Carbon and nitrogen isotopic compositions of particulate organic matter in four large river systems across the United States

Abstract

Abstract:Riverine particulate organic matter (POM) samples were collected bi-weekly to monthly from 40 sites in the Mississippi, Colorado, Rio Grande, and Columbia River Basins (USA) in 1996-97 and analysed for carbon and nitrogen stable isotopic compositions. These isotopic compositions and C : N ratios were used to identify four endmember sources of POM: plankton, fresh terrestrial plant material, aquatic plants, and soil organic material. This large-scale study also incorporated ancillary chemical and hydrologic data to refine and extend the interpretations of POM sources beyond the source characterizations that could be done solely with isotopic and elemental ratios. The ancillary data were especially useful for differentiating between seasonal changes in POM source materials and the effects of local nutrient sources and in-stream biogeochemical processes.Average values of υ 13 C and C : N for all four river systems suggested that plankton is the dominant source of POM in these rivers, with higher percentages of plankton downstream of reservoirs. Although the temporal patterns in some rivers are complex, the low υ 13 C and C : N values in spring and summer probably indicate plankton blooms, whereas relatively elevated values in fall and winter are consistent with greater proportions of decaying aquatic vegetation and/or terrestrial material. Seasonal shifts in the υ 13 C of POM when the C : N remains relatively constant probably indicate changes in the relative rates of photosynthesis and respiration. Periodic inputs of plant detritus are suggested by C : N ratios >15, principally on the Columbia and Ohio Rivers. The υ 15 N and υ 13 C also reflect the importance of internal and external sources of dissolved carbon and nitrogen, and the degree of in-stream processing. Elevated υ 15 N values at some sites probably reflect inputs from sewage and/or animal waste. This information on the spatial and temporal variation in sources of POM in four major river systems should prove useful in future food web and nutrient transport studies.

Distribution and sources of organic matter in surficial sediments on the shelf and slope off Tokachi, western North Pacific, inferred from C and N stable isotopes and C / N ratios

“…Suspended POM in rivers exhibits δ 13 C of -37 to -18‰, δ 15 N of -15 to 14‰ and the molar C/N ratios of 4 to 40 (Meybeck, 1982;Fry and Sherr, 1984;Wada et al, 1987;Mook and Tan, 1991;Bird et al, 1995;Ogawa and Ogura, 1997;Middelburg and Nieuwenhuize, 1998;Kendall et al, 2001;Cloern et al, 2002;Hein et al, 2003;Kao et al, 2003). The values in the Tokachi River (Table 2) were within these ranges.…”

“…δ 13 C-POC in the Tokachi River ranged from -28.8 to -25.2‰ ( Table 2), suggesting that contributions from the C4-based vascular plant organic matter was negligible. The molar C/N ratios in vascular plants, soil, and riverine phytoplankton are generally > 15, 8 to 15, and 5 to 8, respectively (Kendall et al, 2001). Because the C/N ratios of suspended POM at Stn.…”

“…Major sources of suspended POM in rivers are C3 and/or C4 vascular plants, soil organic matter and 11 phytoplankton. According to a review of Kendall et al (2001), the mean and general range of δ 13 C of C3 plants and organic matter in C3 plant-dominated soil, C4 plants and organic matter in C4 plant-dominated soil, and freshwater phytoplankton are -27‰ (-32 to -22‰), -13‰ (-16 to -9‰), and -30‰ (-42 to -24‰), respectively. δ 13 C-POC in the Tokachi River ranged from -28.8 to -25.2‰ ( Table 2), suggesting that contributions from the C4-based vascular plant organic matter was negligible.…”

“…Based on the difference of the values among the sources, stable C and nitrogen (N) isotope ratios and C/N have been widely used to elucidate the source and fate of organic matter in the 4 marine environment (Hedges and Parker, 1976;Peters et al, 1978;Fry and Sherr, 1984;Wada et al, 1987;Thornton and McManus, 1994;Ogawa and Ogura, 1997;Middelburg and Nieuwenhuize, 1998;Tyson, 1995;Naidu et al, 2000;Cloern et al, 2002;Gordon and Goñi, 2003;Goñi et al, 2003). However, it should be noted that C and N isotopic compositions and C/N ratios in marine and riverine particulate organic matter (POM) often vary seasonally (Altabet et al, 1991;Mook and Tan, 1991;Nakatsuka et al, 1992;Tyson, 1995;Kendall et al, 2001;Nakanishi and Minagawa, 2003). While reliable estimates of sedimentary organic matter sources should take into account such seasonal variations, there have only been limited numbers of studies addressing this issue (Cifuentes et al, 1988;Ogawa and Ogura, 1997;Gordon and Goñi, 2003;Goñi et al, 2003).…”

The upside-down river: Reservoirs, algal blooms, and tributaries affect temporal and spatial patterns in nitrogen and phosphorus in the Klamath River, USA

“…Timing of the processing and export of materials may also be disrupted, leading to seasonal changes in downstream productivity (Ahearn et al, 2005). For example, lentic bodies are composed of still waters and are frequently viewed as annual net sinks for phosphorus and nitrogen (Harrison et al, 2009), but are also important sub-annually by influencing the export of nutrients to downstream reaches (Wurtsbaugh et al, 2005;Kendall et al, 2001). The magnitude of these effects shifts with position in the watershed and river network (Jones, 2010;Swanson et al, 1988), as well as with relative proximity to other lentic bodies (Epstein et al, 2012;Kelly, 2001).…”