Baía do Vieira Grande is a remarkable and expansive bay located along the Brazilian coast in the state of Pará, sitting at approximately 0°15′S, 50°42′W within the vast and ecologically rich Amazon Delta. Fed from the south by the Jacaré Grande River and connected to the Amazon through an intricate network of river channels known locally as furos, the bay represents one of the main river channels of the Amazon Delta system. The interplay between freshwater Amazon inputs and Atlantic Ocean tidal influence creates a dynamic, brackish-to-freshwater environment that supports extraordinary aquatic biodiversity. Notably, the bay is now deeper than the main channel of the Amazon itself, having gradually filled and widened over time from what was once a swampy lake — a geological process that continues today. The western edge of the bay is flanked by a chain of islands, the largest being Serraria, which separates the bay from the Amazon's main channel. The presence of phytoplankton species such as Thalassionema nitzschioides reflects the strong Amazonian influence on the bay's waters, signaling a nutrient-rich aquatic system capable of supporting robust fish populations.