Importance of Riparian Forests in Urban Catchments Contingent on Sediment and Hydrologic Regimes

Forested riparian corridors are thought to minimize impacts of landscape disturbance on stream ecosystems; yet, the effectiveness of streamside forests in mitigating disturbance in urbanizing catchments is unknown. We expected that riparian forests would provide minimal benefits for fish assemblages in streams that are highly impaired by sediment or hydrologic alteration. We tested this hypothesis in 30 small streams along a gradient of urban disturbance (1–65% urban land cover). Species expected to be sensitive to disturbance (i.e., fluvial specialists and “sensitive” species that respond negatively to urbanization) were best predicted by models including percent forest cover in the riparian corridor and a principal components axis describing sediment disturbance. Only sites with coarse bed sediment and low bed mobility (vs. sites with high amounts of fine sediment) had increased richness and abundances of sensitive species with higher percent riparian forests, supporting our hypothesis that response to riparian forests is contingent on the sediment regime. Abundances of Etheostoma scotti, the federally threatened Cherokee darter, were best predicted by models with single variables representing stormflow (r² = 0.34) and sediment (r² = 0.23) conditions. Lentic-tolerant species richness and abundance responded only to a variable representing prolonged duration of low-flow conditions. For these species, hydrologic alteration overwhelmed any influence of riparian forests on stream biota. These results suggest that, at a minimum, catchment management strategies must simultaneously address hydrologic, sediment, and riparian disturbance in order to protect all aspects of fish assemblage integrity.     

Emission factors for PCDD/PCDF and dl-PCB from open burning of biomass

The Stockholm Convention on Persistent Organic Pollutants includes in its aims the minimisation of unintentional releases of polychlorinated dibenzo-dioxins and dibenzofurans (PCDD/PCDF) and dioxin like PCB (dl-PCB) to the environment. Development and implementation of policies to achieve this aim require accurate national inventories of releases of PCDD/PCDF/dl-PCB. To support this objective, the Conference of Parties established a process to review and update the UNEP Standardized Toolkit for Identification and Quantification of Dioxin and Furan Releases. An assessment of all emission inventories was that for many countries open burning of biomass and waste was identified as the major source of PCDD/PCDF releases. However, the experimental data underpinning the release estimates used were limited in number and, consequently, confidence in the accuracy of the emissions predictions was low. There has been significant progress in measurement technology since the last edition of the Toolkit in 2005. In this paper we reassess published emission factors for release of PCDD/PCDF and dl-PCB to land and air.In total, four types of biomass and 111 emission factors were assessed. It was found that there are no systematic differences in emission factors apparent between biomass types or fire classes. The data set is best described by a lognormal distribution. The geometric mean emission factors (EFs) for releases of PCDD/PCDF to air for the four biomass classes used in the Toolkit (sugarcane, cereal crops, forest and savannah/grass) are 1.6 μg TEQ (t fuel)⁻¹, 0.49 μg TEQ (t fuel)⁻¹, 1.0 μg TEQ (t fuel)⁻¹ and 0.4 μg TEQ (t fuel)⁻¹, respectively. Corresponding EFs for release of PCDD/PCDF to land are 3.0 ng TEQ (kg ash)⁻¹, 1.1 ng TEQ (kg ash)⁻¹, 1.1 ng TEQ (kg ash)⁻¹ and 0.67 ng TEQ (kg ash)⁻¹. There are now also sufficient published data available to evaluate EFs for dl-PCB release to air for sugarcane, forest and grass/savannah; these are 0.03 μg TEQ (t fuel)⁻¹, 0.09 μg TEQ (t fuel)⁻¹ and 0.01 μg TEQ (t fuel)⁻¹, respectively. The average EF for dl-PCB release to land is 0.19 ng TEQ (kg ash)⁻¹. Application of these EFs to national emissions of PCDD/PCDF for global estimates from open burning will lower previous estimates of PCDD/PCDF releases to air and to land by 85% and 90%, respectively. For some countries, the ranking of their major sources will be changed and open burning of biomass will become less significant than previously concluded.