Seasonal floodplains and pools, sometimes known regionally as paúis (singular: paul) or lagoas temporárias, are critically important Mediterranean wetlands characterized by extreme seasonal shifts. While found predominantly in the Ribatejo, Baixo Alentejo, and Algarve regions, there are smaller examples of this habitat in Central Portugal, for example, along some stretches of the Rio Nabão, a tributary of the Rio Zêzere, within the Tejo catchment.  These ecosystems cycle rapidly between winter aquatic havens and summer terrestrial deserts. This harsh volatility eliminates permanent fish predators, allowing a highly specialized, endemic community to thrive. 

The ecological architecture of these Portuguese ecosystems includes:

Hydrological Profile & Soil Dynamics

• Atlantic-Mediterranean Pulse: Driven by erratic autumn and winter rainfall, filling shallow depressions over impermeable clay or hardpan soils, and sometimes leading rivers and streams to spate and flood surrounding land.

• Extreme Hydric Stress: Water levels usually peak between January and March, followed by rapid evaporation starting in May.

• Summer Baking: By July, the surface completely dries out, leaving cracked, oxygen-depleted, and highly compacted soils.

Flora & Vegetation Zonation

Where natural vegetation dominates, plants organise themselves in strict concentric rings based on how many days the soil stays waterlogged:

• Central Pool (Deepest): Dominated by aquatic ferns like Isoetes velata (quillworts) and Marsilea batardae (a rare, endemic four-leaf clover).

• Intermediate Ring: Characterized by specialized amphibious turf species like Mentha pulegium (pennyroyal) and Illecebrum verticillatum.

• Peripheral Belt: Dominated by rushes (Juncus species) and grasses that transition smoothly into the surrounding Mediterranean scrub.

However, many areas are dominated by rough grasslands used for grazing, and vegetation zonation is not always evident.

Indicator and Endemic Fauna

In the absence of fish, these temporary waters can turn into high-yield biological nurseries:

• Prehistoric Crustaceans: The temporary ponds of Alentejo are strongholds for large branchiopods like Triops vicentinus (an endangered tadpole shrimp endemic to southwest Portugal).

• Amphibian Hotspots: These zones support over a dozen species, including the Iberian painted frog (Discoglossus galganoi) and the marbled newt (Triturus marmoratus).

However, fish are often present, especially invasive species such Gambusia affinis and Carrasius gibelio, along with the invasive crayfish Procambarus clarkii from the USA which is well adapted to this habitat.

• Avian Foraging: Key Portuguese sites like the Paul de Arzila or Paul do Boquilobo host massive seasonal influxes of wading birds, Cormorants and various herons, clearly attracted by fish.

Ecological Threats in Portugal

Today, these habitats are highly endangered in Portugal due to deep plowing for intensive agriculture, coastal tourism development in the Algarve, and groundwater depletion. Many of these habitats are classified as priority conservation structures under the EU Habitats Directive (Habitat 3170*, Mediterranean Temporary Ponds) and the larger sites are generally protected. However, there are many small, unprotected sites which continue to provide amphibians with important breeding sites.

Rio Nabão, near Freixanda, Ourém

The Rio Nabão begins its journey at a limestone spring called Olhos d'Água, located in the parish of Lagarteira within the municipality of Ansião.

• Character: Part of the Sicó-Alvaiázere aquifer system, this upper reach behaves like an "unpredictable river of extremes".

• Seasonality: Because it flows through a heavily karstic, porous limestone landscape, the upper reaches of the river can run thin or dry in the summer months. However, during heavy winter rains, water dramatically erupts from underground systems, frequently bursting past its initial banks and flooding surrounding land.

The photos above were taken in February 2026, just after Storm Kristin, which generated the strongest winds ever recorded in Portugal (208.8 km/h (Soure, Coimbra district)) . In the lower photo you can also see the environmental impact of some extreme weather events that have hit this area in recent years. To the right, on the hill, you can see the remnants of the forest that burnt 3 years previously. On the hill, second right, you can see heavy erosion of the hillside caused by heavy, persistent rain 2 years previouly.