154 
Fishery Bulletin 109(2) 
+ Bay immigrants present 
1000m 
A y 
50 
—\ 1 km 
0 Riverine immigrants present 
1 Riverine immigrants 50% or more 
w 
94° W 
93° W 
92°' W 
91° W 90° W 89° 
W 
Figure 6 
Offshore locations in the Gulf of Mexico where smaller (<125 mm) brown shrimp (Farfantepenaeus aztecus) were 
captured as immigrants from estuaries. Symbols indicate inferred origins of these shrimp. 
Table 2), a triple isotope label associated with higher 
primary productivity (see Discussion section). These 
proventriculus isotope labels were strongest in the 
more inshore of the two groups (Fig. 9), namely the 
mid-shelf hypoxic group, and S 15 N values in the pro- 
ventriculus contents were significantly higher in this 
group than in the offshore and mid-shelf transition 
group (Table 2). When all proventriculus samples 
were considered together, C and S isotopes were sig- 
nificantly (P<0.01) and linearly correlated with N 
isotopes (Fig. 10), consistent with mixing between 
two food sources across the shelf. The correlation of S 
and C isotopes for these samples also was significant 
(P<0.01, data not shown). 
The mid-shelf and offshore station groups (Fig. 8) 
differed in their patterns of trophic enrichment factors 
(TEFs) (the difference between isotopes measured in 
consumers and their diets, i.e., TEF=muscle S- proven- 
triculus 5). Average TEFs for the most offshore group 
were close to expected (Peterson and Fry, 1987) at 
2.8%c and 0.2%e, respectively, for 8 15 N and 5 34 S, but rel- 
atively high at 5.2 %c for S 13 C (Table 2). Inshore groups 
differed significantly from these offshore TEF values, 
notably with significantly lower nitrogen isotope TEF 
values for the mid-shelf groups (Table 2). For the more 
inshore of the two mid-shelf groups, average nitrogen 
isotope TEF values were unexpectedly negative (-2.2%<?) 
because many proventriculus S 15 N values were >10.7%e 
and therefore were higher than the average values 
for offshore resident shrimp (Table 2, Fig. 8, circled 
points). 
Discussion 
There are many reasons to expect strong river sup- 
port of brown shrimp production, ranging from the 
riverine construction of inshore habitats by natural 
long-term delta-building processes to more recent river 
and nutrient-enhanced primary productivity of the off- 
shore ecosystem (Deegan et al., 1986; Bierman et ah, 
1994; Green et al., 2008). Summer surface salinities 
are 20-33 psu across most of the study area owing to 
the enormous freshwater inputs from the Mississippi 
River, so that Louisiana brown shrimp exist in a river- 
influenced marine ecosystem. The river water affects 
the isotope biogeochemistry of receiving waters, adding 
nitrates with high 8 15 N and dissolved inorganic carbon 
with low 8 13 C (Fry and Allen, 2003). Primary productiv- 
ity and the wider shrimp food webs seemed to respond 
to these basal isotope changes in a straightforward way, 
with shrimp having high S 15 N and low S 13 C in the Bird’s 
Foot Delta region that was most influenced by the river. 
This same pattern of a riverine dual isotope label may 
be fairly general in human-influenced estuaries and 
was observed, for example, in shrimp from Galveston 
Bay at a low salinity station in the upper bay (Table 
1) influenced by freshwater inflows from the urban 
Houston Ship Channel. The more negative average 
S 13 C values of -18.5%c or less that characterize these 
systems seem to develop for brown shrimp in planktonic 
bays of the Gulf of Mexico when salinities are <20 psu 
(Fry, 1981, 1983). There is less local information for the 
Gulf of Mexico about the determinants of spatial 8 15 N 
