The dissolving mountains in the Catalan Pyrenees, the Aigüestortes i Estany de Sant Maurici National Park is defined by water and rock.%%CITE_1%%%%CITE_2%%%%CITE_3%% Nearly 200 lakes are set among jagged peaks formed from granite and slate pushed up from the sea during the Alpine orogeny.%%CITE_1%% These granitic rocks, cooled from magma deep underground around 300 million years ago during the Hercynian orogeny, are composed of minerals like feldspar, quartz, and mica.%%CITE_4%%%%CITE_5%%
Normally, these hard rocks weather very slowly. Natural rainwater, made slightly acidic by atmospheric carbon dioxide, reacts with minerals like feldspar in a process called hydrolysis. This chemical reaction slowly breaks down the feldspar into clay minerals, releasing various ions and leaving behind the highly resistant quartz as sand. In the pristine environment of the high Pyrenees, this process unfolds over geological timescales. However, the chemistry of the rain has changed.
Pollutants such as sulfur dioxide and nitrogen dioxide from industrial regions can travel long distances, reacting with water in the atmosphere to form sulfuric and nitric acid. This acid rain falls on the Pyrenees, drastically lowering the pH of the water interacting with the rock. Studies in the Pyrenees show that lakes in granitic basins are highly sensitive to acidification because they lack the limestone (calcium carbonate) that would otherwise neutralize the acid. The increased acidity accelerates the chemical weathering of the granite, dissolving the rock's mineral structure at an unnatural rate.
A toxic chain reaction
The accelerated weathering of granite triggers a toxic cascade in the park's aquatic ecosystems. Aluminum, the most common metallic element in the Earth's crust, is a component of the feldspar minerals in the granite. Under normal pH conditions, this aluminum is locked safely within the mineral's crystal structure. But as acid rain lowers the pH of the lake water, sometimes below 5.5, it liberates aluminum from the minerals into the water as toxic, positively charged ions.
For fish like the native brown trout, this invisible threat is lethal. Aluminum is not required for any cellular function and is highly toxic, especially in its inorganic forms found in acidic water. The primary target is the gills. The dissolved aluminum damages the gill tissues, causing excessive mucus production that clogs the gills and leads to asphyxiation. It also disrupts the fish's ability to regulate ions like sodium and calcium, which is essential for survival. This ionoregulatory failure leads to a loss of electrolytes and, Death. Fish are most sensitive to this aluminum toxicity in water with a pH between 4.5 and 5.5. The result is a decline in fish populations in some of the park's most remote and otherwise untouched alpine lakes.