Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. R&D 1021-EN-01/ Assessment of Zebra Mussel (Dreissena polymorpha) Infestation Risk Using GIS for Water Basins in the North-West Bulgaria; Final Report, November 2006 -November 2007 AbstractIn Bulgaria, zebra mussel (Dreissena polymorpha) occurred originally in the Danube River and the Black Sea coastal lakes and rivers. During the last 10 years a rapid spread of this species in the inland water bodies has been observed. Two of the reservoirs in the North-West Bulgaria have been reported as infested. The goal of the present study was to determine the risk of zebra mussel infestation in this region by combining available biological and environmental data and by subsequent processing and analyzing of these data using Geographic Information System (GIS). This will be the first step in developing an action plan for vulnerable regions in Bulgaria.The evaluation of physical and chemical characteristics of water bodies and their suitability to the zebra mussel requirements was made based on existing data from monitored river stations, spatial data and reservoir field survey. Fifteen reservoirs were sampled to determine which physicochemical variables explain the occurrence and abundance of zebra mussel. The principal component analysis used showed that the first two principal components related mainly to calcium concentration, pH, electroconductivity and Secchi disk transparency, explained the majority of total variance of data. pH, calcium concentration and dissolved oxygen were selected as habitat suitability parameters, and corresponding tolerance ranges based on the environmental requirements of zebra mussels during the larval and early growth stages, were defined. These parameters were divided into three main categories that reflected the infestation potential of zebra mussel: Low, Moderate and High. After all spatial and attributive data were collected, they were processed and analyzed in a GIS environment, which included the GIS database development, spatial and geostatistical interpolation of limiting factors, reclassification and overlay.The results showed that based on water chemistry, 63.81% of the territory can be classified with high potential, 15.45% -with moderate potential, and 20.74% -with low po...