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2021 was a dry year, yet Doñana wetlands hosted abundant and diverse aquatic communities

July 2021

Last April-May, the Long-Term Biodiversity Monitoring Group (ICTS-RBD) carried out the annual spring survey of Doñana’s wetlands. This survey, which started in 2004, collects information on the hydrology, water quality and aquatic biota: (plants, invertebrates, fish and amphibians) in a network of sampling stations distributed over Doñana’s marshland, inflowing streams and ponds.

The low rainfall received this year (over 300 L until March) resulted in scarce flooding. Flooding levels were, however, highly heterogeneous. In the marshland, which receives most of its water from the Northern streams (El Partido and La Rocina), the highest flooding levels could be found in La Vera and the western marshland (Marisma del Rocío, Marisma de Hinojo, and Marismillas); while the areas around the Travieso and Guadiamar channels (locally known as ‘caños’) showed extremely low flooding levels. Permanent ponds showed fairly high flooding, with water depths reaching 130 cm at Laguna Dulce and 80 cm at Laguna del Sopetón – reflecting the effect of winter rainfall on groundwater levels. In contrast, temporary ponds such as El Vivero de la Mata and La Mata de los Domínguez barely conserved water in their “zacayones” (deepened hole in the deepest part of the ponds, excavated to provide water to livestock and wildlife).

Marshland areas with longer flooding developed broad and dense meadows of submerged macrophyte, with diverse mixtures of Charophytes and aquatic angiosperms (Photo). While those with shorter flooding, such as Caño Guadiamar, were almost denuded of submerged vegetation. Pond vegetation ranged from communities of amphibious plants typical of oligotrophic wet meadows (Isolepis fluitans, Juncus heterophyllus), at the most ephemeral ponds, to dense macrophyte meadows (hosting, e.g., Myriophyllum alterniflorum, Potamogeton pectinatus, and/or Zannichellia obtusifolia) in those with permanent or prolonged flooding cucles. Excavated ponds (“zacayones”) with high densities of red swamp crayfish (Procambarus clarkii) were denuded of aquatic plants, which are consumed by this invasive species.

Red swamp crayfish was also found in La Vera’s wetlands, part of the ponds, and the mouths of the streams inflowing the marshes. Among the native aquatic invertebrates, large predatory aquatic beetles (Cybister, Dytiscus, Agabus) were particularly abundant. The fish community has been dominated, during the last two decades, by exotic species; hence, the only native species found in the survey were the eel (Anguilla anguilla) and, more rarely, the tusk fish (Cobitis paludica). Amongst the exotic, catfish (Ameiurus melas) was very abundant in La Rocina stream, where it has displaced the formerly-abundant black-bass (Micropterus salmoides). Western mosquitofish (Gambusia holbrooki) is widely distributed across all aquatic habitats; while of carps (Cyprinus carpio) and crucian carps (Carassius spp.) were less abundant than in previous years.

Despite the scarcity of water, we detected 2,641 individuals belonging to the 11 amphibian species known in Doñana. In the marsh, we found 5 anurans and only 1 newt, the Iberian ribbed newt (Pleurodeles waltl), while in the ponds and streams, we found all of the 11 species present. Species from waterbodies with permanent or prolonged flooding, such as the Iberian ribbed newt and the Western spadefoot toad (Pelobates cultripes), were the most abundant in the survey and presented high numbers of tadpoles. As for the rest of the species, their detection reflected their phenology: early reproducing species that start breeding in winter (Natterjack toad Epidalea calamita, Iberian parsley frog Pelodytes ibericus) were observed as juveniles and metamorphs; intermediate species (Mediterranean tree frog Hyla meridionalis, Southern marbled newt Triturus pygmeaus) were found mainly as tadpoles; while Iberian green frogs (Pelophylax perezi), a late-reproduction species, were observed mainly as reproductive adults.



 

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