink, with unrestored mudflat transitioning to a carbon source. While restored native saltmarshes showed lower atmospheric carbon uptake compared to pre-eradication S. alterniflora levels, they exhibited significantly enhanced carbon sequestration relative to unrestored mudflats. Additionally, the organic carbon density of soil (0–50 cm) and vegetation in restored sites exceeded unrestored areas by >1.4 times, recovering >70% of the carbon storage observed in S. alterniflora communities.Sustained global warming potentials (SGWP) analysis over a 100-year timescale revealed that without post-eradication vegetation restoration, saltmarsh could shift from climate cooling to warming effects. Native saltmarsh restoration effectively mitigated this transition, demonstrating substantial climate change mitigation potential.Synthesis and applications. Our findings not only reveal that native saltmarsh restoration is a blue carbon-friendly ecological restoration approach following S. alterniflora removal, but also highlight the critical trade-offs between carbon losses from invasive species removal and the carbon offset achieved through restoring native vegetation, providing actionable guidance for coastal management. These insights are particularly valuable for regions facing similar invasive species challenges, informing the development of integrated strategies that maximize carbon compensation while enhancing coastal sustainability and climate resilience. Future restoration programmes should prioritise multifunctional outcomes that simultaneously address biodiversity conservation and climate mitigation objectives.</ol
Ni isotope cycling in sediments of highly productive upwelling systems
No full textNickel is a bio-essential micronutrient in the ocean and the element holds significant potential for the reconstruction of paleoenvironmental conditions. Nickel isotopes provide information on processes controlling internal oceanic cycling and the oceanic mass balance. These insights provide the basis for inferences from the sedimentary record. Recent studies have shown that diagenetic Ni cycling can cause isotope fractionation and may affect the global oceanic dissolved pool, but direct data to constrain such processes in detail remain relatively scarce. Here, we present a large Ni isotope dataset for sediments and pore waters at eight different locations along the Namibian and Peruvian margins, some of the most productive and oxygen-depleted regions in the open ocean. These different stations represent a wide range of depositional redox conditions, allowing evaluation of Ni behaviour in different environments and better constraints on sedimentary Ni cycling and the potential of Ni as a paleo-environmental proxy.The sedimentary data presented here reinforce findings from previous studies that have suggested that excess Ni burial in organic-rich, strongly reducing sediments underlying highly productive surface waters is unfractionated from the deep ocean isotope composition (1.33 ± 0.07 ‰), with an average δ60Niexcess of 1.37 ± 0.16 ‰ (1 SD, n = 74) for Ni-enriched samples (fexcess >0.8). This observation suggests that sedimentary records from organic-rich continental margin upwelling settings can serve as an archive for past deep-ocean δ60Ni values. The pore water data provide additional constraints on Ni isotope cycling in different redox conditions and show that the preferential precipitation of isotopically light Ni leaves sulphidic pore waters enriched in isotopically heavy Ni, with an average δ60Ni value of 2.04 ± 0.41. Pore-water Ni concentrations are higher than those in the overlying bottom water, implying a diffusive benthic source in these environments, with an average local flux of -28 μmol m-2 yr-1. Where outputs from the oceanic dissolved pool are defined in terms of the characteristics of deeper sediments, this benthic flux should be viewed as recycled, not as a new net input in the oceanic mass balance. With other recent studies, our data highlight benthic Ni fluxes as a feature of highly productive environments despite sulphidic pore waters. However, the observed benthic fluxes in these studies are relatively small compared to sedimentary Ni accumulation rates, suggesting Ni burial is relatively efficient, with 92 ± 7 % (1SD, n = 11) of deposited Ni being retained in the sediment
Influence of ocean waters in retreat episodes of a West Greenland tidewater outlet glacier
No full textThe behavior of tidewater outlet glaciers is critical for understanding the dynamics of the Greenland Ice Sheet. The retreat of these glaciers has far-reaching implications, with impacts ranging from altering local fjord ecosystems to affecting global sea levels. Here, we integrated hydrographic with glaciological parameters to investigate the rapid retreat of Narsap Sermia (NS)—a West Greenland tidewater outlet glacier located near Greenland\u27s largest city, Nuuk. In just 20 years, from 2003 to 2023, NS retreated by 6.7 km with ice discharge volume increasing by 35%–52%, which was more than double the average increase in glacier discharge observed for Greenland Ice Sheet outlet glaciers during the same period. This retreat occurred in three big episodes: 2004–2005, 2010–2012, and 2019–2021. Three main phases were identified during these retreats: a trigger, a cascade, and a stabilization. Each retreat episode was triggered by a surge in subglacial discharge caused by increased meltwater over the Ice Sheet or drainage of ice-dammed lakes. Importantly, warm ocean conditions were necessary to allow these initial retreats to cascade. Stabilization could occur when the terminus eventually settled on more favorable bed topography or if the glacier was in contact with colder oceanic inflows. Based on the pattern established by these past retreats, NS might retreat further for 30 km in a few decades if there is no topographic stabilization. NS thus highlights the role changing ocean temperatures can play in regulating tidewater outlet glacier processes and, consequently, its impacts on local fjord glacial ice cover, eco-hydrology and nearby communities.</span
