Resilient aquifer stoneflies handle low oxygen well

Abstract

Sweeping through Montana's mighty Rocky Mountains, the majestic Flathead River courses through pristine valleys and canyons. But according to Rachel Malison from The University of Montana, USA, the river extends well beyond the babbling torrent visible at the surface, flooding deep into the gravel- and cobblestone-packed aquifer beneath. And these subterranean waterways are bursting with life. In the 1970s, Jack Stanford from the University of Montana, USA, found that stonefly nymphs – some of the most vulnerable inhabitants of rivers as they require well-oxygenated water – are remarkably content to reside in aquifers, even though the oxygen levels can plunge as low as 0.14 mg l–1. In addition, Amanda DelVecchia had recently discovered that the stonefly occupants of the Flathead River's Nyack aquifer dine on microorganisms that only thrive in deoxygenated environments, deepening the paradox. How could insects that usually perish when oxygen levels dwindle thrive in an aquifer where the oxygen supply can be patchy in places?After travelling through the dramatic landscape to the Nyack floodplain, Malison and intern Hailey Jacobsen pumped water from the wells drilled by Stanford in the 1980s to collect the subterranean stoneflies. ‘Sometimes we didn't get any, sometimes we might get 100’, says Malison, adding that additional creatures, including tiny crustaceans and other microscopic invertebrates, also appeared in the water. Back at the Flathead Lake Biological station, Malison identified three stonefly species (Paraperla frontalis, Isocapnia spp. and Kathroperla perdita) before measuring the insects’ oxygen consumption to calculate their metabolic rate as she steadily reduced their oxygen supply from 12 to 0.5 mg dissolved oxygen l–1.Comparing the metabolic rates of the aquifer stoneflies with stoneflies that had been collected from the river, Malison found that the aquifer residents had a much lower initial metabolic rate than the surface-dwelling insects. In addition, the species in the aquifer coped much better as the oxygen levels fell – their metabolic rate only dropping ∼46% when the oxygen was reduced by 83% – in contrast to the river dwellers’ metabolic rates, which dropped by 66%. And when Malison and Jacobsen checked the insects’ recovery after their oxygen ran out, the species that live in streams never regained their full metabolic rate, while the aquifer residents bounced back to almost normal.Knowing that the aquifer-based species could nip in and out of oxygen-depleted water if they are to dine on the specialised microorganisms that thrive there, Malison and Jacobsen then tested how P. frontalis and Isocapnia fared when the oxygen levels dropped from 12 to 0.5 mg l–1 three times over a 9 h period. Impressively, all of the stoneflies survived oxygen levels that would probably have proved fatal for the river-dwelling species; however, the reduction in P. frontalis’ metabolic rate suggests that visiting deoxygenated regions briefly to dine may be stressful. And when Malison, Julia Cotter and Haley Dole embarked on the Herculean task of continually recording the insects’ metabolic rates over a week when the animals repeatedly went without oxygen overnight after the levels had declined gradually during the day, K. Perdita was most robust (91% survival), although Isocapnia seemed to struggle more (38%).Even though most stonefly species are notoriously sensitive to the oxygen levels in their water, the stoneflies that have descended beneath the ground and made their homes in aquifers seem to be more resilient, tolerating oxygen levels that would be fatal for river dwellers. ‘This tolerance suggests that they can enter zones of low oxygen to forage and exploit hot spots in the underground aquifer where food is abundant’, says Malison.