Hydromechanical stimulation of bioluminescent plankton

The response of the bioluminescent dinoflagellate Pyrocystis fusiformis was investigated for different hydraulic conditions (‘hydromechanical stimulation’). Pipe flow and oscillating shear produced luminescence, whereas changes in hydrostatic pressure were not stimulating. More intense fluid motion led to higher intensity, mainly due to a higher probability of cell response. The organism was also able to emit light in a glucose–salt mixture. The experiments suggest that the cells are effectively stimulated if the flow conditions change in time. Copyright © 2002 John Wiley & Sons, Ltd.     

Do light/dark cycles of medium frequency enhance phytoplankton productivity?

It has been suggested that turbulence with the resultant light/dark cycle and light gradient through which phytoplankton move, enhances their productivity. The stationary bottle incubation technique for estimating rates of primary productivity has mainly been criticized because of bottle effects, the elimination of natural turbulence and the presence of photo-inhibition. In a series of experiments where productivity was measured over static profiles and compared to the productivity in a mixed system, no definite conclusion could be reached regarding the effect of varying light/dark cycles of medium frequency (seconds to minutes). It appeared as though the ratio of the euphotic depth to mixing depth (Z _eu/Z _m) influenced productivity more than the duration of the light/dark cycle. The static bottle incubation method gave higher integral productivities than the mixed samples at low ratio's ofZ _eu/Z _m. It is suggested that mixing has two separate, but synergistic effects i.e. it not only moves the phytoplankton cells through a light/dark cycle, but also decreases the boundary layer, which increases the rate of exchange through the cell wall of nutrients and metabolites. In doing so more nutrients are available and light could be utilized more efficiently and therefore, productivity is increased.     

The effect of turbulent motion on the diffusion of microorganisms

The effect of turbulent fluid motion on the diffusion of simple organisms is discussed. The net reproduction rate and the turbulent flow are assumed to be Gaussian-correlated random variables. For homogeneous istropic turbulence, simple equations for the average concentration of the organisms are derived in terms of the energy density of the fluid. It is shown that the effective diffusivity generated by the motion is positive-definite, and is independent of the helicity of the flow.     

Mini review: Hydrodynamics of larval settlement into fouling communities

Abstract

Flowing water delivers planktonic larvae to surfaces, but also dislodges them. This paper reviews experiments in the field and in laboratory flumes, as well as mathematical models, which have revealed how the interaction of ambient water motion with a developing fouling community affects larval settlement. Although mean current velocities across fouling communities in harbours are low, instantaneous velocities can be much higher due to turbulence and to the velocity oscillations of wind chop and ship wakes. As a fouling community develops, its topography becomes more complex and the range of flow microhabitats on the spatial scale of larvae increases. In spite of the prevalence of waves in shallow coastal habitats, and in spite of the importance to settlement of the fine-scale instantaneous velocities encountered by larvae, most studies of flow effects on larval settlement have focused on unidirectional currents and on temporally- and spatially-averaged aspects of the flow.     

Pollination of lark daisy on roadsides declines as traffic speed increases along an Amazonian highway

Ecological disturbances caused by roadways have previously been reported, but traffic speed has not been addressed. We investigate effects of traffic speed on pollination of Centratherum punctatum (Asteraceae) along an Amazonian highway roadside. We hypothesised that frequency of flower visitors, duration of single visits and pollen deposition on stigmas will vary negatively as traffic speed increases. After measuring vehicle velocities, we classified three road sections as low‐, mid‐ and high‐velocity traffic. The main pollinator bee, Augochlora sp., visited C. punctatum inflorescences with decreasing frequency from low‐ to high‐velocity roadside sections, whereas the nectar thief butterflies did the opposite. Duration of single visits by bees and butterflies was shorter, and arrival of pollen on C. punctatum stigmas was lower, in high‐ than in low‐velocity roadside. Air turbulence due to passing vehicles increases with velocity and disturbed the flower visitors. Overall, results support that traffic velocity negatively affects foraging of flower visitors and the pollination of C. punctatum on roadsides.     

Model studies of the flow in abdominal aortic aneurysms during resting and exercise conditions

Pulsatile flow in abdominal aortic aneurysm (AAA) models has been examined in order to understand the hemodynamics that may contribute to growth of an AAA. The model studies were conducted by experiments (flow visualization and laser Doppler velocimetry) and by numerical simulation using physiologically realistic resting and exercise flow conditions. We characterize the flow for two AAA model shapes and sizes emulating early AAA development through moderate AAA growth (mean and peak Reynolds numbers of 362<Re_mean<1053 and 3308<Re_peak<5696 with Womersley parameter 16.4<<21.2). The results of our investigation indicate that AAA flow can be divided into three flow regimes: (i) Attached flow over the entire cycle in small AAAs at resting conditions, (ii) vortex formation and translation in moderate size AAAs at resting conditions, and (iii) vortex formation, translation and turbulence in moderate size AAAs under exercise conditions. The second two regimes are classified in the medical literature as disturbed flow conditions that have been correlated with atherogenesis as well as thrombogenesis. Thus, AAA disturbed hemodynamics may be a contributing factor to AAA growth by accelerating the degeneration of the arterial wall. Our investigation also concluded that vortex development is considerably weaker in an asymmetric AAA. Furthermore, turbulence was not observed in the asymmetric model. Finally, our investigation suggests a new mode of transition to turbulence: vortex ring instability and bursting to turbulence. The transition process depends on a combination of the pulsatile flow conditions and the tube cross-sectional area change.     

A review of fish swimming mechanics and behaviour in altered flows

Fishes suspended in water are subject to the complex nature of three-dimensional flows. Often, these flows are the result of abiotic and biotic sources that alter otherwise uniform flows, which then have the potential to perturb the swimming motions of fishes. The goal of this review is to highlight key studies that have contributed to a mechanistic and behavioural understanding of how perturbing flows affect fish. Most of our understanding of fish behaviour in turbulence comes from observations of natural conditions in the field and laboratory studies employing controlled perturbations, such as vortices generated in the wake behind simple geometric objects. Laboratory studies have employed motion analysis, flow visualization, electromyography, respirometry and sensory deprecation techniques to evaluate the mechanisms and physiological costs of swimming in altered flows. Studies show that flows which display chaotic and wide fluctuations in velocity can repel fishes, while flows that have a component of predictability can attract fishes. The ability to maintain stability in three-dimensional flows, either actively with powered movements or passively using the posture and intrinsic compliance of the body and fins, plays a large role in whether fish seek out or avoid turbulence. Fish in schools or current-swept habitats can benefit from altered flows using two distinct though not mutually exclusive mechanisms: flow refuging (exploiting regions of reduced flow relative to the earth frame of reference) and vortex capture (harnessing the energy of environmental vortices). Integrating how the physical environment affects organismal biomechanics with the more complex issue of behavioural choice requires consideration beyond simple body motions or metabolic costs. A fundamental link between these two ways of thinking about animal behaviour is how organisms sense and process information from the environment, which determines when locomotor behaviour is initiated and modulated. New data are presented here which show that behaviour changes in altered flows when either the lateral line or vision is blocked, showing that fish rely on multi-modal sensory inputs to negotiate complex flow environments. Integrating biomechanics and sensory biology to understand how fish swim in turbulent flow at the organismal level is necessary to better address population-level questions in the fields of fisheries management and ecology.