Settling of highly porous and impermeable particles in linear stratification: implications for marine aggregates

The settling velocity of porous particles in linear stratification is affected by the diffusive exchange between interstitial and ambient water. The extent to which buoyancy and interstitial mass adaptation alters the settling velocity depends on the ratio of the diffusive and viscous time scales. We conducted schlieren experiments and lattice Boltzmann simulations for highly porous (95 %) but impermeable spheres settling in linear stratification. For a parameter range that resembles marine porous particles, ‘marine aggregates’, i.e. low Reynolds numbers ( $0.05\leq \textit {Re}\leq 10$ ), intermediate Froude numbers ( $0.1\leq \textit {Fr}\leq 100$ ) and Schmidt number of salt ( $\textit {Sc}=700$ ), we observe delayed mass adaptation of the interstitial fluid due to lower-density fluid being dragged by a particle that forms a density boundary layer around the particle. The boundary layer buffers the diffusive exchange of stratifying agent with the ambient fluid, leading to an enhanced density contrast of the interstitial pore fluid. Stratification-related drag enhancement by means of additional buoyancy of dragging lighter fluid and buoyancy-induced vorticity resembles earlier findings for solid spheres. However, the exchange between density boundary layer and pore fluid substantially increases stratification drag for small $\textit {Fr}$ . To estimate the effect of stratification on marine aggregates settling in the ocean, we derived scaling laws and show that small particles ( $\leq$ 0.5 mm) experience enhanced drag which increases retention times by 10 % while larger porous particle (>0.5 mm) settling is dominated by delayed mass adaptation that diminishes settling velocity by 10 % up to almost 100 %. The derived relationships facilitate the integration of stratification-dependent settling velocities into biogeochemical models.

A Novel Measurement-Based Model for Calculating Diffusive Fluxes Across Substrate-Water Interfaces of Marine Aggregates, Sediments and Biofilms

Our understanding of the small-scale processes that drive global biogeochemical cycles and the Earth’s climate is dependent on accurate estimations of interfacial diffusive fluxes to and from biologically-active substrates in aquatic environments. In this study, we present a novel model approach for accurate calculations of diffusive fluxes of dissolved gases, nutrients, and solutes from concentration profiles measured across the substrate-water interfaces using microsensors. The model offers a robust computational scheme for automatized determination of the interface position and enables precise calculations of the interfacial diffusive fluxes simultaneously. In contrast to other methods, the new approach is not restricted to any particular substrate geometry, does not require a priori determination of the interface position for the flux calculation, and, thus, reduces the uncertainties in calculated fluxes arising from partly subjective identification of the interface position. In addition, it is robust when applied to measured profiles containing scattered data points and insensitive to reasonable decreases of the spatial resolution of the data points. The latter feature allows for significantly reducing measurement time which is a crucial factor for in situ experiments.

Teamwork in the viscous oceanic microscale

Nutrient acquisition is crucial for oceanic microbes, and competitive solutions to solve this challenge have evolved among a range of unicellular protists. However, solitary solutions are not the only approach found in natural populations. A diverse array of oceanic protists form temporary or even long-lasting attachments to other protists and marine aggregates. Do these planktonic consortia provide benefits to their members? Here, we use empirical and modeling approaches to evaluate whether the relationship between a large centric diatom, Coscinodiscus wailesii, and a ciliate epibiont, Pseudovorticella coscinodisci, provides nutrient flux benefits to the host diatom. We find that fluid flows generated by ciliary beating can increase nutrient flux to a diatom cell surface four to 10 times that of a still cell without ciliate epibionts. This cosmopolitan species of diatom does not form consortia in all environments but frequently joins such consortia in nutrient-depleted waters. Our results demonstrate that symbiotic consortia provide a cooperative alternative of comparable or greater magnitude to sinking for enhancement of nutrient acquisition in challenging environments.

