Siddharth Talapatra

Algal toxins alter copepod feeding behavior.

Using digital holographic cinematography, we quantify and compare the feeding behavior of free-swimming copepods, Acartia tonsa, on nutritional prey (Storeatula major) to that occurring during exposure to toxic and non-toxic strains of Karenia brevis and Karlodinium veneficum. These two harmful algal species produce polyketide toxins with different modes of action and potency. We distinguish between two different beating modes of the copepod’s feeding appendages–a “sampling beating” that has short durations (<100 ms) and involves little fluid entrainment and a longer duration “grazing beating” that persists up to 1200 ms and generates feeding currents. The durations of both beating modes have log-normal distributions. Without prey, A. tonsa only samples the environment at low frequency. Upon introduction of non-toxic food, it increases its sampling time moderately and the grazing period substantially. On mono algal diets for either of the toxic dinoflagellates, sampling time fraction is high but the grazing is very limited. A. tonsa demonstrates aversion to both toxic algal species. In mixtures of S. major and the neurotoxin producing K. brevis, sampling and grazing diminish rapidly, presumably due to neurological effects of consuming brevetoxins while trying to feed on S. major. In contrast, on mixtures of cytotoxin producing K. veneficum, both behavioral modes persist, indicating that intake of karlotoxins does not immediately inhibit the copepod’s grazing behavior. These findings add critical insight into how these algal toxins may influence the copepod’s feeding behavior, and suggest how some harmful algal species may alter top-down control exerted by grazers like copepods.

Three-dimensional velocity measurements in a roughness sublayer using microscopic digital in-line holography and optical index matching

Microscopic in-line digital holography and particle tracking are used for measuring the 3D flow field in the inner part of a turbulent boundary layer over a rough surface. This paper focuses on procedures, uncertainty and data quality. Experiments are performed for a rectangular channel flow, at Re??= 3520, with the top and bottom surfaces containing uniformly distributed pyramidal elements. Optical accessibility through the acrylic rough walls is attained by matching the optical refractive index of the fluid with that of acrylic using a NaI solution in water. Localized particle injection ensures that the seeding is sufficient for detecting 5000?10?000 particle pairs in each hologram pair within the 3.1?2.1?1.8 mm3 sample volume, which covers the entire roughness sublayer. The data quality is assessed by evaluating how well the data satisfy the continuity equation for varying resolutions and procedures. Mean velocity and Reynolds stress profiles are compared to 2D particle image velocimetry data, showing excellent agreement above one roughness height away from the wall, and discrepancies, some of which can be attributed to spatial resolution and bias, closer to the rough wall. Sample instantaneous flow realizations contain low-lying vortices, some with spanwise orientation, and others aligned parallel to the roughness grooves, flooding the lower part of the sublayer. Quasi-streamwise vortices with vertical inclinations of 50??60? also appear, with a small fraction of them spanning the entire sublayer.

Biological thin layers: history, ecological significance and consequences to oceanographic sensing systems

Thin layers are water column structures that contain concentrations of organisms (or particles) that occur over very small vertical scales (a few meters or less), but with large horizontal scales (e.g. kilometers). Thin layers are now known to be common phenomenon in a wide variety of environments and can be a critical componant in marine ecosystem dynamics and functioning. While knowledge about their dynamics is important to our basic understanding of oceanic processes, thin layers can have significant impacts on both oceanographic and defense related sensing systems, e.g. thin layers can affect underwater visibility, imaging, vulnerability, communication and remote sensing for both optical and acoustic instrumentation. This paper will review the history of thin layers research, their ecological significance, innovations in oceanographic instrumentation and sampling methodologies used in their study, and the consequences of their occurence to oceanographic sensing systems.

Understanding the 3-D Volumetric Flow and Coherent Structure Alignment in the Inner Part of a Rough Channel Using Microscopic Holography

The 3D flow in the inner part of a turbulent boundary layer over a rough surface, with Reτ = 3400, is measured using digital in-line microscopic holography and particle tracking. Experiments are performed in a special facility, in which the optical refractive index of the transparent rough wall is matched with that of the working fluid. Holograms are recorded in a sample volume covering the roughness sublayer. Using localized particle injection, each hologram pair contains 5000–10,000 matched particle traces, providing the 3D velocity field. Profiles of mean velocity are compared to 2D PIV data, recorded under the same flow conditions. Sample instantaneous flow realizations elucidate some of the typical vortical structures encountered in the sublayer, such as low-lying vortices, some with spanwise and others with roughness groove parallel orientations, and quasi-streamwise structures with vertical inclinations of 50°–60°, some of which extend from the surface to the top of the sublayer. Conditional sampling indicates that characteristic structures have a preferred alignment in the spanwise direction close to the wall. However, with increasing elevation, these structures turn towards the streamwise direction due to roughness-induced flow channeling, and then rise in sharp angles of about 55° to the mean flow.Copyright

Three Dimensional Volumetric Velocity Measurements in the Inner Part of a Turbulent Boundary Layer Over a Rough Wall Using Digital HPIV

Microscopic digital Holographic PIV is used to measure the 3D velocity distributions in the roughness sublayer of a turbulent boundary layer over a rough wall. The sample volume extends from the surface, including the space between the tightly packed, 0.45 mm high, pyramidal roughness elements, up to about 5 roughness heights away from the wall. To facilitate observations though a rough surface, experiments are performed in a facility containing fluid that has the same optical refractive index as the acrylic rough walls. Magnified in line holograms are recorded on a 4864×3248 pixel camera at a resolution of 0.67μm/pixel. The flow field is seeded with 2μm silver coated glass particles, which are injected upstream of the same volume. A multiple-step particle tracking procedure is used for matching the particle pairs. In recently obtained data, we have typically matched ∼5000 particle images per hologram pair. The resulting unstructured 3D vectors are projected onto a uniform grid with spacing of 60 μm in all three directions in a 3.2×1.8×1.8 mm sample volume. The paper provides sample data showing that the flow in the roughness sublayer is dominated by slightly inclined, quasi-streamwise vortices whose coherence is particularly evident close to the top of the roughness elements.Copyright

A Study of Grazing Behavior of Copepods Using Digital Holographic Cinematography

Generating proper feeding currents for entraining prey is one of the important features in the grazing behavior of (∼1mm) copepods. These feeding currents vary with the copepod species, as well as with the species or strains and concentration of prey (∼10 μm) dinoflagellates. Calanoid copepods also hover for a while, while slowly sinking, and then intermittently jump to a different location. In our study, we employed high speed digital holographic cinematography to measure elements of the flow field around copepods in an environment seeded with dinoflagellates. In most cases, the flow field and feeding currents were characterized based on the trajectories of the dinoflagellates. However, in some of the tests we also added neutrally buoyant 20 μm particles as independent flow tracers. At low magnifications, we simultaneously recorded two perpendicular views to obtain the same spatial resolution in all directions. Data were recorded at varying magnifications and frame rates. In recent experiments, we exposed the copepods to different strains of the same dinoflagellate species that have varying levels of toxicity, and measured the resulting changes to the grazing behavior of the copepods. Here we present results from two of these experimental setups: Acartia tonsa with Karlodinium veneficum (non toxic strain) and Acartia tonsa in particle seeded flow. Issues such as swimming characteristics, feeding classification (raptorial vs. filter feeding approaches) and copepod response to different environmental settings were addressed.Copyright