Kimberly B. Catton

Quantitative analysis of tethered and free-swimming copepodid flow fields

SUMMARY We quantified the flow field generated by tethered and free-swimming Euchaeta antarctica using the particle image velocimetry (PIV) technique. The streamlines around the free-swimming specimens were generally parallel to the body axis, whereas the streamlines around all of the tethered copepodids demonstrated increased curvature. Differences noted in the streamline pattern, and hence the vorticity, dissipation rate and strain rate fields, are explained by considering the forces on the free-swimming specimen compared to the tethered specimen. Viscous flow theory demonstrates that the force on the fluid due to the presence of the tether irrevocably modifies the flow field in a manner that is consistent with the measurements. Hence, analysis of the flow field and all associated calculations differ for tethered versus free-swimming conditions. Consideration of the flow field of the free-swimming predatory copepodid shows the intensity of the biologically generated flow and the extent of the mechanoreceptive signal quantified in terms of shear strain rate. The area in the dorso-ventral view surrounded by the 0.5 s-1 contour of exy, which is a likely threshold to induce an escape response, is 11 times the area of the exoskeletal form for the free-swimming case. Thus, mechanoreceptive predators will perceive a more spatially extended signal than the body size.

The hydrodynamic disturbances of two species of krill: implications for aggregation structure

SUMMARY Krill aggregations vary in size, krill density and uniformity depending on the species of krill. These aggregations may be structured to allow individuals to sense the hydrodynamic cues of neighboring krill or to avoid the flow fields of neighboring krill, which may increase drag forces on an individual krill. To determine the strength and location of the flow disturbance generated by krill, we used infrared particle image velocimetry measurements to analyze the flow field of free-swimming solitary specimens (Euphausia superba and Euphausia pacifica) and small, coordinated groups of three to six E. superba. Euphausia pacifica individuals possessed shorter body lengths, steeper body orientations relative to horizontal, slower swimming speeds and faster pleopod beat frequencies compared with E. superba. The downward-directed flow produced by E. pacifica has a smaller maximum velocity and smaller horizontal extent of the flow pattern compared with the flow produced by E. superba, which suggests that the flow disturbance is less persistent as a potential hydrodynamic cue for E. pacifica. Time record analysis reveals that the hydrodynamic disturbance is very weak beyond two body lengths for E. pacifica, whereas the hydrodynamic disturbance is observable above background level at four body lengths for E. superba. Because the nearest neighbor separation distance of E. superba within a school is less than two body lengths, hydrodynamic disturbances are a viable cue for intraspecies communication. The orientation of the position of the nearest neighbor is not coincident with the orientation of the flow disturbance, however, which indicates that E. superba are avoiding the region of strongest flow.

The effect of fluid viscosity, habitat temperature, and body size on the flow disturbance of Euchaeta

The spatial extent and temporal decay of copepod-generated hydrodynamic disturbances during cruise and escape behavior were examined using the particle image velocimetry technique combined with theoretical models. Our study compared results for two species in the genus Euchaeta: the larger E. elongata living in colder water of higher viscosity versus the smaller E. rimana living in warmer water of lower viscosity. We expected that body size and viscosity would work in opposite directions in shaping the spatial and temporal properties of the hydrodynamic disturbances generated by these two copepod species. We found that the spatial extent of the copepod-induced hydrodynamic signal in front of the copepods during cruising was equivalent, with the peak strength of the signal to preferred prey showing no significant difference. In contrast, the spatial extent and strength of the hydrodynamic disturbance during escape were larger for E. elongata, although the decay time of the flow disturbance to a threshold value was equivalent between the species. Importantly, the observation of vortex rings during escape for Euchaeta strongly supports the appropriateness of the impulsive stresslet model over the impulsive Stokeslet model. Moreover, our empirical data discount the validity of using a sphere in creeping flow to model copepod–fluid interactions. Rather, these results suggest a complicated interaction of fluid viscosity, body size, and swimming speed for the genus Euchaeta that partially explains the adaptations to the local environmental conditions.

A geographical assessment of vegetation carbon stocks and greenhouse gas emissions on potential microalgae-based biofuel facilities in the United States

The microalgae biofuels life cycle assessments (LCA) present in the literature have excluded the effects of direct land use change (DLUC) from facility construction under the assumption that DLUC effects are negligible. This study seeks to model the greenhouse gas (GHG) emissions of microalgae biofuels including DLUC by quantifying the CO2 equivalence of carbon released to the atmosphere through the construction of microalgae facilities. The locations and types of biomass and Soil Organic Carbon that are disturbed through microalgae cultivation facility construction are quantified using geographical models of microalgae productivity potential including consideration of land availability. The results of this study demonstrate that previous LCA of microalgae to biofuel processes have overestimated GHG benefits of microalgae-based biofuels production by failing to include the effect of DLUC. Previous estimations of microalgae biofuel production potential have correspondingly overestimated the volume of biofuels that can be produced in compliance with U.S. environmental goals.