ПІДВИЩЕННЯ НАДІЙНОСТІ СУДНОВИХ ЕНЕРГЕТИЧНИХ УСТАНОВОК ШЛЯХОМ УСУНЕННЯ НЕГАТИВНОГО ВПЛИВУ РОЗЦЕНТРОВОК ОСЕЙ З'ЄДНУВАЛЬНИХ ВАЛІВ

The influence of skew axes of the connecting shafts of marine aggregates on the efficiency and reliability of ship power plants is investigated. The reasons for the occurrence of centerings of axes of connecting shafts of marine aggregates are considered and their classification is offered. Failures of ship power plants are classified according to their impact on the efficiency of the installation (according to the consequences), according to the methods of their elimination, and by their nature and are divided into the groups according to the classes. It is shown that during operation, in the conditions of axial skew, traditional designs of the toothed clutch are characterized by non-uniform distribution of forces between pair of gears, which leads to their overload and, as a consequence, reduced load capacity, increased contact stresses, deterioration of lubrication surfaces of gears, increase in power consumption for friction, reduction of efficiency and deterioration of vibroacoustic characteristics due to shock load of gear. Ways to increase the efficiency of ship power plants by providing parameters of reliability of the main power plants by eliminating the negative impact of the centering axes of the connecting shafts of marine aggregates through the use of compensating devices are considered. It is proposed to use as compensating devices, the design of high-efficiency clutch, which in the conditions of axial skew, are characterized by a more uniform distribution of forces between pair of gears than existing ones, ie work almost as a perfect hinge. Regarding the proposed toothed couplings, a previously unknown objectively existing property of the toothed clutch operating at the skew of the axes of the connecting shafts is established, which is that under a certain nonlinear law changes in the generating side surfaces of the outer teeth evenly between all conjugate pairs of teeth. The use of a high-efficiency clutch in the ship power plants is considered and the basic and functional (structural) scheme of the main marine aggregate with a series connection of elements is made to assess the impact on reliability parameters.

Microrheology reveals microscale viscosity gradients in planktonic systems

Microbial activity in planktonic systems creates a dynamic and heterogeneous microscale seascape that harbors a diverse community of microorganisms and ecological interactions of global significance. In recent decades great effort has been put into understanding this complex system, particularly focusing on the role of chemical patchiness, while overlooking a physical parameter that governs microbial life and is affected by biological activity: viscosity. Here we reveal spatial heterogeneity of viscosity in planktonic systems by using microrheological techniques that allow measurement of viscosity at length scales relevant to microorganisms. We show the viscous nature and the spatial extent of the phycosphere, the region surrounding phytoplankton. In ∼45% of the phytoplankton cells analyzed we detected increases in viscosity that extended up to 30 µm away from the cell with up to 40 times the viscosity of seawater. We also show how these gradients of viscosity can be amplified around a lysing phytoplankton cell as its viscous contents leak away. Finally, we report conservative estimates of viscosity inside marine aggregates, hotspots of microbial activity, more than an order of magnitude higher than in seawater. Since the diffusivities of dissolved molecules, particles, and microorganisms are inversely related to viscosity, microheterogeneity in viscosity alters the microscale distribution of microorganisms and their resources, with pervasive implications for the functioning of the planktonic ecosystem. Increasing viscosities impacts ecological interactions and processes, such as nutrient uptake, chemotaxis, and particle encounter, that occur at the microscale but influence carbon and nutrient cycles at a global scale.

Single-cell metabarcoding reveals biotic interactions of the Arctic calcifier Neogloboquadrina pachyderma with the eukaryotic pelagic community

Isotopic and trace-element signals in the calcite shells of the planktonic foraminifera Neogloboquadrina pachyderma represent key proxies to reconstruct past climatic conditions in northern high latitudes. A correct interpretation of these chemical signals requires knowledge of the habitat and trophic interactions of the species. Direct observations on the biological interactions of N. pachyderma in polar environments are lacking and to date no consensus exists on the trophic behaviour of this species. Here we use single-cell metabarcoding to characterise the interactions of 39 specimens of N. pachyderma from two sites in the Baffin Bay with the local eukaryotic pelagic community. Our results show that the eukaryotic interactome of the foraminifera is dominated by diatoms, accounting for > 50% of the reads in 17 of the samples, but other groups such as Crustacea and Syndiniales are also present. The high abundance Syndiniales suggests that these parasites could infect N. pachyderma and may play an important role in its population dynamics. Moreover, the strong but taxonomically non-specific association with algae, existing irrespective of depth and occurring in specimens collected far below the photic zone indicates that opportunistically grazed diatom-fuelled marine aggregates likely represent the main interaction substrate of N. pachyderma.

