Solutions are analysed from large-eddy simulations of the phase-averaged equations for oceanic currents in the surface planetary boundary layer (PBL), where the averaging is over high-frequency surface gravity waves. These equations have additional terms proportional to the Lagrangian Stokes drift of the waves, including vortex and Coriolis forces and tracer advection. For the wind-driven PBL, the turbulent Langmuir number, Latur = (U∗/Us)1/2, measures the relative influences of directly wind-driven shear (with friction velocity U∗) and the Stokes drift Us. We focus on equilibrium solutions with steady, aligned wind and waves and a realistic Latur = 0.3. The mean current has an Eulerian volume transport to the right of the wind and against the Stokes drift. The turbulent vertical fluxes of momentum and tracers are enhanced by the presence of the Stokes drift, as are the turbulent kinetic energy and its dissipation and the skewness of vertical velocity. The dominant coherent structure in the turbulence is a Langmuir cell, which has its strongest vorticity aligned longitudinally (with the wind and waves) and intensified near the surface on the scale of the Stokes drift profile. Associated with this are down-wind surface convergence zones connected to interior circulations whose horizontal divergence axis is rotated about 45° to the right of the wind. The horizontal scale of the Langmuir cells expands with depth, and there are also intense motions on a scale finer than the dominant cells very near the surface. In a turbulent PBL, Langmuir cells have irregular patterns with finite correlation scales in space and time, and they undergo occasional mergers in the vicinity of Y-junctions between convergence zones.

Langmuir turbulence in the ocean

A new extension of digestion theory and re-interpretation of published empirical evidence suggest that the principal pathway of dissolved organic carbon (DOC) from phytoplankton to bacteria is through the byproducts of animal ingestion and digestion rather than via excretion of DOC directly from intact phytoplankton. Simple model calculations reveal that for a substance with diffusion coefficient equalling 10−5 cm2 s−1, excess (over ambient) concentrations of solute in a fecal pellet of typical size (diam. ⩽ 1 mm) are lost rapidly; ⩾ 50% of any excess is diffused out of the pellet within 5 min—even in a stagnant water column and without particle sinking. Reasons for rapid loss and its insensitivity to fluid dynamic conditions are small size of the pelletal reservoir and the sharp concentration gradient between pelletal and ambient concentrations upon pellet release. As a consequence, most solutes initially contained in fecal pellets of zooplankton generally will remain in the 10–100 m thick water layer within which the pellets initially are deposited. Focus on animal-caused organic release over these very short time scales may help to resolve some of the growing paradoxes of DOC standing stocks and fluxes in the upper ocean.

Closing the microbial loop: dissolved carbon pathway to heterotrophic bacteria from incomplete ingestion, digestion and absorption in animals

Microelectrode studies demonstrate the existence of persistent oxygen and pH gradients around flocculent, macroscopic marine particles known as marine snow. Oxygen is partially, but continuously, depleted within and around marine snow in the dark and can be completely depleted within large fecal pellets. Boundary layers hundreds of micrometers thick are maintained despite advection of fluid past the particles. The existence of chemical microhabitats on the scale of millimeters around macroscopic particles in the pelagic zone may significantly influence the distribution and activity of marine microorganisms and permit processes requiring low oxygen, including denitrification.

https://science.sciencemag.org/content/sci/235/4789/689.full.pdf

Can microscale chemical patches persist in the sea? Microelectrode study of marine snow, fecal pellets.

Models of particle encounter efficiency borrowed from aerosol fil- tration theory have revolutionised ideas about mechanisms of suspension-feeding (a form of hydrosol filtration), although they continue to be under-utilised in generating predictions concerning feeding ecology and morphological evolution and in guiding quantitative experimentation . As a complementary tool we model encounter rate, demonstrated with parameterisations ofthe same particle-encounter mechanisms . The rate models include specific encounter geometry, absent from the prevalent efficiency indices, which is used to refine estimates of the absolute and relative effectiveness of the various encounter mechanisms . Using rate (as opposed to effi- ciency and in accord with optimal foraging theory) as the dependent variable yields very different predictions for optimisation schemes . For example, a larger collector radius reduces encounter efficiency for most mechanisms, but it either increases or leaves unaffected encounter rate . Similarly, an increase in advection velocity often reduces encounter efficiency, but it increases encounter rate for most mechanisms

Physical mechanisms and rates of particle capture by suspension-feeders

A multi-scale asymptotic theory is derived for the evolution and interaction of currents and surface gravity waves in water of finite depth, under conditions typical of coastal shelf waters outside the surf zone. The theory provides a practical and useful model with which wave–current coupling may be explored without the necessity of resolving features of the flow on space and time scales of the primary gravity-wave oscillations. The essential nature of the dynamical interaction is currents modulating the slowly evolving phase of the wave field and waves providing both phase-averaged forcing of long infra-gravity waves and wave-averaged vortex and Bernoulli-head forces and hydrostatic static set-up for the low-frequency current and sea-level evolution equations. Analogous relations are derived for wave-averaged material tracers and density stratification that include advection by horizontal Stokes drift and by a vertical Stokes pseudo-velocity that is the incompressible companion to the horizontal Stokes velocity. Illustrative solutions are analysed for the special case of depth-independent currents and tracers associated with an incident surface wave field and a vortex with O(1) Rossby number above continental shelf topography.

An asymptotic theory for the interaction of waves and currents in coastal waters

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1987.32.5.1177

Wave interactions and fluid flows

Slowly sinking particles exchange mass with their liquid environment if dissolution processes within the particles maintain a surface concentration excess or deficiency x0 of a soluble substance above background. If the exchange is controlled by difIi.tsion in the liquid phase, the total mass loss is proportional to x0, diffusivity D, and particle diameter d. If advection associated with the sinking of the particle dominates over diffusion, the exchange rate varies as QY,d2 and is weakly affected by particle density and fluid viscosity. This is the case for spherical particles of a diameter z70 pm. The resulting dissolution rate per unit depth of sinking is constant for large particles, but increases sharply once the particles are small and slow enough for mass exchange to be controlled by diffusion. With oxygen supply supposed the rate-controlling process of particle decomposition, a realistic depth of decomposition of order 1 km is calculated for 200~pm-diameter spherical particles. The generation of an oxygen minimum at such depths then requires the production near the sea surface of sufficiently large organic particles in sufficient numbers. Small biogenic particles play an important role in the chemical balance of the ocean. Their decomposition produces a layer of oxygen minimum and layers of maxima of nitrate, phosphate, and silicate. In the Sargasso Sea the O2 minimum layer lies at about 700-m depth, nutrient maximum layers somewhat deeper. In the Slope Sea (between the Gulf Stream and the continental shelf north of Cape Hatteras) the same layers are found some 500 m shallower. In a note supporting some ideas of Rossby (1936), Redfield (1936) suggested that the Slope Sea minimum and maximum layers must be maintained by along-isopycnal ad

/pdf/mass-transfer-to-and-from-small-particles-in-the-sea1-2-m09m383ag7.pdf

G. T. Csanady

Papers

Wave interactions and fluid flows

Mass transfer to and from small particles in the sea1,2