Interannual variation in stable carbon and nitrogen isotope biogeochemistry of the Mattaponi River, Virginia

Seasonal and interannual variation of the stable carbon (C) and nitrogen (N) isotope composition of suspended particulate organic matter (POM) was measured in the brackish and tidal freshwater regions of the Mattaponi River, a tributary of the York River, Virginia, and a pristine end member on a continuum of anthropogenic modification within Chesapeake Bay. A principal components analysis indicated that seasonal variation was related to physical mixing and river discharge. Freshwater POM had high C : N (.12), depleted particulate organic carbon isotopic composition (dCPOC, 226% to 230%), and depleted particulate nitrogen isotopic composition (dNPN, 2–10%) compared to brackish water POM, which had lower C : N and enriched dCPOC (224% to 227%) and dNPN (7–15%). During high discharge events, the dCPOC was enriched, the dNPN depleted, and the C : N high relative to low discharge periods, indicating a large contribution from terrestrial-derived material. Within tidal freshwater, POM was comprised of humic-rich sediment, vascular plant matter, and phytoplankton produced in situ. Nonconservative mixing behavior was observed. Endogenously produced phytoplankton increased POC concentrations in tidal freshwater and oligohaline portions during base flows. Where estuarine and riverine POM mixed, the isotopic composition of the POM was homogenized, blurring source-specific characters observed upriver and thereby emphasizing the need to characterize the freshwater end member of estuaries carefully in order to identify POM sources. In estuaries, identifying the origin of particulate organic matter (POM) is difficult because POM is received from multiple sources, including riparian vegetation, adjacent marsh vegetation, submerged and emergent aquatic vegetation and associated epiphytes, and phytoplankton produced in situ. Early research using the stable isotope composition of estuarine POM to identify its dominant origins and fates led investigators to conclude that estuarine phytoplankton and terrestrial material were the major contributors to estuarine organic matter (OM; dissolved and particulate fractions) and that these sources fueled the heterotrophic food webs within estuaries (Parker 1964; Haines 1976). Since that time, research has revealed that many processes may contribute to the stable isotope composition of POM in aquatic ecosystems, potentially complicating the interpretation of stable isotope data. For example, the isotopic composition of the POM pool can be altered by the decomposition of vegetation due to the preferential removal of select compounds (Benner et al. 1987) or by short-term changes in the concentration and isotopic composition of nutrient pools, which can affect phytoplankton fractionation (Cifuentes et al. 1988). In complex ecosystems, such as estuarine–riverine complexes, overlapping stable isotope compositions and seasonal variability will further hinder identification of sources (e.g., Cloern et al. 2002). Despite these difficulties, it has been possible to infer major biogeochemical processes and sources of OM using stable isotopes, particularly when these processes are tracked through an entire season (e.g., Cifuentes et al. 1988; Hellings et al. 1999) or watershed (e.g., Cai et al. 1988). At the watershed scale, differences exist between the stable carbon isotope composition (d13C) of upland vegetation and marine phytoplankton because these plants utilize different carbon (C) pools that have distinct isotopic compositions (Mook and Tan 1991). The d13C of total dissolved CO2 (DIC) in the ocean is about 0% (Mook et al. 1974) because of the equilibrium fractionation of atmospheric CO2 (d13C 5 27%; Deines 1980) across the atmosphere–ocean boundary. The isotope fractionation by C3 plants is about 221%, which results in a marine phytoplankton d13C of about 221% (Tan and Strain 1983), though fractionation can vary as a result of DIC concentration, phytoplankton growth rate, and nutrient availability (O’Leary 1981; Farquhar et al. 1982; Rau et al. 1982). In contrast, riparian plants that utilize the C3 pathway have a d13C of about 228% (Smith and Epstein 1 Corresponding author. Present address: U.S. Environmental Protection Agency, National Health and Ecological Effects Research Laboratory, Mid-Continent Ecology Division, 6201 Congdon Boulevard, Duluth, Minnesota 55804 (Hoffman.Joel@ epa.gov). Acknowledgments We thank Liz Canuel for advice on sampling methodology. She and two anonymous reviewers provided helpful comments on the manuscript. We also thank David Evans for advice on statistical methods; Brian Watkins, Patricia Crewe, and Demetria Christo for field assistance; and David Harris for assistance with preparation and analysis of stable isotope samples. Robert and Pat Stephens, Cecky Ropelewski and Jerry Walker, and Rose Mary Zellner kindly allowed use of their private piers. Supported under a National Science Foundation Graduate Research Fellowship to J.C.H. and sponsored in part by NOAA Office of Sea Grant, U.S. Department of Commerce, under grant NA03OAR4170084 to the Virginia Graduate Marine Science Consortium and Virginia Sea Grant College Program, with additional support from the Wallop-Breaux program of the U.S. Fish and Wildlife Service through the Marine Recreational Fishing Advisory Board of the Virginia Marine Resources Commission (grants F-116-R-6 and 7). This is contribution 2734 of the Virginia Institute of Marine Science, The College of William and Mary. Limnol. Oceanogr., 51(5), 2006, 2319–2332 E 2006, by the American Society of Limnology and Oceanography, Inc.