Artificial barriers and estuarine squeeze: A novel assessment of estuarine vulnerability to climate change and sea level rise
No full textEstuarine ecosystems are threatened globally by changes in climate and catchment land use, with upper estuarine tidal freshwater and low-salinity zones being particularly vulnerable, yet the most poorly understood. These zones play a pivotal role in estuarine structure and functioning but are overlooked in assessments of vulnerability to sea-level rise and climate change. Commonly the tidal limits or landward boundaries of these zones are defined by in-stream barriers, such as weirs and sluices. These barriers restrict the natural inland migration of estuaries, intensifying the risk of saline intrusion as sea levels rise and summer river flows decline – a phenomenon known as ‘estuarine squeeze’. This study provides the first estuarine squeeze vulnerability assessment for mainland England and Wales. Using an extensive dataset of salinity and electrical conductivity measurements, we delineate for the first time, tidal freshwater, oligohaline, brackish and marine zones across 85 estuaries. Of these, 59 (69 %) are constrained by in-stream barriers, and 45 (53 %) contain tidal freshwater and oligohaline zones. Nineteen of these 45 estuaries are bound by barriers at their tidal limits, making them susceptible to estuarine squeeze. These estuaries account for 64 % of all tidal fresh and oligohaline waters in mainland England and Wales. The Medway, Exe and Ouse estuaries in the south of England are identified as being most at risk. These zones are vital gateways, supplying and exchanging energy, matter, and organisms to the lower brackish estuary and upper non-tidal freshwater river. Their loss underscores the urgent need for their assessment, monitoring and management. However, it also presents an opportunity to compensate for their loss through for habitat creation, such as tidal freshwater marshes, offering ecosystem benefits and bolstering resilience against climate and other human-induced changes.</span
Behaviour of cadmium isotopes in sulfidic waters and sediments of the Black Sea: Implications for global cadmium cycling and the application of cadmium isotopes as a paleo-oceanographic proxy
No full textThe cadmium isotope system has found use as a tracer for biological productivity, redox and organic carbon burial in the oceans. There are, however, very few observational constraints on Cd isotopic behaviour in modern sulfidic marine conditions, limiting our understanding of the modern Cd cycle, and our ability to use Cd isotope measurements of ancient sedimentary deposits as a paleoceanographic proxy. Here we study the behaviour of dissolved Cd in the water column of the Black Sea and its incorporation into sediments on the basin floor. The isotopic composition of dissolved Cd in the upper ∼50 m of the Black Sea water column is controlled by a combination of biological uptake and regeneration along with mixing of river water and Mediterranean seawater. Cadmium declines to <2 % of its peak subsurface concentration within the zone of nitrate-reduction but above the sulfide chemocline. The isotopic composition of dissolved Cd below the sulfide chemocline evolves in a manner that is consistent with experimental Cd sulfide fractionation factors, providing field-based support for these earlier studies. In contrast, the stability of the dissolved Cd isotopic signature above the chemocline, despite a large reduction in dissolved Cd concentrations, is explained by diffusion towards the Cd-sulfide sink below the chemocline. Sediments accumulating in the deep Black Sea below the sulfide chemocline have isotopic compositions that are similar to dissolved Cd in its near-surface aqueous concentration maximum. Sediments accumulating at shallower depths have lighter isotopic compositions that are likely affected by non-sulfidic burial pathways mediated by macrofaunal mixing and diagenesis of shallow burial phases. Our study shows that the isotopic composition of bulk sediments accumulating under sulfidic conditions is similar to dissolved Cd in the upper water column, supporting its use as a paleo-chemical proxy for ancient seawater as long as basin-scale controls on open ocean seawater sources are considered. We show that Cd burial in the deep Black Sea, predominantly as CdS, accounts for ∼0.5–15 % of total annual Cd burial in the global ocean.</span
Matching planting methods to species and site conditions is key in seed-based intertidal seagrass restoration
