Microbial faecal pollution patterns along large rivers

World-wide, large rivers are used for a variety of anthropogenic purposes. On the one hand, they are important transport routes, they are used for recreational purposes, their water is abstracted for agricultural and industrial use, and with highest significance to human health they are used for drinking water production from river bank filtration. On the other hand, large rivers serve as important receiving waters for treated or untreated wastewater. Despite the implementation of state-of-the-art wastewater treatment plants in most industrialized countries, large rivers receiving wastewater often exhibit very high levels of microbial faecal pollution, imposing threats to all kinds of water usage. Despite of ist high importance, information on pollution microbiology of large rivers (spatial and temporal patterns, origin of fecal pollution) regarding their whole transnational context ("from the mouth to the sea") is still extremely limited. More knowledge is needed to develop a conceptual understanding of microbial pollution patterns in large rivers. Based on a list of defined knowledge gaps and own investigations from the past decade we formulated two main hypotheses on microbial pollution patterns in large rivers that shall be tested within the proposed study during the upcoming Joint Danube Survey 3 in 2013 and an annual time-series analysis. The "Longitudinal-Continuum-vs.-Lateral-Discontinuum" hypothesis states that depending on the intensity of pollution the lateral areas of large rivers display strong variations in their microbial faecal load. In contrast, microbial faecal load in the midstream of large rivers displays slowly changing continuous trends due to the fact that transversal mixing of faecal contaminated water masses from the river side is a slow process. The "Transversal-Selection-in-Large-Rivers" hypothesis states that microbial faecal pollution patterns of midstream and lateral river zones differ not only in quantitative but also in qualitative aspects. Lateral zones harbour both sensitive and persistent populations of faecal microorganisms, while a shift to more persistent forms is observed towards the midstream zones (along the transversal mixing processes) due to differential persistence and die-off. The Joint Danube Survey 2013 is the worlds biggest river expedition and a unique opportunity to test the newly formulated hypotheses and concepts along the whole length (> 2400 km) of a large and highly international river. In addition to the determination of standard faecal indicators, microbial source tracking tools based on qPCR and NGS analysis, a large environmental data set will be available allowing to create a comprehensive picture of faecal pollution patterns in the Danube and to formulate general principles and a conceptual model that shape/explain microbial faecal pollution patterns of large rivers. The new insights will be the basis to further develop new modelling tools for the prediction of microbial faecal pollution in large rivers. Such efforts will be crucial for future water safety management covering transnational boundaries on whole river scale.

Knowledge of the pathways and patterns of microbial-faecal pollution in large rivers is of paramount importance for all forms of water use, but also for a general understanding of ecosystem functions. In this study, a nested concept for the spatio-temporal resolution of pollution patterns was developed and applied based on holistic, interdisciplinary analyses using the Danube as an example. The investigations included a total river inspection in combination with an annual inspection at selected sites, in the middle and at the left and right riverbanks. A detailed microbiological water quality map of the entire Danube and its main tributaries was prepared using a new colour/symbol code and taking into account the prevailing hydrological situation. In addition to local pollution events, this map shows that the microbial-hygienic water quality in the middle of the river reflects the general pollution situation in a particular section of the river. Nevertheless, detailed river cross- section investigations should be carried out for local use-oriented questions. The use of host-specific faecal markers along a 2600 km long flow path made it possible for the first time to identify the main sources of faecal pollution of a large river and its main tributaries. For the Danube it turned out that human faecal inputs are by far the dominant source of pollution and entries of cattle and pigs are at most of local importance. The high-resolution combination of traditional standard methods and modern, intelligent faecal markers thus provides valuable additional information for better management of the microbiological water quality of large rivers with regard to potential uses (drinking water production, use as bathing water, irrigation for agriculture, etc.). The study also provided initial basic insights into the distribution of antimicrobial resistance in important clinically relevant bacterial groups along the Danube. In addition to the detection of several multi-resistant isolates with high clinical relevance, similar or higher levels of resistance were found in the upper sections compared to the lower sections, in which the wastewater inflow and antibiotic use are significantly higher. In addition to the analysis of microbial pollution patterns, the development of bacterial communities along the Danube was investigated for the first time in such a large river using high-throughput sequencing methods and new bioinformatic and statistical tools. Contrary to expectations, the bacterial communities gradually developed from large groundwater and soil communities to typical lake communities, with smaller, less active cells, despite increasing wastewater discharges and inflows in the middle and lower reaches of the Danube. These results lead to a completely new understanding of the basic microbiological processes and the functioning of large river ecosystems.