Hydrological and ecological drivers of leaf retention under urbanization in a Central Amazon Headwater stream

Retained organic matter is the primary energy source for aquatic biota in forested ecosystems, serving as an indicator of ecosystem integrity through the retention of allochthonous inputs. The composition and seasonal dynamics of riparian vegetation directly influence the quantity and quality of this essential resource. This study evaluated the leaf retention capacity of the urbanized Bindá Stream, located in central Amazonia (Manaus, Brazil). Leaf release and capture experiments were conducted monthly over a six-month period (November 2017 to April 2018) along a 30-meter reach, with two replicates per month (n = 12). Riparian plant species richness was also surveyed. During each trial, depth, width, flow velocity, discharge, and the proportion of physical obstacles were recorded. The Bindá Stream showed low leaf retention capacity, with less than 33% of the leaves retained. Across the study period, depth ranged from 0.22 to 0.38 m, flow velocity from 0.27 to 0.45 m s⁻¹, channel width from 2.87 to 3.48 m, and discharge from 0.24 to 0.46 m³ s⁻¹. These hydrological variations influenced retention rates, as shown by leaf transport curves. The low retention capacity reflects poor conservation status resulting from significant anthropogenic impacts. Combined with water pollution, this limitation in organic matter retention weakens aquatic food webs by reducing resource availability for aquatic organisms. Our findings underscore the urgent need for further studies and ecological restoration efforts in Manaus’ urban streams to safeguard their ecological integrity and ecosystem functions.