Silfra, a rift in the mid-Atlantic Range, is the only place on Earth where it’s possible to snorkel between two tectonic plates. Glacial meltwater flows continuously through the fissure, which is situated on the rim of Thingvellir Lake. Having percolated through volcanic rock, this water is exceptionally pure and gin-clear, with visibility of up to 100 metres. But as I waded in, immersed my masked face and gazed into the blue abyss, the underwater landscape appeared barren, a labyrinth of rock and boulder piles. I was clad in layers of thermals, a drysuit and a neoprene hood, but within seconds my exposed lips and cheeks began to sear with cold. With a constant temperature of between 2-4C, it seemed an inhospitable environment for any organism.
Though biodiversity is limited, Silfra does contain life, most notably Crymostygius thingvallensis, an endemic species of groundwater amphipod crustacean. Many of the fissure’s other inhabitants are microinvertebrates, invisible to the human eye. As the current carried me through the Big Crack and into a wider section known as Silfra Hall, I began to notice that the rocks were draped with translucent creamy-beige and reddish-brown biofilm mats, which I discovered were matrices of these imperceptible creatures, cyanobacteria and benthic diatoms (a form of microalgae). They had the ragged appearance of degrading plastic bags and were dotted here and there with tufts of neon green algae known locally as troll hair.
As I ventured deeper into the fissure, entering Silfra Cathedral, a breathtaking open chamber with soaring lava walls, there was a momentary flash of scales in my peripheral vision. A 10cm-long, olive-green, speckled-flanked Arctic char (Salvelinus alpinus) darted out of an inky crevice. In contrast with the UK, where the species is predominantly restricted to upland lakes and lochs, and is at risk of extinction due to acidification, pollution and rising water temperatures, they are the most common and widespread freshwater fish in Iceland. Thingvellir Lake supports four morphological variants, which have evolved to exploit different ecological niches. While the benthic, piscivorous and planktivorous char occasionally stray into the fissure during the August to September mating season, the diminutive dwarf morph is the only fish to inhabit it year-round.
• Country diary is on Twitter at @gdncountrydiary
