Thank you very much for your question. I think I may have identified this species, but I have forwarded your photograph to a scientist in Madrid to see if he can confirm it.
There has been research done on several species of amphibians that have some tolerance to brackish or salt water habitats. Here's a link to a paper on this subject:
As soon as I hear back from the scientist in Madrid, I will be back in touch. Thank you for your patience and for contacting Ask an Expert.
I received a reply from my contact at the Museo Nacional de Ciencias Naturales, CSIC in Madrid. He said that your frog is called Perez's Frog, scientific name Pelophylax perezi. They are also known as the Iberian waterfrog, Iberian green frog and the Coruna frog.
He said they are the most common and abundant frog species in Spain. They are very strong and resistant and are found in just about all types of water bodies including ones that are polluted and salty water, montane areas in excess of 2000 meters and water bodies with introduced predators (fish and crayfish).
Here are some links where you can find additional information on this species:
I hope this answers your question, and thank you for contacting Ask an Expert.