tinsed with distilled water, and dried in a oven at 60°C. Then, the white muscle of each individual was ground 

 to a fine powder with a mortar and pesde to homogenize the sample. Individuals were used as samples except 

 where shrimp were too small to obtain sufficient tissue for accurate 6"C and 6'^N analyses. For larval shrimp, 

 6 or 8 individuals (10-11 mm size length) from the same sampling occasion were combined. Some of the brown 

 shrimp collected and analysed for 6"C were also measured for 6'^N. Zooplankton was collected with a (176 

 |Jm) mesh net in Nueces Bay near Rincon mouth (Fig.l). Samples of zooplankton were acidified (10% HCl) to 

 remove any residual carbonates from cuticules, rinsed with distilled water, dried in a oven at 60°C and ground 

 to a fine powder with a mortar and pesde to homogenize the sample. 



Corixidae Corix sp and mysids Mysidopsis almyra were sampled at Up Marsh at different occasions, between April 

 1995 and February 1996, by using the same procedure as for brown shrimp. Composite samples of corixidae (8 

 individuals) and mysids were prepared in the same way as for zooplankton and brown shrimp, respectively. 



For sampHng particulate organic matter (POM), 15 1 of water were collected, then filtered on precombusted 

 Whatman GF/F glass fiber filters under moderate vacuum within five hours after collection. Samples were 

 acidified (10% HCl) to remove carbonates, dried at 60°C and kept frozen (-80°C) vintil analysis. For sedimented 

 organic matter (SOM) analysis, sediment samples were taken in the Nueces River at "River site" and in the 

 Rincon Bayou marsh at "Up Marsh" and "Down Marsh" by scraping the upper 1 cm of mud. At the laboratory, 

 samples were homogeneized, dried at 60°C, ground using a mortar and a pesde, and then acidified (10% HCl) 

 to remove any inorganic carbon. These samples were not rinsed to prevent any loss of dissolved organics. They 

 were dried overnight at 50°C under a slight vacuum to evaporate the acid. Once dried, the sediment was mixed 

 with Milli-Q water, freeze-dried, ground again to a fine powder and kept frozen (-80°C) until analysis. 



At "Rincon mouth", leaves and twigs of the two dominant marine phanerogames, Spartina altemlflora and 

 Salicomia sp, were collected. For samples of terrestrial organic matter, leaves of the dominant vascular plants, 

 namely Salix sp, Fraxinus sp and the switchgtass Panicum virgatum, were collected along the Nueces River up 

 "River site". Most samples within Rincon Bayou marsh were the Gulf Cord Grass Spartina spartinae which 

 dominates along the Rincon Bayou channel. These plant samples were cleaned of epibionts, and prepared 

 similarly to shrimp muscle tissue. Blue green algal mats were collected in the Rincon Bayou channel and 

 acidified (10% HCl), rinsed with distilled water, freeze-dried, and firozen until analysis. Benthic diatoms were 

 also sampled from muddy sediments near the Rincon Bayou channel, and separated using a procedure from 

 Couch (1989) as modified by Riera & Richard (1996). Briefly, the surficial sediments with dense microalgal mats 

 was scraped and brought into the laboratory where it was spread on flat trays to a depth of about 1 cm. A 

 nylon screen (63 p.m mesh) was laid upon the sediment surface and covered with a 4 to 5 mm layer of 

 combusted silica powder (60-210 p.m). After 12 hours, the top 2 mm of the sihca powder was gendy scraped 

 and then filtered on previously combusted glass fiber filters, acidified (10% HCl), rinsed with distilled water, 

 freeze-dried, and frozen (-80''C). 



Stable Isotope Analysis 



Samples for isotope analyses were combusted at 900°C using CuO as an oxydant in evacuated quartz tubes 

 (Stump & Frazer 1973). Samples for isotope analyses were prepared as in Boutton (1991). Before the 

 purification of CO^, N^ was trapped on silica gel granules in a stopcock sample ampule and analyzed immediady 

 after CO, collection (Mariotti 1982). The carbon and nitrogen isotope ratios were measured using a Sigma 200 

 (CJS Sciences) double inlet, triple collector isotope ratio mass spectrometer. Data are expressed in the standard 

 d unit notation where 6 X = [(R,^piyR„fe^cc)-l] x 10\ widi R = '^C/''C for carbon and '*N/"N for nitrogen, 

 and reported relative to the Pee Dee Belemnite standard (PDB) for carbon and to air N^ for nitrogen. The 

 typical precision of the overall procedure (i.e., preparation plus analysis) was ± 0.1 %o for carbon and ± 0.2 %o 

 for nitrogen. 



Results 

 6"C AND 6'^15N OF POM, SOM, Sources and Invertebrates 



O'^C and o'*N of POM, SOM, plant sources and invertebrates are presented in Table 1. There was a gradient in 

 carbon isotope values from the sea to the river. At Aransas Pass, POM 6"C values were 6'^befween -24.8%o 



Appendix E ♦ E-3 



