[en] Highlights: • In the Couette laminar flow the D. magna filtration increases linearly with shear. • Turbulent flow completely inhibits D. magna filtration. • Turbulent regime restricts D. magna swimming capacity. • D. magna filtration is predicted to increase with the third power of the body length. Daphnia are filter feeder organisms that prey on small particles suspended in the water column. Since Daphnia individuals can feed on wastewater particles, they have been recently proposed as potential organisms for tertiary wastewater treatment. However, analysing the effects of hydrodynamics on Daphnia individuals has scarcely been studied. This study focuses then, on quantifying the filtration and swimming velocities of D. magna individuals under different hydrodynamic conditions. Both D. magna filtration and movement responded differently if the flow was laminar or if it was turbulent. In a laminar-dominated flow regime Daphnia filtration was enhanced up to 2.6 times that of a steady flow, but in the turbulent-dominated flow regime D. magna filtration was inhibited. In the laminar flow regime D. magna individuals moved freely in all directions, whereas in the turbulent flow regime they were driven by the streamlines of the flow. A model based on Daphnia-particle encountering revealed that the filtration efficiency in the laminar regime was driven by the length of the D. magna individuals and the shear rate imposed by the system.

[en] Highlights: • The filtration capacity of D. magna is enhanced in sheared flows. • High MP concentrations lead to a reduction in D. magna filtration capacity. • The amount of exposure time to MP reduces the filtration capacity of D. magna. -- Abstract: There is consensus on the need to study the potential impact microplastics (MP) have on freshwater planktonic organisms. It is not yet fully understood how MP enter the aquatic food web or the effect they have on all the trophic levels. As a result of the potential for MP to accumulate throughout food webs, there is increasing interest in evaluating their fate in a variety of environmental conditions. This study investigated the variability in the ingestion of MP to food ratios and the exposed time of MP to Daphnia magna in non-sheared and sheared conditions. The sheared environment provided Daphnia magna with the conditions for optimal filtering capacity. Regardless of the ratios of MP concentration to food concentration (MP:Food), the filtration capacity of the Daphnia magna was enhanced in the sheared experiments. In both the sheared and non-sheared experiments, filtration capacity decreased when the ratios of MP to food concentration and the exposure times to MP were increased. Mortality was mainly enhanced in the non-sheared conditions at higher MP concentrations and exposure times to MP. No mortality was found in the sheared conditions for the exposure times studied. Therefore, in aquatic systems that undergo constant low sheared conditions, Daphnia magna can survive longer when exposed to MP than in calm conditions, provided food concentrations do not limit their capacity to filter.

[en] Aquatic plants, turbulence and sediment fluxes interact with each other in a complex, non-linear fashion. While most studies have considered turbulence as being generated primarily by mean flow, it can, however, also be generated by the action of the wind or by the night cooling convection at the surface of the water column. Here, we study turbulent interaction with vegetation and the effects it has on sediment suspension, in the absence of mean flow. In a water tank containing a base layer of sediment, turbulence was generated by oscillating a grid with the main objective being to determine the differences in sediment resuspension in sediment beds over a wide range of consolidation times (1 h–3 days), for a set of model canopies with different structural characteristics: density and flexibility, and for three types of sediment beds. The greater the consolidation time was, the lower the sediment resuspension. For bed consolidation times below 6 h, the concentration of resuspended sediment was approximately constant and had no dependence on turbulence intensity. However, for higher bed consolidation times, between 6 and 3 days, the resuspension of the sediment beds increased with turbulence intensity (defined in terms of turbulent kinetic energy; TKE hereafter). The TKE within the sparse flexible canopies was higher than that in the sparse rigid canopies, while within the dense flexible canopies it was below that of the rigid canopies. Therefore, the sediment resuspension in the sparse flexible canopies was greater than that of the sparse rigid canopies. In contrast, the sediment resuspension in the dense flexible canopies was lower than that of the dense rigid canopies. Using different sediment types, the results of the study indicate that sediments with greater concentrations of small particles (muddy beds) have higher concentrations of resuspended sediment than sediment beds that are composed of larger particle sizes (sandy beds).

[en] Highlights: • A minimum seagrass patch size is needed for successful canopy restoration • The response of seagrass patches depends on wave velocity and canopy density. • Under low velocities seagrasses do not interact with waves, but attenuate seabed TKE. • Under moderate to high wave velocities seagrasses interact with waves producing TKE. For the past two centuries coastal zones have been suffering seagrass loss resulting in a network of vegetated patches which are barely interconnected and which may compromise the ecological services provided by the canopy. To optimize management efforts for successful restoration strategies, questions need to be addressed about what appropriate canopy architectural considerations are required under certain hydrodynamic conditions. In this study, a set of laboratory experiments were conducted in which hydrodynamic conditions, plant densities and vegetated patch lengths were varied to determine minimum patch lengths for successful management strategies. Based on the TKE production, this study finds two possible canopy behaviours of seagrasses under oscillating flows: one where plants do not interact with the flow and the other where they interact with waves and produce TKE. A threshold from the first to second behaviour occurs for ${\left[C_{D-\mathit{Patch}}\frac{n{d}^2}{2\left(1-\mathrm{\varphi }\right)}\right]}^{\frac{1}{3}}{U}_w$ = 2, where C_D is the drag of the vegetated patch, n is the number of stems per m², d is the stem diameter and ϕ is the solid plant fraction. Therefore, high canopy densities, large patches of vegetation or moderate wave velocities will produce plant-wave interaction, whereas low canopy densities, small vegetation patches or slow wave velocities will produce a behaviour akin to the non-vegetated cases.

[en] This article studies the capacity of biofuels to reduce motor fuel price fluctuations. For this purpose, we study dependence between crude oil and biodiesel blend prices in Spain. Copula models are used for this purpose. Results suggest that the practice of blending biodiesel with diesel can protect consumers against extreme crude oil price increases. - Highlights: ► We study the capacity of biofuels to reduce fuel price fluctuations. ► We focus on Spanish biodiesel market. ► Biodiesel and crude oil price dependence is studied using copula functions. ► Biodiesel can protect consumers against extreme crude oil price increases.