Small-scale entrainment in inclined gravity currents

We investigate the effect of buoyancy on the small-scale aspects of turbulent entrainment by performing direct numerical simulation of a gravity current and a wall jet. In both flows, we detect the turbulent/nonturbulent interface separating turbulent from irrotational ambient flow regions using a range of enstrophy iso-levels spanning many orders of magnitude. Conform to expectation, the relative enstrophy isosurface velocity \(v_n\) in the viscous superlayer scales with the Kolmogorov velocity for both flow cases. We connect the integral entrainment coefficient E to the small-scale entrainment and observe excellent agreement between the two estimates throughout the viscous superlayer. The contribution of baroclinic torque to \(v_n\) is negligible, and we show that the primary reason for reduced entrainment in the gravity current as compared to the wall-jet are 1) the reduction of \(v_n\) relative to the integral velocity scale \(u_T\); and 2) the reduction in the surface area of the isosurfaces.     

Identification of microcystins in cyanobacteria from the Bleiloch former drinking-water reservoir (Thuringia, Germany).

The presence of microcystins in cyanobacterial samples collected from the Bleiloch reservoir, formerly an important drinking-water supply in Thuringia, Germany, was proven by application of a combination of recently developed analytical methods. The raw extracts were cleaned by size-exclusion chromatography (SEC) or solid-phase extraction (SPE). The determination of microcystins was achieved by different HPLC separation followed by the application of alternative detection methods (UV, diode array detection (DAD), and mass spectrometry (MS), respectively). Furthermore, the different results of clean-up by SPE and SEC are demonstrated. The identity of microcystins was verified by MS/MS measurements. In the cyanobacterial sample from 1998, microcystin-RR, -LR and -YR were found, whereas in 1999 only microcystin-LR and -YR were detectable. In addition to detection of cell-bound microcystins, in 1999 traces of dissolved microcystins in water from the Bleiloch reservoir were detected. It can be assumed that not only the Bleiloch reservoir is contaminated with hepatotoxins but also many similar lakes still used for drinking water supply.     

Effect of prior disturbances on the extent and severity of wildfire in Colorado subalpine forests

Disturbances are important in creating spatial heterogeneity of vegetation patterns that in turn may affect the spread and severity of subsequent disturbances. Between 1997 and 2002 extensive areas of subalpine forests in northwestern Colorado were affected by a blowdown of trees, bark beetle outbreaks, and salvage logging. Some of these stands were also affected by severe fires in the late 19th century. During a severe drought in 2002, fires affected extensive areas of these subalpine forests. We evaluated and modeled the extent and severity of the 2002 fires in relation to these disturbances that occurred over the five years prior to the fires and in relation to late 19th century stand-replacing fires. Occurrence of disturbances prior to 2002 was reconstructed using a combination of tree-ring methods, aerial photograph interpretation, field surveys, and geographic information systems (GIS). The extent and severity of the 2002 fires were based on the normalized difference burn ratio (NDBR) derived from satellite imagery. GIS and classification trees were used to analyze the effects of prefire conditions on the 2002 fires. Previous disturbance history had a significant influence on the severity of the 2002 fires. Stands that were severely blown down (> 66% trees down) in 1997 burned more severely than other stands, and young (approximately 120 year old) postfire stands burned less severely than older stands. In contrast, prefire disturbances were poor predictors of fire extent, except that young (approximately 120 years old) postfire stands were less extensively burned than older stands. Salvage logging and bark beetle outbreaks that followed the 1997 blowdown (within the blowdown as well as in adjacent forest that was not blown down) did not appear to affect fire extent or severity. Conclusions regarding the influence of the beetle outbreaks on fire extent and severity are limited, however, by spatial and temporal limitations associated with aerial detection surveys of beetle activity. Thus, fire extent in these forests is largely independent of prefire disturbance history and vegetation conditions. In contrast, fire severity, even during extreme fire weather and in conjunction with a multiyear drought, is influenced by prefire stand conditions, including the history of previous disturbances.     

