Turbulence structure in temperature inversions and in convection fields as observed by doppler sodar

The structure of turbulence in an inversion layer and in an homogeneous convective field of the planetary boundary layer is described. In the first part of the paper, we validate the sodar estimates of turbulent dissipation, by using measurements with an hot-wire anemometric system in situ. Limitations of an ε measurement technique using structure function calculations are given, taking account of atmospheric properties and acoustic Doppler instrumental effects. By comparison between isopleths of backscattering intensity and of turbulent dissipation rates, we observe that in the early morning, turbulence is advected by mechanical turbulence generated by wind shear. The same mechanism seems to be operating in the case of an inversion layer capping thermal instability, when the convective activity is not too greatly developed. A turbulent kinetic energy budget is examined using aircraft, sodar, and tower measurements. This indicates a constant turbulent dissipation profile through a deep convective layer.     

Effects of Oxygen Loss on Carbon Processing and Heterotrophic Prokaryotes from an Estuarine Ecosystem: Results from Stable Isotope Probing and Cytometry Analyses

Many aquatic ecosystems are experiencing a decline in their oxygen (O₂) content and this is predicted to continue. Implications of this change on several properties of bacterioplankton (heterotrophic prokaryotes) remain however are poorly known. In this study, oxic samples (～170 μM O₂?=?controls) from an oligohaline region of the Scheldt Estuary were purged with N₂ to yield low-O₂ samples (～69 μM O₂?=?treatments); all were amended with ¹³C-glucose and incubated in dark to examine carbon incorporation and cell size of heterotrophic prokaryotes, and relationships between organic matter (OM) degradation and phosphate (P) availability in waters following O₂ loss. Stable isotope (¹³C) probing of phospholipid fatty acids (PLFA) and flow cytometry were used. In samples that have experienced O₂ loss, PLFA biomass became higher, prokaryotic cells had significantly larger size and higher nucleic acid content, but P concentrations was lower, compared to controls. P concentration and OM degradation were positively related in controls, but uncoupled in low-O₂ samples. Moreover, the dominant PLFA 16:1ω7c (likely mainly from Gram-negative bacteria) and the nucleic acid content of heterotrophic prokaryotic cells in low-O₂ samples explained (62–72 %) differences between controls and low-O₂ samples in P amounts. Shortly after incubations began, low-O₂ samples had consistently lower bacterial PLFA ¹³C-enrichments, suggesting involvement of facultatively anaerobic metabolism in carbon incorporation, and supporting the view that this metabolic pathway is widespread among pelagic bacteria in coastal nutrient-rich ecosystems. Estimates based on ¹³C-enrichment of PLFAs indicated that grazing by protozoa on some bacteria was stronger in low-O₂ samples than in controls, suggesting that the grazing pressure on some heterotrophic prokaryotes may increase at the onset of O₂ deficiency in nutrient-rich aquatic systems. These findings also suggest that physiological responses of heterotrophic prokaryotes to O₂ loss in such ecosystems include increases in cell activity, high carbon incorporation, and possibly phosphorus retention by cells that may contribute to reduce phosphate availability in waters.