A desert that is not a desert
The Lençóis Maranhenses National Park in northeastern Brazil covers 1,550 square kilometers of sweeping white dunes. At first glance, it appears to be a vast desert. This landscape, however, receives approximately 1,200 millimeters of rain annually, far exceeding the less than 250 millimeters that technically defines a desert. Most of this rainfall occurs between January and June.
The park's geology results of river, ocean, and wind. The Parnaíba and Preguiças rivers carry quartz sand from the continent's interior to the Atlantic coast. Ocean currents then deposit this sediment along the shoreline, where powerful winds drive the sand up to 50 kilometers inland. This process forms massive barchanoid dunes, or crescent-shaped dunes, that can reach heights of up to 40 meters.
Beneath the porous sand lies a layer of impermeable rock. This subterranean feature prevents the heavy rainwater from draining away. Instead, the water collects in the interdunal valleys, forming thousands of temporary freshwater lagoons that can be up to 100 meters long and 3 meters deep. These pools, with a pH between 4.9 and 6.2 and water temperatures from 27.5°C to 32°C, create a massive, temporary aquatic ecosystem.
Life finds a way
A feature of these ephemeral lagoons is the sudden appearance of fish. Though the pools evaporate completely during the dry season, losing up to one meter of depth per month, life persists. When the rains return, so do the fish.
One of the most resilient residents is the wolf fish (Hoplias malabaricus). This carnivorous, predatory fish has a remarkable survival strategy. As the lagoons shrink, the wolf fish burrows into the remaining wet mud and enters a dormant state called aestivation. It remains there through the dry season, emerging only when the annual rains rehydrate its environment.
Other fish arrive through different means. During the wet season's peak, some lagoons become interconnected with each other and with local rivers, like the Rio Negro. This allows fish to migrate into the newly formed dune pools. The park's ecosystem also supports 112 species of birds and 42 species of reptiles, including the endemic Brazilian slider turtle (Trachemys adiutrix). The area is a transition zone between three major Brazilian biomes—the Cerrado, Caatinga, and Amazon—contributing to its unique biodiversity.