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Recent studies show that spherical motile micro-organisms in turbulence subject to gravitational torques gather in down-welling regions of the turbulent flow. By analysing a statistical model we analytically compute how shape affects the dynamics, preferential sampling, and small-scale spatial clustering. We find that oblong organisms may spend more time in up-welling regions of the flow, and that all organisms are biased to regions of positive fluid-velocity gradients in the upward direction. We analyse small-scale spatial clustering and find that oblong particles may either cluster more or less than spherical ones, depending on the strength of the gravitational torques.PACS numbers: 47.63.Gd,92.20.jf Patchiness in suspensions of micro-organisms is frequently observed on a range of spatial scales. The underlying mechanisms differ, depending on the properties of the micro-organisms, and upon the spatial scale. Patchiness can be caused by density stratification and vertical shears [1], by predator-prey cycles, or by interactions between the organisms and water-column gradients -in light, chemistry, turbulence, and in hydrostatic pressure [2]. Patchiness is important because many biological processes (mating, feeding, predation) rely on individual encounters [3], and the encounter rate is strongly influenced by small-scale number-density fluctuations.Gravitaxis may cause such inhomogeneities in the spatial distribution of motile micro-organisms. Density-or drag-asymmetries of the body give rise to torques affecting the swimming direction [4][5][6]. When the effects of gyrotactic torques and fluid-velocity gradients balance, inhomogeneities may form in the spatial distribution, as shown by the micro-alga Chlamydomonas nivalis swimming up against a down-welling pipe flow. The microalgae gather in the centre of the pipe where the downwelling velocity is largest [7]. Gyrotaxis may trap motile organisms in macroscopic shear gradients [8,9], and fluctuating vorticity may cause patchiness [10]. This is confirmed by recent direct numerical simulations (DNS) of motile, spherical micro-organisms in turbulence [11] revealing that the organisms are more likely to be found in down-welling regions of the turbulent flow, they 'preferentially sample' such regions.These results raise three fundamental questions that we address and answer in this Letter. First, how does shape affect the dynamics in turbulence of motile microorganisms subject to gyrotaxis? In Ref.[11] the organisms were assumed to be spherical. Non-spherical or- * Version accepted for publication (postprint) in Phys. Rev. Lett. 116, 108104 (2016) ganisms respond not only to turbulent vorticity but also to turbulent strain [12][13][14][15]. This causes passive rods to exhibit intricate orientational patterns on the surface of turbulent and other complex flows [16][17][18]. Also, shape strongly affects the trajectories of active particles in model flows [19][20][21], and recent DNS indicate that prolate gyrotactic organisms cluster less than spherical ones when...

The influence of vegetation on the urban climate was studied in the subtropical city Gaborone, the rapidly expanding capital of Botswana with approximately 200 000 inhabitants. Temperature records from an urban and a rural station were analysed for the period 1985-96. In an attempt to explain possible seasonal change in vegetation, NOAA satellite normalized difference vegetation index imagery was analysed. The present urban influence was investigated with temperature loggers at selected urban and rural sites. In addition, mobile measurements revealed spatial patterns in temperature and humidity for different land uses.Seasonal patterns of urban-rural differences in minimum temperatures emerge during the period 1994-96, especially during the winter when the heat island effect is largest. It is shown that differences in urban and rural vegetation over the year partly explain this variation. Mobile measurements reveal a weak nocturnal heat island of 2-3°C during clear and calm nights. There are intra-urban temperature differences that are in the same range as the urban-rural relationship due to the role of vegetation. Evapotranspiration lowers the temperature, which was detected by high humidity in areas of lush vegetation. This becomes apparent at midday, when densely vegetated areas were up to 2°C cooler than rural sites. An oasis effect, therefore, only exists on a highly local basis. In contrast, parts of the city with sparse vegetation are warmer than the countryside.There is an apparent opposed effect of rural and urban vegetation, whereby the former is hindering the temperature from falling and the latter is cooling the environment through evapotranspiration. This can be explained by the overwhelming amounts of imported water in the city promoting evaporative cooling.

Surface–atmosphere energy exchanges in Ouagadougou, Burkina Faso, located in the West African Sahel, were investigated during February 2003. Basic knowledge of the impact of land cover changes on local climate is needed to understand and forecast the impacts of rapid urbanization predicted for the region. Previously collected data showed a large dry season urban heat island (UHI), which dramatically decreased with the onset of the rainy season and corresponding changes to the natural land cover thermal and radiative properties. Observations of local-scale energy balance fluxes were made over a residential district, and building surface temperatures were measured in three separate locations. Net all-wave radiation showed an increase with urbanization owing to the higher albedo, lower heat capacity, and thermal conductivity of the bare dry soil compared to the urbanized surface. The combination of material and geometry resulted in a decrease in albedo toward the urban center. Despite the higher albedo, surface temperatures of bare undisturbed soil could exceed surface temperatures in the residential area and urban center by 15°–20°C due to differences in thermal characteristics. Turbulent heat exchange measured over a residential area was dominated by sensible heat flux. Latent heat fluxes were greater than expected from the amount of vegetation but in accordance with water use in the area. An urban land surface scheme reproduced fluxes in agreement with measurements. The results point toward an intensification of the dry season urban heat island in Ouagadougou, given increased urbanization.

Abstract-The effect of foraging behavior on predation risk was studied by exposing the two small Calanoid copepods Acartia clausi and Acartia tonsa to 0 or 1 ppm (-1,500 cells ml-l) of the diatom Thalassiosira weissfogii and to presence of the predator Pureuchaeta norvegica. In filtered water, predation rate was the same on the two species. In algal suspension, predation rate on A. clausi was half that in filtered water and half that on A. tonsa. Video observations revealed distinct differences in motility of Acartia depending on algal concentration. Both species performed frequent short feeding bouts in algal suspension; nonfeeding copepods in filtered water alternately sank or adjusted their vertical distribution by stronger jumps. Jump frequency nearly doubled for A. clausi in filtered water, but no significant difference was observed for A. tonsa. To explain the predation, assuming that P. norvegica is a rheotactic predator, WC developed a model of potential hydrodynamic disturbance associated with each foraging behavior. Increased encounter rate with P. nowegica caused by frequent strong jumps by A. clausi in the absence of algae could explain >40% of the observed increase in predation rate. For A. tonsa, jump frequencies and predation rates were similar in both food treatments, which is in accordance with the model.