Energy flow in a floodplain aquifer ecosystem

We developed an energy budget for the hyporheic invertebrate community of a large floodplain aquifer, based on invertebrate populations, closed chamber respirometry, in situ community respiration, mesocosm and microcosm experiments, stable isotope analysis and invertebrate gut contents. The invertebrate respiration scaling exponent was 0.474 (+/- 0.068 95% CI) across six orders in body mass, which was significantly lower than the ¾ power scaling predicted by metabolic theory. Cyclopoid copepods dominated invertebrate production, followed by the harpacticoid Bryocamptus hiemalis, Stygobromus amphipods, and amphibiont stoneflies. Invertebrate production ranged from 10.7 to 1680 mg C/m³ sediment/year, and showed a U-shaped response to mean dissolved oxygen (high production at both low and high oxygen concentrations). Production declined exponentially with depth for most sites, but there was an exponential increase at the oxycline below low areas of low dissolved oxygen. Estimated microbial community production ranged from 1210 to 2020 mg C/ m³ sediment/year, also showing a U-shaped response to oxygen. System RQ ranged from ≈ 0 to 9.5, indicating a significant contribution of anaerobic production to system energy flow. POM was by far the largest carbon reservoir in the aquifer at ≈ 10⁸ mg C/ m³ sediment (ranging as high as 10¹⁰). Carbon loading from the estimated rate of breakdown of POM was more than sufficient to explain microbial and invertebrate production in the aquifer. Carbon stable isotope signatures showed strong levels of depletion for invertebrates (δ¹³C -25‰ to -70‰). These results suggest a significant anaerobic subsidy of aerobic food webs in the subsurface, and a potential methane subsidy of 10% to 99% of invertebrate energy flow. Low oxygen hotspots corresponded with the migration of stonefly nymphs 100’s of meters from the river into the aquifer.