HR: 16:45h
AN: H14B-06    [Abstracts]
TI: Potential Contribution of Particulate Organic Matter in the Mississippi River to Hypoxia in the Gulf of Mexico
AU: * Kendall, C
EM: ckendall@usgs.gov
AF: USGS, Menlo Park, CA, United States
AU: Silva, S R
EM: srsilva@usgs.gov
AF: USGS, Menlo Park, CA, United States
AB: Episodic hypoxia in the Gulf of Mexico has drawn attention to high levels of nitrate in the Mississippi Basin that may be fueling algal productivity in the Gulf and resulting in low levels of dissolved oxygen. However, about half the N being transported in the Mississippi River is in the form of dissolved organic matter, and if much of it is bioavailable, it may be an important contributor to hypoxia in the Gulf. Since autochthonous organic matter is probably more bioavailable than terrestrial detritus, determination of (1) the relative contributions of different sources of particulate organic matter (POM) and (2) how controls on POM sources, characteristics, and lability vary both temporally and spatially, may provide critical information about other factors contributing to hypoxia. Therefore, riverine POM samples were collected bi-weekly to monthly from 30 USGS NASQAN sites in the Mississippi River Basin from 1996-2004, and analyzed for d13C, d15N, and C:N. These samples demonstrate that there is considerable temporal and spatial variation in the composition of fine-sized POM. A four-source mixing model (plankton, fresh terrestrial plant material, macrophytes, and soil organic material) is used to differentiate general sources of POM using d13C, d15N, and C:N ratios. Average values of d13C and C:N indicate that plankton and heterotrophic bacteria account for approximately half of the POM in the Basin, with higher percentages of plankton downstream of reservoirs and lower percentages in the mainstem of the river. Temporal patterns of d13C are complex but low d13C and C:N values in spring and summer suggest the occurrence of plankton blooms, whereas relatively elevated values in fall and winter are consistent with greater proportions of decaying macrophytes and/or terrestrial material. Many sites also show seasonal trends in d15N. Although soil and macrophyte detritus have overlapping d13C and C:N values, their relative contributions can be distinguished at many sites by (1) differences in d15N values and (2) strong correlations of d13C and C:N values with discharge and discharge-related parameters consistent with soil-derived materials in runoff. Periodic inputs of fresh terrestrial plant detritus at a few sites, principally on the Ohio River sites, are suggested by C:N ratios greater than 15. The d15N and d13C of POM also reflect the importance of internal and external sources of dissolved carbon and nitrogen, and the degree of in-stream processing.
DE: 1010 Chemical evolution
DE: 1045 Low-temperature geochemistry
DE: 1803 Anthropogenic effects
DE: 1806 Chemistry of fresh water
SC: Hydrology [H]
MN: 2005 Joint Assembly