Floodplain aquifer chemotrophy: Food web dependence on abiogenic and biogenic methane
River floodplains are among the most biodiverse, productive, and threatened ecosystems globally. In expansive alluvial floodplains, the hyporheic zone often encompasses the entire alluvial aquifer. These aquifers are recharged by river water and tend to be well oxygenated with extremely carbon limited (<2mg/L) production. Despite these conditions, the aquifers contain diverse and productive communities dominated by obligate large-bodied hyporheic stonefly nymphs. This is exemplified by the Nyack Floodplain (Middle Fork of the Flathead River, Montana), where we recently found that biomass of hyporheic stoneflies is predominately composed of methane-derived carbon. We asked: what is the source of methane and how is it cycled in the aquifer food web?
We sampled the aquifer via monitoring wells mainly in Nyack and also in two other floodplains where groundwater stoneflies are abundant: Kalispell (MT) and Methow (WA). We used stable isotope analysis of three stonefly species, particulate organic matter (13C and15N), and methane gas (13C and 2H), radiocarbon dating of stonefly biomass and dissolved methane, and routine measurement of dissolved oxygen and dissolved methane to infer sources of methane, fractional contributions of methane to stonefly biomass and the extent to which direct grazing on methanotrophic biofilm might drive 13C depletion.
Stoneflies were 13C depleted despite low dissolved methane concentrations, implying that methane is rapidly assimilated: Nyack stoneflies averaged δ13C -49.22 ± 2.59 ‰, Kalispell -41.66 ± 2.66 ‰, and Methow -39.87 ± 2.56 ‰, but methane concentrations (excluding two wells on the Nyack) averaged 0.67 ± 0.19 µmol/L, 0.13 ± 0.05 µmol/L, and 0.30 ± 0.03 µmol/L, respectively. Our conservative estimate of Nyack mean methane dependence calculated using a two-source mixing model was 51.95 ± 7.0 % but ranged to >100% (methane δ13C of -67 ‰, POM δ13C of -30 ‰). Methane sources in high concentration wells (means 5.43 ± 0.45 µmol/L and 9.89 ± 1.14 µmol/L) varied between acetoclastic methanogenesis and thermogenic outgassing as shown by: δ13C -69.46 ± 0.64 and δ2H averaging -289.2 ± 8.2 with age <1000 years, and δ13C -40.50 ± 0.30 ‰ and δ2H -63.7 ± 0.6 ‰ with age >7000 years. We found significant negative linear relationships between δ15N and methane dependence; slopes varied significantly by wells. This suggests that the most methane dependent individuals might using methanotrophic biofilm directly, and that variation in food sources is only possible under specific conditions. Thermogenic and biogenic methane therefore subsidize and/or drive the hyporheic food web.