Shoaling internal waves may reduce gravity current transport

Gravity currents descending along slopes have typically been studied in quiescent environments, despite the fact that in many geophysical settings there is significant externally driven motion. Here we investigate how the head of a gravity current is influenced by interfacial internal waves at the pycnocline of a two-layer ambient water column. Our experimental measurements show that larger amplitude internal waves, interacting with the gravity current, reduce both the mass transport by the gravity current and its thickness. These results suggest that the ambient internal wave field should be considered when estimating transport by gravity currents in geophysical settings with strong internal waves, such as lakes and the coastal ocean.     

Mating in bighorn sheep: frequent male reproduction via a high-risk “unconventional” tactic

Rocky Mountain bighorn rams use three distinct tactics in competition for mates. Two tactics (tending and blocking) feature defense and cooperative mating over a relatively prolonged consort period (up to 3 days). In the coursing tactic, subordinate rams fight dominants for temporary copulatory access (lasting seconds) to defended ewes. By combining population-wide genetic (microsatellite) exclusion of paternity, behavioral data and a model of bighorn reproductive competition, we estimated that coursing rams fathered 44% of 142 lambs assigned paternity in two natural populations. In one population, the probability of successful defense against coursing was lowest among rams that had many female consorts and held highest dominance rank. Even so, per capita annual male reproductive success was positively associated with social rank in both herds when measured in terms of fall conceptions. The proportion of coursing versus defending ram matings in each population (0.36 and 0.39) was similar to the corresponding fraction of lambs (0.43 and 0.47) fathered by coursing rams, suggesting that sperm competition approximated a fair lottery. Male traits important in gaining social status and obtaining cooperative consorts with ewes were different and potentially in conflict with those needed to defend against (and practise) coursing. Although the concussive weapons (horns) of rams are less dangerous than, for example, the piercing weapons of other bovids, injury from falls and horn blows during coursing brawls may cause death, handicap future mating competition or increase the risk of predation. Coursing is a rare example of an unconventional alternative mating tactic that is high-gain and high-risk. Received: 17 April 1996 / Accepted after revision: 15 March 1997     

Rates of nitrogen fixation on coral reefs across the continental shelf of the central Great Barrier Reef

This study was undertaken in 1981 to determine whether there were major variations in potential rates of nitrogen fixation on apparently bare coralline substrate from reefs across the continental shelf of the central Great Barrier Reef. Nitrogen fixation, measured as rates of ethylene production (nmol cm^-2h^-1), was significantly lower on substrata from two inner-shelf reefs, (0.46 and 1.07) than on two middle-shelf reefs (2.10 and 2.97) and on two outer-shelf reefs (3.20 and 3.81). By contrast, algal biomass (mg cm^-2) on experimental substrate was significantly higher on inner-shelf reefs (80.8 and 59.4) than on middleshelf (27.1 and 23.8) and outer-shelf reefs (26.4 and 22.4). The rate of nitrogen fixation was positively correlated with the proportion of bare substratum and significantly higher concentrations of dissolved inorganic nitrogen were found in waters over the reefs than in water flowing onto those reefs. The abundance of algal-grazing fishes was reported previously to be significantly lower on inner-shelf reefs. It is suggested that this cross-shelf variation in the activity of algal-grazing fishes may be a determinant of the observed cross-shelf variations in potential nitrogen fixation.Contribution No. 233 from the Australian Institute of Marine Science     

Stomatal uptake of O3 in aspen and aspen-birch forests under free-air CO2 and O3 enrichment

Rising atmospheric carbon dioxide (CO₂) may alleviate the toxicological impacts of concurrently rising tropospheric ozone (O₃) during the present century if higher CO₂ is accompanied by lower stomatal conductance (g_s), as assumed by many models. We investigated how elevated concentrations of CO₂ and O₃, alone and in combination, affected the accumulated stomatal flux of O₃ (AFst) by canopies and sun leaves in closed aspen and aspen-birch forests in the free-air CO₂-O₃ enrichment experiment near Rhinelander, Wisconsin. Stomatal conductance for O₃ was derived from sap flux data and AFst was estimated either neglecting or accounting for the potential influence of non-stomatal leaf surface O₃ deposition. Leaf-level AFst (AFst_l) was not reduced by elevated CO₂. Instead, there was a significant CO₂ × O₃ interaction on AFst_l, as a consequence of lower values of g_s in control plots and the combination treatment than in the two single-gas treatments. In addition, aspen leaves had higher AFst_l than birch leaves, and estimates of AFst_l were not very sensitive to non-stomatal leaf surface O₃ deposition. Our results suggest that model projections of large CO₂-induced reductions in g_s alleviating the adverse effect of rising tropospheric O₃ may not be reasonable for northern hardwood forests.     

Energetics of mixing for the filling box and the emptying-filling box

The mixing efficiency of a plume in a filling box and an emptying-filling box is calculated for both transient and steady states. The mixing efficiency of a plume in a filling box in an asymptotic state is 1/2, independent of the details of this state or how the plume is modelled. The mixing efficiency of a plume in an emptying-filling box in steady state is \(1 - \xi \), where \(\xi = h/H\), the depth of the ambient layer h normalised by the height of the box H. A deeper mixed layer therefore corresponds to a higher mixing efficiency. These results shed light on the interpretation of mixing efficiencies of open and closed systems.

     

CULTURAL ASPECTS OF WATER RESOURCE DEVELOPMENT PAST,PRESENT, AND FUTURE1

ABSTRACT Attitudes toward the development of the American West have undergone important changes over the past century just as the nature of water resources as factors in development have changed. Viewing these changes processually, stages for water resources definition and use can be identified in the total process of western cultural development. The first stage involves the value of water resource development as a stimulus to population and economic growth in the West. The second stage, still in process', adopts a dominant cultural norm which sees water resource development as inevitable if not necessary to keep up with growth. A third stage to this evolutionary process is incipient. Future cultural values and thinking with respect to water resource development will be to look at development as a means for controlling or managing both the location and quantity of population and economic growth. To this end planners will have to become concerned with the questions of human adaptation. Concern will have to be given to the problems of getting a living which enables individuals to meet the subsistence needs of self and family, to establishing community which provides for cooperation among individuals and the management of conflict, to establishing improved communication which promotes interpersonal interaction, and for fostering innovation which provides the new ideas necessary to adapt to new environmental situations.,     

Cystic lung lesions – prenatal diagnosis and management

With increasing use of fetal ultrasound comes an increase in the detection of clinically silent ‘abnormalities’ which pose diagnostic and management dilemmas for perinatologists and paediatricians. Congenital thoracic malformations (CTMs) (excluding congenital diaphragmatic hernia) are one such example, where a few cases are symptomatic in early life and management options are clear, but the majority are clinically asymptomatic, giving rise to difficulties in defining postnatal management of the well child with a sonographic or radiological lesion. Here, we will outline the prenatal presentation and natural history of CTMs that are not congenital diaphragmatic herniae and briefly discuss the approach to postnatal management, which is covered in more detail in the review by Laje and Liechty in this issue. Copyright © 2008 John Wiley & Sons, Ltd.