Adaptations to hypoxia and anoxia are important because oxygen is essential for sustaining intracellular bioenergetics for most species on earth. Aquatic insects cope with hypoxia and anoxia using a variety of behavioral and physiological responses. Most stoneflies (Plecoptera) occur in highly oxygenated flowing water, yet some live underground in alluvial aquifers which can contain zones of low dissolved oxygen concentration (DO). We documented DO dynamics in six floodplain wells over seven years from a continuous monitoring system that recorded oxygen hourly in a floodplain of the Flathead River, Montana. In the same floodplain we collected stoneflies by pumping aquifer wells. We compared the oxygen-dependence of metabolic rates of 174 nymphs from aquifer and benthic sites in the same floodplain. We measured mass-specific metabolic rates at different oxygen concentrations (12, 8, 6, 4, 2, 0.5 mg/L, and during recovery) and tolerance of aquifer nymphs to repeated exposure to hypoxia over two time periods (day and week) using intermittent respirometry experiments. Individual stonefly nymphs were placed in glass chambers and oxygen levels were recorded every second during measurement periods. Metabolic rates were estimated from raw data on changes in oxygen concentrations.
Results/Conclusions
Hypoxia regularly occurred in two floodplain wells on the Nyack floodplain and nymphs of Paraperla frontalis were collected from these wells during and just prior to the onset of hypoxia. Metabolic rates of aquifer taxa were similar from 12 down to 2 mg/L (P > 0.437), but the metabolic rates of benthic taxa dropped significantly with declining oxygen concentrations (P < 0.0001; 2.9× lower at 2 vs. 12 mg/L). All tested aquifer individuals survived short-term repeated exposure to extreme hypoxia. Metabolic rates during recovery were similar to initial rates measured for Isocpania spp. (P = 0.0855) and recovery rates for P. frontalis were similar to rates measured at normoxia for repeated exposure levels 2 and 3. In contrast, longer-term repeated exposure (>12 hr hypoxia) suggests limits on their tolerance. Survival rates ranged from 38-91% following week long repeated exposures and there were significant differences in metabolic rates with repeated exposure period for all taxa (P < 0.0295). Tolerance to short-term repeated exposure likely enables aquifer stoneflies to exploit the rich methane-derived carbon sources found in/near anoxic zones, and helps explain how unconventional carbon sources in alluvial aquifers of river floodplains may be fundamental in supporting diverse and productive food webs.