Options for Managing Hypoxic Blackwater in River Systems: Case Studies and Framework

Hypoxic blackwater events occur when large amounts of organic material are leached into a water body (e.g., during floodplain inundation) and rapid metabolism of this carbon depletes oxygen from the water column, often with catastrophic effects on the aquatic environment. River regulation may have increased the frequency and severity of hypoxic blackwater events in lowland river systems, necessitating management intervention to mitigate the impacts of these events on aquatic biota. We examine the effectiveness of a range of mitigation interventions that have been used during large-scale hypoxic blackwater events in the Murray–Darling Basin, Australia and that may be applicable in other environments at risk from hypoxic blackwater. Strategies for hypoxia mitigation include: delivery of dilution flows; enhancement of physical re-aeration rates by increasing surface turbulence; and diversion of blackwater into shallow off-channel storages. We show that the impact of dilution water delivery is determined by relative volumes and water quality and can be predicted using simple models. At the dilution water inflow point, localized oxygenated plumes may also act as refuges. Physical re-aeration strategies generally result in only a small increase in dissolved oxygen but may be beneficial for local refuge protection. Dilution and natural re-aeration processes in large, shallow lake systems can be sufficient to compensate for hypoxic inflows and water processed in off-channel lakes may be able to be returned to the river channel as dilution flows. We provide a set of predictive models (as electronic supplementary material) for estimation of the re-aeration potential of intervention activities and a framework to guide the adaptive management of future hypoxic blackwater events.     

Measuring natural resource scarcity: Theory and practice

Concern over the implications of natural resource availability for economic well-being has been among the most long-lived policy issues involving economists. Scarcity and Growth by Barnett and Morse has remained the most influential evaluation of these issues. The purpose of this paper is to evaluate the Barnett-Morse methodology. Using the results from comparative static and dynamic models three scarcity indexes are evaluated for their consistency in signaling changes in the availability of natural resources. In addition, the paper reports a reconsideration of the movements in the relative prices of extractive outputs to 1973. This analysis indicates that the empirical evidence alone is not sufficient to arrive at a conclusion as to whether there is increasing scarcity of natural resources.     

Land clearance not dieback continues to drive tree loss in a Tasmanian rural landscape

Tree cover has declined markedly since European settlement in the agricultural areas of Australia due to land clearing and natural and stress-induced senescence of remnant trees combined with widespread regeneration failure. These decreases in tree cover have seldom been quantified on a landscape scale, but are important in understanding losses in carbon storage and biodiversity across large areas of the continent. We used historical aerial photography and satellite imagery to study changes in tree canopy cover in the southern Tasmanian Midlands, one of the oldest agricultural regions in Australia. Tree cover at 1,000 random sites was assessed from imagery captured on four occasions between 1947 and 2010. Tree cover was stable between 1947 and the 1970s, but decreased from 33 % in the 1970s to 21 % in 2010. The decrease appeared driven by transition from some cover to no cover, rather than from more cover to less cover, because sites with tree cover in 2010 showed little net change during the study period. Statistical modelling showed soils derived from igneous rocks lost more cover than did the less fertile sedimentary soils, and contrary to the expectations, if drought was the cause of cover loss, sun-exposed north-facing sites did not lose more cover than south-facing ones. We argue that these temporal and spatial patterns of tree cover change are consistent with tree clearing rather than loss of remnant isolated trees due to drought-induced dieback.     

Water striders (family Gerridae): mercury sentinels in small freshwater ecosystems

To circumvent some of the previous limitations associated with contaminant-monitoring programs, we tested the suitability of the water strider (Hemiptera: Gerridae) as a mercury sentinel by comparing total mercury concentrations in water striders and brook trout (Salvelinus fontinalis) from a variety of stream sites in New Brunswick, Canada. There was a strong association between the two variables across sites (r(2)=0.81, P<0.001) in systems where both atmospheric deposition and a point source (an abandoned gold mine) were likely contributing to ambient mercury levels. In a small stream draining the gold mine tailings pile, water striders had mercury concentrations an order of magnitude higher than those from reference locations. Temporal variation at three southern New Brunswick stream sites was non-significant. These results suggest that water strider mercury levels accurately quantify food chain entry of the element. The use of sentinel species holds great potential for expanding contaminant-monitoring programs.     

Prenatal determination of dihydropteridine reductase in a normal fetus at risk for malignant hyperphenylalaninemia

Assay of dihydropteridine reductase in amniotic cells from a fetus at risk for malignant hyperphenylalaninemia is described. A normal result was obtained, this was confirmed after delivery. On two-dimensional polyacrylamide gel electrophoresis the enzyme from amniotic cells showed the same mobility as the mature liver reductase.     

General equilibrium benefits for environmental improvements: projected ozone reductions under EPA's Prospective Analysis for the Los Angeles air basin

This research demonstrates how locational equilibrium models can be used for benefit measurement with the detail required to match EPA's benefit analysis for the first Prospective Analysis. Using the projected changes in ozone concentrations for 2000 and 2010 together with the Sieg et al. (Int. Econ. Rev., forthcoming) estimates for household preferences for housing, education, and air quality, this paper measures general equilibrium willingness to pay for the policy scenarios developed for the Prospective study as they relate to households in the Los Angeles area. Benefits are evaluated taking account (at the household level) of initial air quality conditions, relocation based on changes in ozone, and price changes. The framework generalizes the partial equilibrium/general equilibrium comparisons available with conventional computable general equilibrium and property capitalization models. Estimated general equilibrium gains from the policy range from $33 to $2400 annually at a household level (in 1990 dollars).     

Secondary materials and international trade

The paper develops the international trade dimensions of recycling. It begins by outlining the sources of demand and supply that determine the volume, composition and direction of trade in waste materials. The model is intended to describe factors affecting international flows of any type of secondary material, although its usefulness in an empirical context is restricted in many cases by data limitations and complex lag structures on the supply side. Parts of the model are then applied to the case of waste paper, focusing on observed differences in national recycling rates and the role of trade in reconciling those differences.