 |
|
A winning proposal for the Innovative Research Program, 2007: The Fate of Old Carbon in Stream EcosystemsInvestigators: James H. McCutchan, Jr.1 and William M. Lewis, Jr.2. Background: In forested streams, carbon pathways are influenced by strong linkages between the terrestrial and aquatic environments. Shading by riparian forests limits photosynthesis by algae in many streams, but forests also provide a subsidy of organic matter (i.e., from terrestrial vascular plants) to stream food webs. Although some heteroptrophs in shaded streams depend on algal carbon, most of the organic matter available in forested streams originates from terrestrial vascular plants (McCutchan and Lewis 2002). Carbon derived from terrestrial vascular plants enters streams in particulate form (e.g., leaf litter and large woody debris) and as dissolved organic carbon (DOC). Some DOC is produced within streams, but recent studies have shown that much of the DOC in streams comes from the watershed and this DOC often is very old (i.e., it is derived from plants that grew thousands of years ago; Raymond et al. 2004). Furthermore, the flux of DOC from the watershed far exceeds primary production in many streams (Webster and Meyer 1997). Few consumers in streams can assimilate DOC directly, but DOC assimilated by heterotrophic microbes can support production of higher trophic levels (e.g., invertebrates and fish). Stream ecologists have assumed that terrestrial DOC is unimportant to aquatic consumers. It is possible, however, that this abundant form of carbon in streams is an unrecognized source of trophic support for aquatic food webs. Studies of soil carbon suggest that the lability of terrestrial DOC is strongly affected by temperature (e.g., Carrasco et al. 2006). Thus, the relative importance of terrestrial DOC to stream food webs may vary with latitude and with elevation. In addition, changes in climate and land use may affect the flux of DOC to streams, the quality of terrestrial DOC, and ultimately the carbon base for stream food webs. Tests of atomic weapons greatly increased the level of 14C in the atmosphere and this signal has been incorporated into plant biomass over the past several decades. Much older DOC exported to streams does not have the strong 14C signal that is present in recently-fixed carbon, and 14C decays over time. Thus, 14C provides a means of tracing the fate of old versus new (recently fixed) carbon in aquatic ecosystems. Traditional methods of carbon dating require large samples, but new analytical techniques (i.e., accelerator mass spectrometry) have made it possible to analyze very small samples (e.g., <25 µMol C) as would be required for analyses of DOC, microbial biofilms, and small invertebrates. Objectives: The objectives of the proposed study are 1) to measure the 14C-age of DOC from streams that span a range of latitude, elevation, land use, and disturbance history, 2) to test the assumption that "old" DOC of terrestrial origin contributes to heterotrophic production in running waters, and 3) to support the preparation of an NSF proposal for a larger study of the fate of "old" carbon in aquatic ecosystems. Importance: Changes in climate and land use can affect the cycling of carbon in soils and also the quantity and quality of DOC delivered to running waters (Raymond et al. 2004). If terrestrial DOC represents an unrecognized source of carbon for stream consumers, accelerated export of DOC to streams and changes in the lability of DOC may have a significant effect on stream food webs. Accelerated export of DOC to streams also may greatly affect the flux of CO2 from running waters and the export of organic carbon to the world's oceans. If results of the proposed study support the hypothesis that old terrestrial carbon contributes substantially to aquatic food webs, the results of the study will form the basis for a proposal to NSF. Innovation: Recent studies of 14C by accelerator mass spectrometry demonstrate the potential to use 14C as a tracer to follow carbon pathways in aquatic ecosystems, but 14C has not previously been used for this purpose. The proposed research would build on recent advances in terrestrial biogeochemistry and would apply this new information to better understand the source and fate of terrestrial carbon in running waters. Research plan: The proposed research will be divided into three parts: 1) sample collection, 2) carbon dating of bulk DOC, and 3) carbon dating of additional samples from selected sites. Part 1 - Collection and processing of samples: Samples of water, microbial biofilms, and benthic invertebrates will be collected from 15 streams in Alaska, Colorado, Maryland, and North Carolina. Sampling locations will span a range of latitude and elevation, and also a range of watershed characteristics (e.g., land use and disturbance history). Streams in AK, MD, and NC will be sampled once during summer; streams in CO will be visited twice (once during early summer and once during fall). Bulk DOC will be isolated from each water sample, and DOC also will be separated into humic and non-humic fractions with XAD-8 resin (Kaushal and Lewis 2003). Samples of biofilm and invertebrate tissue from each site will be dried and homogenized; these samples, along with the DOC fractions, will be stored pending results from Part 2. Part 2 - Carbon dating of bulk DOC: Bulk DOC from each site will be analyzed for 14C content (Δ14C) by accelerator mass spectrometry (AMS). Results of the preliminary analyses will be used to identify a subset of seven sites for additional analyses. Part 3 - Carbon dating of DOC fractions, biofilm, and invertebrates: For each of seven sites identified in Part 2, the following samples will be analyzed: humic and non-humic DOC fractions, microbial biofilm, and three samples of benthic invertebrates. References
|
216 UCB