Temporary Mediterranean ponds

Mediterranean temporary ponds, a habitat of priority interest in Europe, are small wetlands that dry up during the spring. They are the source of a veritable explosion of life and species… where algae, zooplankton, branchiopods, aquatic insects, amphibians, dragonflies, aquatic ferns and other emergent plants thrive. It is really hard to find a habitat richer in biodiversity.

The surface and depth of the water body, the temperature of the water column, and the concentrations of oxygen and mineral salts are factors that vary more or less in synergy with the duration of the flood. Add in the geology of the soil (which can be very salty) and the year’s rainfall to create an incredible variability of ecological conditions. Temporary ponds thus go through several phases from flooding to drying(ecological succession).

The more specialized and often threatened fauna and flora require a period of pond drying before hatching the following winter.

Step 1: Winter rains

Stage 1. Typical year: December – January. After prolonged rainfall throughout the winter, the depressions in the ground gradually fill with water and form puddles or even lagoons, depending on the size of the catchment area.

These water bodies occupy grasslands with mostly sparse terrestrial vegetation, holes dug for livestock, field bottoms, unprotected lagoon drainage ditches, roadside ruts, abandoned quarries or even basins on granite blocks (‘marmites’).

Although it goes completely unnoticed in the dry period, a seed bank of aquatic plants and branchiopod eggs is waiting for the right conditions to arise.

Step 2: After a few days of flooding…

Stage 2: Typical year: January – February. At this stage, the water bodies are poorly mineralized, cool at night and lack vegetation and animal life.

And yet… After only a few days of flooding, the gill plates eggs hatch in large numbers and the larvae develop massively, feeding on algae and organic debris.

Soon after, small planarians appear. Finally, the first adult winged insects (aquatic beetles and bugs) and amphibians colonize the ponds and begin a reproductive cycle. The earliest species that depend on the temporary ponds are Spotted Toads(Pelodytes spp.) and Natterjack toads(Bufo calamita).

Stage 3: the explosion of life

Typical year: February – March. Stage 3: Highly variable period depending on factors such as water volume, physicochemical conditions and rainfall regime. Heavy rains return the pond to its previous state while high temperatures accelerate its evolution.

Amphibians eventually colonize the ponds, in particular, southern frogs(Hyla meridionalis) and Iberianribbed newts(Pleurodles waltl) are abundant in deeper conditions. Most of the branchiopods are adults, and the saline species typical of deep water develop later.

In shallow ponds, the water warms up, but does not cool down as much during the night. A veritable explosion of life fills the ponds with organisms: aquatic plants have grown vigorously, a new generation of predatory insect larvae and tadpoles have become very abundant. Aquatic ferns(Marsilea, Isoetes) grow in the wet soils at the edge of the pond.

Waterfowl are frequent visitors to the wetlands: storks, Cattle Egrets and Little Egrets prey on abundant prey, especially amphibian larvae and Triops.

Step 4: The first heat

Typical year: March – April. Stage 4: The pond has passed its peak. and it is starting to dry out… Noticeably lower water level, warm temperature and signs of eutrophication.

Branchiopods have become very rare, as they cannot withstand high temperatures and predation. However, a few individuals survive and reach large sizes. On the other hand, the growth of other aquatic organisms is accelerated.

Aquatic insects and amphibians have metamorphosed in large numbers, plants have completed their life cycle. Dragonflies generally need prolonged flooding. New rains could prolong this stage but the end of a season is already perceived.

Step 5: low water level and (temporary) end of ponding

Typical year: late April through May (rainy years; large ponds). Stage 5: the pond eventually dries out. At first, persistent moisture in the soil favors dense herbaceous vegetation (it is different in saline soils). Domestic animals take advantage of this herbaceous bonanza when the environment is already withered. Quickly, aridity gains ground and the soil becomes arid again and hides its riches… until the next rains.

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