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In groundwater ecosystems, in situ primary production is low, and metabolism depends on organic matter inputs from other regions of the catchment. Heterotrophic metabolism and biogeochemistry in the floodplain groundwater of a headwater catchment (Rio Calaveras, New Mexico, USA) were examined to address the following questions: (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?

At Rio Calaveras, surface discharge and water table elevation increased at the onset of spring snowmelt. Groundwater biogeochemical changes in response to snowmelt included increases in dissolved oxygen and dissolved organic carbon (DOC) concentrations. Dissolved organic carbon concentration then 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 mass, and ∼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 may be important ecologically if the growth and abundance of groundwater organisms are limited by DOC availability.

The influence on groundwater heterotrophic metabolism of DOC availability, inorganic nitrogen (N), inorganic phosphorus (P), temperature, and season were assessed using laboratory manipulations of aquifer sediments and seasonal measurements in field microcosms. Augmentation with DOC (10 mgC/L above background) nearly doubled respiration rate during base flow but did not influence respiration during snowmelt. In contrast, addition of N and P did not influence respiration at any time. Respiration rate during snowmelt was significantly higher than at base flow and was not influenced by any combination of DOC, N, P, or temperature. The hypothesis that groundwater metabolism is limited by DOC availability during base flow was supported. Hydrologic linkage between soils and groundwater represents a critical flux of DOC that supports metabolism in unconfined alluvial aquifers.

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