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Microbial Source Tracking in Aquatic Ecosystems: The State of the Science and an Assessment of Needs

Workshop Summary

Fecal pollution of aquatic ecosystems is a significant problem in many areas across Canada. This pollution can come from diverse sources such as municipal wastewater effluents, failing on-site sewage systems, livestock manure and wildlife droppings. Its impacts on human health, aquatic ecosystems and local economies can be significant through waterborne disease outbreaks, boil water advisories, contamination of irrigation waters, and beach and shellfish closures.

There is a growing need for a science-based approach to determine the source of fecal pollution in aquatic ecosystems. Identifying the correct fecal pollution source could help mediate conflicts between stakeholders in communities, and help target appropriate, cost-effective, corrective actions to prevent future pollution. Microbial source tracking (MST) is an emerging field that offers considerable promise for determining the source of fecal pollution contaminating aquatic ecosystems. In general, the MST approach is based on comparing the similarity of microorganisms collected from aquatic ecosystems to microorganisms collected from nearby fecal pollution sources in order to make inferences about the likely source of fecal contamination. However, the field is still under development. There are no standardized methods and there have been few field studies to test the reliability and accuracy of MST techniques. Nevertheless, there is considerable interest in this field of research, along with high expectations for tools to help resolve fecal pollution problems.

Consequently, a MST workshop was held March 7-8, 2005 , in Toronto, Ontario, to review the state of MST science and its readiness for widespread application and to assess the fecal pollution source tracking needs in communities across Canada. The workshop was hosted by Environment Canada's National Water Research Institute and Ontario Region, the City of Toronto, the Ontario Ministry of the Environment, Health Canada, and Agriculture and Agri-Food Canada. Seventy participants attended from federal, provincial and municipal government agencies, non-government organizations, and from universities in Canada, the United States and Australia. Participants represented the scientific research community, and the water policy and program management perspectives relevant to MST in Canada. The workshop is another in the Linking Water Science to Policy Workshop Series that brings together leading researchers with policy analysts and program managers to communicate recent science and to solicit feedback on research drivers and needs. The report is also another in the NWRI Scientific Assessment Report Series that publishes national scientific assessments of priority freshwater issues in Canada, serving as an authoritative review of current scientific knowledge, trends, and information and program needs to assist water science decision-makers, resource managers, and the research community in setting research priorities and in developing sound management policies and practices.

State of the Science

Presentations provided an overview of the state of MST science based upon recent scientific publications and case studies. Early MST studies in the late 1990's raised considerable expectations for MST tools to resolve problems in fecal pollution source tracking. More recent scientific publications have pointed to limitations in MST methods, particularly for studies of large watersheds with complex sources of fecal contamination. Library-based methods, such as those based upon E. coli, were seen to suffer from high misclassification rates and the need to have increasingly larger libraries to represent the diversity of potential E. coli isolates from fecal sources. Non-library-based methods, such as those based upon Bacteroides sp., require additional host-specificity evaluation and remain to be well tested in field studies.

While some of these recent MST studies were perceived to have placed a "wet blanket" on the field, cases of successful applications of MST in field studies continue to occur. In some cases, MST methods have identified unexpected fecal pollution sources, and were instrumental in leading to effective remedial clean-up actions. In other cases, results from MST field studies have been consistent with other lines of evidence in identifying predominant fecal pollution sources. Additional presentations at the workshop showed that the field of MST is still evolving. Novel tools such as DNA microarrays and protozoan genotyping methods could help enlarge the existing MST toolbox. Other tools based on chemical tracers (e.g., coprostanol) or DNA markers for host animal cells sloughed off in feces may also prove useful for fecal pollution source tracking.

The Workshop recognized that while many methods exist in the current MST toolbox, there is no "silver bullet",universally accepted best method. While some methods have achieved a level of maturity where they could be considered for standardization, others are still experimental or research-grade tools. Additional research is required to understand more fully the advantages and limitations of microbial source tracking methods, and wider application of the tools will require standard methods and careful consideration of appropriate experimental designs. Library-dependent MST methods require a library of an appropriate size and representation, which still remains to be determined. Library-independent MST methods require validation of host-specific markers in field studies. The Workshop placed emphasis on applying MST methods as part of multiple lines of evidence to resolve fecal pollution source tracking problems. While MST tools can be relevant to tackle such problems, it will be important to ensure there are realistic expectations among the end-user communities for MST studies in the near future.

MST Needs and Drivers/Policy and Program Relevance

With aging municipal wastewater infrastructure, urbanization and increasingly intensive livestock production capacity across Canada, the need for science-based tools like MST methods to resolve existing fecal pollution problems and prevent future ones can be anticipated to grow. Microbial source tracking has been slower to develop in Canada than in the United States. The U.S. Clean Water Act requirements for calculating Total Maximum Daily Loads for fecal contaminants in watersheds have been driving much of the application of MST studies in the United States. Canada does not have equivalent legislation, and applications of MST have been driven more by a "bottom-up" approach from local community concerns for beach and shellfish closures. MST studies may prove useful in helping to meet federal, provincial, territorial or municipal microbial water quality guidelines, objectives and regulations. In particular, identifying the source of fecal pollution is invaluable at better targeting cost-effective abatement options. Other drivers for conducting MST studies include the need to provide guidance for development of source water protection plans and to help target microbial pollution education and awareness initiatives at local levels. Ongoing MST research activities, including the Health Canada-Agriculture and Agri-Food Canada MST study under Canada 's Agricultural Policy Framework, will help raise awareness of the advantages and limitations of MST methods for water programs across Canada.