5,101 research outputs found
Differential fates of Emiliania huxleyi-derived fatty acids and alkenones in coastal marine sediments: Effects of the benthic crustacean Palaemonetes pugio
In order to examine how benthic crustaceans affect the fates of phytoplankton-derived lipid biomarkers (fatty acids and alkenones) in coastal marine sediments, we incubated Emiliania huxleyi cells in microcosms (pre-sieved sediment cores with and without the grass shrimp Palaemonetes pugio ) over six weeks. Crustacean, transport of surface sediments, and distributions of algal lipids were followed during incubations. Crustacean activities enhanced degradation of algal fatty acids (2β4Γ faster) but had a small impact on algal alkenone degradation (\u3c1.4Γ) compared to the controls. During the first few days of incubations, alkenone concentrations were enriched while algal fatty acid concentrations were depleted in suspended particles in the overlying water of cores, indicating that P. pugio selectively grazed algal material from sediments and preferentially assimilated fatty acids over alkenones through digestion. Unlike algal fatty acids, alkenones were degraded primarily by microbial processes rather than by crustacean grazing. A substantial fraction (20β30%) of algal lipids was moved downward to the subsurface of sediments by P. pugio but algal fatty acids were more rapidly (3β6Γ) degraded than alkenones. In the presence of P. pugio, fatty acids bound in cell membrane and intracellular storage components degraded similarly, indicating that the crustacean activities minimized the effects of structural associations on fatty acid decomposition. Furthermore, there was no preferential degradation of 37:3 and 37:2 alkenones in both crustacean and control cores, suggesting that the U37k\u27 index (a paleotemperature indicator) was not significantly altered by P. pugio\u27s grazing or microbial decomposition
Unsteady Aerodynamic Interaction Between Rotor and Ground Obstacle
The mutual aerodynamic interaction between rotor wake and surrounding obstacles is complex, and
generates high compensatory workload for pilots, degradation of the handling qualities and performance,
and unsteady force on the structure of the obstacles. The interaction also affects the minimum distance
between rotorcrafts and obstacles to operate safely. A vortex-based approach is then employed to
investigate the complex aerodynamic interaction between rotors and ground obstacle, and identify the
distance where the interaction ends, and this is also the objective of the GARTEUR AG22 working group
activities. In this approach, the aerodynamic loads of the rotor blades are described through a panel method,
and the unsteady behaviour of the rotor wake is modelled using a vortex particle method. The effects of the
ground plane and obstacle are accounted for via a viscous boundary model. The method is then applied to a
βLargeβ and a βWeeβ rotor near the ground and obstacle, and compared with the earlier experiments carried
out at the University of Glasgow. The results show that the predicted rotor induced inflow and flow field
compare reasonably well with the experiments. Furthermore, at certain conditions the tip vortices are pushed
up and re-injected into the rotor wake due to the effect of the obstacle resulting in a recirculation. Moreover,
contrary to without the obstacle case, the peak and thickness of the radial outwash near the obstacle is
smaller due to the barrier effect of the obstacle, and an up-wash is observed. Additionally, as the rotor closes
to the obstacle, the rotor slipstreams impinge directly on the obstacle, and the up-wash near the obstacle is
faster, indicating a stronger interaction between the rotor wake and the obstacle. Also, contrary to the case
without the obstacle, the fluctuations of the rotor thrust, rolling and pitching moments are obviously
strengthened. When the distance between the rotor and the obstacle is larger than 3R, the effect of the
obstacle is small
Diagenesis of planktonic fatty acids and sterols in Long Island Sound sediments: Influences of a phytoplankton bloom and bottom water oxygen content
Diagenesis of organic matter in coastal sediments from Long Island Sound (LIS) was investigated by measuring fatty acids and sterols in (1) a time-series of surface sediment samples over a spring phytoplankton bloom; and (2) sediment cores collected during and after a bloom at two sites with distinctively different bottom-water oxygen contents. Time-dependent distributions of sedimentary fatty acids and sterols in LIS were strongly affected by pulsed inputs from the overlying water column, variations in benthic community, and redox-related degradation processes. The phytoplankton bloom delivered an intense pulse of unsaturated fatty acids (e.g., 16:1(Ο7) and 20:5) to the surface sediments. Continuous increases of cholesterol and diunsaturated sterols after the bloom were related to zooplankton grazing processes and increase in benthic faunal abundance. High inventories of planktonic fatty acids and sterols in the upper 5 cm sediments were observed at the low oxygen site during summer, probably caused by a combination of higher input, reduced degradation rates and lower macrofaunal activity under anoxic conditions compared to oxic conditions
Spatial and temporal distributions of sedimentary chloropigments as indicators of benthic processes in Long Island Sound
Regular spatial and seasonal distribution patterns of sedimentary chloropigments occurred at 19 subtidal stations located in Long Island Sound (LIS) from MAY 1988 to APR 1989. Inventories of chloropigments were higher (2β10x) in western than central LIS, mirroring patterns of phytoplankton production in surface waters. Shallow water sediments (\u3c25 m) received more chloropigments than deep stations (\u3e25 m). Lateral resuspension and redistribution of particles, as shown by 234Th inventories, are partly responsible for these patterns. Seasonal variations of Chl-a inventories in LIS sediments follow the production pattern in the water column: higher values occur in spring and lower values in summer and fall. A time lag (about 1β2 months) exists between maximum Chl-a (during early spring) and maximum phaeopigment sedimentary inventories (during late spring). Vertical profiles of chloropigments often exhibited exponential decreases with depth, implying that degradation processes significantly affect chloropigment distributions. Based on temperature-dependent first-order decomposition rate constants, reactive Chl-a inventories were converted into planktonic carbon fluxes across the water-sediment interface. These agreed reasonably well (within a factor of 2) with total benthic O2 uptake at the same sites. Particle reworking rates were estimated by using chl-a profiles combined with diagenetic models of decomposition. The seasonal patterns and magnitudes of DB (sediment mixing coefficients) derived from chl-a distributions are similar (Β±2β3x) to those estimated from 234Th distributions
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