Multiple determinants of community structure in shallow marsh habitats,Cape Fear River estuary,North Carolina,USA

The nekton of tidal creeks was studied at 17 sampling localities from September 1977 through August 1978, in the Cape Fear River estuary, North Carolina, USA. Prior to these dates, collections were made at 9 stations beginning in January 1977; these data were used to supplement conclusions drawn from the larger effort. Species recruited from the ocean utilized marsh habitats only temporarily and dominated the catches with over 70% of the total abundance. Their distribution was influenced by salinity gradients and to a lesser extent by substrate characteristics. In addition, temporal habitat partitioning with associated size differences of related species played an important role in structuring marsh nekton communities. A clearly defined ecotone was associated with the mesohaline-polyhaline transition zone, in slainities between 14 and 21 S. Numerous marine stenohaline forms were restricted to salinities above 16 S, thus increasing species richness in high salinity marshes. Despite differences in freshwater flows in 1977 and 1978, major features of the various marsh communities (species associations and relative abundances) exhibited little change throughout the Cape Fear estuary, indicating that these communities were relatively persistent in time. Standing crops for ocean-spawned species at the end of the growing season indicated that considerable annual export in the form of living biomass of fish and shellfish takes place from the marshes. Since most individuals of these species return to the ocean in the fall, an important energy link between the marshes and nearshore marine environment is demonstrated.     

Secondary metabolites of the cyanobacterium Microcoleus lyngbyaceus and the sea hare Stylocheilus longicauda: palatability and toxicity

The cyanobacterium Microcoleus lyngbyaceus (Kütz.) Crouan (Lyngbya majuscula Gomont) produces a variety of nitrogen-containing secondary metabolites. M. lyngbyaceus is eaten by the specialist sea hare Stylocheilus longicauda, which sequesters secondary metabolites from its diet and transforms some sequestered metabolites into related compounds. We examined the palatability and toxicity of the metabolites malyngamide A, malyngamide B, and a mixture of majusculamides A and B (from M. lyngbyaceus), and malyngamide B acetate (from S. longicauda, derived from malyngamide B), in order to explore a series of hypotheses about why M. lyngbyaceus produces secondary metabolites and why S. longicauda sequesters and transforms them. All three M. lyngbyaceus metabolites significantly reduced feeding by omnivorous pufferfish (Canthigaster solandri) and crabs (Leptodius spp.). Direct comparisons indicated that neither malyngamide A nor the majusculamide mixture differed significantly in palatability from malyngamide B. The S. longicauda metabolite malyngamide B acetate did not significantly deter feeding by either consumer. Direct comparisons indicated that pufferfish found malyngamide B acetate more palatable than malyngamide B, and that crabs showed a non-significant trend in this direction. Palatability of the metabolites did not correlate with toxicity. Although no significant differences were observed among the M. lyngbyaceus metabolites in their palatability, two toxicity assays consistently ranked malyngamide A more toxic than malyngamide B, which was more toxic than the majusculamide mixture. The majusculamide mixture did not significantly differ in toxicity from malyngamide B acetate, even though the two differed strongly in palatability.     

Maximum nitrogen removal in the step-feed activated sludge process.

This paper presents a mathematical framework that can be used to determine the flow distributions for a step-feed activated sludge process that result in maximum nitrogen removal. The model indicates that nitrogen removal efficiency in a step-feed activated sludge process is highly dependent on the ultimate biochemical oxygen demand (BOD(L))-to-total Kjeldahl nitrogen (TKN) ratio of the wastewater. For typical domestic wastewater, which has a relatively high BOD(L)-to-TKN ratio, the step-feed process will outperform the Modified Ludzack-Ettinger process for nitrogen removal, when the flow to each step is optimally distributed. Using plant-specific water quality data and operating conditions from a 1-year period, nitrogen removal performance for four step-feed activated sludge plants operated by the Sanitation Districts of Los Angeles County (California) was calculated using the developed model. The calculated nitrogen removal efficiencies match well with the actual plant performance data. These results validate the model as a useful tool for predicting nitrogen removal in a step-feed activated sludge process. Other analyses revealed that improvements in nitrogen removal at existing facilities are achievable by adjusting the split of primary effluent flow to each anoxic zone several times during the day. The timing of the adjustments and the optimal flow splits can be determined from data on diurnal fluctuations in BOD(L) and TKN concentrations. An example is provided to illustrate the application of such an operating strategy and the potential enhancement of nitrogen removal.