No full textSeagrass meadows continue to be lost and degraded globally. Restoration is one promising and emerging conservation strategy to combat such losses and place seagrass on a pathway to net gain. However, successful restoration methods remain limited to a few species, and geographically constrained, with few experimental trials comparing planting methods across species and seagrass bioregions. This study trialled three seed-based seagrass restoration planting methods in two seagrass bioregions (the temperate north Atlantic and temperate southern oceans). Using two seagrass species Zostera marina and Zostera muelleri this research investigated seed-based planting methods and their influence on the likelihood of seedling emergence, shoot emergence, and seedling growth (i.e. leaf length). Seagrass emergence was observed at 50 % of the experimental sites, with the likelihood of seagrass emergence largely influenced by local site conditions. Each planting method performed variably in relation to species and environmental conditions. Dispenser injection seeding resulted in the highest shoot emergence efficiency of the three methods for Z. marina while biodegradable planting pots and hessian bags were the more favourable methods for use with Z. muelleri seeds. Despite all chosen sites deemed suitable for restoration from habitat suitability models, low seedling emergence suggests that site conditions including wind fetch, redox boundary depth and mud- dominant sediments present specific bottlenecks to seed germination and retention. This work demonstrates the importance of matching seed planting methods to site conditions and species life history traits and highlights the need for greater understanding of mechanisms to overcome germination and emergence bottlenecks in seed-based restoration.</span
Ocean acidification and elevated temperatures alter the behavior of a sub-Antarctic fish
No full textThe interaction of multiple climate change stressors can affect the behavior of marine fish. While these effects have been reported in tropical and temperate species, much less is known for fish inhabiting high latitudes. We analyzed the combined effects of ocean acidification and the highest and lowest seasonal temperatures on the activity level and boldness of Eleginops maclovinus, an ecologically and commercially important notothenioid fish from the subantarctic area. Juveniles were acclimated for one month to two temperatures (T = 4 and 10 °C) and two pCO2 levels (∼500 and ∼1800 μatm) in a full factorial design. In an open field test, the time spent active was significantly affected by temperature, with fish at 10 °C 1.63 times more active than those at 4 °C, but not by pCO2 or the interaction (T × pCO2). No differences were observed in the average swimming velocity measured when active, nor in the time spent in the inner zone of the tank. A refuge emergence test indicated increased boldness under near-future pCO2 levels with fish emerging 2.06 (4 °C) and 1.23 (10 °C) times faster than those acclimated to present-day pCO2 levels. The disruptions of these fundamental behaviors by these climate-driven stressors could have consequences for foraging and predator-prey interactions, with likely detrimental effects on the interactions among sympatric subantarctic fishes under projected climate change scenarios.</span
Determining physiological responses of mussels (<i>Mytilus edulis</i>) to hypoxia by combining multiple sensor techniques
No full textIntertidal bivalves survive longer without oxygen when aerially exposed during low tide than when submerged in hypoxic water. To understand this, we combined three biosensors to continuously monitor responses of individual blue mussels (Mytilus edulis) to aerial exposure in simulated low-tide conditions and during aqueous hypoxia. A valve sensor, heart rate monitor, and an in-shell oxygen microsensor simultaneously recorded behavioural and physiological responses. During aerial exposure, which often occurs in the intertidal, all individuals immediately closed their valves, rapidly depleted in-shell oxygen, and decreased their heart rate. This suggested a shift to anaerobic metabolism and reduced activity as mechanisms to save energy and survive in-shell anoxia during ‘low-tide’ conditions. At the onset of exposure to hypoxic (<1 mg O2/L) water, however, all mussels fully opened their valves, with 75% of the individuals increasing valve activity for at least 1 hour (the duration of our measurements), possibly in an attempt to collect more oxygen by increasing filtration activity. Only 25% of the mussels closed their valves after about 40 minutes of aqueous hypoxia, shifting to the energy efficient strategy used during aerial exposure. As the valves of most individuals remained open during hypoxia, a mussel does not appear to need to close its valve to begin the transition to anaerobic metabolism. Interindividual variation in responses was much lower after exposure to air compared to aqueous hypoxia when the heart rate of most mussels either steadily declined or became highly erratic. Differences in energy expenditure during these different types of exposures likely explains why most mussels, at least from the population we studied, can survive longer during exposure to air compared to aqueous hypoxia, a situation that could occur under situations of elevated temperature in waters with high nutrient loads