Analysis of long-term data from (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009) in four springs that discharge from the Upper Floridan aquifer into the Flint River (southwestern Georgia, United States) indicate aquifer and surface-water susceptibility to nutrient loading. Nitrate-N concentrations ranged from 1.74 to 3.30 mg ⁄ l, and exceeded historical levels reported for the Upper Floridan aquifer (0.26-1.52 mg ⁄ l). Statistical analyses suggest increasing nitrate-N concentration in groundwater discharging at the springs (n = 146 over eight years) and that nitrate-N concentration is influenced by a dynamic interaction between depth to groundwater (an indicator of regional hydrologic conditions) and land use. A one-time synoptic survey of 10 springs (6 springs in addition to the 4 previously mentioned) using stable isotopes generated d 15 N-NO 3 ) values (4.8-8.4& for rural springs and 7.7-13.4& for developed ⁄ urban springs) suggesting mixed sources (i.e., fertilizer, animal waste, and soil organic nitrogen) of nitrate-N to rural springs and predominantly animal ⁄ human waste to urban springs. These analyses indicate a direct relation between nitrate-N loading since the 1940s and intensification of agricultural and urban land use. This study demonstrates the importance of evaluating long-term impacts of land use on water quality in groundwater springs and in determining how rapidly these changes occur.
We examined isolated wetland food webs using stable C and N isotopes to understand resource-consumer relationships and controls on baseline isotopic signatures. Marshes were usually more 13 C-enriched than cypress savannas and cypress gum swamps. Analysis of coarse particulate organic matter (CPOM) fractions indicated that C 3 plants contributed the majority of organic matter to isolated wetlands. Individual wetlands of the same type were sometimes significantly different in δ 13 C and/or δ 15 N, suggesting that differences in stable isotopic signatures were related to baseline isotopic values rather than feeding preferences. Bi-plots were normalized using baseline values and resulting resource-consumer relationships were similar among the three wetland types. Food webs appeared to be supported by basal resources including periphyton and conditioned particulate organic matter, although indirect evidence suggested phytoplankton as an important basal resource. Differences in baseline isotopic signatures may have been partly a function of vascular plant decomposition based on significant positive relationships between the median δ 13 C of individual wetland food webs, the δ 13 C of dissolved inorganic carbon (DIC), dissolved organic carbon (DOC) and CPOM pools, and a strong positive relationship between δ 13 C-DIC and δ 13 C-DOC. Mixing model calculations showed that other mechanisms were also important for establishing baseline isotopic signatures.
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