Water pollution drives environmental degradation in a seasonally influenced Neotropical coastal river.

Aquatic and coastal ecosystems are increasingly altered by human activities, particularly through the removal of riparian vegetation, pollution, urbanization, and biological invasions. This study aimed to assess environmental conditions and identify a gradient of human disturbance in the Guaraguaçu River, located on the coastal plain of southern Brazil and subjected to multiple stressors. To achieve this, we evaluated land use, water and sediment quality, and the dominance of invasive macrophytes, and applied a rapid physical habitat assessment protocol quarterly throughout 2024 at 22 sites along the main channel of the Guaraguaçu River and its two tributaries, the Pery and Pombas Rivers. Data were summarized using principal coordinate analysis (PCoA), and the axis scores were used to define the environmental gradient. The results revealed a clear gradient in environmental conditions, likely driven by anthropogenic pressures across the watershed. Among the various stressors analyzed, water pollution emerged as the primary driver of environmental degradation in the Guaraguaçu River Basin. Conductivity, salinity, ammonia, and orthophosphate contributed most significantly to the observed gradient. The highest values for these parameters were recorded in the Pery River and downstream from its confluence with the Guaraguaçu River, indicating a decline in environmental quality in this tributary and, consequently, in the lower basin sector. The results showed that environmental conditions varied along a gradient significantly influenced by both seasonality and river zone, suggesting that seasonal dynamics are a key driver of this pattern. This influence of seasonality was also supported by the PCoA results, which reflected clear temporal patterns in environmental variability. Furthermore, human-induced changes to the Pery River, particularly water pollution and hydrological modifications, have impaired ecological conditions and intensified environmental degradation in downstream areas of the Guaraguaçu River basin. These findings highlight the need for targeted management strategies, including improved wastewater treatment, to mitigate cumulative stressors and enhance the resilience of this coastal river ecosystem.