Lagoons of Lençóis Maranhenses: Freshwater Amid White Dunes

A seeming desert sea

From the air, the dune landscapes of Lençóis Maranhenses National Park look like endless fields of sand. With the onset of the rainy season, the picture changes: clear water collects in hollows between the dunes, shimmering in blues and greens. For visitors the scene seems paradoxical — lagoons in the middle of the sand.

How the lagoons form

The phenomenon depends on the interplay of rainfall, sand dynamics and the subsurface. During the rainy season the region receives heavy precipitation. Water accumulates in natural depressions but remains unusually long there: the sand’s grain sizes and layering, sometimes over a less permeable soil layer, slow infiltration. As long as rainfall exceeds evaporation, the basins stay filled.

The lagoons contain freshwater. There is no permanent connection to the sea; coastal barriers and sandbanks shield the area most of the time. However, during exceptionally high floods or strong tides, channels can open temporarily, allowing saltwater to enter and mix with percolating water.

A view from space

Satellite imagery makes the annual rhythm visible. Optical sensors show the strong contrast between bright, highly reflective sand and darker-appearing water; vegetation shows up in greens. Time series document when and where lagoons form, how large they grow — and when they disappear again.

Long-term analyses provide clues to trends: shifts in the start and end of filling phases, changes in extent or repeated saltwater intrusions can be detected in the data and related to rainfall patterns and coastal dynamics.

Habitats in flux

The ponds are a dynamic habitat. Species adapted to the cycle of wet and dry seasons take advantage of the temporary availability of freshwater: aquatic insects, amphibians and certain fish colonize the pools, and plants establish along the shores. For local people the lagoons are fishing spots and a tourist draw.

At the same time the system is fragile. Dunes are sensitive to disturbance: foot traffic, uncontrolled vehicles and construction can undermine stability and alter natural storage mechanisms. Changes in precipitation regimes affect the length of the water season and thus biological productivity.

Risks from climate and coastal change

Projections for northeastern Brazil indicate greater precipitation variability: extreme events could become more frequent and dry spells longer. In a landscape that depends on the seasonal balance of rainfall and evaporation, this tends to shorten lagoon lifespans. Heavy rains can increase erosion and favor temporary connections to the sea.

Rising sea levels raise the risk of saltwater incursions. That changes the chemical conditions in the basins and puts freshwater organisms under pressure. Protecting the dunes as a barrier is therefore an ecological and hydrological necessity.

Monitoring for protection

Satellite data enable continuous monitoring that is difficult to achieve solely on the ground. Researchers map seasonal water surfaces, track vegetation changes and record erosion events. This information supports management: for example, guiding visitor flows, protecting against overuse, or setting up early-warning systems for unusual hydrological situations.

For long-term observation, remote sensing is particularly valuable. Image series can reveal trends that spot field studies might miss — such as a systematic shortening of filling phases or an increasing frequency of saline flooding.

Between tourism and preservation

The spectacular landscape brings income to local communities when visitor numbers are managed. Unregulated tourism, however, harms the fragile ecosystem: trampling destroys vegetation, vehicles leave ruts, and additional infrastructure can alter natural water flows.

Outlook

The lagoons of Lençóis Maranhenses illustrate how closely climate, sediment dynamics and coastal processes are intertwined. Remote sensing makes these changes visible — and provides the data needed to better balance use and protection.