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Harvard Forest Symposium Abstract 2007

• Title: Carbon dynamics in a hemlock-dominated stream ecosystem
• Primary Author: William Sobczak (College of the Holy Cross)
• Abstract:

  Headwater streams make up greater than 80% of total channel length in the United States and play important roles in regulating nutrient, organic matter, and sediment fluxes from terrestrial to downstream ecosystems. Headwater streams are common features of many upland-forested watersheds in New England, yet are not explicitly factored into forest water, carbon, and nutrient budgets. Here we report on recent efforts to examine terrestrial and stream ecosystem linkages in carbon biogeochemistry in a hemlock-dominated watershed. A prototype stream biogeochemical system (SBS) was tested at the Harvard Forest LTER. The SBS allows a suite of stream water properties to be characterized in real-time over extended periods of time thus allowing questions to be asked at a wide range of time scales. The prototype SBS was field-tested in Bigelow Brook West on the Prospect Hill research tract at Harvard Forest. Bigelow Brook West was selected because the watershed’s hydrology, stream ecology, forest composition, land-use history, and carbon sequestration have been characterized, thus aiding the interpretation of SBS data and facilitating multi-disciplinary research. The prototype system monitors stream and air temperature, pCO2 , colored dissolved organic matter, total suspended sediments, PAR, water depth, pH, and dissolved O2. Independent estimates of organic and inorganic water chemistry were used to validate and interpret SBS data.
  
  
  
  Here we report on preliminary findings from the pilot SBS at Harvard Forest to highlight the usefulness of headwater stream chemistry data to a wide-diversity of ecosystem scientists. 1) Low pH constrains dissolved inorganic carbon solute fluxes to downstream ecosystems. 2) CO2 concentrations were always supersaturated in Bigelow Brook, averaging about 6x atmospheric values in summer, thus the stream is an important source of CO2 to the atmosphere. Variation in CO2 appears to be linked to multiple mechanisms including changes in stream temperature, soil CO2, and in-stream processes. 3) Diurnal variation in stream dissolved oxygen concentrations is linked to both in situ stream processes and watershed processes, such as evapotranspiration. 4) Colored dissolved organic matter (CDOM) concentrations were tightly linked to discharge with concentrations increasing with discharge. The surprising additive effect of increasing DOC concentration with discharge cause DOC fluxes to increase non-linearly with water fluxes in Bigelow Brook. 5) Low inorganic N and P concentrations suggest that organic N and P may account for a disproportionate fraction of N and P fluxes to downstream ecosystems. Preliminary SBS data have already altered our conceptual model regarding terrestrial and aquatic linkages in the Bigelow Brook watershed and have illuminated a fundamental lack of understanding of the mechanisms regulating diurnal and storm-driven stream water chemistry in hemlock-dominated watersheds.
  
  

• Research Category: Watershed Ecology