Disruption Behavior of Aggregates in a Rotating/Oscillating Cylindrical Tank and Implications for Particle Transport in the Ocean

Particle size and settling speed determine the rate of particulate mass transfer from the ocean surface to the sea bed. Turbulent shear in the ocean can act on large, faster-settling flocculated particles to break them into slower-settling primary particles or sub-aggregates. However, it is difficult to understand the disruption behavior of aggregates and their response to varying shear forces due to the complex ocean environment. A study was conducted to simulate the disruption behavior of marine aggregates in the mixed layer of the ocean. The breakup process was investigated by aggregating and disrupting flocs of bentonite clay particles in a rotating and oscillating cylindrical tank 10 cm in diameter filled with salt water. This laboratory tank, which operated based on an extension of Stokes’ second problem inside a cylinder, created laminar oscillating flow superimposed on a constant rotation. This motion allowed the bentonite particles to aggregate near the center of the tank but also exposed large aggregates to high shear forces near the wall. A high-speed camera system was used, along with particle tracking measurements and image processing techniques, to capture the breakup of the large particle aggregates and locate their radial position. The breakup response of large aggregates and the sizes of their daughter particles after breakup were quantified using the facility. The disruption strength of the aggregated particles is presented and discussed relative to their exposure to varying amounts of laminar shear.

New observations of the distribution, morphology and dissolution dynamics of cryogenic gypsum in the Arctic Ocean

To date, observations on a single location indicate that cryogenic gypsum (Ca[SO4]⚫2H2O) may constitute an efficient but hitherto overlooked ballasting mineral enhancing the efficiency of the biological carbon pump in the Arctic Ocean. In June–July 2017 we sampled cryogenic gypsum under pack ice in the Nansen Basin north of Svalbard using a plankton net mounted on a remotely operated vehicle (ROVnet). Cryogenic gypsum crystals were present at all sampled stations, which suggested a persisting cryogenic gypsum release from melting sea ice throughout the investigated area. This was supported by a sea ice backtracking model, indicating that gypsum release was not related to a specific region of sea ice formation. The observed cryogenic gypsum crystals exhibited a large variability in morphology and size, with the largest crystals exceeding a length of 1 cm. Preservation, temperature and pressure laboratory studies revealed that gypsum dissolution rates accelerated with increasing temperature and pressure, ranging from 6 % d−1 by mass in polar surface water (−0.5 ∘C) to 81 % d−1 by mass in Atlantic Water (2.5 ∘C at 65 bar). When testing the preservation of gypsum in formaldehyde-fixed samples, we observed immediate dissolution. Dissolution at warmer temperatures and through inappropriate preservation media may thus explain why cryogenic gypsum was not observed in scientific samples previously. Direct measurements of gypsum crystal sinking velocities ranged between 200 and 7000 m d−1, suggesting that gypsum-loaded marine aggregates could rapidly sink from the surface to abyssal depths, supporting the hypothesized potential of gypsum as a ballasting mineral in the Arctic Ocean.

