The alkaline engine
The Laguna de Pitillas is an endorheic, or terminal, saline lake—it retains water and allows no outflow. Situated in a basin of Cenozoic-era sedimentary rock rich in salts, the lagoon's water is naturally saline and alkaline. Its most remarkable feature is a pH that can reach 9.5, a level maintained by a powerful biogeochemical process. The primary drivers of this system are immense populations of cyanobacteria, including species like Anathece minutissima and Synechococcus sp..
These photosynthetic microorganisms consume vast quantities of dissolved carbon dioxide (CO2) from the water. This rapid removal of CO2 significantly shifts the water's carbonate equilibrium. As carbonic acid levels drop, the concentration of hydroxide ions increases, causing the pH to rise to highly alkaline levels. When the pH becomes sufficiently high, dissolved calcium and carbonate ions in the water reach a supersaturation point. They then precipitate out of the solution, forming carbonate minerals like calcite. This precipitation effectively removes carbonate from the system, which in turn places a limit on how high the pH can rise. The entire process functions as a self-regulating feedback loop, where microscopic life dictates the lake's large-scale geochemistry.
An oasis for extremophiles and birds
Despite its extreme chemistry, the 216-hectare lagoon supports a robust and specialized ecosystem. The water chemistry that favors cyanobacteria also supports specific submerged aquatic plants, such as those from the Chara genus, which can tolerate the high salinity and pH. The dense reed beds surrounding the water are dominated by Phragmites australis. This vegetation is an important habitat for a huge diversity of birdlife, the lagoon is one of Navarra's significant wetlands.
The site is a designated Natural Reserve, a Special Protection Area for Birds (ZEPA), and a Ramsar wetland of international importance. Over 100 bird species have been documented here. The lagoon is an important nesting site for the Great Bittern (Botaurus stellaris) and the Marsh Harrier (Circus aeruginosus). During migratory periods, it becomes an important stopover for species like the Greylag Goose and Common Crane. The foundation of this bright food web is the extreme microbial life, which thrives in conditions that would be inhospitable to most other organisms. The productivity of the cyanobacteria and other microorganisms directly or indirectly feeds the invertebrates, amphibians, and fish that the large bird populations depend on.