Showing papers by "Peter A. Raymond published in 2001"
TL;DR: It is found that much of the young dissolved organic carbon can be selectively degraded over the residence times of river and coastal waters, leaving an even older and more refractory component for oceanic export.
Abstract: Global riverine discharge of organic matter represents a substantial source of terrestrial dissolved and particulate organic carbon to the oceans. This input from rivers is, by itself, more than large enough to account for the apparent steady-state replacement times of 4,00-6,000 yr for oceanic dissolved organic carbon. But paradoxically, terrestrial organic matter, derived from land plants, is not detected in seawater and sediments in quantities that correspond to its inputs. Here we present natural 14C and 13C data from four rivers that discharge to the western North Atlantic Ocean and find that these rivers are sources of old (14C-depleted) and young (14C-enriched) terrestrial dissolved organic carbon, and of predominantly old terrestrial particulate organic carbon. These findings contrast with limited earlier data that suggested terrestrial organic matter transported by rivers might be generally enriched in 14C from nuclear testing, and hence newly produced. We also find that much of the young dissolved organic carbon can be selectively degraded over the residence times of river and coastal waters, leaving an even older and more refractory component for oceanic export. Thus, pre-ageing and degradation may alter significantly the structure, distributions and quantities of terrestrial organic matter before its delivery to the oceans.
540 citations
TL;DR: In this article, a comparison of gas transfer velocity measurements in estuaries and rivers reveals a general lack of agreement among studies and physically-based predictive models and that it will be difficult to use gas exchange in rivers or estusaries to accurately mass-balance gases of interest.
Abstract: We are writing this comment to call attention to large uncertainties in the estimates of CO2 flux from rivers and estuaries, a topic that has been receiving considerable attention recently (Raymond et al. 1997, 2000; Cai and Wang 1998; Frankignoulle et al. 1998). It is our view that there are too few direct measurements of the physical component of gas exchange (e.g., the gas transfer velocity, piston velocity, gas exchange coefficient, or k) for rivers and estuaries. While studies in streams, lakes, and marine systems have progressed to the point where the gas transfer velocity can be partially predicted from physical forcing functions (O’Connor and Dobbins 1958; Cole and Caraco 1998; Wanninkhof and McGillis 1999), this is not yet the case for estuaries and rivers. A comparison of gas transfer velocity measurements in estuaries and rivers reveals a general lack of agreement among studies and physically-based predictive models. Until we have a better understanding of the magnitude and causes of variation in estuarine gas transfer velocity estimates, it will be difficult to use gas exchange in rivers or estuaries to accurately mass-balance gases of interest. The exchange of CO2 between an aquatic ecosystem and the overlying atmosphere is an area of intense interest for several reasons: aquatic systems can be significant CO2 sources or sinks on a global or regional scale (Kling et al. 1991; Quay et al. 1992; Sarmiento and Sundquist 1992; Cole et al. 1994); the magnitude and direction of an ecosystems CO2 flux can provide important clues about metabolism in a given system (Depetris and Kempe 1993; Gattuso et al. 1993; Hamilton et al. 1995;
527 citations
TL;DR: In this article, the authors summarized the available data on 14C and 13C measurements in dissolved and particulate organic carbon (DOC and POC, respectively) in these systems, and concluded that through the use of paired 14C-13C measurements, a more robust interpretation of sources, sinks, and residence times of organic matter may be attained than by using either isotope separately.
408 citations
TL;DR: In this paper, the authors measured dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and their corresponding D 14 C and d 13 C values in order to study the sources and fates of DOC in the York River Estuary (Virginia, U.S.A.).
Abstract: We measured dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), and their corresponding D 14 C and d 13 C values in order to study the sources and fates of DOC in the York River Estuary (Virginia, U.S.A.). The D 14 C and d 13 C values of DOC and DIC at the freshwater end-member indicate that during periods of moderate to high flow, riverine DOC entering the York was composed of decadal-aged terrestrially organic matter. In nearly all cases, DOC concentrations exceeded conservative mixing lines and were therefore indicative of a net DOC input flux from within the estuary that averaged 1.2 m ML 21 d 21 . The nonconservative behavior of DOC in the York River Estuary was also apparent in carbon isotopic mixing curves and the application of an isotopic mixing model. The model predicted that 20‐38% of the DOC at the mouth of the estuary was of riverine (terrestrial 1 freshwater) origin, while 38‐56% was added internally, depending on the isotopic values assigned to the internally added DOC. Measurements of D 14 C and d 13 C of DOC and DIC and marsh organic matter suggest that the internal sources originated from estuarine phytoplankton and marshes. The isotopic mixing model also indicates a significant concomitant loss (27‐45%) of riverine DOC within the estuary. Changes in DOC concentration, D 14 C-DOC, and d 13 C-DOC were also measured during incubation experiments designed to quantify the amounts, sources, and ages of DOC supporting the carbon demands of estuarine bacteria. Results of these experiments were consistent with an estuarine source of phytoplankton and marsh DOC and the preferential utilization of young ( 14 C-enriched) DOC in the low-salinity reaches of the York. However, the average removal of riverine DOC by bacteria accounts for only ;4‐19% of the riverine pool; therefore, other significant sinks for DOC exist within the estuary.
188 citations