Marine aggregates deposits in Greece

Η παρούσα διδακτορική διατριβή με τίτλο «Υποθαλάσσιες αποθέσεις (offshore placers) αδρανών υλικών στην Ελλάδα: ιζηματολογική – ορυκτολογική & γεωχημική μελέτη ─ Δυνατότητες αξιοποίησης τους» έχει ως αντικείμενο τη συστηματική διερεύνηση του δυναμικού των πιθανών Υποθαλάσσιων Κοιτασμάτων Αδρανών Υλικών (ΥΚΑΥ) στην Ελλάδα, σε σχέση με τη βιομηχανική χρήση των πρώτων, αυτών, υλών. Η παρούσα διατριβή επικεντρώνεται στην έρευνα σύγχρονων υπεράκτιων ιζημάτων στην ελληνική υφαλοκρηπίδα εξετάζοντας τα ιζηματολογικά, ορυκτολογικά και γεωχημικά χαρακτηριστικά τους, δίνοντας έμφαση στην έρευνα χαλαζιακής άμμου. Οι, υπο μελέτη, υπεράκτιες περιοχές προτεραιοποιούνται ως πιθανές θέσεις ΥΚΑΥ βάσει των ποιοτικών χαρακτηριστικών των ιζημάτων καθώς και βασικών περιβαλλοντικών χαρακτηριστικών τους. Πιο συγκεκριμένα, στο πλαίσιο της παρούσας διατριβής, μελετήθηκαν τέσσερις υπεράκτιες περιοχές στην ελληνική υφαλοκρηπίδα σαν πιθανές θέσεις ΥΚΑΥ, οι οποίες βρίσκονται στη ΝΑ ν. Εύβοια, στη ΒΔ ν. Κρήτη, στη νότια και ανατολική ν. Λέσβο και στη ΒΑ ν. Ρόδο. Εν συνεχεία, μελετήθηκε υλικό από επιλεγμένες παράκτιες περιοχές, οι οποίες βρίσκονται στην κεντρική/ανατολική ν. Εύβοια, στη δυτική ν. Κρήτη, στη βόρεια και ανατολική Πελοπόνησο, στη ν. Μήλο, στη ν. Σάμο και στη ν. Κεφαλονιά, ώστε να προσδιοριστεί η δυνατότητα για περαιτέρω υπεράκτια έρευνα. Επιπλέον, μελετήθηκαν βιομηχανικά δείγματα ως υλικά για σύγκριση και αξιολόγηση των υπο μελέτη ιζημάτων. Ο βαθμός καταλληλότητας των υπο μελέτη ιζημάτων ως αδρανή σκυροδέματος προσδιορίστηκε, επιπροσθέτως, από εξειδικευμένες τεχνικές δοκιμές.Για την επιτευξη των ως άνω στόχων, συλλέχθηκαν επιφανειακά υπεράκτια και παράκτια ιζήματα, καθώς και υπεράκτιοι πυρήνες βαρύτητας. Τα ποιοτικά χαρακτηριστικά των ιζημάτων προσδιορίστηκαν σύμφωνα με: την κοκκομετρία τους καθώς και των στατιστικών κοκκομετρικών παραμέτρων της, την πυκνότητα, την ορυκτολογία με στερεσκοπία, XRD και SEM/EDS, τη γεωχημεία με τη μέθοδο XRF, των χλωρϊόντων και του ανθρακικού ασβεστίου. Χρησιμοποιήθηκαν οι τεχνικές δοκιμές του ισοδύναμου άμμου και μπλε του μεθυλενίου, ώστε να προσδιοριστεί η καταλληλότητα των προτεινόμενων υλικών ως αδρανή σκυροδέματος. Τέλος, στις προτεινόμενες περιοχές εφαρμόστηκαν περιβαλλοντικοί χωρικοί περιορισμοί που αφορούν στις ζώνες Natura του είδους Posidonia oceanica, στο closure depth καθώς και στις κατάλληλες αποστάσεις από την ακτογραμμή. Η πιο υποσχόμενη υπεράκτια περιοχή βρίσκεται στη ΝΑ ν. Εύβοια, η οποία προτείνεται πως φιλοξενεί ΥΚΑΥ που εκτείνονται σε 2,03 km2 μέχρι την ισοβαθή των 50 m. Η επόμενη πιο ελπιδοφόρα περιοχή βρίσκεται στη ΒΔ ν. Κρήτη, στον κόλπο της Κισσάμου, η οποία προτείνεται πως φιλοξενεί ΥΚΑΥ που εκτείνεται σε 4,85 km2 μέχρι την ισοβαθή των 50 m. Είναι η πρώτη φορά που πραγματοποιείται μια ολοκληρωμένη μελέτη βασισμένη σε ποιοτικά και περιβαλλοντικά κριτήρια και αφορά ΥΚΑΥ στην ελληνική υφαλοκρηπίδα. Τα προτεινόμενα ΥΚΑΥ συμβάλλουν στην Εθνική Πολιτική για τον στρατηγικό σχεδιασμό και την εκμετάλλευση των Ορυκτών Πρώτων Υλών. Ωστόσο, για την τελική οριοθέτηση και εκμετάλλευση των προτεινόμενων ΥΚΑΥ, θα πρέπει να εξεταστούν τα ποσοτικά τους χαρακτηριστικά (ωκεανογραφική μελέτη) καθώς και να ληφθεί υπόψη ο τοπικός χωροτακτικός σχεδιασμός, ώστε να καλύπονται οι πυλώνες της βιώσιμής ανάπτυξης.