Fine particulate matter plays an important role in stream ecological functioning, being a main driver of productivity and metabolism of fluvial ecosystems. A large portion of this fine particulate matter is deposited and accumulated in the hyporheic zone (the interface between groundwater and surface water). Therefore, it is important to understand the factors that determine the spatial distribution of this fine particulate matter and its bioreactivity. This is especially relevant to rivers that receive waste water treatment plant (WWTP) effluents because they are a constant source of fine particles, which can profoundly affect their metabolism. The objective of this work was to study the influence of the WWTP effluent of Santa Maria de Palautordera on the quantity, quality and bioreactivity of fine particulate matter in the receiving stream. In addition, we explored the physical and hydrological factors that determine its spatial distribution along a 100 m section downstream of the effluent inlet. The study was conducted at a time of storm flow conditions, which is an extreme hydrological moment, but at the same time very little studied. The results showed the same amount of fine particulate matter upstream and downstream of the effluent inlet from the WWTP, but higher percentage of organic matter downstream. The fine particulate matter along the reach was distributed heterogeneously, and we detected the presence of hot spots as a result of various factors, such as the spatial distribution of hydrological variables, as well as the presence of physical obstacles such as trunks and macrophyte patches. Microbial metabolic activity associated with fine particulate matter increased downstream of the WWTP effluent, especially at those points where fine particles rich in organic matter accumulated. This study shows that the influence of the WWTP on fine particle standing stocks and their bioreactivity is low during high flow conditions, when the dilution factor is high. However, fine particulate matter still tends to accumulate in specific stream areas (hot spots) even during moments of maximum advection and transport
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