| It is commonly believed that metabolism in groundwater ecosystems is limited by organic carbon availability. Because in situ primary production is low, metabolism depends on dissolved or particulate organic matter inputs from other regions of the catchment. This project examined several questions concerning heterotrophic metabolism and biogeochemistry in the groundwater of Rio Calaveras, a headwater catchment in New Mexico: (1) How do groundwater metabolism and biogeochemistry vary spatially and temporally? (2) What factors influence groundwater metabolism? (3) What is the energy source for groundwater metabolism?;The hydrograph of many mountain catchments is dominated by spring snowmelt. At Rio Calaveras, surface discharge and water table elevation increased at the onset of spring snowmelt. Groundwater biogeochemical changes in response to snowmelt included an increase in dissolved oxygen and dissolved organic carbon (DOC) concentration. DOC concentration decreased exponentially with time, suggesting that newly saturated floodplain sediments were a major source of DOC. Organic matter content in seasonally saturated sediments averaged 3% by weight, and about 0.05 mg C/g dry sediment was water soluble. Microorganisms from these sediments were able to consume an average of 45% of the leached DOC. These results show that snowmelt imports DOC to groundwater and that a substantial amount can be consumed by biota. These results are important ecologically because the growth and abundance of groundwater organisms may be limited by DOC availability.;The influence of DOC availability, inorganic nitrogen (N), inorganic phosphorus (P), temperature and season on groundwater heterotrophic metabolism was assessed using laboratory manipulations of aquifer sediments, field microcosms, and in situ addition of DOC to the subsurface. Addition of DOC to 10 mgC/L above background nearly doubled respiration rate during baseflow, but did not influence respiration during snowmelt. Addition of N and P did not influence respiration. During snowmelt, respiration rate was significantly higher compared to baseflow, and was not influenced by any combination of DOC, N, P, or temperature. Results support the hypothesis that groundwater metabolism is limited by DOC availability during baseflow, and that seasonally saturated sediments overlying permanent groundwater are a major source of DOC for metabolism. |
