and -22.0%o while at the Nueces River site, POM 6"C ranged from -28.8 to -26.3 %o. POM 6"C at Rincon 

 Bayou mouth ranged from -27.2%o to -20.8%o. ^X'ithin Rincon Bayou marsh, POM exhibited a large range for 

 6"C, from -26.3%o to -17.4%o at Down Marsh, and from -24.1%o to -17.r/oo at Up Marsh. Corresponding 6'*N 

 values for POM ranged from 3 to 11 %o at Aransas Pass, from 8.8 to 10.8 %o at Nueces River and from 9.2 to 

 9.4 %o at Rincon Bayou mouth. Within Rincon Bayou marsh, POM 6'^N ranged from 2.6 to 9.3 %o. 6"C for 

 SOM (i.e., including benthic algae) ranged from -24.0 to -22.2%o at Nueces River and from -21.8 to -20.2 %o in 

 Rincon Bayou marsh. SOM 6'^N values were between 7.1 to 8.2 %o in Rincon Bayou marsh. At Rincon Bayou 

 mouth, o'^C values ranged from -27.8 to -26.3 %o for SaUcomia sp and from -16.2 to -13.7 %o for Spartina 

 altemifhra, tj-pical of 6'^C for C3 and C4 plants, respectively (Fry & Sherr 1984, Currin et al. 1995) and close to 

 values previously observed for Spartina sp and Salicomia sp by Creach (1997). Within Rincon Bayou marsh, 6"C 

 for Spartina spartinae ranged from -16.8 to -14.5 %o. Benthic diatoms inhabiting muddy sediments near the 

 Rincon Bayou channel had 6'^C values from -18.5 to -16.3 %o. Blue green algae had 6'^C from -15.7 to -15.9 

 %o typical of "C-enriched surfidal blue-green algal mats from Texas (Calder & Parker 1973). Blue green algae 

 were the most '*N-depleted primary producers with 6'^N from -0.7 to 1.7 %o. 6"C values for leaves of the 

 most common terrestrial vegetation along Nueces River, from -30.3 to -27.6 %o, for Fraxinus sp and Sa&x sp, 

 were typical of terrestrial C3 plants (Degens 1969). 6"C oi Panicum virgatum ranged from -15.9 to -14.6 %o 

 indicating a C4 photosynthetic pathway (Fr)' & Sherr 1984). At Rincon Bayou mouth zooplankton was "C- 

 depleted (from -26.3 to -25.6 %o) and '^N-enriched (1 1.7 %o). Within Rincon Bayou marsh Corix sp and 

 Mysidopsis almyra showed 6'^C from -20.8 to -16.2 %o and from -21.6 to -20.5 %o, respectively. Corresponding 

 6'*N values were -0.2%o for Corix sp and from 10.5 to 10.8 %o for Mysidopsis almyra. 



Size Length, 6"C and S'^N of Penaeus aztecus 



Early in the study, only small larval shrimp were found in the Aransas Pass during their migration into the bay 

 (Fig. 2). Shrimp length increased from 10-11 mm (larvae) injanviaiy 1996 to 80-90 mm (subadults) in July 1996 

 when they migrated seaward, out of the Pass. Shrimp size increased from about 40 mm to 60 mm in the nursery 

 habitat (Fig. 2). Isotope values of 6"C and 6'*N for Penaeus a^ecus also differed between i\ransas Pass and 

 Nueces River and within sites over the period Januar)' to July 1996 (Table 2). >\lthough the sampling procedure 

 was similar at each sampling occasion, the number of shrimp collected was unequal at the different sites and 

 dates, owing to abundance fluctuations throughout the migration. In particular, on the different sampUng dates, 

 brown shrimps were not found at every site (Table 2) confirming the migratory pattern of the juvenile brown 

 shrimp previously observed along the South Texas coast (Moffet 1970, Fry 1981). However, the sampling 

 procedure did not allow an accurate quantitative evaluation of shrimp abundance at the different sites over the 

 sampUng period. 



Shrimp feeding habitats and assimilated carbon and nitrogen were different in the animals at various stages of 

 the migratory life cycle. At i\ransas Pass, 6'^C values for brown shrimps varied between -21.7 and -20.7 %o for 

 larval stages (10-11 mm) in winter and from -16.4 to -12.5 %o for sub-adults (80-90 mm) in summer. 6''C 

 values for juvenile shrimps ranged from -20.6 to -14.8 %o at Rincon Bayou mouth, from -21.3 to -14.5 %o at 

 Down Marsh and from -18.4 to -15.3 %o at Up Marsh. At the Nueces River site, brown shrimp were collected 

 between mid-May and mid-lune and their 6'^C ranged from -25.2 to -20.4 %o. 6"C for Penaeus a^ecus^e.r& 

 significantly different among the sampling sites (Kruskal-WalHs test=35.3, df=4, p<0,001). At Aransas Pass, 

 o'^N values were between 5.5 and 7.7 %o for larval shrimp and between 3.3 and 9.7 %o for sub-adults. o'*N 

 ranged from 4.5 to 13.1 %o at Rincon Bayou mouth, from 1.0 to 9.9 %o at Down Marsh and from 1.2 to 5.1 %o 

 at Up Marsh. At the Nueces River site, 6"N values ranged between 6.6 and 13.9 %o. 6'^N values were 

 significandy different among the sampling sites either (Kruskal-Wallis test=36.8, df=4, p<0,001). 



DISCUSSION 



6"C Variations of Penaeus aztecus Throughout the Migration 



spatial and temporal O'^C of migrating brown shrimp exhibited variation throughout migration (Fig.3) 

 confirming the migratory pattern of juvenile brown shrimp in Texas bays previously observed (Moffet 1970, 

 Ft)' 1981). The range of 6''C values for Peneaus amicus (from -25.2 to -12.5 %o) is similar to the range of o"C 

 observed for the main sources of organic matter (from -30.3 to -13.7 %o). This observation is consistent with 



E-4 ^ Utilis^ation ofEstuarine Organic Matter During Growth 

 and Migration by juvenile Broom Shrimp 



