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Spatio-temporal variability of physicochemical conditions in the headwaters of neotropical streams

Abstract:

Ecuador is one of Earth's biodiversity hotspots and is characterized by a great diversity of aquatic ecosystems. Despite its rich biodiversity, the Ecuadorian coastal region is severely impacted by human development. However, there are few studies on the water quality of its rivers or streams. In order to contribute to this gap in knowledge, the objectives of this study were to: i) describe the physical-chemical characteristics of mountain streams in the coastal region of Ecuador, ii) analyze the patterns of spatial and temporal variation in the physical-chemical properties of those streams, and iii) compare the results with those of existing studies on streams in Ecuador and the neotropical region. The physicochemical parameters and hydromorphological characteristics of four headwater streams were monitored during the dry season of 2020 (summer) and the wet season of 2021 (winter). The water temperature was highly constant (19.74–22.72 °C); the dissolved oxygen concentration was low in all four streams (1.90–5.53 mg/L); all the streams were alkaline (pH 8.06–9.14) and mineralized (electrical conductivity 216.38–1582.75 μS/cm); there were few nitrogen species in any of the streams (<1 mg/L NO3, <0.04 mg/L NO2, <0.2 mg/L NH3), while phosphate concentrations were high (0.57–2.57 mg/L). The first two axes of the Principal Component Analysis (PCA) explained 42% of the accumulated variance, of which axis 1 represented a mineralization gradient, while axis 2 separated the winter/summer samples obtained from the four streams. Most of the physicochemical variables (ammonia, pH, dissolved oxygen, electrical conductivity, temperature, and current velocity) were significantly different among streams and between seasons. The spatio-temporal variability in water chemistry, both among streams and between seasons, could be explained principally by natural factors such as the geology of the watershed, the rainfall regime, and the riparian conditions (high amounts of litter and shading), although it is also supposed that agriculture (maize crops) contributes to the high phosphate levels and conductivity of some streams.