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.

harmful algal blooms (Donaghay & Osborn 1997, McManus et al. 2008). Early studies of the patchiness in physical and biochemical patterns focused on large scales, typically tens of meters or larger (e.g. Cassie 1963, Haury et al. 1978). It has been recognized that fine-scale patchiness is critical to the interaction between physicochemical parameters and biology (Valiela 1995, Ryan et al. 2010). A particular phenomenon that has received substantial attention is formation of vertically layered, thin patches of © Inter-Research 2013 • www.int-res.com

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 Reτ = 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.

Microscopic holographic PIV performed in an optically index-matched facility resolves the three-dimensional flow in the inner part of a turbulent channel flow over a rough wall at Reynolds number Re τ = 3520. The roughness consists of uniformly distributed pyramids with normalized height of k + s = 1.5k + = 97. Distributions of mean flow and Reynolds stresses agree with two-dimensional PIV data except very close to the wall (<0.7k) owing to the higher resolution of holography. Instantaneous realizations reveal that the roughness sublayer is flooded by low-lying spanwise and groove-parallel vortical structures, as well as quasi-streamwise vortices, some quite powerful, that rise at sharp angles. Conditional sampling and linear stochastic estimation (LSE) reveal that the prevalent flow phenomenon in the roughness sublayer consists of interacting U-shaped vortices, conjectured in Hong et al. (J. ). Their low-lying base with primarily spanwise vorticity is located above the pyramid ridgeline, and their inclined quasi-streamwise legs extend between ridgelines. These structures form as spanwise vorticity rolls up in a low-speed region above the pyramid's forward face, and is stretched axially by the higher-speed flow between ridgelines. Ejection induced by interactions among legs of vortices generated by neighbouring pyramids appears to be the mechanism that lifts the quasistreamwise vortex legs and aligns them preferentially at angles of 54 • -63 • to the streamwise direction.

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