Energy Flow in a Floodplain Aquifer Ecosystem

We developed an energy budget to identify energy sources for the invertebrate community of a large 20 km2 floodplain aquifer, based on biomass distributions, organismal respirometry, in situ community respiration, mesocosm and microcosm experiments, stable isotopes and invertebrate gut contents. The invertebrate respiration scaling exponent was 0.474 (+/- 0.068, 95% CI) across six orders in body mass, which is significantly lower than the ¾ power scaling predicted by metabolic theory. Invertebrate production was dominated by copepods (Diacyclops, Acanthocyclops, Bryocamptus), Stygobromus amphipods, and amphibiont stoneflies, and ranged from 26.9 to 4200 mg C/m3 sediment/year. Production and density showed a U-shaped response to dissolved oxygen (high production at both low and high oxygen concentrations). Production declined exponentially with depth for most sites, but at sites with orthograde oxygen profiles there was an exponential increase at the oxycline. Aerobic microbial community production ranged from 1210 to 2020 mg C/m3 sediment/year, also showing a U-shaped response to oxygen. System respiratory quotient (RQ) ranged from ≈ 0 to 9.5, indicating a significant contribution of anaerobic production to system energy flow. We documented multiple lines of evidence for DOC (soil, river) and buried POM carbon sources, however POM was by far the largest carbon reservoir in the aquifer at ≈ 108 (to 1010) mg C/ m3 sediment. Energy from POM breakdown was the only source sufficient to explain microbial and invertebrate production. 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. Oxygen showed high, non-random, spatial and temporal variation across the aquifer, with a large scale decline in oxygen along the axis of the floodplain, and distinct hotspots of low oxygen. Low oxygen hotspots corresponded with migration of stonefly nymphs 100’s of meters into the aquifer. The U-shaped responses and biogeochemical trends suggest a major threshold at bulk oxygen concentrations of 3-5 mg/l. Collectively, these findings indicate the role of dissolved oxygen as a key variable in groundwater ecosystems.

Name thesis:degree_name

Doctor of Philosophy (PhD)

Grantor dc:publisher

University of Montana

Year

2007

Author dc:creator

• Reid, Brian

• Aquifer Organic Matter
• Community Respiration
• Dissolved Oxygen Sytematics
• Energy Flow
• Hyporheic Mesocosm
• Hyporheic Zone
• Metabolic Scaling

Repository record dc:identifier

https://scholarworks.umt.edu/etd/372

OAI identifier oai:identifier

oai:scholarworks.umt.edu:etd-1391