Organic carbon supply and metabolism in a shallow groundwater ecosystem

In groundwater ecosystems, in situ primary production is low, and metabolis m depends on organic matter inputs from other regions of the catchment. Het erotrophic metabolism and biogeochemistry in the floodplain groundwater of a headwater catchment (Rio Calaveras, New Mexico, USA) were examined to add ress the following questions: (1) How do groundwater metabolism and biogeoc hemistry vary spatially and temporally? (2) What factors influence groundwa ter 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 respons e to snowmelt included increases in dissolved oxygen and dissolved organic carbon (DOC) concentrations. Dissolved organic carbon concentration then de creased exponentially with time, suggesting that newly saturated floodplain sediments were a major source of DOG. Organic matter content in seasonally saturated sediments averaged 3% by mass, and similar to0.05 mg C/g dry sed iment was water soluble. Microorganisms from these sediments were able to c onsume an average of 45% of the leached DOG. These results show that snowme lt imports DOC to groundwater and that a substantial amount can be consumed by biota. These results may be important ecologically if the growth and ab undance 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 w ere assessed using laboratory manipulations of aquifer sediments and season al measurements in field microcosms. Augmentation with DOC (10 mgC/L above background) nearly doubled respiration rate during base flow but did not in fluence respiration during snowmelt. In contrast, addition of N and P did n ot influence respiration at any time. Respiration rate during snowmelt was significantly higher than at base flow and was not influenced by any combin ation of DOG, N, P, or temperature. The hypothesis that groundwater metabol ism is limited by DOC availability during base flow was supported. Hydrolog ic linkage between soils and groundwater represents a critical flux of DOC that supports metabolism in unconfined alluvial aquifers.