The life in mediterranean temporary ponds
Mediterranean temporary ponds, a priority habitat in Europe, are small wetlands that dry up during the spring. Seats of a veritable explosion of life and species… where algae, zooplankton, branchiopods, aquatic insects, amphibians, dragonflies, aquatic ferns and other emerging plants thrive. It is really difficult to find more biodiversity in the surroundings.
Depth and surface of the pool, temperature of the water column, oxygen and minerals concentrations, all are factors that vary more or less in synergy with the duration of the flooding. Alltogether with the geology (soils can be very salty) and the rainfall of the year, this creates an incredible variety of ecological conditions. Temporary ponds thus go through several phases from flooding to drying up, exemplifying a didactic ecological succession.
The most specialized and often threatened fauna and flora require a period of drying up of the ponds before hatching the following winter.
Step 1: winter impoundment
Stage 1. After prolonged rains throughout the winter, land depressions are gradually loaded with water and form pools, or even lagoons, depending on the size of the catchment area.
These bodies of water occupy grassy land with mostly sparse terrestrial vegetation, holes dug for livestock, bottoms of fields, drainage ditches of unprotected lagoons, ruts along the roadside, abandoned quarries, or even basins on granite blocks (‘marmites’).
Although it goes completely unnoticed in the dry period, a bank of aquatic plant seeds and branchiopod eggs is just waiting for the right conditions to emerge.
Typical year: December – January.
Step 2: After a few days of flooding…
Stage 2: At this stage, these habitats are little more than bodies of low mineral water, cooling overnight and devoid of vegetation.
And yet… After only a few days of flooding, branchiopod eggs hatch in large numbers and larvae develop massively, feeding on algae and organic debris.
Shortly after, small planarians appear. Finally, the first adult winged insects (water beetles and aquatic bugs) and amphibians colonize the ponds and begin a reproductive cycle. The early species and most dependent on temporary pools are Parsley frogs (Pelodyte ibericus) and Natterjack Toads (Bufo calamita).
Typical year: January – February.
Stage 3: aquatic wildlife flourishing
Stage 3: Highly variable period depending on factors such as water volume, physico-chemical conditions and precipitation regime. Heavy rains bring the pond back to the previous stage while high temperatures accelerate its evolution.
Amphibians end up colonizing the ponds, in particular Mediterranean tree frogs (Hyla meridionalis) and Ribbed-newt (Pleurodles waltl) are abundant in deeper conditions. Most branchiopods are adults, with the typical deep-water, saline species developing later.
In shallow ponds, the water warms up, but does not cool as much during the night. A real explosion of life fills the ponds with organisms: aquatic plants have grown vigorously, a new generation of larvae of predatory insects and tadpoles has become very abundant. Aquatic ferns (Marsilea, Isoetes) grow on the wet soils at the edge of the lagoon.
Waterfowl frequently visit wetlands. Storks, cattle egrets and egrets prey on abundant prey, especially larvae of amphibians and Triops.
Typical year: February – March.
Step 4: The first heat
Stage 4: The pond has passed its peak and is beginning to dry out…notably lower water level, warmer temperature, and signs of eutrophication.
Branchiopods have become very rare, not supporting high temperatures or predation. Nevertheless, a few individuals survive and reach large sizes. On the other hand, warmth accelerates the growth of other aquatic organisms.
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 perceptible.
Typical year: March – April.
Step 5: low water level… and the (temporary) pond’s end
Stage 5: the pond eventually dries up. At first, the persistent humidity in the soil favors dense herbaceous vegetation (situation is different in saline soils). Domestic cattle often take advantage of this wealth of herbs when the surroundings are already withered. Quickly, the aridity gains ground and the soil becomes arid again and hides its riches…until the next rains.
Typical year: Late April to May (rainy years; large lagoons).
How long can the eggs last in the substrate?
What is the preference of the species depending on the depth of the water column and salinity?
Why is there especially bright coloration on the abdomen of females?
How many species can live in the same pond?
How do these species colonize new bodies of water, by air and by land?