[1] Internal waves of depression were observed propagating along-shelf and into northern Monterey Bay, California (CA) on the inner shelf. These waves had amplitudes approximately equal to the thermocline depth (∼4 m), and were unstable to shear and mix the thermocline. Isopycnal gradient spectra showed that the wave packets lead to an elevated mean dissipation rate of " = 2.63 × 10 −5 m 3 s −2 for up to 2 hours after wave passage. The proximity to the surface created strong surface convergences that can actively transport buoyant material, such as plankton, back into the bay. The wave packets were observed regularly over the upwelling season across multiple years suggesting they may have large effects on the documented spatial variation of phytoplankton and larvae on the inner shelf. The timing of the waves suggests they are not formed by tides interacting with bathymetry, but are generated by buoyant plume propagation.

No abstract

Inhalant flows draw fluid into an orifice from a reservoir and are ubiquitous in engineering and biology. Surprisingly, there is a lack of quantitative information on viscous inhalant flows. We consider here laminar flows (Reynolds number Re≤100) developing after impulsive inhalation begins. We implement finite element simulations of flows with varying Re and extraction height h (orifice height above a bottom bed). Numerical results are experimentally validated using particle image velocimetry measurements in a physical model for a representative flow case in the middle of the Re-h parameter space. We use two metrics to characterize the flow in space and time: regions of influence (ROIs), which describe the spatial extent of the flow field, and inhalation volumes, which describe the initial distribution of inhaled fluid. The transient response for all Re features an inviscid sinklike component at early times followed by a viscous diffusive component. At lower Re, diffusion entrains an increasing volume of fluid over time, enlarging the ROI indefinitely. In some geometries, these flows spatially bifurcate, with some fluid being inhaled through the orifice and some bypassing into recirculation. At higher Re, inward advection dominates outward viscous diffusion and the flow remains trapped in a sinklike state. Both ROIs and inhalation volumes are strongly dependent on Re and extraction height, suggesting that organisms or engineers could tune these parameters to achieve specific inhalation criteria.

Particles that float on the surface of a 3D incompressible turbulent flow are exposed to non-divergencefree properties that result in clustering and unmixing, a reversal of how turbulence normally acts to mix and dilute scalars. Particle clustering is dominated by Lagrangian processes that depend on the time history of the flow; this suggests that Lagrangian coherent structures (LCS) might serve as templates for cluster formation. In this study, non-divergence-free clustering is examined both experimentally and numerically to elucidate the role of LCS in the formation of particle clusters and voids. Experiments are performed on the free-surface of a water-filled tank with turbulence driven by the random pulsing of centrifugal pumps on the tank bottom. Clustering is quantified by imaging fluorescent, buoyant particles that are placed in an initially random distribution on the freesurface. Within clusters, concentrations are observed to increase by an order of magnitude, with the likelihood of observing enhanced concentrations increasing by two orders of magnitude. LCS, obtained from velocity fields utilizing particle image velocimetry, are shown to act as templates for cluster formation. In addition, LCS are shown to possess a dilatation component in non-divergence-free flows that is responsible for unmixing. Numerically, a non-divergence-free chaotic model consisting of interacting Taylor vortices is utilized to investigate processes responsible for cluster formation seen in the experiments. The model results support the experimental finding that LCS act as templates for particle clusters, with scalar unmixing driven by the dilatation component.

In a physical model of a thin planktonic layer, the estuarine copepod Acartia tonsa strongly avoided weakly stratified layers of dissolved chemical compounds from the harmful dinoflagellate Karenia brevis. Chemical‐induced changes in swimming kinematics allowed copepods to effectively avoid the layer and surrounding volume, highlighting the relevance of harmful alga‐grazer interactions at a distance that involve dissolved chemical signals. Avoidance increased significantly with increasing chemical concentration representative of a range of ecologically relevant bloom conditions (1–104 cells/mL equivalent). Under mid to maximal bloom conditions, Acartia displayed visually and hydrodynamically conspicuous avoidance jumps featuring large, rapid displacements with swimming speeds near those reported for predatory escape reactions. Previous findings show K. brevis is both toxic and nutritionally inadequate to A. tonsa when ingested and that exposure to dissolved chemical compounds likely rapidly suppresses effective sampling and grazing behaviors. Thus, copepods have strong incentive to sense and avoid nearby, spatially discrete patches of toxic algae in order to improve fitness by avoiding exposure and/or ingestion and associated negative impacts. Our results suggest harmful alga not only produce deleterious physiological effects in copepod grazers, but chemical‐induced behavioral responses also likely alter grazer distributions and top‐down control via avoidance reactions (reduced harmful alga‐grazer encounter rates). Additionally, predator‐prey encounter rates at higher trophic levels are likely enhanced via significant changes in copepod swimming kinematics. These combined mechanisms could protect and sustain harmful blooms contained in subsurface thin layers until blooms reach critical mass and produce widespread impacts at the ecosystem level, the “cryptic bloom” effect.