Appendix C. Individual values of PO13C and PO15N (‰) vs. salinity (psu) for Barataria Basin (left panels) and Breton Sound (right panels).
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| FIG. C1. Individual values of PO13C and PO15N (‰) vs. salinity (psu) for Barataria Basin (left panels) and Breton Sound (right panels). For ease of comparison, the regression line for Barataria Basin data (calculated after excluding data where salinities were less than 0.1 psu) is repeated with the Breton Sound results to illustrate the differences between the two areas. Note that the influence of the Mississippi River appears in Breton Sound at low salinities (<4 psu) as elevated PO13C and PO15N values. |
With respect to PO13C (upper panels), we found the typical trend towards higher values with increasing salinities, which reflects the mixing of freshwater dissolved inorganic carbon with low δ13C (DI13C) values with high marine DI13C (Ogawa and Ogura 1997, Hellings et al. 1999), combined with an approximately 20‰ fractionation between DI13C and phytoplankton (Canuel et al. 1995, Chanton and Lewis 1999). Both absolute values and seasonal variability of PO13C in Barataria Basin were very similar to values shown for estuaries in Europe (Hellings et al. 1999), Asia (Ogawa and Ogura 1997), and North America (Coffin et al. 1994). PO15N values in Barataria Basin were usually between 4 and 6‰, a range that has been reported for several other estuaries as well (Deegan and Garritt 1997, Riera 1998, Kurata et al. 2001, Bouillon et al. 2002). Hence, stable carbon and nitrogen isotope values of POM in Barataria Basin were comparable to published literature values. Our Barataria Basin reference site generally reflected typical patterns of PO13C and PO15N values for estuarine systems. However, we observed some deviations from these general patterns, which could generally be explained by the effects of biological activity. For example, the freshest stations along Bayou Chevreuil west of Lake des Allemands occasionally had very low PO13C values. These conditions occurred during drainage events of the surrounding freshwater cypress swamps after heavy local rainfall (Wissel, personal observation). Most likely, respired CO2 that was depleted in δ13C values entered the DIC pool (Del Giorgio and France 1996) and resulted in overall lower PO13C values. Overall, one would assume that due to the close vicinity of our two study areas (only separated by the Mississippi River) strong deviations from these general patterns could be caused by the altered hydrology and freshwater sources. This expectation was met especially at the freshwater end of the Breton Sound estuary most impacted by the Mississippi River diversion (right panels, low salinity data <4 psu).
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